Category: Astronomy

Life near the center of our galaxy never had a chance. Every 20 million years on average, gas pours into the galactic center and slams together, creating millions of new stars. The more massive stars soon go supernova, exploding violently and blasting the surrounding space with enough energy to sterilize it completely. This scenario is detailed by astronomer Antony Stark (Harvard-Smithsonian Center for Astrophysics) and colleagues in the October 10, 2004, issue of The Astrophysical Journal Letters.

The team’s discovery was made possible using the unique capabilities of the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). It is the only observatory in the world able to make large-scale maps of the sky at submillimeter wavelengths.

The gas for each starburst comes from a ring of material located about 500 light-years from the center of our galaxy. Gas collects there under the influence of the galactic bar-a stretched oval of stars 6,000 light-years long rotating in the middle of the Milky Way. Tidal forces and interactions with this bar cause the ring of gas to build up to higher and higher densities until it reaches a critical density or “tipping point.” At that point, the gas collapses down into the galactic center and smashes together, fueling a huge burst of star formation.

“A starburst is star formation gone wild,” says Stark.

Astronomers see starbursts in many galaxies, most often colliding galaxies where lots of gas crashes together. But starbursts can happen in isolated galaxies too, including our own galaxy, the Milky Way.

The next starburst in the Milky Way is coming relatively soon, predicts Stark. “It likely will happen within the next 10 million years.”

That assessment is based on the team’s measurements showing that the gas density in the ring is nearing the critical density. Once that threshold is crossed, the ring will collapse and a starburst will blaze forth on an unimaginably huge scale.

Some 30 million solar masses of matter will flood inward, overwhelming the 3 million solar mass black hole at the galactic center. The black hole, massive as it is, will be unable to consume most of the gas.

“It would be like trying to fill a dog dish with a firehose,” says Stark. Instead, most of the gas will form millions of new stars.

The more massive stars will burn their fuel quickly, exhausting it in only a few million years. Then, they will explode as supernovae and irradiate the surrounding space. With so many stars packed so close together as a result of the starburst, the entire galactic center will be impacted dramatically enough to kill any life on an Earth-like planet. Fortunately, the Earth itself lies about 25,000 light-years away, far enough that we are not in danger.

The facility used to make this discovery, AST/RO, is a 1.7-meter-diameter telescope that operates in one of the most challenging environments on the planet-the frigid desert of Antarctica. It is located at the National Science Foundation’s Amundsen-Scott Station at the South Pole. The air at the South Pole is very dry and cold, so radiation that would be absorbed by water vapor at other sites can reach the ground and be detected.

“These observations have helped advance our understanding of star formation in the Milky Way,” says Stark. “We hope to continue those advancements by collaborating with researchers who are working on the Spitzer Space Telescope’s Legacy Science Program. AST/RO’s complementary observations would uniquely contribute to that effort.”

Stark’s co-authors on the paper announcing this finding are Christopher L. Martin, Wilfred M. Walsh, Kecheng Xiao and Adair P. Lane (Harvard-Smithsonian Center for Astrophysics), and Christopher K. Walker (Steward Observatory).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

Image credit: NASA
Three powerful blasts from three wholly different regions in space have left scientists scrambling. The blasts, which lasted only a few seconds, might be early alert systems for star explosions called supernovae, which could start appearing any day now.

The first two blasts, called X-ray flashes, occurred on September 12 and 16. These were followed by a more powerful burst on September 24 that seems to be on the cusp between an X-ray flash and a full-fledged gamma-ray burst, a discovery interesting in its own right. If these signals lead to supernovae, as expected, scientists would have a tool to predict star explosions and then watch them go off from start to finish.

A team led by Dr. George Ricker of the Massachusetts Institute of Technology detected the explosions with NASA’s High-Energy Transient Explorer (HETE-2). Science teams around the world using space- and ground-based observatories have joined in, torn and conflicted over which burst region to track most closely.

“Each burst has been beautiful,” said Ricker. “Depending on how these evolve, they could support important theories about supernovae and gamma-ray bursts. These past two weeks have been like ‘cock, fire, reload.’ Nature keeps on delivering, and our HETE-2 satellite keeps on responding flawlessly.”

Gamma-ray bursts are the most powerful explosions known other than the Big Bang. Many appear to be caused by the death of a massive star collapsing into a black hole. Others might be from merging black holes or neutron stars. In either case, the event likely produces twin, narrow jets in opposite directions, which carry off tremendous amounts of energy. If one of jets points to Earth, we see this energy as a “gamma-ray” burst.

