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Chandra Independently Measures the Hubble Constant

(Added 08/16/06) A critically important number that specifies the expansion rate of the Universe, the so-called Hubble constant, has been independently determined using NASA's Chandra X-ray Observatory. This new value matches recent measurements using other methods and extends their validity to greater distances, thus allowing astronomers to probe earlier epochs in the evolution of the Universe.

"The reason this result is so significant is that we need the Hubble constant to tell us the size of the Universe, its age, and how much matter it contains," explained Max Bonamente from the University of Alabama in Huntsville and NASA's Marshall Space Flight Center (MSFC) in Huntsville, AL, lead author on the paper describing the results. "Astronomers absolutely need to trust this number because we use it for countless calculations."

The Hubble constant is calculated by measuring the speed at which objects are moving away from us and dividing by their distance. Most of the previous attempts to determine the Hubble constant have involved using a multi-step, or distance ladder, approach in which the distance to nearby galaxies is used as the basis for determining greater distances.

The most common approach has been to use a well-studied type of pulsating star known as a Cepheid variable, in conjunction with more distant supernovae to trace distances across the Universe. Scientists using this method and observations from the Hubble Space Telescope were able to measure the Hubble constant to within 10%. However, only independent checks would give them the confidence they desired, considering that much of our understanding of the Universe hangs in the balance.

By combining X-ray data from Chandra with radio observations of galaxy clusters, the team determined the distances to 38 galaxy clusters ranging from 1.4 billion to 9.3 billion light years from Earth. These results do not rely on the traditional distance ladder. Bonamente and his colleagues find the Hubble constant to be 77 kilometers per second per megaparsec (a megaparsec is equal to 3.26 million light years), with an uncertainty of about 15%.

This result agrees with the values determined using other techniques. The Hubble constant had previously been found to be 72, give or take 8, kilometers per second per megaparsec based on Hubble Space Telescope observations. The new Chandra result is important because it offers the independent confirmation that scientists have been seeking and fixes the age of the Universe between 12 and 14 billion years. "These new results are entirely independent of all previous methods of measuring the Hubble constant," said team member Marshall Joy also of MSFC.

The astronomers used a phenomenon known as the Sunyaev-Zeldovich effect, where photons in the cosmic microwave background (CMB) interact with electrons in the hot gas that pervades the enormous galaxy clusters. The photons acquire energy from this interaction, which distorts the signal from the microwave background in the direction of the clusters. The magnitude of this distortion depends on the density and temperature of the hot electrons and the physical size of the cluster. Using radio telescopes to measure the distortion of the microwave background and Chandra to measure the properties of the hot gas, the physical size of the cluster can be determined. From this physical size and a simple measurement of the angle subtended by the cluster, the rules of geometry can be used to derive its distance. The Hubble constant is determined by dividing previously measured cluster speeds by these newly derived distances.

This project was championed by Chandra's telescope mirror designer, Leon Van Speybroeck, who passed away in 2002. The foundation was laid when team members John Carlstrom (University of Chicago) and Marshall Joy obtained careful radio measurements of the distortions in the CMB radiation using radio telescopes at the Berkeley-Illinois-Maryland Array and the Caltech Owens Valley Radio Observatory. In order to measure the precise X-ray properties of the gas in these distant clusters, a space-based X-ray telescope with the resolution and sensitivity of Chandra was required.

Adapted from the information on

Black Hole Paradox Is Solved

(Added 08/16/06) Black holes are lighting up the Universe, and now astronomers may finally know how. New data from NASA's Chandra X-ray Observatory show for the first time that powerful magnetic fields are the key to these brilliant and startling light shows.

It is estimated that up to a quarter of the total radiation in the Universe emitted since the Big Bang comes from material falling towards supermassive black holes, including those powering quasars, the brightest known objects. For decades, scientists have struggled to understand how black holes, the darkest objects in the Universe, can be responsible for such prodigious amounts of radiation.

New X-ray data from Chandra give the first clear explanation for what drives this process: magnetic fields. Chandra observed a black hole system in our galaxy, known as GRO J1655-40 (J1655, for short), where a black hole was pulling material from a companion star into a disk.

"By intergalactic standards J1655 is in our backyard, so we can use it as a scale model to understand how all black holes work, including the monsters found in quasars," said Jon M. Miller of the University of Michigan, Ann Arbor, whose paper on these results appears in this week's issue of Nature.

Gravity alone is not enough to cause gas in a disk around a black hole to lose energy and fall onto the black hole at the rates required by observations. The gas must lose some of its orbital angular momentum, either through friction or a wind, before it can spiral inward. Without such effects, matter could remain in orbit around a black hole for a very long time.

Scientists have long thought that magnetic turbulence could generate friction in a gaseous disk and drive a wind from the disk that carries angular momentum outward allowing the gas to fall inward.

Using Chandra, Miller and his team provided crucial evidence for the role of magnetic forces in the black hole accretion process. The X-ray spectrum, the number of X-rays at different energies, showed that the speed and density of the wind from J1655's disk corresponded to computer simulation predictions for magnetically-driven winds. The spectral fingerprint also ruled out the two other major competing theories to winds driven by magnetic fields.

"In 1973, theorists came up with the idea that magnetic fields could drive the generation of light by gas falling onto black holes," said co-author John Raymond of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "Now, over 30 years later, we finally may have convincing evidence."

This deeper understanding of how black holes accrete matter also teaches astronomers about other properties of black holes, including how they grow.

"Just as a doctor wants to understand the causes of an illness and not merely the symptoms, astronomers try to understand what causes phenomena they see in the Universe," said co-author Danny Steeghs also of the Harvard-Smithsonian Center for Astrophysics. "By understanding what makes material release energy as it falls onto black holes, we may also learn how matter falls onto other important objects."

In addition to accretion disks around black holes, magnetic fields may play an important role in disks detected around young sun-like stars where planets are forming, as well as ultra-dense objects called neutron stars.

Adapted from the information on

Neutron Star Is Spewing Debris

Neutron Star J0617 in IC 443(Added 08/15/06) A long observation with NASA's Chandra X-ray Observatory has revealed important new details of a neutron star that is spewing out a wake of high-energy particles as it races through space. The deduced location of the neutron star on the edge of a supernova remnant, and the peculiar orientation of the neutron star wake, pose mysteries that remain unresolved.

