Current Events Archive

Current Events

Archive: 2003 | 2004 | 2005 | 2006

(Added 12/30/05) Astronomers have determined laid to resta problem in galactic astronomy: How far away is the Earth's closest spiral arm? New measurements that were made with the planet-wide consortium of radio telescopes, the VLBA, has found that the Perseus arm lies 1.95±0.04 kpc (6.36±0.13 thousand light-years) away from Earth.

Previous techniques involve modeling how fast stars and nebulae within the Galaxy should travel based upon their distances from the center. But, this method has many problems associated with it, with the largest being that the measured velocity from Earth can be a faulty indicator of true speed, and some clusters can have anomalously high or low velocities.

The Perseus arm, the closest spiral arm of our Galaxy to Earth, has previously had its distance measured by observing ultra-luminous stars known as Type O. The distances derived from these are approximately 7.2 thousand light-years. However, measurements based upon the velocities of material observed in the arm result in a distance of 27.7 thousand light-years.

This study used a massive star-forming region known as W3OH in the Perseus arm. The astronomers used the VLBA to measure the slight shift in its apparent location based upon the movement of Earth, known as "parallax." This is the phenomenon of aligning your index finger with a distant target, closing one eye and then switching, and the object appears to move. This is how your brain determines the 3-D nature of the world arround you, and by very careful measurements of this shift, astronomers can measure distances in our Galaxy.

The extremely accurate parallax measurement of W3OH puts the distance to the Perseus arm near the minimum, with the O stars measurements. Other implications are, based upon the accuracy of these measurements, the VLBA could be used to measure parallaxes out to a distance of about 33 thousand light-years, which is a factor of 100 better than the most accurate and comprehensive study to-date, which was made by the Hipparcos satellite.

Adapted from the information in: Xu, Y. et al. "The Distance to the Perseus Spiral Arm in the Milky Way" (2005), which is available at http://lanl.arxiv.org/pdf/astro-ph/0512223.

(Added 12/26/05) One of the thousands of minor planets orbiting the Sun has been found to have its own mini planetary system. Astronomer Franck Marchis (University of California, Berkeley) and his colleagues at the Observatoire de Paris (France) have discovered the first triple asteroid system - two small asteroids orbiting a larger one, where the main asteroid has been known since 1866 as 87 Sylvia.

"Since double asteroids seem to be common, people have been looking for multiple asteroid systems for a long time," said Marchis. "I couldn't believe we found one."

The discovery was made with Yepun, one of ESO's 8.2-m telescopes of the Very Large Telescope Array at Cerro Paranal (Chile), using the outstanding image sharpness provided by the adaptive optics NACO instrument. Via the observatory's proven "Service Observing Mode," Marchis and his colleagues were able to obtain sky images of many asteroids over a six-month period without actually having to travel to Chile.

One of these asteroids was 87 Sylvia, which was known to be double since 2001 from observations made by Mike Brown and Jean-Luc Margot with the Keck telescope. The astronomers used NACO to observe Sylvia on 27 occasions over a two-month period. On each of the images, the known small companion was seen, allowing Marchis and his colleagues to precisely compute its orbit. But on 12 of the images, the astronomers also found a closer and smaller companion, indicating that the asteroid was not double, but a triple system.

Because 87 Sylvia was named after Rhea Sylvia, the mythical mother of the founders of Rome, Marchis proposed naming the twin moons after those founders: Romulus and Remus. The International Astronomical Union approved the names.

Sylvia's moons are considerably smaller than itself, orbiting in nearly circular paths and in the same plane and direction. The closest and newly discovered moonlet, orbiting about 710 km from Sylvia, is Remus, a body only 7 km across and circling Sylvia every 33 hours. The second, Romulus, orbits at about 1360 km in 87.6 hours and measures about 18 km across.

The asteroid 87 Sylvia is one of the largest known from the main asteroid belt, and it is located about 3.5 times further away from the Sun than Earth. The wealth of details provided by the NACO images show that 87 Sylvia is shaped like a lumpy potato, measuring 380 x 260 x 230 km. It is spinning at a rapid rate, once every 5 hours and 11 minutes.

The observations of the moonlets' orbits allow the astronomers to precisely calculate the mass and density of Sylvia. With a density only 20% higher than the density of water, it is likely composed of water ice and rubble from a primordial asteroid. "It could be up to 60% empty space," remarked co-discoverer Daniel Hestroffer (Observatoire de Paris, France).

"It is most probably a 'rubble-pile' asteroid," Marchis added. These asteroids are loose aggregations of rock, presumably the result of a collision. Two asteroids smacked into each other and got disrupted. The new rubble-pile asteroid formed later by accumulation of large fragments while the moonlets are probably debris left over from the collision that were captured by the newly formed asteroid and eventually settled into orbits around it. "Because of the way they form, we expect to see more multiple asteroid systems like this."

Marchis and his colleagues reported their discovery in the August 11 issue of the journal Nature, simultaneously with an announcement that day at the Asteroid Comet Meteor conference in Armação dos Búzios, Rio de Janeiro state, Brazil.

Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-21-05.html.

(Added 12/26/05) On April 30, an international team of astronomers reported confirmation of the discovery of a giant planet, approximately five times the mass of Jupiter, that is gravitationally bound to a young brown dwarf. This puts an end to a year-long discussion on the nature of this object, which started with the detection of a red object close to the brown dwarf.

In February and March of this year, the astronomers took new images of the young brown dwarf and its giant planet companion with the NACO instrument on ESO's Very Large Telescope in northern Chile. The planet is near the southern constellation of Hydra and approximately 200 light-years from Earth.

"Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system," explained Gael Chauvin, astronomer at ESO and leader of the team of astronomers who conducted the study.

"The two objects - the giant planet and the young brown dwarf - are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding," remarked Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA's Astrobiology Institute, and a member of the team. "I'm more than 99% confident." The separation between the planet and the brown dwarf is 55 times the separation of the Earth and Sun.

Anne-Marie Lagrange, another member of the team from the Grenoble Observatory in France, looked towards the future: "Our discovery represents a first step towards one of the most important goals of modern astrophysics: To characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."

Last September, the same team of astronomers reported a faint reddish speck of light in the close vicinity of a young brown dwarf. The feeble object, now called 2M1207b, is more than 100 times fainter than the brown dwarf, 2M1207A. The spectrum of 2M1207b presents a strong signature of water molecules, confirming that it must be cold. Based on the infrared colours and the spectral data, evolutionary model calculations led to the conclusion that 2M1207b is a 5 Jupiter-mass planet. Its mass can be estimated also by use of a different method that focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than 5 Jupiters.

At the time of its discovery in April 2004, it was impossible to prove that the faint source is not a background object (such as an unusual galaxy or a peculiar cool star with abnormal infrared colours), even though this appeared very unlikely. Observations with the Hubble Space Telescope, obtained in August 2004, corroborated the VLT/NACO observations, but were taken too soon after the NACO ones to conclusively demonstrate that the faint source is a planet.

The new observations show with high confidence that the two objects are moving together and hence are gravitationally bound. "Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," says Gael Chauvin. "Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust."

Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-12-05.html.

(Added 12/26/05) Stars are generally born in small groups, mostly in open clusters that typically contain a few hundred stars. From a wide range of observations, astronomers infer that the Sun itself was born in one such cluster, some 4.5 billionyears ago. In some active ("starburst") galaxies, scientists have observed violent episodes of star formation, leading to the development of super star clusters, each containing several million stars. Such events were common during the Milky Way's childhood, more than 12 billion years ago: The many galactic globular clusters - which are nearly as old as our Galaxy - are thought to be the remnants of early super star clusters.

All super star clusters so far observed in starburst galaxies are very distant. It is not possible to distinguish their individual stars, even with the most advanced technology. This dramatically complicates their study and astronomers have therefore long been eager to find such clusters in our neighbourhood in order to probe their structure in much more detail. Now, a team of European astronomers has finally succeeded in doing so, using several of ESO's telescopes at the La Silla observatory (Chile).

The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970-74). This cluster is behind a huge interstellar cloud of gas and dust which blocks most of its visible light. The dimming factor is more than 100,000 -- and this is why it has taken so long to uncover the true nature of this particular cluster.

In 2001, the team of astronomers identified more than a dozen extremely hot and peculiar massive stars in the cluster called "Wolf-Rayet" stars. They have since studied Westerlund 1 extensively with various ESO telescopes. From these observations, they were able to identify about 200 cluster member stars. To establish the true nature of these stars, the astronomers then performed spectroscopic observations of about one quarter of them. These observations have revealed a large population of very bright and massive, quite extreme stars. Some would fill the solar system space within the orbit of Saturn, and others are as bright as a million Suns.

Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population and a true astronomical bonanza. All stars identified are evolved and very massive, spanning the full range of stellar oddities from Wolf-Rayet stars, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns) and Luminous Blue Variables (similar to the exceptional Eta Carinae object).

All stars so far analysed in Westerlund 1 weigh at least 30-40 times more than the Sun. Because such stars have a rather short life - astronomically speaking - Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years.

"If the Sun were located at the heart of Westerlund 1, the sky would be full of stars, many of them brighter than the full Moon", commented Ignacio Negueruela of the Universidad de Alicante in Spain and member of the team. The large quantity of very massive stars implies that Westerlund 1 must contain a huge number of stars. "In our Galaxy," explained Simon Clark of the University College London (UK) and one of the authors of this study, "there are more than 100 solar-like stars for every star weighing 10 times as much as the Sun. The fact that we see hundreds of massive stars in Westerlund 1 means that it probably contains close to half a million stars, but most of these are not bright enough to peer through the obscuring cloud of gas and dust". This is ten times more than any other known young cluster in the Milky Way.