The lower-energy X-ray flashes might be gamma-ray bursts viewed slightly off angle from the jet direction, somewhat similar to how a flashlight is less blinding when viewed at an angle. The majority of light particles from X-ray flashes, called photons, are X rays — energetic, but not quite as powerful as gamma rays. Both types of bursts last only a few milliseconds to about a minute. HETE-2 detects the bursts, studies their properties, and provides a location so that other observatories can study the burst afterglow in detail.

The trio of bursts from the past few weeks has the potential of settling two long-standing debates. Some scientists say that X-ray flashes are different beasts all together, not related to gamma-ray bursts and massive star explosions. Detecting a supernova in the region where the X-ray flash appeared would refute that belief, instead confirming the connection between the two. Follow-up observations of the September 24 burst, named GRB040924 for the date it was observed, are already solidifying the theory of a cosmic explosion continuum from X-ray flashes up through gamma-ray bursts.

More interesting for supernova hunters is the fact that X-ray flashes are closer to Earth than gamma-ray bursts are. While the connection between gamma-ray bursts and supernovae has been made, these supernovae are too distant to study in detail. X-ray flashes might be signals for supernovae that scientists can actually sink their teeth into and observe in detail. Yet for now, it is just watch and wait.

“Last year the discovery of GRB030329 by HETE-2 sealed the connection between gamma-ray bursts and massive supernovae,” said Prof. Stanford Woosley of the University of California at Santa Cruz, who has championed several theories concerning the physics of star explosions. “These two September bursts may be the first time we see an X-ray flash lead to a supernova. We might know very soon.”

In addition to all of this, GRB040924 goes on record as generating the fastest response ever for a gamma-ray burst satellite. HETE-2 detected the burst and relayed information through the NASA-operated Gamma-ray Burst Coordinates Network in under 14 seconds, which led to an optical detection about 15 minutes later with the Palomar 60-inch telescope, just north of San Diego. Dr. Derek Fox of Caltech was the lead on this observation.

“We all expect much more of this type of exciting science to come after the launch of Swift,” said Dr. Anne Kinney, director of NASA’s Universe Division. Swift, to launch in October, contains three telescopes (gamma ray, X ray and UV/optical) for quick burst detection, swift relay of information, and immediate follow-up observations of the afterglow.

HETE was built by MIT as a mission of opportunity under the NASA Explorer Program, collaboration among U.S. universities, Los Alamos National Laboratory, and scientists and organizations in Brazil, France, India, Italy and Japan.

Additional information about the physics of star explosions:
While many scientists say that X-ray flashes are gamma-ray bursts viewed slightly off angle, another theory is that the star explosion that causes the X-ray flash is rich in baryons (a family of particles that includes protons and neutrons), as opposed to leptons (particles that include electrons). A baryon-dominated blast would produce more X rays, and a lepton-dominated blast would produce more gamma rays. This is because the baryons move more slowly than leptons; and slower moving matter would make a softer (lower-energy) burst at all angles.

According to Dr. Stanford Woosley, the supernova / gamma-ray burst connection is this: When a massive star runs out of nuclear fuel, its core will collapse, yet without the star’s outer part knowing. A black hole forms inside surrounded by a disk of accreting matter, and, within a few seconds, this launches a jet of matter away from the black hole that ultimately makes the gamma-ray burst. The jet pierces the outer shell of the star about nine seconds after its creation. The jet of matter, in conjunction with vigorous winds of newly forged radioactive nickel-56 blowing off the disk inside, shatters the star within seconds. This shattering represents the supernova event, and the amount of radioactive nickel-56 gives its brightness. However, from our vantage point, we will not see the supernova until about two weeks after the gamma-ray burst because the region is enshrouded by gas and dust, blocking light.

Original Source: NASA News Release

With ESA?s XMM-Newton observatory, an international team of scientists has observed a nearby head-on collision of two galaxy clusters that has smashed together thousands of galaxies and millions upon millions of stars. It is one of the most powerful events ever witnessed. Such collisions are second only to the Big Bang in total energy output.

The event details what the scientists are calling the ?perfect cosmic storm?: galaxy clusters that collided like two high-pressure weather fronts and created hurricane-like conditions, tossing galaxies far from their paths and churning shock waves of 100-million-degree gas through intergalactic space.