"Like a kite flying in the wind, the behavior of this neutron star and its wake tell us what sort of gas it must be plowing through," said Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., and lead author of a paper accepted to The Astrophysical Journal. "Yet we're still not sure how the neutron star got to its present location."

The neutron star, known as CXOU J061705.3+222127, or J0617 for short, appears to lie near the outer edge of an expanding bubble of hot gas associated with the supernova remnant IC 443. Presumably, J0617 was created at the time of the supernova -- approximately 30,000 years ago -- and propelled away from the site of the explosion at about 500,000 miles per hour.

However, the neutron star's wake is oriented almost perpendicularly to the direction expected if the neutron star were moving away from the center of the supernova remnant. This apparent misalignment had previously raised doubts about the association of the speeding neutron star with the supernova remnant.

Gaensler and his colleagues provide strong evidence that J0617 was indeed born in the same explosion that created the supernova remnant. First, the shape of the neutron star's wake indicates it is moving a little faster than the speed of sound in the remnant's multimillion-degree gas. The velocity that one can then calculate from this conclusion closely matches the predicted pace of the neutron star. In contrast, if the neutron star were outside the confines of the remnant, its inferred speed would be a sluggish 20,000 miles per hour. Also, the measured temperature of the neutron star matches that of one born at the same time of the supernova remnant.

What then, could cause the misaligned, or wayward, neutron star wake?

The authors speculate that perhaps the doomed progenitor star was moving at a high speed before it exploded, so that the explosion site was not at the observed center of the supernova remnant. Fast moving gusts of gas inside the supernova remnant have further pushed the neutron star's wake out of alignment.

Observations of J0617 in the next 10 years should put this idea to the test. "If the neutron star was born off-center and if the wake is being pushed around by cross-winds, the neutron star should be moving close to vertically, away from the center of the supernova remnant. Now we wait and see," said Gaensler.

Another group, led by Margarita Karovska, also of the CfA, has concentrated on other, previously unnoticed intriguing features of J0617. At a recent conference on neutron stars in London, England, they announced their findings, which include a thin filament of cooler gas that appears to extend from the neutron star along the long axis of its wake, and a second point-like feature embedded in the X-ray nebula around the neutron star.

"There are a number of puzzling observational features associated with this system crying out for longer observations," said Karovska.

Adapted from the information on

Gas Halo Around Ordinary Spiral Galaxy Galaxy NGC 5746 in optical (DSS) and X-ray (Chandra)

(Added 08/15/06) Scientists using NASA's Chandra X-ray Observatory have detected an extensive halo of hot gas around a quiescent spiral galaxy. This discovery is evidence that galaxies like our Milky Way are still accumulating matter from the gradual inflow of intergalactic gas.

"What we are likely witnessing here is the ongoing galaxy formation process," said Kristian Pedersen of the University of Copenhagen, Denmark, and lead author of a report on the discovery.

Chandra observations show that the hot halo extends more than 60,000 light years on either side of the disk of the galaxy known as NGC 5746. The detection of such a large halo alleviates a long-standing problem for the theory of galaxy formation. Spiral galaxies are thought to form from enormous clouds of intergalactic gas that collapse to form giant, spinning disks of stars and gas.

One prediction of this theory is that large spiral galaxies should be immersed in halos of hot gas left over from the galaxy formation process. Hot gas has been detected around spiral galaxies in which vigorous star formation is ejecting matter from the galaxy, but until now hot halos due to infall of intergalactic matter have not been detected.

"Our observations solve the mystery of the missing hot halos around spiral galaxies," said Pedersen. "The halos exist, but are so faint that an extremely sensitive telescope such as Chandra is needed to detect them."

NGC 5746 is a massive spiral galaxy about a 100 million light years from Earth. Its disk of stars and gas is viewed almost edge-on. The galaxy shows no signs of unusual star formation, or energetic activity from its nuclear region, making it unlikely that the hot halo is produced by gas flowing out of the galaxy.

"We targeted NGC 5746 because we thought its distance and orientation would give us the best chance to detect a hot halo caused by the infall of intergalactic gas," said Jesper Rasmussen of the University of Birmingham, United Kingdom and a coauthor of the report. "What we found is in good agreement with computer simulations in which galaxies are built up gradually from the merger of smaller clouds of hot gas and dark matter."

The computer simulations were done by Jesper Sommer-Larsen (also a coauthor of the report) and collaborators at the University of Copenhagen. The paper describing these results will be published in the April issue of the journal New Astronomy. Other researchers on this project were Sune Toft, Yale University; Andrew Benson, University of Oxford, United Kingdom; and Richard Bower, University of Durham, United Kingdom.

Adapted from the information on

Hourglass Magnetic Field Required for Starbirth

(Added 08/15/06) Long predicted by theory, the Smithsonian's Submillimeter Array has found the first conclusive evidence of an hourglass-shaped magnetic field in a star formation region. Measurements indicate that material in the interstellar cloud is dense enough to allow it to gravitationally collapse, warping the magnetic field in the process.

Astronomers Josep Girart (Institute of Space Studies of Catalonia, Spanish National Research Council), Ramprasad Rao (Institute of Astronomy and Astrophysics, Academia Sinica), and Dan Marrone (Harvard-Smithsonian Center for Astrophysics) studied the protostellar system designated NGC 1333 IRAS 4A. This system of two protostars is located approximately 980 light-years from Earth in the direction of the constellation Perseus. "We selected this system because previous work had offered tantalizing hints of an hourglass-shaped magnetic field," explained Marrone. "The Submillimeter Array offered the resolution and sensitivity we needed to confirm it."

NGC 1333 IRAS 4A is part of the Perseus molecular cloud complex - a collection of gas and dust holding as much mass as 130,000 suns. This region is actively forming stars. Its proximity to Earth and young age make the Perseus complex an ideal laboratory for studying star formation.

Theorists predict that collapsing molecular cloud cores - the seeds of star formation - have to overcome the support provided by their magnetic field in order to form stars. In the process, the competition between gravity pulling inward and magnetic pressure pushing outward was expected to produce a warped, hourglass pattern to the magnetic field within these collapsed cores.

