Category: Astronomy

Current theories may not describe our Universe very accurately. Image credit: Brussels Museum of Fine Arts, and Space Telescope Institute. Click to enlarge
A Chinese astronomer from the University of St Andrews has fine-tuned Einstein’s groundbreaking theory of gravity, creating a ‘simple’ theory which could solve a dark mystery that has baffled astrophysicists for three-quarters of a century.

A new law for gravity, developed by Dr Hong Sheng Zhao and his Belgian collaborator Dr Benoit Famaey of the Free University of Brussels (ULB), aims to prove whether Einstein’s theory was in fact correct and whether the astronomical mystery of Dark Matter actually exists. Their research was published on February 10th in the US-based Astrophysical Journal Letters. Their formula suggests that gravity drops less sharply with distance as in Einstein, and changes subtly from solar systems to galaxies and to the universe.

Theories of the physics of gravity were first developed by Isaac Newton in 1687 and refined by Albert Einstein’s general theory of relativity in 1905 to allow light bending. While it is the earliest-known force, gravity is still very much a mystery with theories still unconfirmed by astronomical observations in space.

The ‘problem’ with the golden laws of Newton and Einstein is whilst they work very well on earth, they do not explain the motion of stars in galaxies and the bending of light accurately. In galaxies, stars rotate rapidly about a central point, held in orbit by the gravitational attraction of the matter in the galaxy. However astronomers found that they were moving too quickly to be held by their mutual gravity – so not enough gravity to hold the galaxies together instead stars should be thrown off in all directions!

The solution to this, proposed by Fritz Zwicky in 1933, was that there was unseen material in the galaxies, making up enough gravity to hold the galaxies together. As this material emits no light astronomers call it ‘Dark Matter’. It is thought to account for up to 90% of matter in the Universe. Not all scientists accept the Dark Matter theory however. A rival solution was proposed by Moti Milgrom in 1983 and backed up by Jacob Bekenstein in 2004. Instead of the existence of unseen material, Milgrom proposed that astronomers understanding of gravity was incorrect. He proposed that a boost in the gravity of ordinary matter is the cause of this acceleration.

Milgrom’s theory has been worked on by a number of astronomers since and Dr Zhao and Dr Famaey have proposed a new formulation of his work that overcomes many of the problems previous versions have faced.

They have created a formula that allows gravity to change continuously over various distance scales and, most importantly, fits the data for observations of galaxies. To fit galaxy data equally well in the rival Dark Matter paradigm would be as challenging as balancing a ball on a needle, which motivated the two astronomers to look at an alternative gravity idea.

Legend has it that Newton began thinking about gravity when an apple fell on his head, but according to Dr Zhao, “It is not obvious how an apple would fall in a galaxy. Mr Newton’s theory would be off by a large margin – his apple would fly out of the Milky Way. Efforts to restore the apple on a nice orbit around the galaxy have over the years led to two schools of thoughts: Dark Matter versus non-Newtonian gravity. Dark Matter particles come naturally from physics, with beautiful symmetries and explain cosmology beautifully; they tend to be everywhere. The real mystery is how to keep them away from some corners of the universe. Also Dark Matter comes hand- in-hand with Dark Energy. It would be more beautiful if there were one simple answer to all these mysteries”.

Dr Zhao, a PPARC Advanced Fellow at University of St Andrews, School of Physics and Astronomy, and member of the Scottish Universities Physics Alliance (SUPA), continued “There has always been a fair chance that astronomers might rewrite the law of gravity. We have created a new formula for gravity which we call ‘the simple formula’, and which is actually a refinement of Milgrom’s and Bekenstein’s. It is consistent with galaxy data so far, and if its predictions are further verified for solar system and cosmology, it could solve the Dark Matter mystery. We may be able to answer common questions such as whether Einstein’s theory of gravity is right and whether the so-called Dark Matter actually exists”.

“A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function. It is ready for fellow scientists to falsify. It is time to keep an open mind for new fields predicted in our formula while we continue our search for Dark Matter particles.”

The new formula will be presented to an international workshop at Edinburgh’s Royal Observatory in April, which will be given the opportunity to test and debate the reworked theory. Dr Zhao and Dr Famaey will demonstrate their new formula to an audience of Dark Matter and gravity experts from ten different countries.