This unprecedented view of a merger in action crystallises the theory that the Universe built its magnificent hierarchal structure from the ?bottom up? – essentially through mergers of smaller galaxies and galaxy clusters into bigger ones.

“Here before our eyes we see the making of one of the biggest objects in the Universe,” said Dr Patrick Henry of the University of Hawaii, who led the study. “What was once two distinct but smaller galaxy clusters 300 million years ago is now one massive cluster in turmoil.?

Henry and his colleagues, Alexis Finoguenov and Ulrich Briel of the Max-Planck Institute for Extraterrestrial Physics in Germany, present these results in an upcoming issue of the Astrophysical Journal. The forecast for the new super-cluster, they said, is ‘clear and calm’ now that the worst of the storm has passed.

Galaxy clusters are the largest gravitationally bound structures in Universe, containing hundreds to thousands of galaxies. Our Milky Way galaxy is part of a small group of galaxies but is not gravitationally bound to the closest cluster, the Virgo Cluster. We are destined for a collision in a few thousand million years, though.

The cluster named Abell 754 in the constellation Hydra has been known for decades. However, to the scientists’ surprise, the new observation reveals that the merger may have occurred from the opposite direction than what was thought. They found evidence for this by tracing the wreckage today left in the merger’s wake, spanning a distance of millions of light years. While other large mergers are known, none has been measured in such detail as Abell 754.

For the first time, the scientists could create a complete ?weather map? of Abell 754 and thus determine a forecast. This map contains information about the temperature, pressure and density of the new cluster. As in all clusters, most the ordinary matter is in the form of gas between the galaxies and not locked up in the galaxies or stars themselves. The massive forces of the merging clusters accelerated intergalactic gas to great speeds. This resulted in shock waves that heat the gas to very high temperatures, which then radiated X-ray light, far more energetic than the visible light our eyes can detect. XMM-Newton, in orbit, detects this type of high-energy light.

The dynamics of the merger revealed by XMM-Newton point to a cluster in transition. “One cluster has apparently smashed into the other from the ‘north-west’ and has since made one pass through,” said Finoguenov. “Now, gravity will pull the remnants of this first cluster back towards the core of the second. Over the next few thousand million of years, the remnants of the clusters will settle and the merger will be complete.”

The observation implies that the largest structures in the Universe are essentially still forming in the modern era. Abell 754 is relatively close, about 800 million light years away. The construction boom may soon be over in a few more thousand million years though. A mysterious substance dubbed ‘dark energy’ appears to be accelerating the Universe’s expansion rate. This means that objects are flying apart from each other at an ever-increasing speed and that clusters may eventually never have the opportunity to collide with each other.

X-ray observations of galaxy clusters such as Abell 754 will help to better define dark energy and also dark matter, an ?invisible? and mysterious substance that appears to comprise over 80 percent of a galaxy cluster’s mass.

This observation was announced at a NASA Internet press conference today. A paper describing these results, by Patrick Henry and his collaborators, will be published in the Astrophysical Journal.

Original Source: ESA News Release

A mystery lurking at the centre of our own Milky Way galaxy – an object radiating high-energy gamma rays – has been detected by a team of UK astronomers working with international partners. Their research, published today (September 22nd) in the Journal Astronomy and Astrophysics, was carried out using the High Energy Stereoscopic System (H.E.S.S.), an array of four telescopes, in Namibia, South-West Africa.

The Galactic Centre harbours a number of potential gamma-ray sources, including a supermassive black hole, remnants of supernova explosions and possibly an accumulation of exotic ‘dark matter’ particles, each of which should emit the radiation slightly differently. The radiation observed by the H.E.S.S. team comes from a region very near Sagittarius A*, the black hole at the centre of the galaxy. According to most theories of dark matter, it is too energetic to have been created by the annihilation of dark matter particles. The observed energy spectrum best fits theories of the source being a giant supernova explosion, which should produce a constant stream of radiation.

Dr. Paula Chadwick of the University of Durham said, “We know that a giant supernova exploded in this region 10,000 years ago. Such an explosion could accelerate cosmic gamma rays to the high energies we have seen – a billion times more energy than the radiation used for X-rays in hospitals. But further observations will be needed to determine the exact source.”