This super star cluster now provides astronomers with a unique perspective towards one of the most extreme environments in the Universe. Westerlund 1 will certainly provide new opportunities in the long-standing quest for more and finer details about how stars, and especially massive ones, do form.

Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-08-05.html.

(Added 12/26/05) The supermassive black hole at the center of the Milky Way has surprisingly helped spawn a new generation of stars, according to observations from NASA's Chandra X-ray Observatory. This novel mode of star formation may solve several mysteries about the supermassive black holes that reside at the centers of nearly all galaxies.

"Massive black holes are usually known for violence and destruction," remarked Sergei Nayakshin of the University of Leicester, United Kingdom, and coauthor of a paper on this research in an upcoming issue of the Monthly Notices of the Royal Astronomical Society. "So it's remarkable that this black hole helped create new stars, not just destroy them."

Black holes have earned their fearsome reputation because any material -- including stars -- that falls within the event horizon is never seen again. However, these new results indicate that the immense disks of gas known to orbit many black holes at a "safe" distance from the event horizon can help nurture the formation of new stars.

This conclusion came from new clues that could only be revealed in x-rays. Until the latest Chandra results, astronomers have disagreed about the origin of a mysterious group of massive stars discovered by infrared astronomers to be orbiting less than a light year from the Milky Way's central black hole, AKA Sagittarius A*, or Sgr A*. At such close distances to Sgr A*, the standard model for star formation predicts that gas clouds from which stars form should have been ripped apart by tidal forces from the black hole.

Two models to explain this puzzle have been proposed. In the disk model, the gravity of a dense disk of gas around Sgr A* offsets the tidal forces and allows stars to form; in the migration model, the stars formed in a star cluster far away from the black hole and migrated in to form the ring of massive stars. The migration scenario predicts about a million low mass, sun-like stars in and around the ring, whereas in the disk model, the number of low mass stars could be much less.

Nayakshin and his coauthor, Rashid Sunyaev of the Max Plank Institute for Physics in Garching, Germany, used Chandra observations to compare the x-ray glow from the region around Sgr A* to the x-ray emission from thousands of young stars in the Orion Nebula star cluster. They found that the Sgr A* star cluster contains only about 10,000 low mass stars, thereby ruling out the migration model.

"We can now say that the stars around Sgr A* were not deposited there by some passing star cluster, rather they were born there," explained Sunyaev. "There have been theories that this was possible, but this is the first real evidence. Many scientists are going to be very surprised by these results." Because the Galactic Center is shrouded in dust and gas, it has not been possible to look for the low-mass stars in optical observations. In contrast, X-ray data have allowed astronomers to penetrate the veil of gas and dust and look for these low mass stars.

The results suggest that the "rules" of star formation change when stars form in the disk of a giant black hole. Because this environment is very different from typical star formation regions, there is a change in the proportion of stars that form. For example, there is a much higher percentage of massive stars in the disks around black holes. And, when these massive stars explode as supernovae, they will "fertilize" the region with heavy elements such as oxygen. This may explain the large amounts of such elements observed in the disks of young supermassive black holes.

Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_101305.html.

(Added 12/26/05) NASA's Chandra X-ray Observatory survey of nearby sun-like stars suggests there is nearly three times more neon in the sun and local universe than previously believed. If true, this would solve a critical problem with understanding how the sun works.

"We use the sun to test how well we understand stars and, to some extent, the rest of the universe," explained Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "But in order to understand the sun, we need to know exactly what it is made of," he added. It is not well known how much neon the sun contains. This is critical information for creating theoretical models of the sun. Neon atoms, along with carbon, oxygen and nitrogen, play an important role in how quickly energy flows from nuclear reactions in the sun's core to its edge, where it then radiates into space.

The rate of this energy flow determines the location and size of a crucial stellar region called the Convection Zone. The zone extends from near the sun's surface inward approximately 125,000 miles. The zone is where the gas undergoes a rolling, convective motion much like the unstable air in a thunderstorm. "This turbulent gas has an extremely important job because nearly all of the energy emitted at the surface of the sun is transported there by convection," Drake described.

The accepted amount of neon in the sun has led to a paradox. The predicted location and size of the solar convection zone disagree with those deduced from solar oscillations. Solar oscillations - known also as "helioseismology" - is a technique astronomers previously relied on to probe the sun's interior. Several scientists have noted the problem could be fixed if the abundance of neon is in fact about three times larger than currently accepted.

Attempts to measure the precise amount of neon in the Sun have been frustrated by a quirk of nature; neon atoms in the Sun give off no signatures in visible light. However, in a gas heated to millions of degrees, neon shines brightly in x-rays. Stars like the sun are covered in this super-heated gas that is betrayed by the white corona around them during solar eclipses. However, observations of the sun's corona are very difficult to analyze.

To probe the neon content, Drake and his colleague Paola Testa of the Massachusetts Institute of Technology in Cambridge, AM, observed 21 sun-like stars within a distance of 400 light-years from Earth. These local stars and the sun should contain about the same amount of neon when compared to oxygen. However, these close stellar kin were found to contain on average almost three times more neon than is believed for the sun. "Either the sun is a freak in its stellar neighborhood, or it contains a lot more neon than we think," Testa said.

These Chandra results reassured astronomers the detailed physical theory behind the solar model is secure. Scientists use the model of the sun as a basis for understanding the structure and evolution of other stars, as well as many other areas of astrophysics.

"If the higher neon abundance measured by Drake and Testa is right, then it is a simultaneous triumph for Chandra and for the theory of how stars shine," said John Bahcall of the Institute for Advanced Study, Princeton, NJ. Bahcall is an expert in the field who was not involved in the Chandra study. Drake is lead author of the study published in this week's issue of the journal Nature.

Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_072705.html.

(Added 12/26/05) New results from NASA's Chandra X-ray Observatory imply that X-ray super-flares torched the young Solar System. Such flares likely affected the planet-forming disk around the early Sun, and may have enhanced the survival chances of Earth.

By focusing on the Orion Nebula (M42) almost continuously for 13 days, a team of scientists used Chandra to obtain the deepest X-ray observation ever taken of this or any star cluster. The Orion Nebula is the nearest rich stellar nursery, located just 1,500 light years away.

These data provide an unparalleled view of 1400 young stars, 30 of which are prototypes of the early Sun. The scientists discovered that these young suns erupt in enormous flares that dwarf - in energy, size, and frequency - anything seen from the Sun today.

"We don't have a time machine to see how the young Sun behaved, but the next-best thing is to observe Sun-like stars in Orion," explained Scott Wolk of Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "We are getting a unique look at stars between one and 10 million years old - a time when planets form."

A key result is that the more violent stars produce flares that are a hundred times as energetic as the more docile ones. This difference may specifically affect the fate of planets that are relatively small and rocky, like the Earth. "Big X-ray flares could lead to planetary systems like ours where Earth is a safe distance from the Sun," remarked Eric Feigelson of Penn State University in University Park, and principal investigator for the international Chandra Orion Ultradeep Project. "Stars with smaller flares, on the other hand, might end up with Earth-like planets plummeting into the star."

Affects of a Flare on a Protoplanetary Disk

According to recent theoretical work, X-ray flares can create turbulence when they strike planet-forming disks, and this affects the position of rocky planets as they form. Specifically, this turbulence can help prevent planets from rapidly migrating towards the young star. "Although these flares may be creating havoc in the disks, they ultimately could do more good than harm," said Feigelson. "These flares may be acting like a planetary protection program."

About half of the young suns in Orion show evidence for disks, likely sites for current planet formation, including four lying at the center of proplyds (proto-planetary disks) imaged by Hubble Space Telescope. X-ray flares bombard these planet-forming disks, likely giving them an electric charge. This charge, combined with motion of the disk and the effects of magnetic fields should create turbulence in the disk.

The numerous results from the Chandra Orion Ultradeep Project appeared in a dedicated issue of The Astrophysical Journal Supplement in October, 2005. The team contains 37 scientists from institutions across the world including the US, Italy, France, Germany, Taiwan, Japan and the Netherlands.

Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_051005.html.

(Added 12/26/05) The most massive stars in our galaxy weigh as much as 100 small stars like the Sun. How do such monsters form? Do they grow rapidly by swallowing smaller protostars within crowded star-forming regions? Some astronomers thought so, but a new discovery suggests instead that massive stars develop through the gravitational collapse of a dense core in an interstellar gas cloud via processes similar to the formation of low mass stars.

"In the past, theorists have had trouble modeling the formation of high-mass stars and there has been an ongoing debate between the merger versus the accretion scenarios." explained astronomer Nimesh Patel of the Harvard-Smithsonian Center for Astrophysics (CfA). "We've found a clear example of an accretion disk around a high-mass protostar, which supports the latter while providing important observational constraints to the theoretical models."

Patel and his colleagues studied a young protostar 15 times more massive than the Sun, located more than 2,000 light-years away in the constellation Cepheus. They discovered a flattened disk of material orbiting the protostar. The disk contains 1 to 8 times as much gas as the Sun and extends outward for more than 30 billion miles - eight times farther than Pluto's orbit.

The existence of this disk provides clear evidence of gravitational collapse, the same gradual process that built the Sun. A disk forms when a spinning gas cloud contracts, growing denser and more compact. The angular momentum of the spinning material forces it into a disk shape. The planets in our solar system formed from such a disk 4.5 billion years ago.