Using the Array, Marrone and his colleagues observed dust emission from IRAS 4A. Because the magnetic field aligns the dust grains in the cloud core, the team could measure the magnetic field's geometry and estimate its strength by measuring the polarization of the dust emission.

"With the special polarization capabilities of the SMA we see the shape of the field directly. This is the first textbook example of theoretically predicted magnetic structure," remarked Rao. The data indicate that, in the case of IRAS 4A, magnetic pressure is more influential than turbulence in slowing star formation within the cloud core. The same likely is true for similar cloud cores elsewhere.

Despite the moderating influence of the magnetic field, IRAS 4A is dense enough for gravitational collapse to continue. Approximately a million years in the future, two sunlike stars will shine where only a dust-cloaked cocoon lies today.

Adapted from the information on They reported their findings in the August 11 issue of the journal Science.

Giant Stellar Explosion

(Added 08/15/06) On February 12, 2006, skygazers spotted a nova that appeared when a faint star brightened dramatically, becoming visible to the unaided eye. The cause of the brightening was a thermonuclear explosion that blasted off a white dwarf star's outer layers while leaving the core unscathed.

"This nova was more exciting to astronomers than any fireworks display," remarked Jennifer Sokoloski (Harvard-Smithsonian Center for Astrophysics). Yet the eruption was minuscule compared to what will come. Astronomers predict that the star in question may eventually explode violently as a supernova in the distant future, ripping itself apart and scattering its gaseous remains across space. Similar explosions are bright enough to be seen across billions of light-years of space. This nearby system within the Milky Way offers astronomers a unique opportunity to refine their physical understanding of one type of rare star system that can generate such powerful blasts.

"Astronomers use such supernovae to measure the expansion of the universe, so it's important for us to understand how the star systems that generate those explosions evolve prior to their demise," said Sokoloski.

The star system under study, RS Ophiuchi, is located about 5,000 light-years from Earth in the direction of the constellation Ophiuchus. RS Ophiuchi consists of a dense, white dwarf star (a stellar core about the size of the Earth but containing more mass than the Sun) and a bloated red giant star. The red giant companion emits a stellar wind that spills material onto the white dwarf. When enough of that material has accumulated, theorists say, a gigantic thermonuclear explosion occurs.

Interestingly, the white dwarf star orbits inside the extended gaseous envelope of its companion. The material ejected from the white dwarf during the nova plows into this surrounding material, creating a shock wave that both heats gas to emit energetic X-rays and accelerates electrons to emit radio waves.

"What we could infer from the X-ray data, we could image with the radio telescopes," explained Michael Rupen (National Radio Astronomy Observatory), who studied RS Ophiuchi using the National Science Foundation's Very Long Baseline Array.

Using satellites and ground-based telescopes, independent teams studied RS Ophiuchi at multiple wavelengths. Their observations showed that the explosion was more complex than scientists generally assumed. Standard computer models presume a spherical explosion with matter ejected in all directions equally. Observations of RS Ophiuchi showed evidence for two opposing jets of matter and a possible ring-like structure. "The radio images represent the first time we've ever seen the birth of a jet in a white dwarf system," said Rupen. "We literally see the jet 'turn on.'"

Systems such as RS Ophiuchi eventually may produce a vastly more powerful explosion - a supernova - when the white dwarf accumulates enough mass to cause it to collapse and explode violently. Because such supernova explosions (called Type 1a supernovae by astronomers) all are triggered as the white dwarf reaches the same mass, they are thought to be nearly identical in their intrinsic brightness. This makes them extremely valuable as "standard candles" for measuring distances in the universe.

With the Rossi X-ray Timing Explorer, the scientists calculated the mass of the white dwarf to be close to 1.4 times that of the Sun - nearly as massive as a white dwarf can become before collapsing.

Adapted from the information on

A Dusty Andromeda Galaxy

M31 - Andromeda Galaxy in Infrared (IR) Light, Observed by Spitzer(Added 08/15/06) The Andromeda galaxy, named for the mythological princess who almost fell prey to a sea monster, appears tranquil in a new image from NASA's Spitzer Space Telescope. The mesmerizing infrared mosaic shows red waves of dust over a blue sea of stars.

"What's really interesting about this view is the contrast between the galaxy's smooth, flat disk of old stars and its bumpy waves of dust heated by young stars," said Pauline Barmby of the Harvard-Smithsonian Center for Astrophysics (CfA). Barmby and her colleagues recently observed Andromeda using Spitzer.

Barmby presented the Spitzer image of Andromeda today in a press conference at the 208th meeting of the American Astronomical Society.

Barmby and her team used the Spitzer data to make drastically improved measurements of Andromeda's infrared brightness. They found that the galaxy shines with the same amount of energy as about 4 billion suns. Based on these measurements, the astronomers confirmed that there are roughly one trillion stars in the galaxy. Our Milky Way galaxy is estimated to house about 400 billion stars.

"This is the first time the stellar population of Andromeda has been determined using the galaxy's infrared brightness," said Barmby. "It's reassuring to know our numbers are in agreement with previous estimates of the mass of the stars based on the stars' motion."

The new false-colored portrait also provides astronomers with the best look yet at the dust-drenched spiral arms that swirl out of the galaxy's center, a region hidden by bright starlight in visible-light images. Dust and gas are the building materials of stars. They are clumped together throughout the spiral arms, where new stars are forming.

"The Spitzer data trace with startling clarity the star-forming material all the way into the inner part of the galaxy," described George Helou, deputy director of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena. "The challenge is to understand what shapes the distribution of this gas and dust, and what modulates the star formation at different locations."

Spitzer's infrared array camera captured infrared light emanating from both older stars (blue) and dust made up of molecules called polycyclic aromatic hydrocarbons (red). These carbon-containing molecules are warmed by sunlight and glow at infrared wavelengths. They are often associated with dense clouds of new stars, and can be found on Earth in barbecue pits and car exhaust, among other places.

The Andromeda galaxy, also known by astronomers as Messier 31, is located 2.5 million light-years away in the constellation Andromeda. It is the closest major galaxy to the Milky Way, making it the ideal specimen for carefully examining the nature of galaxies. On a clear, dark night, the galaxy can be spotted with the naked eye as a fuzzy blob. The disk of Andromeda spans about 260,000 light-years, which means that a light beam would take 260,000 years to travel from one end of the galaxy to the other. By comparison, the Milky Way is about 100,000 light-years across. When viewed from Earth, Andromeda occupies a portion of the sky equivalent to seven full moons.