Professor Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council (PPARC) which funds UK involvement in H.E.S.S. said; “Science continues to throw out the unexpected as we push back the frontiers of knowledge.” Halliday added “The centre of our Galaxy is a mysterious place, home to exotic phenomena such as a black hole and dark matter. Finding out which of these sources produced the gamma-rays will tell us a lot about the processes taking place in the very heart of the Milky Way.”

However, the team’s theory doesn’t fit with earlier results obtained by the Japanese /Australian CANGAROO instrument or the US Whipple instrument. Both of these have detected high-energy gamma rays from the Galactic Centre in the past (observations from 1995-2002), though not with the same precision as H.E.S.S, and they were unable to pinpoint the exact location as H.E.S.S. has now done, making it harder to deduce the source. These previous results have different characteristics to the H.E.S.S. observations. It is possible that the gamma-ray source at the Galactic Centre varies over the timescale of a year, suggesting that the source is in fact a variable object, such as the central black hole.

The H.E.S.S. team hopes to unravel the mystery with further observations of the Galactic Centre over the next year or two. The full array of four telescopes will be inaugurated on September 29th 2004.

Original Source: PPARC News Release

The Universe has experienced far fewer collisions among galaxies than previously thought, according to a new analysis of Hubble Space Telescope data by an ANU researcher.

Astronomer Dr Alister Graham, from the Research School of Astronomy and Astrophysics, analysed a sample of galaxies located 100 million light years away ? and discovered that the number of violent encounters between large galaxies is around one-tenth of the number earlier studies had suggested.

Although theoretical models predict that fewer collisions were involved in the evolution of the universe, Dr Graham?s observations are the first that confirm these theories.

?The new result is in perfect agreement with popular models of hierarchical structure formation in our universe,? Dr Graham said. ?Galactically speaking, things appear to be a little safer out there.?

For years, astronomers have known the collision and merger of galaxies resulted in the formation of larger galaxies. The biggest of these galaxies appear largely devoid of stars at their cores, a phenomenon believed to result from the damage caused by the ?supermassive? black holes from the smaller galaxies as they merge near the centre of the new galaxy.

However, rather than requiring multiple mergers to clear away the stars from the heart of a galaxy, Dr Graham has shown just one collision between two galaxies is sufficient.

Using images from Hubble’s Wide Field Planetary Camera 2, Dr Graham was able to examine galaxies 100 million light years away, whose cores had not been depleted of stars, providing an important insight into star distributions before any major collisions occurred. By considering the overall galaxy structure, he was able to more accurately measure the sizes of the depleted cores in the galaxies.

The result: the mass of the deficit of stars at the galaxies centres equalled rather than exceeded the mass of the black hole.

?If there had been 10 mergers, we would have found a star deficit 10 times the mass of the central black hole. Many galaxies have large central black holes but no depleted cores. It is therefore not the case that every black hole is formed by gobbling up its surrounding stars. Instead, we?re observing the demolished cores of galaxies after the union of two massive cosmic wrecking balls.?

Although small satellite galaxies have been captured by our galaxy, the Milky Way, it has not experienced a recent major merger. If it had, the plane of its disk, visible as a faint wide ribbon in the night sky, would have been scattered and dispersed across the heavens. Such a fate is expected in about three billion years when the Milky Way collides with a neighbouring spiral galaxy, Andromeda.

The research was conducted during Dr Graham’s tenure at the University of Florida and was funded by NASA via a grant from the Space Telescope Science Institute in Baltimore. Dr Graham?s research will appear in the September 20 edition of Astrophysical Journal Letters.

Original Source: ANU News Release

For more than 400 years, astronomers both professional and amateur have taken a special interest in observing Mira stars, a class of variable red giants famous for pulsations that last for 80-1,000 days and cause their apparent brightness to vary by a factor of ten times or more during a cycle.

An international team of astronomers led by Guy Perrin from the Paris Observatory/LESIA (Meudon, France) and Stephen Ridgway from the National Optical Astronomy Observatory (Tucson, Arizona, USA) has used interferometric techniques to observe the close environments of five Mira stars, and were surprised to find that the stars are surrounded by a nearly transparent shell of water vapor, and possibly carbon monoxide and other molecules. This shell gives the stars a deceptively large apparent size. By penetrating through this layer using the combined light of several telescopes, the team found that Mira stars are likely only half as large as formerly believed.