Evidence in favor of high-mass accretion has been elusive since massive stars are rare and evolve quickly, making them tough to find. Patel and his colleagues solved this problem using the Submillimeter Array (SMA) telescope in Hawaii, which offers much sharper and highly sensitive imaging capabilities compared to single-dish submillimeter telescopes. SMA is currently a unique instrument that makes such studies possible by allowing astronomers to directly image the dust emission at submillimeter wavelengths and also to detect emission from highly excited molecular gas.

The team detected both molecular gas and dust in a flattened structure surrounding the massive protostar HW2 within the Cepheus A star formation region. SMA data also showed a velocity shift due to rotation, supporting the interpretation that the structure is a gravitationally bound disk. Combined with radio observations showing a bipolar jet of ionized gas, a type of outflow often observed in association with low-mass protostars, these results support theoretical models of high-mass star formation via disk accretion rather than by the merging of several low-mass protostars.

"Merging low-mass protostars wouldn't form a circumstellar disk and a bipolar jet," said co-author Salvador Curiel of the National Autonomous University of Mexico (UNAM), who is on sabbatical leave at CfA. "Even if they had circumstellar disks and outflows before the merger, those features would be destroyed during the merger."

Adapted from the information on http://cfa-www.harvard.edu/press/pr0527.html.

(Added 12/26/05) In 1987, earthbound observers saw a star explode in the nearby dwarf galaxy called the Large Magellanic Cloud. Astronomers eagerly studied this supernova - the closest seen in the past 300 years - and have continued to examine its remains. Although its blast wave has lit up surrounding clouds of gas and dust, the supernova appears to have left no core behind. Astronomers now report that even the sharp eyes of the Hubble Space Telescope failed to locate the black hole or ultracompact neutron star they believe was created by the star's death 18 years ago.

"We think a neutron star was formed. The question is: Why don't we see it?" asked astronomer Genevieve Graves of UC Santa Cruz, first author on the paper announcing these results.

When a massive star explodes, it leaves behind some sort of compact object, either a city-sized ball of subatomic particles called a neutron star, or a black hole. The outcome depends on the mass of the progenitor star. Smaller stars form neutron stars while larger stars form black holes.

The progenitor of supernova (SN) 1987A weighed 20 times as much as the sun, placing it right on the dividing line and leaving astronomers uncertain about what type of compact object it produced. All observations to date have failed to detect a light source in the center of the supernova remnant, leaving the question of the outcome unanswered.

Detecting a black hole or neutron star is challenging. A black hole can be detected only when it swallows matter because the matter heats up and emits light as it falls into the black hole. A neutron star at the distance of the Large Magellanic Cloud can be detected only when it emits beams of radiation as a pulsar, or when it accretes hot matter like a black hole.

"A neutron star could just be sitting there inside SN 1987A, not accreting matter and not emitting enough light for us to see," mused astronomer Peter Challis (CfA), second author on the study.

Observations have ruled out the possibility of a pulsar within SN 1987A. Even if the pulsar's beams were not aimed at Earth, they would light the surrounding gas clouds. However, theories predict that it can take anywhere from 100 to 100,000 years for a pulsar to form following a supernova because the neutron star must gain a sufficiently strong magnetic field to power the pulsar beam. SN 1987A may be too young to hold a pulsar.

As a result, the only way astronomers might detect the central object is to search for evidence of matter accreting onto either a neutron star or a black hole. That accretion could happen in one of two ways: Spherical accretion in which matter falls in from all directions, or disk accretion in which matter spirals inward from a disk onto the compact object.

The Hubble data rule out spherical accretion because light from that process would be bright enough to detect. If disk accretion is taking place, the light it generates is very faint, meaning that the disk itself must be quite small in both mass and radial extent. Also, the lack of detectable radiation indicates that the disk accretion rate must be extremely low - less than about one-fifth the mass of the Moon per year.

In the absence of a definitive detection, astronomers hope to learn more about the central object by studying the dust clouds surrounding it. That dust absorbs visible and ultraviolet light and re-radiates the energy at infrared wavelengths. "By studying that reprocessed light, we hope to find out what's powering the supernova remnant and lighting the dust," explained Graves. Future observations by NASA's Spitzer Space Telescope should provide new clues to the nature of the hidden object. Additional observations by Hubble also could help solve the mystery. "Hubble is the only existing facility with the resolution and sensitivity needed to study this problem," said Kirshner.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0515.html, and based upon a research paper online at http://arxiv.org/abs/astro-ph?0505066.

(Added 12/26/05) When the distant planetoid Sedna was discovered on the outer edges of our solar system, it posed a puzzle to scientists. Sedna appeared to be spinning very slowly compared to most solar system objects, completing one rotation every 20 days. Astronomers hypothesized that this world possessed an unseen moon whose gravity was slowing Sedna's spin. Yet Hubble Space Telescope images showed no sign of a moon large enough to affect Sedna.

New measurements by Scott Gaudi, Krzysztof (Kris) Stanek and colleagues at the Harvard-Smithsonian Center for Astrophysics (CfA) have cleared up this mystery by showing that a moon wasn't needed after all. Sedna is rotating much more rapidly than originally believed, spinning once on its axis every 10 hours. This shorter rotation period is typical of planetoids in our solar system, requiring no external influences to explain. "We've solved the case of Sedna's missing moon. The moon didn't vanish because it was never there to begin with," remarked Gaudi.

Sedna is an odd world whose extreme orbit takes it more than 45 billion miles from the Sun, or more than 500 A.U. (where one astronomical unit (A.U.) is the average Earth-Sun distance of 93 million miles). Sedna never approaches the Sun any closer than 80 A.U., and it takes 10,000 years to complete one orbit. In comparison, Pluto's 248-year-long oval orbit takes it between 30 and 50 A.U. from the Sun.

"Up until now, Sedna appeared strange in every way it had been studied. Every property of Sedna that we'd been able to measure was atypical," said Gaudi. "We've shown that Sedna's rotation period, at least, is entirely normal."

Sedna appears unusual in other ways besides its orbit. First and foremost, it is one of the largest known "minor planets," with an estimated size of 1,000 miles compared to Pluto's 1,400 miles. Sedna also displays an unusually red color that is still unexplained.

Initial measurements indicated that Sedna's rotation period was also extreme - extremely long compared to other solar system residents. By measuring small brightness fluctuations, scientists estimated that Sedna rotated once every 20-40 days. Such slow rotation likely would require the presence of a nearby large moon whose gravity could apply the brakes and slow Sedna's spin. As a result of this interpretation, artist's concepts released when Sedna's discovery was announced showed a companion moon. One month later, images taken by NASA's Hubble Space Telescope demonstrated that no large moon existed.

In true detective fashion, Gaudi and his colleagues re-investigated the matter by observing Sedna using the new MegaCam instrument on the 6.5-meter-diameter MMT Telescope at Mount Hopkins, Ariz. They measured Sedna's brightness looking for telltale, periodic brightening and dimming that would show how fast Sedna rotates. As noted by Matthew Holman, one of the members of the CfA team, "The variation in Sedna's brightness is quite small and could have been easily overlooked." Their data fits a computer model in which Sedna rotates once every 10 hours or so. The team's measurements definitively rule out a rotation period shorter than 5 hours or longer than 10 days.

While these data solve one mystery of Sedna, other mysteries remain. Chief among them is the question of how Sedna arrived in its highly elliptical, eons-long orbit. "Theorists are working hard to try to figure out where Sedna came from," said Gaudi. Astronomers will continue to study this strange world for some time to come.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0510.html, and based upon a research paper online at http://arxiv.org/abs/astro-ph/0503673.

(Added 12/26/05) Two teams of astronomers announced that they have directly detected light from two known planets orbiting distant stars. This discovery opens a new frontier in the study of extrasolar planets. Researchers now can directly measure and compare such planetary characteristics as color, reflectivity, and temperature.

A team led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics (CfA) published their detection of the planet TrES-1 in the June 20 issue of The Astrophysical Journal. A team led by Drake Deming of the Goddard Space Flight Center (GSFC) published their observations of the planet HD 209458b in the March 22 online issue of Nature.

"It's an awesome experience to realize we are seeing the glow of distant worlds," said Charbonneau. "When I first saw the data, I was ecstatic." Each of the two target planets periodically crosses in front of and behind its star. When in front, the planet partially eclipses the star and blocks a small portion of the star's light. Similarly, the system dims slightly when the planet disappears behind its star since the star blocks the planet's light. By observing this "secondary eclipse," astronomers can tease out the faint signal of the planet from the overwhelming light of the nearby star.

Charbonneau and his colleagues used the Infrared Array Camera (IRAC), a Smithsonian-developed instrument aboard NASA's Spitzer Space Telescope, to observe TrES-1 in the infrared region of the spectrum. Deming and his associates used Spitzer's Multiband Imaging Photometer for Spitzer (MIPS) to observe HD 209458b.

"Planets like TrES-1 are tiny and faint compared to their stars, but the one thing they can't hide is their heat," explained Charbonneau. "We are like detectives. Previous clues told us the planet must be there, so we put on our 'infrared goggles' and suddenly, it popped into view." Infrared offers an advantage because the star outshines the planet by a factor of 10,000 in visible light, while in the infrared the star is only about 400 times brighter, making it easier to pick out a planet's feeble light. Astronomers compare the challenge to trying to spot a firefly buzzing next to a searchlight.