Dr Famaey commented “It is possible that neither the modified gravity theory, nor the Dark Matter theory, as they are formulated today, will solve all the problems of galactic dynamics or cosmology. The truth could in principle lie in between, but it is very plausible that we are missing something fundamental about gravity, and that a radically new theoretical approach will be needed to solve all these problems. Nevertheless, our formula is so attractively simple that it is tempting to see it as part of a yet unknown fundamental theory. All galaxy data seem to be explained effortlessly”.

Original Source: PPARC News Release

Artist’s impression of Integral observing Earth. Image credit: ESA Click to enlarge
Cosmic space is filled with continuous, diffuse high-energy radiation. To find out how this energy is produced, the scientists behind ESA’s Integral gamma-ray observatory have tried an unusual method: observing Earth from space.

During a four-phase observation campaign started on 24 January this year, continued until 9 February, Integral has been looking at Earth. Needing complex control operations from the ground, the satellite has been kept in a fixed orientation in space, while waiting for Earth to drift through its field of view.

Unusually, the main objective of these observations is not Earth itself, but what can be seen in the background when Earth moves in front of the satellite. This is the origin of the diffuse high-energy radiation known as the ‘cosmic X-ray background’.

Until now with Integral, this was never studied simultaneously with such a broad band of energy coverage since the 1970s, and certainly not with such advanced instruments.

Astronomers believe that the ‘cosmic X-ray background’ is produced by numerous supermassive and accreting black holes, distributed throughout deep space. These powerful monsters attract matter, which is then hugely accelerated and so emit high energy in the form of gamma- and X-rays.

X-ray observatories such as ESA’s XMM-Newton and NASA’s Chandra have been able to identify and directly count a large number of individual sources ? likely black holes ? that already account for more than 80 percent of the measured cosmic diffuse X-ray background.

However, very little is known about the origin of the highest energy band of this cosmic radiation, above the range of these two satellites. This is spread out in the form of high-energy X- and gamma-rays, within the reach of Integral.

It is believed that most of the gamma-ray background emission is produced by individual supermassive black holes too, but scientists need to couple this emission with clearly identified sources to make a definitive statement. In fact, other sources such as far-away galaxies or close weak sources could be also be responsible.

Identifying the individual sources in the gamma-ray range that make up the diffuse cosmic background is much more difficult than counting the individual X-ray sources. In fact, the powerful gamma-rays cannot be focused with lenses or mirrors, because they simply pass straight through.

So to produce a gamma-ray image of a source, Integral uses a ‘mask’ technique – an indirect imaging method that consists of detecting the shadow of a mask placed on top of the telescope, as projected by a gamma-ray source.

During the observations, the scientists used Earth’s disk as an ‘extra mask’. Earth naturally blocks, or shades, the highest energy flux from millions of distant black holes.

Their combined flux can be accurately measured in an indirect way, that is by measuring the amplitude and the energy spectrum of the energy drop when Earth passes through Integral’s field of view. Once this is known, scientists can eventually try to connect the radiation to individual sources.

All the observations were very successful, as all the gamma-ray and X-ray instruments on board Integral (IBIS, SPI and JEM-X) recorded clear and unambiguous signals in line with expectations.

The Integral scientists are already proceeding with the analysis of the data. The aim is to ultimately understand the origin of the highest energy background radiation and, possibly, provide new clues on the history of growth of super-massive black holes since the early epochs of the Universe.

Original Source: ESA Portal

An illustration comparing the size of a gargantuan star and its dusty disk with our solar system. Image credit: NASA/JPL Click to enlarge
NASA’s Spitzer Space Telescope has identified two huge “hypergiant” stars circled by monstrous disks of what might be planet-forming dust. The findings surprised astronomers because stars as big as these were thought to be inhospitable to planets.

“These extremely massive stars are tremendously hot and bright and have very strong winds, making the job of building planets difficult,” said Joel Kastner of the Rochester Institute of Technology in New York. “Our data suggest that the planet-forming process may be hardier than previously believed, occurring around even the most massive stars that nature produces.”

Kastner is first author of a paper describing the research in the Feb. 10 issue of Astrophysical Journal Letters.

Dusty disks around stars are thought to be signposts for present or future planetary systems. Our own sun is orbited by a thin disk of planetary debris, called the Kuiper Belt, which includes dust, comets and larger bodies similar to Pluto.