?This discovery resolves nagging inconsistencies between observations of the size of Mira stars, and models describing their composition and pulsations, which now can be seen to generally agree with each other,? Ridgway explains. ?The revised picture is that Mira stars are very luminous yet relatively normal stars of the asymptotic giant branch, but they have a resonant pulsation that drives their large variability.?

Mira stars are particularly interesting since they are similar in size to the Sun and they are undergoing a late stage of the same evolutionary path that all one-solar mass stars, including the Sun, will experience. Therefore, these stars illustrate the fate of our Sun five billion years from now. If such a star, including its surrounding shell, were located at the Sun?s position in our solar system, its vaporous shell would extend beyond the orbit of Mars.

Although they are really very large in diameter (up to a few hundred solar radii), red giant stars are point-like to unaided human eyes on Earth, and even the largest telescopes fail to distinguish their surfaces. This challenge can be overcome by combining signals from separate telescopes using a technique called astronomical interferometry that makes it possible to study very small details in the close surroundings of Mira stars. Ultimately, images of the observed stars can be reconstructed.

Mira stars are named after the first such known object, Mira (or Omicron Ceti). One possible explanation for their significant variability is that large amounts of material, including dust and molecules, are produced during each cycle. This material blocks much of the outgoing stellar radiation, until the material becomes diluted by expansion. The close environment of Mira stars is therefore very complex, and the characteristics of the central object are difficult to observe.

To study the close environment of these stars, the team led by Perrin and Ridgway carried out observations at the Infrared-Optical Telescope Array (IOTA) of the Smithsonian Astrophysical Observatory in Arizona. IOTA is a Michelson stellar interferometer, with two arms forming an L-shaped array. It operates with three collectors that can be located at different stations on each arm. In the present study, observations were made at several wavelengths using different telescope spacings ranging from 10 to 38 meters.

From these observations, the team was able to reconstruct the variation of the stellar brightness across the surface of each star. Details down to about 10 milli-arcseconds can be detected. For comparison, at the Moon?s distance, this would correspond to seeing features down to 20 meters in size.

The observations were made at near-infrared wavelengths that are of particular interest for the study of water vapor and carbon monoxide. The role played by these molecules was suspected some years ago by the team and independently confirmed by observations with the Infrared Space Observatory. The new observations using IOTA clearly demonstrate that Mira stars are surrounded by a molecular layer of water vapor and, in at least some cases, of carbon monoxide. This layer has a temperature of about 2,000 K and extends to about one stellar radius above the stellar photosphere, or roughly 50 percent of the observed diameter of the Mira stars in the sample.

Previous interferometric studies of Mira stars led to estimates of star diameters that were biased by the presence of the molecular layer, and were thus much overestimated. This new result shows that the Mira stars are about one-half as large as previously believed.

The new observations presented by the team are interpreted in the framework of a model that bridges the gap between observations and theory. The space between the star?s surface and the molecular layer very likely contains gas, like an atmosphere, but it is relatively transparent at the observed wavelengths. In visible light, the molecular layer is rather opaque, giving the impression that it is a surface, but in the infrared, it is thin and the star can be seen through it.

This model is the first ever to explain the structure of Mira stars over a wide range of spectral wavelengths from the visible to the mid-infrared and to be consistent with the theoretical properties of their pulsation. However, the presence of the layer of molecules far above the stellar surface is still somewhat mysterious. The layer is too high and dense to be supported purely by atmospheric pressure. The pulsations of the star probably play a role in producing the molecular layer, but the mechanism is not yet understood.

As Mira stars represent a late evolutionary stage of Sun-like stars, it will be very interesting to better describe the processes that occur in and around them, as a foreshadowing of the Sun?s own expected fate in the distant future. Mira stars eject large amounts of gas and dust into space, typically about one-third Earth mass per year, thus providing more than 75 percent of the molecules in the galaxy. The carbon, nitrogen, oxygen and other elements of which we are made were mostly produced in the interior of such stars (with heavier elements coming from supernovae), and are then returned to space via this mass loss to become part of new stars and planets. The maturing technique of interferometry is revealing details of the Mira atmosphere, bringing scientists close to observing and understanding the production and ejection of molecules and dust, as these stars recyle their contents on an astronomical scale.

The paper ?Unveiling Mira stars behind the molecules: Confirmation of the molecular layer model with narrow band near-infrared interferometry,? by Perrin et al., will appear in an upcoming issue of the journal Astronomy & Astrophysics.