Using Spitzer data combined with previous measurements, Charbonneau and his colleagues confirmed that TrES-1, which orbits its star at a distance of 4 million miles, has a temperature of about 1,450 °F (1060 K). They also calculated that the planet has a reflectivity of only 31%, meaning it absorbs the majority of the star's light that falls on it.

CfA researcher Guillermo Torres modeled the dynamics of the TrES-1 system to constrain the planet's orbit. He determined that the orbit has been made very nearly circular by the tidal effect of the nearby star, as expected.

Charbonneau is quick to point out that the achievement of directly detecting an extrasolar planet's light is only the beginning. "We've caught our first 'firefly.' Now we want to study a swarm of them." Astronomers expect the Trans-Atlantic Exoplanet Survey (TrES) network, which spotted TrES-1, to locate additional "hot Jupiters." That ground-based network is designed to spot planets orbiting bright stars, which can be more easily studied with Spitzer and other instruments. By comparing many "hot Jupiter" planets, researchers hope to determine what gases their atmospheres contain and how their composition was affected by when and how they formed.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0509.html.

(Added 12/26/05) A monstrous cosmic explosion last December showed that Earth is in more danger from real-life space threats than from hypothetical alien invasions. The gamma-ray flare, which briefly outshone the full moon, occurred within the Milky Way galaxy. Even at a distance of 50,000 light-years, the flare disrupted Earth's ionosphere. If such a blast happened within 10 light-years of Earth, it would destroy the much of the ozone layer, causing extinctions due to increased radiation.

"Astronomically speaking, this explosion happened in our backyard. If it were in our living room, we'd be in big trouble!" explained Bryan Gaensler (Harvard-Smithsonian Center for Astrophysics), lead author on a paper describing radio observations of the event. Gaensler headed one of two teams reporting on this eruption at a special press event today at NASA headquarters. A multitude of papers are planned for publication.

The giant flare detected on December 27, 2004, came from an isolated, exotic neutron star within the Milky Way. The flare was more powerful than any blast previously seen in our galaxy. "This might be a once-in-a-lifetime event for astronomers, as well as for a neutron star," remarked David Palmer of Los Alamos National Laboratory, lead author on a paper describing space-based observations of the burst. "We know of only two other giant flares in the past 35 years, and this December event was one hundred times more powerful."

NASA's newly launched Swift satellite and the NSF-funded Very Large Array (VLA) were two of many observatories that observed the event, arising from neutron star SGR 1806-20, about 50,000 light years from Earth in the constellation Sagittarius.

Neutron stars form from collapsed stars. They are dense, fast-spinning, highly magnetic, and only about 15 miles in diameter. SGR 1806-20 is a unique neutron star called a magnetar, with an ultra-strong magnetic field capable of stripping information from a credit card at a distance halfway to the Moon. Only about 10 magnetars are known among the many neutrons stars in the Milky Way.

"Fortunately, there are no magnetars anywhere near the Earth. An explosion like this within a few trillion miles could really ruin our day," said graduate student Yosi Gelfand (CfA), a co-author on one of the papers. The magnetar's powerful magnetic field generated the gamma-ray flare in a violent process known as magnetic reconnection, which releases huge amounts of energy. The same process on a much smaller scale creates solar flares. "This eruption was a super-super-super solar flare in terms of energy released," added Gaensler.

Using the VLA and three other radio telescopes, Gaensler and his team detected material ejected by the blast at a velocity three-tenths the speed of light. The extreme speed, combined with the close-up view, yielded changes in a matter of days. Spotting such a nearby gamma-ray flare offered scientists an incredible advantage, allowing them to study it in more detail than ever before. "We can see the structure of the flare's aftermath, and we can watch it change from day to day. That combination is completely unprecedented," said Gaensler.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0506.html.

(Added 12/26/05) Using the MMT Observatory in Tucson, AZ, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) are the first to report the discovery of a star leaving our galaxy, speeding along at over 1.5 million miles per hour (2.4 million km/hr). This incredible speed likely resulted from a close encounter with the Milky Way's central black hole, which flung the star outward like a stone from a slingshot. So strong was the event that the speedy star eventually will be lost altogether, traveling alone in the blackness of intergalactic space.

"We have never before seen a star moving fast enough to completely escape the confines of our galaxy," said co-discoverer Warren Brown (CfA). "We're tempted to call it the outcast star because it was forcefully tossed from its home." The star, catalogued as SDSS J090745.0+24507, once had a companion star. However, a close pass by the supermassive black hole at the galaxy's center trapped the companion into orbit while the speedster was violently flung out. Astronomer Jack Hills proposed this scenario in 1998, and the discovery of the first expelled star seems to confirm it. "Only the powerful gravity of a very massive black hole could propel a star with enough force to exit our galaxy," explained Brown.

While the star's speed offers one clue to its origin, its path offers another. By measuring its line-of-sight velocity, it suggests that the star is moving almost directly away from the galactic center. Its composition and age provide additional proof of the star's history. The fastest star contains many elements heavier than hydrogen and helium, which astronomers collectively call metals. Less than 80 million years were needed for the star to reach its current location, which is consistent with its estimated age.

The star is traveling twice as fast as galactic escape velocity, meaning that the Milky Way's gravity will not be able to hold onto it. Like a space probe launched from Earth, this star was launched from the galactic center onto a never-ending outward journey. It faces a lonely future as it leaves our galaxy, never to return.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0505.html.

(Added 12/26/05) Using the Spitzer Space Telescope, a team of astronomers led by Kevin Luhman (Harvard-Smithsonian Center for Astrophysics) has discovered a protoplanetary disk around a surprisingly low-mass brown dwarf. This remarkable finding raises the possibility of planet formation around objects that themselves have planetary masses. Moreover, the presence of a disk suggests that terrestrial planets could form and thrive while orbiting an object too small to shine via nuclear fusion.

"It's an exciting possibility -- one that hasn't been explored extensively because this is the first evidence for the building blocks of planets around such a small object," explained Luhman. The team's findings were published in the February 10 issue of The Astrophysical Journal Letters.

The brown dwarf in question, OTS 44, is located approximately 500 light-years away in the southern constellation Chamaeleon. OTS 44 weighs in at around 15 Jupiter masses, placing it near the dividing line between brown dwarfs (generally defined as objects of 15-70 Jupiter masses) and planets. At a temperature of 3,600 °F (2300 Kelvin), OTS 44 is the coolest and least massive brown dwarf known to have a circumstellar disk.

Although the team cannot measure the total mass of the disk, it likely contains enough matter to make one small gas giant or several Earth-sized planets. "This brown dwarf and its disk could eventually evolve into a miniature version of our solar system," remarked Luhman. Due to the brown dwarf's low temperature, an Earth-sized world would have to orbit much closer to the brown dwarf than the Earth from the Sun in order to be as warm as Earth. Theorists estimate that liquid water could exist on the surface of a planet about 1 to 4 million miles from the brown dwarf. The disk of OTS 44 extends beyond both sides of this "habitable zone."

Without nuclear fusion to sustain it, the brown dwarf will gradually cool and dim. If an Earth-sized world forms near the brown dwarf, it will be scorching at first, then grow cooler and more hospitable over time. Since the brown dwarf cools more slowly as it gets older, such a planet could remain in the habitable zone for an extended time, raising the intriguing possibility that life might evolve.

The researchers plan to search for similar disks around other nearby brown dwarfs. Spitzer revealed the disk of OTS 44 in only 20 seconds of observing time. Further searches may locate similar disks around even smaller central objects of 10 Jupiter masses or less. The team detected OTS 44's circumstellar disk using Spitzer's Infrared Array Camera, or IRAC. IRAC data showed an excess of infrared emission at long wavelengths-the signature of a dusty disk that absorbs radiation from the brown dwarf, heats up, and re-radiates the energy in the infrared.

Adapted from the information on http://cfa-www.harvard.edu/press/pr0504.html.

(Added 12/23/05) To the surprise of astronomers, NASA's Hubble Space Telescope has photographed a pair of new rings around the distant planet Uranus. The largest is twice the diameter of the planet's previously known rings. The new rings are so far away that they are being called Uranus's "second ring system."

In addition, Hubble has spied two small satellites, one sharing its orbit with one of the newly discovered rings. Even more surprisingly, precise analysis of the data reveals that the orbits of Uranus's family of inner moons have changed significantly in the last decade. Collectively, these new discoveries mean that Uranus has a densely packed, rapidly changing, and possibly unstable dynamical system of orbiting bodies. "The new discoveries dramatically demonstrate that Uranus has a youthful and dynamic system of rings and moons," remarked Mark Showalter of the SETI Institute. "Until now, nobody had a clue the rings were there, we had no right to expect them."

Uranian Rings R/2003 U1 and U2, and Moons Mab and Cupid

The image above shows two sets of images taken with Hubble, one in 2003 and the other in 2005. The fully annotated image shows the two new rings, given the temporary designations R/2003 U1 and U2, and the two new moons, which have already been named Mab and Cupid.

Since dust in such an orbit is expected to be depleted by spiraling away, the rings must be continually replenished with fresh material. Showalter and collaborator Jack Lissauer of the NASA Ames Research Center propose that the outermost ring is replenished by a 12-mile-wide companion satellite, named Mab, which they first saw in 2003 using Hubble. Meteoroid impacts continually blast dust off the surface of Mab, and the dust then spreads out into a ring around Uranus. Mab's ring receives a fresh infusion of dust from each impact. In this way, nature "balances the books" by keeping the ring supplied with new dust while older dust spirals away or bangs back into the moon.