Spitzer's wide field of view allowed the telescope to capture a complete snapshot of the Andromeda galaxy, though the task wasn't easy. The final mosaic consists of 3,000 or so individual picture frames stitched together seamlessly.

Adapted from the information on

Stars Are Lonelier than Was Thought

(Added 08/15/06) Common wisdom among astronomers holds that most star systems in the Milky Way are multiple, consisting of two or more stars in orbit around each other. Common wisdom is wrong. A new study by Charles Lada of the Harvard-Smithsonian Center for Astrophysics (CfA) demonstrates that most star systems are made up of single stars. Since planets probably are easier to form around single stars, planets also may be more common than previously suspected.

Astronomers have long known that massive, bright stars, including stars like the sun, are most often found to be in multiple star systems. This fact led to the notion that most stars in the universe are multiples. However, more recent studies targeted at low-mass stars have found that these fainter objects rarely occur in multiple systems. Astronomers have known for some time that such low-mass stars, also known as red dwarfs or M stars, are considerably more abundant in space than high-mass stars.

By combining these two facts, Lada came to the realization that most star systems in the Galaxy are composed of solitary red dwarfs.

"By assembling these pieces of the puzzle, the picture that emerged was the complete opposite of what most astronomers have believed," said Lada.

Among very massive stars, known as O- and B-type stars, 80% of the systems are thought to be multiple, but these very bright stars are exceedingly rare. Slightly more than half of all the fainter, sun-like stars are multiples. However, only about 25% of red dwarf stars have companions. Combined with the fact that about 85% of all stars that exist in the Milky Way are red dwarfs, the inescapable conclusion is that upwards of two-thirds of all star systems in the Galaxy consist of single, red dwarf stars.

The high frequency of lone stars suggests that most stars are single from the moment of their birth. If supported by further investigation, this finding may increase the overall applicability of theories that explain the formation of single, sun-like stars. Correspondingly, other star-formation theories that call for most or all stars to begin their lives in multiple-star systems may be less relevant than previously thought.

"It's certainly possible for binary star systems to 'dissolve' into two single stars through stellar encounters," said astronomer Frank Shu of National Tsing Hua University in Taiwan, who was not involved with this discovery. "However, suggesting that mechanism as the dominant method of single-star formation is unlikely to explain Lada's results."

Lada's finding implies that planets also may be more abundant than astronomers realized. Planet formation is difficult in binary star systems where gravitational forces disrupt protoplanetary disks. Although a few planets have been found in binaries, they must orbit far from a close binary pair, or hug one member of a wide binary system, in order to survive. Disks around single stars avoid gravitational disruption and therefore are more likely to form planets.

Adapted from the information on

Fastest-Spinning Pulsar Found Pulsar Diagram

(Added 01/16/06) Astronomers using the National Science Foundation's Robert C. Byrd Green Bank Telescope have discovered the fastest-spinning neutron star ever found, a 20-mile-diameter superdense pulsar whirling faster than the blades of a kitchen blender. Their work yields important new information about the nature of one of the most exotic forms of matter known in the Universe.

"We believe that the matter in neutron stars is denser than an atomic nucleus, but it is unclear by how much. Our observations of such a rapidly rotating star set a hard upper limit on its size, and hence on how dense the star can be," explained Jason Hessels, a graduate student at McGill University in Montreal.

Pulsars are spinning neutron stars that sling "lighthouse beams" of radio waves or light around as they spin. A neutron star is what is left after a massive star explodes at the end of its "normal" life. With no nuclear fuel left to produce energy to offset the stellar remnant's weight, its material is compressed to extreme densities. The pressure squeezes together most of its protons and electrons to form neutrons; hence, the name "neutron star."

"Neutron stars are incredible laboratories for learning about the physics of the fundamental particles of nature, and this pulsar has given us an important new limit," explained Scott Ransom, an astronomer at the National Radio Astronomy Observatory and one of Hessels' collaborators on this work.

The scientists discovered the pulsar, named PSR J1748-2446ad, in a globular cluster of stars called Terzan 5, located some 28,000 light-years from Earth in the constellation Sagittarius. The newly-discovered pulsar is spinning 716 times per second, or at 716 Hertz (Hz), readily beating the previous record of 642 Hz from a pulsar discovered in 1982. For reference, the fastest speeds of common kitchen blenders are 250-500 Hz.

The scientists say the object's fast rotation speed means that it cannot be any larger than about 20 miles across. According to Hessels, "If it were any larger, material from the surface would be flung into orbit around the star." The scientists' calculation assumed that the neutron star contains less than two times the mass of the Sun, an assumption that is consistent with the masses of all known neutron stars.

The spinning pulsar has a companion star that orbits it once every 26 hours. The companion passes in front of the pulsar, eclipsing the pulsar about 40 percent of the time. The long eclipse period, probably due to bloating of the companion, makes it difficult for the astronomers to learn details of the orbital configuration that would allow them to precisely measure the masses of the pulsar and its companion.

"If we could pin down these masses more precisely, we could then get a better limit on the size of the pulsar. That, in turn, would then give us a better figure for the true density inside the neutron star," explained Ingrid Stairs, an assistant professor at the University of British Columbia and another collaborator on the work.

Competing theoretical models for the types and distributions of elementary particles inside neutron stars make widely different predictions about the pressure and density of such an object. "We want observational data that shows which models fit the reality of nature," Hessels said.

If the scientists can't use PSR J1748-2446ad to do that, they are hopeful some of its near neighbors will yield the data they seek. Using the GBT, the astronomers so far have found 30 new fast "millisecond pulsars" in the cluster Terzan 5, making 33 pulsars known in the cluster in total. This is the largest number of such pulsars ever found in a single globular cluster.

Dense globular clusters of stars are excellent places to find fast-rotating millisecond pulsars. Giant stars explode as supernovae and leave rotating pulsars which gradually slow down. However, if a pulsar has a companion star from which it can draw material, that incoming material imparts its spin, or angular momentum, to the pulsar. As a result, the pulsar spins faster. "In a dense cluster, interactions between the stars will create more binary pairs that can yield more fast-rotating pulsars," Ransom said.