Original Source: NOAO News Release

Scientists have obtained their best measurement yet of the size and contents of a neutron star, an ultra-dense object containing the strangest and rarest matter in the Universe.

This measurement may lead to a better understanding of nature’s building blocks — protons, neutrons and their constituent quarks — as they are compressed inside the neutron star to a density trillions of times greater than on Earth.

Dr. Tod Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleague, Adam Villarreal, a graduate student at the University of Arizona, present these results today during a Web-based press conference in New Orleans at the meeting of the High Energy Astrophysics Division of the American Astronomical Society.

They said their best estimate of the radius of a neutron star is 7 miles (11.5 kilometers), plus or minus a stroll around the French Quarter. The mass appears to be 1.75 times that of the Sun, more massive than some theories predict. They made their measurements with NASA’s Rossi X-ray Timing Explorer and archived X-ray data

The long-sought mass-radius relation defines the neutron star’s internal density and pressure relationship, the so-called equation of state. And this, in turn, determines what kind of matter can exist inside a neutron star. The contents offer a crucial test for theories describing the fundamental nature of matter and energy and the strength of nuclear interactions.

“We would really like to get our hands on the stuff at the center of a neutron star,” said Strohmayer. “But since we can’t do that, this is about the next best thing. A neutron star is a cosmic laboratory and provides the only opportunity to see the effects of matter compressed to such a degree.”

A neutron star is the core remains of a star once bigger than the Sun. The interior contains matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth. The neutron star in today’s announcement is part of a binary star system named EXO 0748-676, located in the constellation Volans, or Flying Fish, about 30,000 light-years away, visible in southern skies with a large backyard telescope.

In this system, gas from a “normal” companion star plunges onto the neutron star, attracted by gravity. This triggers thermonuclear explosions on the neutron star surface that illuminate the region. Such bursts often reveal the spin rate of the neutron star through a flickering in the X-ray light emitted, called a burst oscillation. (Refer to Items 1 – 6 for an artist’s concept of this process. A movie and a detailed caption can be found in the blue column on the right.)

The scientists detected a 45-hertz burst oscillation frequency, which corresponds to a neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars, which are often seen spinning over 300 times per second.

The scientists next capitalized on EXO 0748-676 observations with the European Space Agency’s XMM-Newton satellite from 2002, led by Dr. Jean Cottam of NASA Goddard. Cottam’s team had detected spectral lines emitted by hot gas, similar in look to the lines of a cardiogram. These lines had two features. First, they were Doppler shifted. This means the energy detected was an average of the light spinning around the neutron star, moving away from us and then towards us. Second, the lines were gravitationally redshifted. This means that gravity pulled on the light as it tried to escape the region, stealing a bit of its energy.

Strohmayer and Villarreal determined that the 45-hertz frequency and the observed line widths from Doppler shifting are consistent with a neutron star radius between 9.5 and 15 kilometers, with the best estimate at 11.5 kilometers. The relationship among burst frequency, Doppler shifting and radius is that the velocity of gas swirling around the star’s surface depends on the star’s radius and its spin rate. In essence, a faster spin corresponds to a wider spectral line (a technique similar to how a state trooper can detect speeding cars).

Cottam team’s gravitational redshift measurement offered the first measure of a mass-radius ratio, albeit without knowledge of a mass and radius. This is because the degree of redshifting (strength of gravity) depends on the mass and radius of the neutron star. Some scientists had questioned this measurement, for the spectral lines detected seemed too narrow. The new results strengthen the gravitational redshift interpretation of the Cottam team’s spectral lines (and thus the mass-radius ratio) because a slower-spinning star can easily produce such relatively narrow lines.

So, ever more confident of the mass-radius ratio and now knowing the radius, the scientists could calculate the neutron star’s mass. The value was between 1.5 and 2.3 solar masses, with the best estimate at 1.75 solar masses.

The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed into neutrons, which swirl about as a superfluid, a liquid that flows without friction. Yet the matter isn’t packed so tightly that quarks are liberated, a so-called quark star.

“Our results are really starting to put the squeeze on the neutron star equation of state,” said Villareal. “It looks like equations of state which predict either very large or very small stars are nearly excluded. Perhaps more exciting is that we now have an observational technique that should allow us to measure the mass-radius relations in other neutron stars.”

A proposed NASA mission called the Constellation X-ray Observatory would have the ability to make such measurements, but with much greater precision, for a number of neutron star systems.