Showalter and Lissauer have measured numerous changes to the orbits of Uranus's inner moons since 1994, when their motions were derived from earlier Hubble and Voyager observations. "This appears to be a explained or chaotic process, where there is a continual exchange of energy and angular momentum between the moons," says Lissauer. "The changes in the last ten years are small, but the thing about chaos is that small changes build up exponentially with time. As a result, this suggests that the entire system is orbitally unstable." Lissauer's calculations predict that that moons would begin to collide within a few million years, which is extraordinarily short compared to the 4.5 billion year age of the Uranian system. Perhaps the most unstable moon of all is tiny Cupid, whose orbit brings it within 500 miles (800 km) of the moon Belinda.

Showalter and Lissauer propose that their discovery of a second ring, which orbits closer to the planet, provides further evidence for collisional evolution of the system. This ring orbits in the midst of the moons but has no visible body to re-supply it with dust. "This ring may be the telltale sign of an unseen belt of bodies a few feet to a few miles in size," said Showalter. He proposes that the collisional disruption of a moon in Uranus's past could have produced the debris ring they now observe.

Hubble's exquisite sharpness and sensitivity uncovered the rings in a series of 80 four-minute-long exposures of Uranus taken in August 2004. The team later recognized the faint new rings in 24 similar images taken a year earlier. Recent images from September 2005 reveal them more clearly than ever.

Showalter also found the rings in archival images taken during Voyager 2's flyby of Uranus in 1986. Uranus' first nine rings were discovered in 1977 during stellar occultation observations of the planet's atmosphere. During the Voyager encounters, two other inner rings and ten moons were discovered. However, no one noticed the new outer rings because they are extremely faint and much farther from the planet than anyone had expected. Showalter was able to find them by a careful analysis of nearly 100 Voyager images.

Because the new rings are nearly transparent, they will be easier to see when they are tilted more edge-on. This means that the new rings will increase in brightness every year as Uranus approaches its equinox, when the Sun will shine directly over Uranus' equator. When that happens, in 2007, all of the rings will be tilted edge-on to Earth and the new rings will be much easier to study.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/33/.

(Added 12/23/05) For astronomers, it has always been a source of frustration that the nearest white dwarf star is buried in the glow of the brightest star in the nighttime sky. This burned-out stellar remnant is a faint companion of the brilliant blue-white Dog Star, Sirius, located in the winter constellation Canis Major. Now, an international team of astronomers has used the keen eye of NASA's Hubble Space Telescope to isolate the light from the white dwarf, Sirius B.

The new results allow them to measure precisely the white dwarf's mass based on how its intense gravitational field alters the wavelengths of light emitted by the star. Such spectroscopic measurements of Sirius B taken with a telescope looking through the Earth's atmosphere have been severely contaminated by scattered light from the very bright Sirius.

"Studying Sirius B has challenged astronomers for more than 140 years," remarked Martin Barstow of the University of Leicester, U.K., who is the leader of the observing team. "Only with Hubble have we at last been able to obtain the observations we need, uncontaminated by the light from Sirius, in order to measure its change in wavelengths."

Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. The sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions that are used because of their brightness to measure distances to distant galaxies and the expansion rate of the universe. Measurements based on Type Ia supernovae are fundamental to understanding ‘dark energy,' a dominant repulsive force stretching the universe apart. Also, the method used to determine the white dwarf's mass relies on one of the key predictions of Einstein's theory of General Relativity; that light loses energy when it attempts to escape the gravity of a compact star.

Sirius B has a diameter of 7,500 miles, less than the size of Earth, but it is much more dense. Its powerful gravitational field is 350,000 times greater than Earth's, meaning that a 150-pound person would weigh 50 million pounds standing on its surface. Light from the surface of the hot white dwarf has to climb out of this gravitational field and is stretched to longer, redder wavelengths of light in the process. This effect, predicted by Einstein's theory of General Relativity in 1916, is called gravitational redshift, and is most easily seen in dense, massive, and hence compact objects whose intense gravitational fields warp space near their surfaces.

Based on the Hubble measurements of the redshift, made with the Space Telescope Imaging Spectrograph in February 2004, the team found that Sirius B has a mass that is 98% that of the sun. Sirius itself has a mass of two times that of the sun and a diameter of 1.5 million miles. The Hubble observations have also refined the measurement of Sirius B's surface temperature to be 44,900° Fahrenheit. Sirius itself has a surface temperature of 18,000° Fahrenheit.

At 8.6 light-years away, Sirius is one of the nearest known stars to Earth. Stargazers have watched Sirius since antiquity. Its diminutive companion, however, was not discovered until 1862, when it was first glimpsed by astronomers examining Sirius through one of the most powerful telescopes of that time.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/36/.

(Added 11/20/05) Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope are taking advantage of a once-in-a-lifetime opportunity to watch an old star suddenly stir back into new activity after coming to the end of its normal life. Their surprising results have forced them to change their ideas of how such an old, white dwarf star can re-ignite its nuclear furnace for one final blast of energy. Computer simulations had predicted a series of events that would follow such a re-ignition of fusion reactions, but the star didn't follow the script; events moved 100 times more quickly than the simulations predicted.

"We've now produced a new theoretical model of how this process works, and the VLA observations have provided the first evidence supporting our new model," said Albert Zijlstra, of the University of Manchester in the United Kingdom. Zijlstra and his colleagues presented their findings in the April 8 issue of the journal Science.

The astronomers studied a star known as V4334 Sgr, in the constellation Sagittarius. It is better known as "Sakurai's Object," after Japanese amateur astronomer Yukio Sakurai, who discovered it on February 20, 1996, when it suddenly burst into new brightness. At first, astronomers thought the outburst was a common nova explosion, but further study showed that Sakurai's Object was anything but common.

The star is an old white dwarf that had run out of hydrogen fuel for nuclear fusion reactions in its core. Astronomers believe that some such stars can undergo a final burst of fusion in a shell of helium that surrounds a core of heavier nuclei such as carbon and oxygen. However, the outburst of Sakurai's Object is the first such blast seen in modern times. Stellar outbursts observed in 1670 and 1918 may have been caused by the same phenomenon.

Astronomers expect the Sun to become a white dwarf in about five billion years. A white dwarf is a dense core left after a star's normal, fusion-powered life has ended. A teaspoon of white dwarf material would weigh about 10 tons. White dwarfs can have masses up to 1.4 times that of the Sun; larger stars collapse at the end of their lives into even-denser neutron stars or black holes.

Computer simulations indicated that heat-spurred convection (or "boiling") would bring hydrogen from the star's outer envelope down into the helium shell, driving a brief flash of new nuclear fusion. This would cause a sudden increase in brightness. The original computer models suggested a sequence of observable events that would occur over a few hundred years.

"Sakurai's object went through the first phases of this sequence in just a few years - 100 times faster than we expected - so we had to revise our models," Zijlstra explained. The revised models predicted that the star should rapidly reheat and begin to ionize gases in its surrounding region. "This is what we now see in our latest VLA observations," Zijlstra added. "It's important to understand this process. Sakurai's Object has ejected a large amount of the carbon from its inner core into space, both in the form of gas and dust grains. These will find their way into regions of space where new stars form, and the dust grains may become incorporated in new planets. Some carbon grains found in a meteorite show isotope ratios identical to those found in Sakurai's Object, and we think they may have come from such an event. Our results suggest this source for cosmic carbon may be far more important than we suspected before."

The scientists continue to observe Sakurai's Object to take advantage of the rare opportunity to learn about the process of re-ignition. They are making new VLA observations just this month. Their new models predict that the star will heat very quickly, then slowly cool again, cooling back to its current temperature about the year 2200. They think there will be one more reheating episode before it starts its final cooling to a stellar cinder.

Adapted from the information on http://www.nrao.edu/pr/2005/sakurai/.

(Added 11/20/05) Astronomers using the National Science Foundation's Very Long Baseline Array (VLBA) have measured the motion across the sky of a galaxy nearly 2.4 million light-years from Earth. While scientists have been measuring the motion of galaxies directly toward or away from Earth for decades, this is the first time that the transverse motion (called proper motion by astronomers) has been measured for a galaxy that is not a satellite of our own Milky Way Galaxy.

An international scientific team analyzed VLBA observations made over two and a half years to detect minuscule shifts in the sky position of the spiral galaxy M33. Combined with previous measurements of the galaxy's motion toward Earth, the new data allowed the astronomers to calculate M33's movement in three dimensions for the first time. M33 is a satellite of the larger galaxy M31, the well-known Andromeda Galaxy that is the most distant object visible to the naked eye. Both are part of the Local Group of galaxies that includes the Milky Way.

"A snail crawling on Mars would appear to be moving across the surface more than 100 times faster than the motion we measured for this galaxy," said Mark Reid, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. In addition to measuring the motion of M33 as a whole, the astronomers also were able to make a direct measurement of the spiral galaxy's rotation. Both measurements were made by observing the changes in position of giant clouds of molecules inside the galaxy. The water vapor in these clouds acts as a natural maser, strengthening, or amplifying, radio emission the same way that lasers amplify light emission. The natural masers acted as bright radio beacons whose movement could be tracked by the ultra-sharp radio "vision" of the VLBA.

Reid and his colleagues plan to continue measuring M33's motion and also to make similar measurements of M31's motion. This will allow them to answer important questions about the composition, history and fates of the two galaxies as well as of the Milky Way. "We want to determine the orbits of M31 and M33. That will help us learn about their history, specifically, how close have they come in the past?" Reid explained. "If they have passed very closely, then maybe M33's small size is a result of having material pulled off it by M31 during the close encounter," he added.