Last year, astronomers using Spitzer reported finding a dust disk around a miniature star, or brown dwarf, with only eight one-thousandths the mass of the sun ( http://www.spitzer.caltech.edu/Media/happenings/20051129/). Disks have also been spotted before around stars five times more massive than the sun.

The new Spitzer results expand the range of stars that sport disks to include the “extra large.” The infrared telescope detected enormous amounts of dust around two positively plump stars, R 66 and R 126, located in the Milky Way’s nearest neighbor galaxy, the Large Magellanic Cloud. Called hypergiants, these blazing hot stars are aging descendents of the most massive class of stars, referred to as “O” stars. They are 30 and 70 times the mass of the sun, respectively. If a hypergiant were located at the sun’s position in our solar system, all the inner planets, including Earth, would fit comfortably within its circumference.

Astronomers estimate that the stars’ disks are also bloated, spreading all the way out to an orbit about 60 times more distant than Pluto’s around the sun. The disks are probably loaded with about ten times as much mass as is contained in the Kuiper Belt. Kastner and his colleagues say these dusty structures might represent the first or last steps of the planet-forming process. If the latter, then the disks can be thought of as enlarged versions of our Kuiper Belt.

“These disks may be well-populated with comets and other larger bodies called planetesimals,” said Kastner. “They might be thought of as Kuiper Belts on steroids.”

Spitzer detected the disks during a survey of 60 bright stars thought to be wrapped in spherical cocoons of dust. According to Kastner, R 66 and R 126 “stuck out like sore thumbs” because their light signatures, or spectra, indicated the presence of flattened disks. He and his team believe these disks whirl around the hypergiant stars, but they say it is possible the giant disks orbit unseen, slightly smaller companion stars.

A close inspection of the dust making up the disks revealed the presence of sand-like planetary building blocks called silicates. In addition, the disk around R 66 showed signs of dust clumping in the form of silicate crystals and larger dust grains. Such clumping can be a significant step in the construction of planets.

Stars as massive as R 66 and R 126 don’t live very long. They burn through all of their nuclear fuel in only a few million years, and go out with a bang, in fiery explosions called supernovae. Their short life spans don’t leave much time for planets, or life, to evolve. Any planets that might crop up would probably be destroyed when the stars blast apart.

“We do not know if planets like those in our solar system are able to form in the highly energetic, dynamic environment of these massive stars, but if they could, their existence would be a short and exciting one,” said Charles Beichman, an astronomer at NASA’s Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena.

Other authors of this work include Catherine L. Buchanan of the Rochester Institute of Technology, and B. Sargent and W. J. Forrest of the University of Rochester, N.Y.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech. Spitzer’s infrared spectrograph, which made the new observations, was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell.

An artist concept of a hypergiant and its disk, plus additional graphics and information, are available at http://www.spitzer.caltech.edu/spitzer.

Original Source: NASA News Release

The central part of Messier 12. Image credit: ESO Click to enlarge
Based on observations with ESO’s Very Large Telescope, a team of Italian astronomers reports that the stellar cluster Messier 12 must have lost to our Milky Way galaxy close to one million low-mass stars.

“In the solar neighbourhood and in most stellar clusters, the least massive stars are the most common, and by far”, said Guido De Marchi (ESA), lead author of the study. “Our observations with ESO’s VLT show this is not the case for Messier 12.”

The team, which also includes Luigi Pulone and Francesco Paresce (INAF, Italy), measured the brightness and colours of more than 16,000 stars within the globular cluster Messier 12 with the FORS1 multi-mode instrument attached to one of the Unit Telescopes of ESO’s VLT at Cerro Paranal (Chile). The astronomers could study stars that are 40 million times fainter than what the unaided eye can see (magnitude 25).

Located at a distance of 23,000 light years in the constellation Ophiuchus (The Serpent-holder), Messier 12 got its name by being the 12th entry in the catalogue of nebulous objects compiled in 1774 by French astronomer and comet chaser Charles Messier. It is also known to astronomers as NGC 6218 and contains about 200,000 stars, most of them having a mass between 20 and 80 percent of the mass of the Sun.