The great sensitivity of the giant, 100-meter diameter GBT, along with a special signal processor, called the Pulsar Spigot, made possible the discovery of so many millisecond pulsars in Terzan 5. "We think there are many more pulsars to be found in Terzan 5 and other clusters, and given that the fast ones are often hidden by eclipses, some of them may be spinning even faster than this new one," Ransom said.

"We're excited about using this outstanding new telescope to answer some important questions about fundamental physics," he said. In addition to Hessels, Ransom and Stairs, the research team includes Paulo Freire of Arecibo Observatory in Puerto Rico, Victoria Kaspi, of McGill University, and Fernando Camilo, of Columbia University. Their report is being published in Science Express, the online version of the journal Science.

Adapted from the information on

The Milky Way Blows a Bubble

(Added 01/16/06) Astronomers using the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT) have discovered a huge "superbubble" of hydrogen gas rising nearly 10,000 light-years above the plane of our Milky Way Galaxy. They believe the gas may be driven by supernova explosions and the intense stellar winds from an unseen cluster of young stars in one of our Galaxy's spiral arms.

"This giant gas bubble contains about a million times more mass than the Sun and the energy powering its outflow is equal to about 100 supernova explosions," described Yurii Pidopryhora, of the National Radio Astronomy Observatory (NRAO) and Ohio University. Pidopryhora, along with Jay Lockman of NRAO, and Joseph Shields of Ohio University, presented their results to the American Astronomical Society's meeting in Washington, DC.

The superbubble is nearly 23,000 light-years from Earth. The astronomers discovered it by combining numerous smaller images made with the GBT into one large image. In addition, they added images of ionized hydrogen in the region that were made by a University of Wisconsin optical telescope on Kitt Peak in Arizona.

"We see that all the hydrogen gas in this region of the Galaxy is disturbed, with many smaller outflows closer to the plane of the Galaxy and then a giant plume of gas that forms a sort of cap on the whole thing," Pidopryhora said. The ionized hydrogen, with atoms violently stripped of their electrons, seems to fill the interior of the superbubble while the neutral hydrogen forms its walls and cap.

Our Milky Way Galaxy, about 100,000 light-years across, somewhat resembles a giant dinner plate, with most of its stars and gas residing in a flat disk. "Gas driven outward from the plane of the Galaxy's disk has been seen many times before, but this superbubble is particularly large," Lockman said. "The eruption that drove this much mass so far out of the plane has to have been unusually violent," he added.

The scientists speculate that the gas may be blown outward by the strong stellar winds and supernova explosions from numerous massive young stars in a cluster. "One theoretical model shows that young stars could power an outflow that matches what we see very closely," Pidopryhora said. According to that model, the superbubble probably is 10-30 million years old.

"Finding this superbubble practically in our back yard is quite exciting, because these superbubbles are very important factors in how galaxies evolve," Lockman said. Superbubbles, powered by supernova explosions and young stellar winds, control the way heavy elements, produced only in the cores of stars, are distributed throughout the galaxy, the scientists said. Those heavy elements are then incorporated into the next generation of stars -- and planets -- to form. "The formation of our own Sun and planets probably was heavily influenced, if not triggered, by a nearby supernova explosion."

In addition, if the outflow from superbubbles is energetic enough, it could blow the gas into intergalactic space, never to return to the galaxy. "This would shut down the formation of new stars in the galaxy," Pidopryhora explained.

Adapted from the information on

How to Grow a Black Hole

(Added 01/16/06) Astronomers announced that they have found the first sample of intermediate-mass black holes in active galaxies - a discovery that will help in understanding the early universe. "These are local analogues of the `seed' black holes from which supermassive black holes formed," explained Ms. Jenny E. Greene of the Harvard-Smithsonian Center for Astrophysics (CfA).

"Supermassive black holes (with masses of millions to billions of times the mass of the Sun) are found in the centers of most, if not all, massive galaxies, and the black hole masses scale with the galaxy masses, so that larger black holes reside in larger galaxies," explained Greene. "We want to understand how this connection is established, and more specifically, what role black holes play in the evolution of galaxies."

Black holes probably evolve as material, such as gas, dust, stars and even other black holes, gets sucked in by the strong gravitational pull. "However, we cannot observe the starting conditions of the black holes directly," remarked Ho. "How massive were they? How and when were they made? These are crucial questions to answer if we want to understand how black holes impact the growth of galaxies."

The black hole "seeds" originally may have formed from the explosions of the first stars or from the collapse of clumps of gas in the early universe. Each of these different formation scenarios leads to very different numbers of intermediate-mass black holes left over in the universe today. Until now, few good candidates had been found.

Greene sifted for intermediate-mass black holes in the first data release from the Sloan Digital Sky Survey, a multi-year comprehensive survey of one quarter of the sky. (The first public data release from the SDSS contains information on 50 million objects, including spectra and redshifts for almost 200,000 objects.)

In her thesis work, Greene identified objects with black holes by detecting the light from gas moving at extremely high velocities close to the black hole. Using the speed of the gas, and an estimate of the distance of the gas from the black hole, it was possible to estimate a black hole mass for each galaxy. She then selected all of the objects with masses less than one million solar masses, yielding a total of 19 new black holes.

"This sample provides the only currently available observational constraints on the properties of seed black holes in the early universe," said Greene. Besides the formation of supermassive black holes seen today, this data set may help with another question - the re-ionization of the universe.

Present theory holds that soon after the Big Bang, the universe was filled mostly with hydrogen and helium that was ionized - too hot to remain in a stable state. Over about 300,000 years, the universe expanded and cooled, and the gases began to recombine and stabilize to neutral states. This neutral gas acted as an opaque fog blocking the transmission of light. The universe then entered the dark ages, estimated to have lasted about half a billion years.

At the same time, matter was clumping together to form the first stars, galaxies and quasars. The radiation from these new objects made the opaque gas of the universe become transparent by splitting atoms of hydrogen into free electrons and protons, thus re-ionizing the universe.

"These seed black holes presumably occasionally lit up as 'mini-quasars.' It is still an open question whether the emission from small black holes played an important role in re-ionizing the universe, ending the cosmic dark ages," said Greene. "Our measurements of the light radiated by low-mass black holes will help us decide whether or not black holes in this mass range could have contributed significantly to re-ionization."