Accurate knowledge of the motions of both galaxies also will help determine if there's a collision in their future. In addition, orbital analysis can give astronomers valuable clues about the amount and distribution of dark matter in the galaxies.

The direct measurement of M33's transverse angular spin is the first time such a measurement has been done accurately. In the 1920s, some astronomers thought they had measured the spin of spiral galaxies, but their results proved to be in error. More recently, radio astronomers have measured the Doppler shift of hydrogen gas in galaxies to determine the spin speed, which, when combined with the angular spin, gives a direct estimate of the distance of the galaxy.

The astronomers' task was not simple. Not only did they have to detect an impressively tiny amount of motion across the sky, but they also had to separate the actual motion of M33 from the apparent motion caused by our Solar System's motion around the center of the Milky Way. The motion of the Solar System and the Earth around the Galactic center, some 26,000 light-years away, has been accurately measured using the VLBA over the last decade.

"The VLBA is the only telescope system in the world that could do this work," Reid said. "Its extraordinary ability to resolve fine detail is unmatched and was the absolute prerequisite to making these measurements."

Reid worked with Andreas Brunthaler of the Max Planck Institute for Radioastronomy in Bonn, Germany; Heino Falcke of ASTRON in the Netherlands; Lincoln Greenhill, also of the Harvard- Smithsonian Center for Astrophysics; and Christian Henkel, also of the Max Planck Institute in Bonn. The scientists reported their findings in the March 4 issue of the journal Science.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in orbit. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

Adapted from the information on http://www.nrao.edu/pr/2005/m33motion/.

(Added 11/20/05) A giant flash of energy from a supermagnetic neutron star thousands of light-years from Earth may shed a whole new light on scientists' understanding of such mysterious "magnetars" and of gamma-ray bursts. The blast from an object named SGR 1806-20 came on December 27, 2004, and was first detected by orbiting gamma-ray and X-ray telescopes. It was the brightest outburst ever seen coming from an object beyond our own Solar System, and its energy overpowered most orbiting telescopes. The burst of gamma rays and X-rays even disturbed the Earth's ionosphere, causing a sudden disruption in some radio communications.

While the intensely bright gamma ray burst faded away in a matter of minutes, the explosion's "afterglow" has been tracked by the VLA and other radio telescopes for weeks, providing most of the data needed by astronomers trying to figure out the physics of the blast.

A magnetar is a superdense neutron star with a magnetic field thousands of trillions of times more intense than that of the Earth. Scientists believe that SGR 1806-20's giant burst of energy was somehow triggered by a "starquake" in the neutron star's crust that caused a catastrophic disruption in the magnetar's magnetic field. The magnetic disruption generated the huge burst of gamma rays and "boiled off" particles from the star's surface into a rapidly-expanding fireball that continues to emit radio waves for weeks or months.

The VLA first observed SGR 1806-20 on January 3, and it has been joined by other radio telescopes in Australia, the Netherlands, and India. Scientific papers prepared for publication based on the first month's radio observations report a number of key discoveries about the object. Scientists using the VLA have found:

The fireball of radio-emitting material is expanding at roughly one-third the speed of light.
The expanding fireball is elongated, and it may change its shape quickly.
Alignment of the radio waves (polarization) confirms that the fireball is not spherical.
The flare emitted an amount of energy that represents a significant fraction of the total energy stored in the magnetar's magnetic field.
Of the dozen or so magnetars known to astronomers, only one other has been seen to experience a giant outburst. In 1998, SGR 1900+14 put out a blast similar in many respects to SGR 1806-20's, but much weaker.

The excitement isn't over, either. "The show goes on and we continue to observe this thing and continue to get surprises," said Greg Taylor, an astronomer for NRAO and the Kavli Institute of Particle Astrophysics and Cosmology in Stanford, CA.

One VLA measurement may cause difficulties for scientists trying to fit SGR 1806-20 into a larger picture of gamma ray bursts (GRBs). GRBs, seen regularly from throughout the Universe, come in two main types -- very short bursts and longer ones. The longer ones are generally believed to result when a massive star collapses into a black hole, rather than into a neutron star as in a supernova explosion. The strength and short duration of SGR 1806-20's December outburst has led some astronomers to speculate that a similar event could be seen out to a considerable distance from Earth. That means, they say, that magnetars may be the source of the short-period GRBs.

That interpretation is based to some extent on a previous measurement that indicates SGR 1806-20 is nearly 50,000 light-years from Earth. One team of observers, however, analyzed the radio emission from SGR 1806-20 and found evidence that the magnetar is only about 30,000 light-years distant. The difference, they say, reduces the likelihood that SGR 1806-20 could be a parallel for short-period GRBs. In any case, the wealth of information astronomers have gathered about the tremendous December blast makes it an extremely important event for understanding magnetars and GRBs.

Adapted from the information on http://www.nrao.edu/pr/2005/sgrburst/.

(Added 11/19/05) Using NASA's Hubble Space Telescope to probe the ninth planet in our solar system, astronomers discovered that Pluto may have not one, but three moons.

If confirmed, the discovery of the two new moons could offer insights into the nature and evolution of the Pluto system, Kuiper Belt Objects with satellite systems, and the early Kuiper Belt. The Kuiper Belt is a vast region of icy, rocky bodies beyond Neptune's orbit.

"If, as our new Hubble images indicate, Pluto has not one, but two or three moons, it will become the first body in the Kuiper Belt known to have more than one satellite," explained Hal Weaver of the Johns Hopkins Applied Physics Laboratory, Laurel, MD. He is co-leader of the team that made the discovery. Pluto was discovered in 1930. Charon, Pluto's only confirmed moon, was discovered by ground-based observers in 1978. The planet resides 3 billion miles from the sun in the heart of the Kuiper Belt.

The candidate moons, provisionally designated S/2005 P1 and S/2005 P2, were observed to be approximately 27,000 miles (44,000 kilometers) away from Pluto. The objects are roughly two to three times as far from Pluto as Charon. The team plans to make follow-up Hubble observations in February to confirm that the newly discovered objects are truly Pluto's moons. Only after confirmation will the International Astronomical Union consider names for S/2005 P1 and S/2005 P2.

The Hubble telescope's Advanced Camera for Surveys observed the two new candidate moons on May 15, 2005. "The new satellite candidates are roughly 5,000 times fainter than Pluto, but they really stood out in these Hubble images," said Max Mutchler of the Space Telescope Science Institute and the first team member to identify the satellites. Three days later, Hubble looked at Pluto again. The two objects were still there and appeared to be moving in orbit around Pluto.

"A re-examination of Hubble images taken on June 14, 2002 has essentially confirmed the presence of both P1 and P2 near the predicted locations based on the 2005 Hubble observations," said Marc Buie of Lowell Observatory, Flagstaff, AZ, another member of the research team.

The team looked long and hard for other potential moons around Pluto. "These Hubble images represent the most sensitive search yet for objects around Pluto," said team member Andrew Steffl of the Southwest Research Institute, "and it is unlikely that there are any other moons larger than about 10 miles across in the Pluto system."

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/19/.

(Added 11/19/05) New NASA Hubble Space Telescope images of the distant planet Neptune show a dynamic atmosphere and capture the fleeting orbits of its satellites. Images were taken in 14 different colored filters probing various altitudes in Neptune's deep atmosphere so that scientists can study the haze and clouds in detail.

The natural-color view of Neptune (left), common to naked eye telescopic views by amateur astronomers, reveals a cyan-colored planet. Methane gas in Neptune's atmosphere absorbs most of the red sunlight hitting the planet, making it look blue-green. The image was created by combining images in red, green, and blue light. On April 29 and 30, 2005, Hubble images were taken every 4-5 hours, spaced at about a quarter of Neptune's rotational period.

Neptune's subtle features are more visible in the enhanced-color view (top right). Images taken in special methane filters show details not visible to the human eye (bottom right). The features seen in this enhanced image must be above most of the sunlight-absorbing methane to be detectable through these special filters.

The planet is so dark at the methane wavelengths that long exposures can be taken, revealing some of Neptune's smaller moons. Clockwise from the top (in composite image at left), these moons are Proteus (the brightest), Larissa, Despina, and Galatea. Neptune had 13 moons at last count.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/22/.

(Added 11/19/05) The top view, taken by NASA's Hubble Space Telescope, is the first visible-light image of a dust ring around the nearby, bright young star Fomalhaut (HD 216956). The image offers the strongest evidence yet that an unruly planet may be tugging on the dusty belt. The left part of the ring is outside the telescope's view. The ring is tilted obliquely to our line of sight.

The center of the ring is about 1.4 billion miles (15 A.U.) away from the star. The dot near the ring's center marks the star's location. Astronomers believe that an unseen planet moving in an elliptical orbit is reshaping the ring.

The view at bottom points out important features in the image, such as the ring's inner and outer edges. Astronomers used the Advanced Camera for Surveys' (ACS) coronagraph aboard Hubble to block out the light from the bright star so they could see the faint ring. Despite the coronagraph, some light from the star is still visible in this image, as can be seen in the wagon wheel-like spokes that form an inner ring around Fomalhaut.