Original Source: NASA News Release

A nearby galaxy cluster is facing an intergalactic headwind as it is pulled by an underlying superstructure of dark matter, according to new evidence from NASA’s Chandra X-ray Observatory. Astronomers think that most of the matter in the universe is concentrated in long large filaments of dark matter and that galaxy clusters are formed where these filaments intersect.

A Chandra survey of the Fornax galaxy cluster revealed a vast, swept-back cloud of hot gas near the center of the cluster. This geometry indicates that the hot gas cloud, which is several hundred thousand light years in length, is moving rapidly through a larger, less dense cloud of gas. The motion of the core gas cloud, together with optical observations of a group of galaxies racing inward on a collision course with it, suggests that an unseen, large structure is collapsing and drawing everything toward a common center of gravity.

“At a relatively nearby distance of about 60 million light years, the Fornax cluster represents a crucial laboratory for studying the interplay of galaxies, hot gas and dark matter as the cluster evolves.” said Caleb Scharf of Columbia University in New York, NY, lead author of a paper describing the Chandra survey that was presented at an American Astronomical Society meeting in New Orleans, LA. “What we are seeing could be associated directly with the intergalactic gas surrounding a very large scale structure that stretches over millions of light years.”

The infalling galaxy group, whose motion was detected by Michael Drinkwater of the University of Melbourne in Australia, and colleagues, is about 3 million light years from the cluster core, so a collision with the core will not occur for a few billion years. Insight as to how this collision will look is provided by the elliptical galaxy NGC 1404 that is plunging into the core of the cluster for the first time. As discussed by Scharf and another group led by Marie Machacek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., the hot gas cloud surrounding this galaxy has a sharp leading edge and a trailing tail of gas being stripped from the galaxy.

“One thing that makes what we see in Fornax rather compelling is that it looks a lot like some of the latest computer simulations,” added Scharf. “The Fornax picture, with infalling galaxies, and the swept back geometry of the cluster gas – seen only with the Chandra resolution and the proximity of Fornax – is one of the best matches to date with these high-resolution simulations.”

Over the course of hundreds of millions of years, NGC 1404’s orbit will take it through the cluster core several times, most of the gas it contains will be stripped away, and the formation of new stars will cease. In contrast, galaxies that remain outside the core will retain their gas, and new stars can continue to form. Indeed, Scharf and colleagues found that galaxies located in regions outside the core were more likely to show X-ray activity which could be associated with active star formation.

The wide-field and deep X-ray view around Fornax was obtained through ten Chandra pointings, each lasting about 14 hours. Other members of the research team were David Zurek of the American Museum of Natural History, New York, NY, and Martin Bureau, a Hubble Fellow currently at Columbia.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu
and
http://chandra.nasa.gov

Original Source: Chandra News Release

NASA’s Spitzer Space Telescope has set its infrared sight on a major galactic collision and witnessed not death, but a teeming nest of life.

The colliding galaxies, called the Antennae galaxies, are in the process of merging together. As they churn into each other, they throw off massive streamers of stars and dark clouds of dust. Spitzer’s heat-seeking eyes peered through that dust and found a hidden population of newborn stars.

The new Spitzer image, available at http://www.spitzer.caltech.edu/Media/releases/ssc2004-14/visuals.shtml, is reported in one of 86 Spitzer papers published in the September issue of The Astrophysical Journal Supplement. This special all-Spitzer issue comes just after the one-year anniversary of the observatory’s launch, and testifies to its tremendously successful first year in space.

“This abundance of Spitzer papers just one year after launch shows that the telescope is truly providing a new window on the universe,” said Dr. Michael Werner, project scientist for Spitzer at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “These papers report the earliest results, so the best is yet to come.”

In the latest Antennae galaxies study, Spitzer uncovered a new generation of stars at the site where the two galaxies clash.

“We theorized that there were stars forming at that site, but we weren’t sure to what degree,” said Dr. Zhong Wang, lead author of the new paper and an astronomer at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. “Now we see that the majority of star-forming activity in both galaxies occurs in the overlap regions where the two meet.”

The Antennae galaxies are a classic example of a galactic merger in action. These two spiral galaxies, located 68 million light-years away from Earth, began falling into each other around a common center of gravity about 800 million years ago. As they continue to crash together, clouds of gas are shocked and compressed in a process thought to trigger the birth of new stars. Astronomers believe that the two galaxies will ultimately merge into one spheroidal-shaped galaxy, leaving only hints of their varied pasts.