Gravity waves also could point to an early population of intermediate-sized black holes. When two black holes merge, their coalescence sends out gravity waves, or ripples in space-time. "Gravitational wave experiments, especially the Laser Interferometric Space Antenna (LISA) expect to be very sensitive to the merging of 100,000-solar-mass black holes," said Greene. "The objects we identified will give clues to help the LISA team determine how many black hole collisions they may expect to find."

Adapted from the information on

Spitzer Finds Extra-Solar Cometary Debris?

(Added 01/11/06) NASA's Spitzer Space Telescope has spotted what may be comet dust sprinkled around the white dwarf star G29-38, which died approximately 500 million years ago. The findings suggest the dead star, which most likely consumed its inner planets, is still orbited by a ring of surviving comets and possibly outer planets. This is the first observational evidence that comets can outlive their suns.

"Astronomers have known for decades that stars are born, have an extended middle age, and then wither away or explode. Spitzer is helping us understand how planetary systems evolve in tandem with their parent stars," explained David Leisawitz, NASA's Spitzer program scientist.

Astronomers believe white dwarfs are shrunken skeletons of stars that were once similar to Earth's sun. As the stars aged over billions of years, they grew brighter and eventually swelled in size to become red giants. Millions of years later, the red giants shed their outer atmospheres, leaving behind white dwarfs.

If any planets did orbit in these systems, the red giants would have engulfed at least the inner ones. Only distant outer planets and an orbiting icy outpost of comets would have survived. "The dust seen by Spitzer around G29-38 was probably generated relatively recently when one such outlying comet may have been knocked into the inner region of the system and ripped into dust shreds by the tidal forces of the star," said astronomer William Reach of the Spitzer Science Center at the California Institute of Technology in Pasadena, CA.

Prior to the Spitzer findings, astronomers studying G29-38 noticed an unusual and unknown source of infrared light. Spitzer, with its powerful infrared spectrometer, was able to break this light apart, revealing its molecular makeup. These data told astronomers the light was coming from the same types of dusty minerals found in comets in our solar system.

"We detected a large quantity of very small, dirty silicate grains," described astronomer Marc Kuchner of NASA's Goddard Space Flight Center, Greenbelt, MD. "The size of these grains tells us they are probably from comets and not other planetary bodies."

In our own solar system, comets reside in the cold outer fringes in regions known as the Kuiper Belt and Oort Cloud. Only when something disturbs their orbits, such as another comet or an outer planet, do they begin periodic journeys into the sun's warmer neighborhood. However, these trips to the tropics often end in destruction. Comets slowly disintegrate as they pass close to the sun, or they crash into it. They also occasionally crash into planets, as comet Shoemaker-Levy 9 did when it plunged into Jupiter.

Though the dust seen by Spitzer around the white dwarf is most likely the remains of such a torn-up comet, there may be other explanations. One possibility is that a second wave of planets formed long after the death of the star, leaving a dusty construction zone.

Adapted from the information on, and it was published in the December 20, 2005, issue of The Astrophysical Journal.

Hubble Continues to Shed New Light on Cosmic Wonders Nebula M42 AKA The Great Orion Nebula (or the Orion Nebula) as imaged by NASA's Hubble Space Telescope (HST)

(Added 01/11/06) In one of the most detailed astronomical images ever produced, NASA's Hubble Space Telescope is offering an unprecedented look at the Orion Nebula. This turbulent star-formation region is one of astronomy's most dramatic and photogenic celestial objects.

The crisp image reveals a tapestry of star formation, from the dense pillars of gas and dust that may be the homes of fledgling stars to the hot, young, massive stars that have emerged from their gas-and-dust cocoons and are shaping the nebula with their powerful ultraviolet light. The new picture reveals large-scale structures never seen before, according to C. Robert O'Dell of Vanderbilt University in Nashville, TN. "Only with the Hubble Space Telescope can we begin to understand them," O'Dell said.

In a mosaic containing a billion pixels (reduced in the image here), Hubble's Advanced Camera for Surveys (ACS) uncovered 3,000 stars of various sizes. Some of them have never been spied in visible light. Some are merely 1/100 the brightness of stars seen previously in the nebula.

Among the stars Hubble spotted are possible young brown dwarfs, the first time these objects have been seen in the Orion Nebula in visible light. Brown dwarfs are "failed stars;" these cool objects are too small to be ordinary stars because they cannot sustain nuclear fusion in their cores the way our Sun does.

The Hubble Space Telescope also spied for the first time a small population of possible binary brown dwarfs -- two brown dwarfs orbiting each other. Comparing the characteristics of newborn stars and brown dwarfs in their natal environment provides unique information about how they form.

"The wealth of information in this Hubble survey, including seeing stars of all sizes in one dense place, provides an extraordinary opportunity to study star formation," remarked Massimo Robberto of the Space Telescope Science Institute in Baltimore, MD, and leader of the observations. "Our goal is to calculate the masses and ages for these young stars so that we can map their history and get a general census of the star formation in that region. We can then sort the stars by mass and age and look for trends."

The Orion Nebula is a perfect laboratory to study how stars are born because it is 1,500 light-years away, a relatively short distance within our 100,000 light-year wide galaxy. Astronomers have a clear view into this crowded stellar maternity ward because massive stars in the center of the nebula have blown out most of the dust and gas in which they formed, carving a cavity in the dark cloud.

"In this bowl of stars we see the entire star formation history of Orion printed into the features of the nebula: Arcs, blobs, pillars, and rings of dust that resemble cigar smoke," Robberto said. "Each one tells a story of stellar winds from young stars that impact the stellar environment and the material ejected from other stars. This is a typical star-forming environment. Our Sun was probably born 4.5 billion years ago in a cloud like this one."

This extensive study took 105 Hubble orbits to complete. All imaging instruments aboard the telescope – the ACS, Wide Field and Planetary Camera 2, and Near Infrared Camera and Multi-Object Spectrometer – were used simultaneously to study the nebula. The ACS mosaic covers approximately the apparent angular size of the full moon.