The suspected planet may be orbiting far away from Fomalhaut, near the dust ring's inner edge, between 4.7 billion and 6.5 billion miles (50-70 A.U.) from the star. Only Hubble has the exquisite optical resolution to resolve that the ring's inner edge is sharper than its outer edge, a telltale sign that an object is gravitationally sweeping out material like a plow clearing away snow. The ring is in the Fomalhaut system's frigid outer region, about 12 billion miles (133 A.U.) from the star. This distance is much farther than our outermost planet Pluto is from the Sun. The ring's relatively narrow width, about 2.3 billion miles (25 A.U.), indicates that an unseen planet is keeping the ring from spreading out.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/10/.

(Added 11/19/05) These images reveal the dynamic nature of Saturn's auroras. Viewing the planet's southern polar region for several days, NASA's Hubble Space Telescope snapped a series of photographs of the aurora dancing in the sky. The snapshots show that Saturn's auroras differ in character from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared with Earth, where auroral storms develop in about 10 minutes and may last for a few hours, Saturn's auroral displays always appear bright and may last for several days.

The observations, made by Hubble and the Cassini spacecraft, while en route to the planet, suggest that Saturn's auroral storms are driven mainly by the pressure of the solar wind - a stream of charged particles from the Sun - rather than by the Sun's magnetic field.

The aurora's strong brightening on January 28, 2004, corresponds with the recent arrival of a large disturbance in the solar wind. The image shows that when Saturn's auroras become brighter (and thus more powerful), the ring of light encircling the pole shrinks in diameter.

Seen from space, an aurora appears as a ring of glowing gases circling a planet's polar region. Auroral displays are initiated when charged particles in space collide with a planet's magnetic field. The charged particles are accelerated to high energies and stream into the upper atmosphere. Collisions with the gases in the planet's atmosphere produce flashes of glowing energy in the form of visible, ultraviolet, and infrared light.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/06/.

(Added 01/29/05) NASA's Mars Exploration Rover Opportunity has found an iron meteorite, the first meteorite of any type ever identified on another planet.

The pitted, basketball-size object is mostly made of iron and nickel according to readings from spectrometers on the rover. Only a small fraction of the meteorites fallen on Earth are similarly metal-rich. Others are rockier. As an example, the meteorite that blasted the famous Meteor Crater in Arizona is similar in composition. "This is a huge surprise, though maybe it shouldn't have been," remarked Dr. Steve Squyres of Cornell University, Ithaca, NY, principal investigator for the science instruments on Opportunity and its twin, Spirit.

The meteorite, dubbed "Heat Shield Rock," sits near debris of Opportunity's heat shield on the surface of Meridiani Planum, a cratered flatland that has been Opportunity's home since the robot landed on Mars nearly one year ago. "I never thought we would get to use our instruments on a rock from someplace other than Mars," Squyres added. "Think about where an iron meteorite comes from: a destroyed planet or planetesimal that was big enough to differentiate into a metallic core and a rocky mantle."

Rover-team scientists are wondering whether some rocks that Opportunity has seen atop the ground surface are rocky meteorites. "Mars should be hit by a lot more rocky meteorites than iron meteorites," Squyres explained. "We've been seeing lots of cobbles out on the plains, and this raises the possibility that some of them may in fact be meteorites. We may be investigating some of those in coming weeks. The key is not what we'll learn about meteorites -- we have lots of meteorites on Earth -- but what the meteorites can tell us about Meridiani Planum."

The numbers of exposed meteorites could be an indication of whether the plain is gradually eroding away or being built up. NASA Chief Scientist Dr. Jim Garvin said, "Exploring meteorites is a vital part of NASA's scientific agenda, and discovering whether there are storehouses of them on Mars opens new research possibilities, including further incentives for robotic and then human-based sample-return missions. Mars continues to provide unexpected science 'gold,' and our rovers have proven the value of mobile exploration with this latest finding."

Initial observation of Heat Shield Rock from a distance with Opportunity's miniature thermal emission spectrometer suggested a metallic composition and raised speculation last week that it was a meteorite. The rover drove close enough to use its Moessbauer and alpha particle x-ray spectrometers, confirming the meteorite identification over the weekend.

Opportunity and Spirit successfully completed their primary three-month missions on Mars in April 2004. NASA has extended their missions twice because the rovers have remained in good condition to continue exploring Mars longer than anticipated. They have found geological evidence of past wet environmental conditions that might have been hospitable to life.

Opportunity has driven a total of 2.10 kilometers (1.30 miles). Minor mottling from dust has appeared in images from the rover's rear hazard-identification camera since Opportunity entered the area of its heat-shield debris, said Jim Erickson of NASA's Jet Propulsion Laboratory, Pasadena, CA, rover project manager. The rover team plans to begin driving Opportunity south toward a circular feature called "Vostok" within about a week.

Spirit has driven a total of 4.05 kilometers (2.52 miles). It has been making slow progress uphill toward a ridge on "Husband Hill" inside Gusev Crater.

Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-018.

(Added 01/29/05) On January 14, after its seven-year journey through the solar system on board the Cassini spacecraft, ESA's Huygens probe has successfully descended through the atmosphere of Titan, Saturn's largest moon, and safely landed on its surface.

Huygens is humankind's first successful attempt to land a probe on another world in the outer solar system. "This is a great achievement for Europe and its US partners in this ambitious international endeavor to explore the Saturnian system," said Jean-Jacques Dordain, ESA's Director General.

Following its release from the Cassini mothership on 25 December, Huygens reached Titan's outer atmosphere after 20 days and a 4 million km cruise. The probe started its descent through Titan's hazy cloud layers from an altitude of about 1270 km at 11:13 CET. During the following three minutes Huygens had to decelerate from 18,000 to 1400 km per hour.

A sequence of parachutes then slowed it down to less than 300 km per hour. At a height of about 160 km the probe's scientific instruments were exposed to Titan's atmosphere. At about 120 km, the main parachute was replaced by a smaller one to complete the descent, with an expected touchdown at 13:34 CET. Preliminary data indicate that the probe landed safely, likely on a solid surface.

The probe began transmitting data to Cassini four minutes into its descent and continued to transmit data after landing at least as long as Cassini was above Titan's horizon. The certainty that Huygens was alive came already at 11:25 CET, when the Green Bank radio telescope in West Virginia picked up a faint but unmistakable radio signal from the probe. Radio telescopes on Earth continued to receive this signal well past the expected lifetime of Huygens.

Huygens data, relayed by Cassini, were picked up by NASA's Deep Space Network and delivered immediately to ESA's European Space Operation Centre in Darmstadt, Germany, where the scientific analysis is taking place.

"Titan was always the target in the Saturn system where the need for "ground truth" from a probe was critical. It is a fascinating world and we are now eagerly awaiting the scientific results," says Professor David Southwood, Director of ESA's scientific program.

"The Huygens scientists are all delighted. This was worth the long wait," says Dr Jean-Pierre Lebreton, ESA Huygens Mission Manager. Huygens is expected to provide the first direct and detailed sampling of Titan's atmospheric chemistry and the first photographs of its hidden surface, and will supply a detailed "weather report."

One of the main reasons for sending Huygens to Titan is that its nitrogen atmosphere, rich in methane, and its surface may contain many chemicals of the kind that existed on the young Earth. Combined with the Cassini observations, Huygens will afford an unprecedented view of Saturn's mysterious moon.

"We now have the key to understanding what shapes Titan's landscape," said Dr Martin Tomasko, Principal Investigator for the Descent Imager-Spectral Radiometer (DISR), adding: "Geological evidence for precipitation, erosion, mechanical abrasion and other fluvial activity says that the physical processes shaping Titan are much the same as those shaping Earth."

Titan's Terrain from 20 km Altitude from Huygens Spectacular images captured by the DISR reveal that Titan has extraordinarily Earth-like meteorology and geology. Images have shown a complex network of narrow drainage channels running from brighter highlands to lower, flatter, dark regions. These channels merge into river systems running into lake beds featuring offshore 'islands' and 'shoals' remarkably similar to those on Earth.

Data provided in part by the Gas Chromatograph and Mass Spectrometer (GCMS) and Surface Science Package (SSP) support Dr Tomasko's conclusions. Huygens' data provide strong evidence for liquids flowing on Titan. However, the fluid involved is methane, a simple organic compound that can exist as a liquid or gas at Titan's sub-170 °C temperatures, rather than water as on Earth.

Titan's rivers and lakes appear dry at the moment, but rain may have occurred not long ago.

Deceleration and penetration data provided by the SSP indicate that the material beneath the surface's crust has the consistency of loose sand, possibly the result of methane rain falling on the surface over eons, or the wicking of liquids from below towards the surface.

Heat generated by Huygens warmed the soil beneath the probe and both the GCMS and SSP detected bursts of methane gas boiled out of surface material, reinforcing methane's principal role in Titan's geology and atmospheric meteorology -- forming clouds and precipitation that erodes and abrades the surface.

In addition, DISR surface images show small rounded pebbles in a dry riverbed. Spectra measurements are consistent with a composition of dirty water ice rather than silicate rocks. However, these are rock-like solid at Titan's temperatures.

Titan's soil appears to consist at least in part of precipitated deposits of the organic haze that shrouds the planet. This dark material settles out of the atmosphere. When washed off high elevations by methane rain, it concentrates at the bottom of the drainage channels and riverbeds contributing to the dark areas seen in DISR images.

New, stunning evidence based on finding atmospheric argon 40 indicates that Titan has experienced volcanic activity generating not lava, as on Earth, but water ice and ammonia.

Thus, while many of Earth's familiar geophysical processes occur on Titan, the chemistry involved is quite different. Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere, and instead of lava, Titanian volcanoes spew very cold ice.