Galactic mergers are common throughout the universe and play a key role in determining how galaxies grow and evolve. Our own Milky Way galaxy, for example, will eventually collide with our closest neighbor, the Andromeda galaxy.

Previous images of the Antennae taken by visible-light telescopes show striking views of the swirling duo, with bright pockets of young stars dotting the spiral arms. At the center of the galaxies, however, where the two overlap, only a dark cloud of dust can be seen. In the new false-color Spitzer image, which has been combined with an image from a ground-based, visible-light telescope to highlight new features, this cloud of buried stars appears bright red. The visible-light information, on the other hand, is colored blue and indicates regions containing older stars. The nuclei, or centers, of the two galaxies are white.

“This more complete picture of star-formation in the Antennae will help us better understand the evolution of colliding galaxies, and the eventual fate of our own,” said Dr. Giovanni Fazio, a co-author of the research and an astronomer at the Harvard-Smithsonian Center for Astrophysics.” Fazio is principal investigator for the infrared array camera on Spitzer, which captured the new Antennae image.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. Spitzer’s infrared array camera was built by NASA Goddard Space Flight Center, Greenbelt, Md.

Information about Spitzer can be found at http://www.spitzer.caltech.edu.

Original Source: NASA/JPL News Release

A stunning image released today by the Gemini Observatory captures the graceful interactions of a galactic ballet, on a stage some 300 million light years away, that might better be described as a contortionist’s dance.

The galaxies, members of a famous troupe called Stephan’s Quintet, are literally tearing each other apart. Their shapes are warped by gravitational interactions occurring over millions of years. Sweeping arches of gas and dust trace the interactions and possible ghost-like passage of the galaxies through one another. The ongoing dance deformed their structures while spawning a prolific fireworks display of star formation fueled by clouds of hydrogen gas that were shocked into clumps to form stellar nurseries.

This unprecedented image of the cluster provides a unique combination of sensitivity, high resolution and field of view. “It doesn’t take long to reach an incredible depth when you have an 8-meter mirror collecting light under excellent conditions,” said Travis Rector of the University of Alaska, Anchorage who helped obtain the data with the Gemini North Telescope on Mauna Kea. “We were able to capture these galaxies at many different wavelengths or colors. This allowed us to bring out some remarkable details in the final color image that have never been seen before in one view.”

One striking element of the image is a collection of vibrant red clumps that mark star-forming regions within a galaxy called NGC 7320. Although its relation to the other galaxies in the cluster has been the subject of some controversy, most astronomers now think that the galaxy leads a relatively tranquil existence in the foreground, safely isolated from the violent quarrels of the more distant cluster.

Spectroscopic data show that NGC 7320 has an apparent velocity away from us of about 800 kilometers per second. In contrast, the rest of the group is being carried away from us by the expansion of the universe at over 6,000 kilometers per second. Using current models for the expanding universe, this would put the bulk of the cluster almost 8 times farther away from us than NGC 7320.

The vivid red patches scattered across the spiral arms of NGC 7320 in the new Gemini image provide a dramatic illustration of how these differing apparent velocities can impact our view. NGC 7320 and the other cluster galaxies have regions of intense star formation indicated by glowing clouds of hydrogen gas called HII regions. These areas appear distinctly red because a selective filter was used which only passes a special color of red light, called hydrogen alpha, that is produced in the HII regions. In the higher-velocity members of the cluster, prominent HII clumps dominate around the two closely interacting central galaxies but they do not appear red in the image. In these galaxies, the HII glow was Doppler-shifted beyond the range of the selective filter, and was therefore not detected.

The interacting members of Stephan’s Quintet appear destined to continue their dance for millions more years. Eventually, this dance will probably cause some of the galaxies in the cluster to completely lose their current identity, combining into even fewer objects than we see today.

Stephan’s Quintet was discovered in 1877 by the French astronomer Edouard Stephan using the Foucault 80-centimeter reflector at the Marseilles Observatory. The cluster is listed in the Hickson Compact Group Catalog as number 92. It has been studied extensively at all wavelengths including imaging by the Hubble Space Telescope. Recent observations of star cluster formation near Stephan’s Quintet with Gemini can be found here.

Original Source: Gemini News Release