Adapted from the information on

Vega's Bulging Waistline

(Added 01/11/06) While 60% of Americans are overweight, perhaps it's a little comforting to know that some stars, too, have large bulges around their middles. In particular, new information shows that the 5th-brightest star in the sky, Vega, bulges at its equator so much that it may be a few thousand degrees cooler there than at its poles.

The finding, owing to a phenomena called "gravity darkening," confirms predictions about the star. It could also force scientists to rethink their ideas about the amount of light Vega is shining on a dusty debris ring, called the circumstellar disk, which surrounds the star's equator and is thought to be a potential birthplace for planets.

Vega is located 25 light-years away from Earth in the constellation Lyra. Because Earth's pole wobbles over time, it will take Polaris's place as the North Star in 14,000 AD. Vega makes a full rotation about its axis once every 12.5 hours. The Sun, in comparison, takes 27 days to make one rotation, even though it is much smaller than Vega.

Using Georgia State University's Center for High Angular Resolution Astronomy (CHARA) array in California, researchers confirmed a prediction made by astronomer Richard Gray in 1985 that Vega is spinning at 90% of its critical rotation speed. Stars have a maximum speed, called the "critical rotation," at which they can spin. If stars exceed the critical rotation, the outward force caused by their spinning will overcome the inward gravitational force that keeps the star together.

"If stars get to that limit, they will begin to fly apart," said Jason Aufdenberg, a postdoctoral researcher from the National Optical Astronomy Observatory in Arizona who was involved in the study. One major consequence of Vega's speedy rotation is that it bulges significantly at its equator: The star is nearly 23% fatter than it is tall. Because of the elliptical shape, gravity is not even across the entire surface.

"For an ellipsoid, you actually have lower gravity when you're at the equator because you're farther away from the center of mass," Aufdenberg explained. Gravity in turn affects how much energy a star radiates, a measure known as flux. In regions of the star where gravity is low, like at the equator, the flux will also be low. This sets up a temperature difference between the star's equator and its poles. This is what scientists call gravity darkening.

In the case of Vega, gravity darkening is causing its equator to be cooler by 4,000 °F than at its poles. The finding was presented at the 207th annual meeting of the American Astronomical Society.

Scientists can't observe Vega's equatorial bulge directly because Vega is oriented in such a way that astronomers on Earth have only a pole-on view of the star. The new results, derived from the temperature data, could affect the way scientists think about Vega's circumstellar disk, Aufdenberg said.

Many scientists are interested in the disk of gas and dust because it may be similar to those found around other stars. Circumstellar disks are thought to arise mainly from the collision of rocky asteroid-like objects and to form the starting material for the formation of planets and stars. "Up until now, people have been modeling Vega's spectrum as we see it from Earth, but our measurements make it pretty clear that Vega's circumstellar disk sees a spectrum that is cooler," Aufdenberg told

Based on the new results, the researchers estimate that the amount of light Vega's circumstellar disk receives is only about half of what was previously thought. The change could require scientists to rethink how much energy is being absorbed by the dust and gas that makes up the disk. "If it's being heated up less than it was before, then that changes maybe the number of dust grains that have to be there or the type of dust that is there," Aufdenberg said.

Adapted from the information on

A Galactic Merger at Home

(Added 01/11/06) A huge but very faint structure, containing hundreds of thousands of stars spread over an area nearly 5,000 times the size of a full moon, has been discovered and mapped by astronomers of the Sloan Digital Sky Survey (SDSS-II).

At an estimated distance of 30,000 light years (10 kiloparsecs) from Earth, the structure lies well within the confines of the Milky Way Galaxy. However, it does not follow any of Milky Way's three main components: A flattened disk of stars in which the sun resides, a bulge of stars at the center of the Galaxy and an extended, roughly spherical, stellar halo. Instead, the researchers believe that the most likely interpretation of the new structure is a dwarf galaxy that is merging into the Milky Way. The new dwarf galaxy is found toward the constellation Virgo.

"Some of the stars in this Milky Way companion have been seen with telescopes for centuries," explained Princeton University graduate student Mario Juric, principal author of the findings describing what may well be our closest galactic neighbor. "But because the galaxy is so close, its stars are spread over a huge swath of the sky, and they always used to be lost in the sea of more numerous Milky Way stars. This galaxy is so big, we couldn't see it before."

The result was presented today in a session on The Milky Way at the American Astronomical Society meeting in Washington, D.C.

The discovery was made possible by the unprecedented depth and photometric accuracy of the SDSS, which to date has imaged roughly one-quarter of the northern sky. "We used the SDSS data to measure distances to 48 million stars and build a 3-D map of the Milky Way," explained Zeljko Ivezic of the University of Washington, a co-author of the study. Details of this "photometric parallax" method - using the colors and apparent brightnesses of stars to infer their distances -- are explained in the paper "Milky Way Tomography" submitted to The Astrophysical Journal.

"It's like looking at the Milky Way with a pair of 3-D glasses," said Princeton University co-author Robert Lupton. "This structure that used to be lost in the background suddenly snapped into view."

The new result is reminiscent of the 1994 discovery of the Sagittarius dwarf galaxy, by Rodrigo Ibata and collaborators from Cambridge University. They used photographic images of the sky to identify an excess of stars on the far side of the Milky Way, some 75,000 light-years from Earth. The Sagittarius dwarf is slowly dissolving, trailing streams of stars behind it as it orbits the Milky Way and sinks into the Galactic disk.

In the last decade a new generation of sky surveys using large digital cameras identified a number of streams and lumps of stars in the outer Milky Way. Some of these lumps are probably new Milky Way companions; others may be shreds of the Sagittarius dwarf or of other dissolving dwarf galaxies. Earlier SDSS discoveries include an apparent ring of stars encircling the Milky Way disk that may be the remnant of another disrupted galaxy; and the Ursa Major dwarf, the faintest known neighbor of the Milky Way.

The first hints of an unusually high density of stars in the direction of Virgo were made in 2001 by the QUEST survey, which used a 1-meter telescope in Venezuela to study a class of variable stars called RR Lyrae variables.

"We found a clump of 5 RR Lyrae stars, and speculated that they were they belonged to a small galaxy being cannibalized by the Milky Way," explained Kathy Vivas of the Centro de Investigaciones de Astronomia in Venezuela, who (as a Yale graduate student) was the author of the QUEST discovery paper. "In light of the new SDSS results," Vivas added, "it appears that the stellar stream we detected is itself part of the larger structure identified by Juric and collaborators."