Titan is an extraordinary world having Earth-like geophysical processes operating on exotic materials in very alien conditions.

"We are really extremely excited about these results. The scientists have worked tirelessly for the whole week because the data they have received from Huygens are so thrilling. This is only the beginning, these data will live for many years to come and they will keep the scientists very very busy", said Jean-Pierre Lebreton, ESA's Huygens Project Scientist and Mission manager.

Adapted from the information on http://www.esa.int/export/esaCP/SEMQ1QQ3K3E_index_0.html and http://www.esa.int/export/esaCP/Pr_5_2005_p_EN.html.

(Added 01/17/05) Hubble astronomers have uncovered, for the first time, a population of infant stars in the Milky Way satellite galaxy, the Small Magellanic Cloud (SMC, visible to the naked eye in the southern constellation Tucana), located 210,000 light-years away.

Hubble's exquisite sharpness plucked out an underlying population of infant stars embedded in the nebula NGC 346 that are still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.

Although star birth is common within the disk of our galaxy, this smaller companion galaxy is more primeval in that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion.

Fragmentary galaxies like the SMC are considered primitive building blocks of larger galaxies. Most of these types of galaxies existed far away, when the universe was much younger. The SMC offers a unique nearby laboratory for understanding how stars arose in the early universe. Nestled among other starburst regions with the small galaxy, the nebula NGC 346 alone contains more than 2,500 infant stars.

The Hubble images, taken with the Advanced Camera for Surveys, identify three stellar populations in the SMC and in the region of the NGC 346 nebula -- a total of 70,000 stars. The oldest population is 4.5 billion years, roughly the age of our Sun. The younger population arose only 5 million years ago (about the time Earth's first hominids began to walk on two feet). Lower-mass stars take longer to ignite and become full-fledged stars, so the protostellar population is 5 million years old. Curiously, the infant stars are strung along two intersecting lanes in the nebula, resembling a "T" pattern in the Hubble plot.

Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/04/.

(Added 01/17/05) Astronomers using the National Science Foundation's Very Long Baseline Array (VLBA) have discovered jets of plasma blasted from the cores of distant galaxies at speeds within 99.9% of the speed of light, placing these plasma jets among the fastest objects yet seen in the Universe.

"This tells us that the physical processes at the cores of these galaxies, called blazars, are extremely energetic and are capable of propelling matter very close to the absolute cosmic speed limit," explained Glenn Piner of Whittier College in Whittier, CA.

According to Einstein's Special Theory of Relativity, no object with mass can be accelerated to the speed of light. To get even close to the speed of light requires enormous amounts of energy. "For example, to accelerate a bowling ball to the speed newly measured in these blazars would require all the energy produced in the world for an entire week," Piner described, "and the blobs of plasma in these jets are at least as massive as a large planet".

Blazars are active galactic nuclei -- energetic regions surrounding massive black holes at the centers of galaxies. Material being drawn into the black hole forms a spinning disk called an accretion disk. Powerful jets of charged particles are ejected at high speeds along the poles of accretion disks. When these jets happen to be aimed nearly toward the Earth, the objects are called blazars.

Taking advantage of the extremely sharp radio "vision" of the continent-wide VLBA, the scientists tracked individual features in the jets of three blazars at distances from Earth ranging from 7.3 to 9 billion light-years. A Boston University team led by Svetlana Jorstad earlier had identified the three blazars as having potentially very high jet speeds based on VLBA observations in the mid-1990s. Piner and his colleagues observed the blazars again in 2002 and 2003 with much longer observations, and were able to confirm the high-speed motions in the faint blazar jets.

Their measurements showed that features in the blazar jets were moving at apparent speeds more than 25 times greater than that of light. This phenomenon, called superluminal motion, is not real, but rather is an illusion caused by the fact that the material in the jet is moving at nearly the speed of light almost directly toward the observer. Because the jet features are moving toward Earth at almost the same speed as the radio waves they emit, they can appear to move across the sky at faster-than-light speeds. Scientists can correct for this geometrical effect to calculate a lower limit to the true speed of the features.

"We typically see apparent speeds in blazar jets that are about five times the speed of light, and that corresponds to a true speed of more than 98% of light speed," Piner said. "Now, based on independent confirmation by two groups of astronomers, we see these three blazars with apparent speeds greater than 25 times that of light," Piner added. That apparent speed, the scientists said, corresponds to a true speed of greater than 99.9% of light speed, which is 186,282 miles per second.

Based on other properties of blazars, the scientists believe that their interpretation of the data is accurate and that they have measured the extremely fast speeds in the three blazar jets. However, "we do have to be somewhat careful in interpreting these results, because it is possible that the observed motions represent the motion of some propagating disturbance in the plasma rather than the plasma itself, in the same way that a water wave can move across the surface of the ocean without physically transporting the water," Piner said.

Adapted from the information on http://www.nrao.edu/pr/2005/fastblazars/.

(Added 01/17/05) A swarm of 10,000 or more black holes may be orbiting the Milky Way's supermassive black hole, according to new results from NASA's Chandra X-ray Observatory. This would represent the highest concentration of black holes anywhere in the Galaxy.

These relatively small, stellar-mass black holes, along with neutron stars, appear to have migrated into the Galactic Center over the course of several billion years. Such a dense stellar graveyard has been predicted for years, and this represents the best evidence to date of its existence. The Chandra data may also help astronomers better understand how the supermassive black hole at the center of the Milky Way grows.

The discovery was made as part of Chandra's ongoing program of monitoring the region around Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way. It was announced today by Michael Muno of the University of California, Los Angeles (UCLA) at a meeting of the American Astronomical Society in San Diego, CA.

Among the thousands of x-ray sources detected within 70 light years of Sgr A*, Muno and his colleagues searched for those most likely to be active black holes and neutron stars by selecting only the brightest sources that also exhibited large variations in their x-ray output. These characteristics identify black holes and neutron stars that are in binary star systems and are pulling matter from nearby companion stars. Of the seven sources that met these criteria, four are within three light-years of Sgr A*.

"Although the region around Sgr A* is crowded with stars, we expected that there was only a 20% chance that we would find even one x-ray binary within a three-light-year radius," said Muno. "The observed high concentration of these sources implies that a huge number of black holes and neutron stars have gathered in the center of the Galaxy."

Mark Morris, also of UCLA and a coauthor on the present work, had predicted a decade ago that a process called dynamical friction would cause stellar black holes to sink toward the center of the Galaxy. Black holes are formed as remnants of the explosions of massive stars and have masses of about 10 suns. As black holes orbit the center of the Galaxy at a distance of several light years, they pull on surrounding stars, which pull back on the black holes.

The net effect is that black holes spiral inward, and the low-mass stars move out. From the estimated number of stars and black holes in the Galactic Center region, dynamical friction is expected to produce a dense swarm of 20,000 black holes within three light years of Sgr A*. A similar effect is at work for neutron stars, but to a lesser extent because they have a lower mass.

Once black holes are concentrated near Sgr A*, they will have numerous close encounters with normal stars there, some of which are in binary star systems. The intense gravity of a black hole can induce an ordinary star to "change partners" and pair up with the black hole while ejecting its companion. This process and a similar one for neutron stars are expected to produce several hundreds of black hole and neutron star binary systems.

"If only 1% of these binary systems are x-ray active each year, they can account for the sources we see," said Eric Pfahl of the University of Virginia in Charlottesville and a coauthor of a paper describing these results that has been submitted to the Astrophysical Journal Letters. "Although the evidence is mostly circumstantial, it makes a strong case for the existence of a large population of neutron stars and stellar-mass black holes within three light-years of the center of our Galaxy."

The black holes and neutron stars in the cluster are expected to gradually be swallowed by the supermassive black hole, Sgr A*, at a rate of about one every million years. At this rate, about 10,000 black holes and neutron stars would have been captured in a few billion years, adding about 3% to the mass of the central supermassive black hole, which is currently estimated to contain the mass of 3.7 million suns.

In the meantime, the acceleration of low-mass stars by black holes will eject low-mass stars from the central region. This expulsion will reduce the likelihood that normal stars will be captured by the central supermassive black hole. This may explain why the central regions of some galaxies, including the Milky Way, are fairly quiet even though they contain a supermassive black hole.

The region analyzed in this research near Sgr A* has been observed 16 times between 1999 and 2004 using Chandra's Advanced CCD Imaging Spectrometer (ACIS) instrument. Other members of the research team include Frederick K. Baganoff (Massachusetts Institute of Technology), Niel Brandt (Penn State), Andrea Ghez and Jessica Lu (UCLA).

Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_011005.html.

(Added 01/17/05) A dense globular star cluster near the center of our Milky Way Galaxy holds a buzzing beehive of rapidly-spinning millisecond pulsars, according to astronomers who discovered 21 new pulsars in the cluster using the National Science Foundation's 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The cluster, called Terzan 5, now holds the record for pulsars, with 24, including three known before the GBT observations.

"We hit the jackpot when we looked at this cluster," announced Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, VA. "Not only does this cluster have a lot of pulsars -- and we still expect to find more in it -- but the pulsars in it are very interesting. They include at least 13 in binary systems, two of which are eclipsing, and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender," Ransom added. Ransom and his colleagues reported their findings to the American Astronomical Society's meeting in San Diego, CA, and in the online journal Science Express.

The star cluster's numerous pulsars are expected to yield a bonanza of new information about not only the pulsars themselves, but also about the dense stellar environment in which they reside and probably even about nuc