"With so much irregular structure in the outer Galaxy, it looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it," said Juric.

While the SDSS was originally designed to study the distant universe, its wide area, high precision maps of faint stars have made it an invaluable tool for studying the Milky Way and its immediate neighborhood. The 3-D map created by Juric and his collaborators also provides strong new constraints on the shape and extent of the Milky Way's disk and stellar halo.

"The SDSS has already told us surprising things about the Milky Way, but the most exciting discoveries should lie just ahead."

Adapted from the information on A paper of the results is available in preprint form at

The North Star Has Some Company Polaris and Companions Polaris b and Polaris ab

(Added 01/11/06) We tend to think of the North Star, Polaris, as a steady, solitary point of light that guided sailors long ago. But there is more to the North Star than meets the eye: The North Star is actually a triple star system. And while one companion can be seen easily through small telescopes, the other hugs Polaris so tightly that it has never been seen -- until now.

By stretching the capabilities of NASA's Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

"The star we observed is so close to Polaris that we needed every available bit of Hubble's resolution to see it," explained Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arcsecond from Polaris -- an incredibly tiny angle, equivalent to the apparent diameter of a quarter located 19 miles away. At the system's distance of 430 light-years, that translates into a separation of about 2 billion miles (a distance from the Sun to half-way between Saturn and Uranus).

"The brightness difference between the two stars made it even more difficult to resolve them," stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. "With Hubble, we've pulled the North Star's companion out of the shadows and into the spotlight."

By watching the motion of the companion star, Evans and her colleagues expect to learn not only the stars' orbits but also their masses. Measuring the mass of a star is one of the most difficult tasks facing stellar astronomers. The researchers plan to continue observing the Polaris system for several years. In that time, the movement of the small companion in its 30-year orbit around the primary should be detectable.

Astronomers want to determine the mass of Polaris accurately, because it is the nearest Cepheid variable star. Cepheids' brightness variations are used to measure the distances of galaxies and the expansion rate of the universe, so it is essential to understand their physics and evolution. Knowing their mass is the most important ingredient in this understanding.

"Studying binary stars is the best available way to measure the masses of stars," remarked science team member Gail Schaefer of STScI. "We only have the binary stars that nature provided us," added Bond. "With the best instruments like Hubble, we can push farther into space and study more of them up close."

Adapted from the information on

Using a Star to Learn about Pluto's Charon

(Added 01/11/06) Observing a very rare occultation of a star by Pluto's satellite Charon from three different sites, astronomers were able to determine with great accuracy the radius and density of the main satellite of the farthest planet. The density, 1.71 that of water, is indicative of an icy body with slightly more than half of rocks. The observations also put strong constraints on the existence of an atmosphere around Charon.

Since its discovery in 1978, Charon and Pluto have appeared to form a double planet, rather than a planet-satellite couple. Actually, Charon is about twice as small as Pluto in size, and about eight times less massive. However, there have been considerable discussions concerning the precise radii of Pluto and Charon, as well as about the presence of a tenuous atmosphere around Charon.

In August 2004, Australian amateur astronomer Dave Herald predicted that the 15th magnitude star UCAC2 26257135 should be occulted by - pass behind as seen from Earth - Charon on July 11, 2005. The occultation would be observable from some parts of South America; because of the precise geometry and the vast distance to Charon from Earth, it would not be visible from everywhere on Earth.

Stellar occultations have proved to be powerful tools to both measure sizes - at km-level accuracy, i.e. a factor ten better than what is feasible with other techniques - and detect very tenuous atmosphere - at microbar levels or less. Unfortunately, in the case of Charon, such occultations are extremely rare, owing to the very small angular diameter of the satellite on the sky.

This explains why only one occultation by Charon was ever observed before 2005, namely on April 7, 1980, by Alistair Walker, from the South Africa Astronomical Observatory. Similarly, only in 1985, 1988, and 2002 could astronomers observe stellar occultations by Pluto. Quite surprisingly, the 2002 event showed that Pluto's atmospheric pressure had increased by a factor of two in four years.

"Several factors, however, have boosted our odds for witnessing occultations of Charon," said Bruno Sicardy, from Paris Observatory (France) and lead author of the paper reporting the results. "First, larger telescopes now give access to fainter stars, thus multiplying the candidates for occultations. Secondly, stellar catalogues have become much more precise, allowing us to do better predictions. And, finally, the Pluto-Charon system is presently crossing the Milky Way, thereby increasing the likelihood of an occultation."

The July 2005 event was eventually observed from Paranal with Yepun, the fourth Unit Telescope of the VLT, equipped with the adaptive optics instrument NACO, as well as with the 0.5 m "Campo Catino Austral Telescope" at San Pedro de Atacama (Chile), and with the 2.15 m "Jorge Sahade" telescope at Cerro El Leoncito (Argentina).

An accurate timing of the occultation seen at the three sites provides the most accurate measurement of Charon's size: its radius is found to be 603.6 km, with an error of the order of 5 km. This accuracy now allows astronomers to pin Charon's density down to 1.71 that of water Quite remarkably, Charon's density is now measured with much more precision than Pluto's.

Thanks to these observations, Sicardy and his collaborators could determine that if an tenuous atmosphere exists on Charon, linking it to the freezing -220 °C or so surface, its pressure has to be less than one tenth of a millionth that at the surface of the Earth, or 0.1 microbar, assuming that it is constituted entirely of nitrogen. A similar upper limit is derived for a gas like carbon monoxide. This is more than a factor one hundred smaller than Pluto's surface pressure, which is estimated to be in the range 10-15 microbars. The observations also indicate that methane ice, if present, should be restricted to very cold regions of the surface. Similarly, nitrogen ice would be confined at best to high northern latitudes or permanently shadowed regions of Charon.

As Pluto and its satellite sweep across the Milky Way, observations of more occultations will be tempted from the ground, while the NASA's Pluto-Kuiper Belt Mission, to be launched in January 2006, will be traveling towards the planet, that it should reach in July 2015.

Adapted from the information on, and it was published in the January 5, 2006 issue of the journal Nature.

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