“It is however clear that Messier 12 is surprisingly devoid of low-mass stars”, said De Marchi. “For each solar-like star, we would expect roughly four times as many stars with half that mass. Our VLT observations only show an equal number of stars of different masses.”

Globular clusters move in extended elliptical orbits that periodically take them through the densely populated regions of our Galaxy, the plane, then high above and below, in the ‘halo’. When venturing too close to the innermost and denser regions of the Milky Way, the ‘bulge’, a globular cluster can be perturbed, the smallest stars being ripped away.

“We estimate that Messier 12 lost four times as many stars as it still has”, said Francesco Paresce. “That is, roughly one million stars must have been ejected into the halo of our Milky Way.”

The total remaining lifetime of Messier 12 is predicted to be about 4.5 billion years, i.e. about a third of its present age. This is very short compared to the typical expected globular cluster’s lifetime, which is about 20 billion years.

The same team of astronomers had found in 1999, another example of a globular cluster that lost a large fraction of its original content (see ESO PR 04/99).

The scientists hope to discover and study many more clusters like these, since catching clusters while being disrupted should clarify the dynamics of the process that shaped the halo of our home galaxy, the Milky Way.

High resolution images and their captions are available on this page.
A press release on this is also issued by INAF in Italian and is available at www.inaf.it/comunicati_stampa/cs070206/Inaf-04-06.html.

Original Source: ESO News Release

The massive spiral galaxy NGC 5746. Image credit: NASA Click to enlarge
Chandra observations of the massive spiral galaxy NGC 5746 revealed a large halo of hot gas (blue) surrounding the optical disk of the galaxy (white). The halo extends more than 60,000 light years on either side of the disk of the galaxy, which is viewed 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. Computer simulations and Chandra data show that the likely origin of the hot halo is the gradual inflow of intergalactic matter left over from the formation of the galaxy.

Spiral galaxies are thought to form from enormous clouds of intergalactic gas that collapse to form spinning disks of stars and gas. One prediction of this theory is that massive 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 had not been detected. Indeed, the extensive hot gas halo around NGC 5746 is faint and would be very difficult to detect without a powerful X-ray telescope such as Chandra. Also, the galaxy’s special orientation and large mass increased the chance of detection.

The discovery of a hot halo around NGC 5746 was welcome news to astronomers because it shows that the “missing” hot halos predicted by computer models in fact exist.

Original Source: Chandra X-Ray Observatory

An artist’s illustration of a rocky planet orbiting around a red dwarf star. Image credit: ESO Click to enlarge
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 percent 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 percent of red dwarf stars have companions. Combined with the fact that about 85 percent 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.

Interestingly, astronomers recently announced the discovery of a rocky planet only five times more massive than Earth. This is the closest to an Earth-size world yet found, and it is in orbit around a single red dwarf star.

“This new planet may just be the tip of the iceberg,” said Lada. “Red dwarfs may be a fertile new hunting ground for finding planets, including ones similar in mass to the earth.”

“There could be many planets around red dwarf stars,” stated astronomer Dimitar Sasselov of CfA. “It’s all in the numbers, and single red dwarfs clearly exist in great numbers.”

“This discovery is particularly exciting because the habitable zone for these stars – the region where a planet would be the right temperature for liquid water – is close to the star. Planets that are close to their stars are easier to find. The first truly Earth-like planet we discover might be a world orbiting a red dwarf,” added Sasselov.

This research has been submitted to The Astrophysical Journal Letters for publication and is available online at http://arxiv.org/abs/astro-ph/0601375

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

M15 has a double neutron star system that will eventually merge violently. Image credit: NOAO Click to enlarge
Gamma-ray bursts are the most powerful explosions in the universe, emitting huge amounts of high-energy radiation. For decades their origin was a mystery. Scientists now believe they understand the processes that produce gamma-ray bursts. However, a new study by Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues, Simon Portegies Zwart (Astronomical Institute, The Netherlands) and Stephen McMillan (Drexel University), suggests a previously overlooked source for some gamma-ray bursts: stellar encounters within globular clusters.

“As many as one-third of all short gamma-ray bursts that we observe may come from merging neutron stars in globular clusters,” said Grindlay.

Gamma-ray bursts (GRBs) come in two distinct “flavors.” Some last up to a minute, or even longer. Astronomers believe those long GRBs are generated when a massive star explodes in a hypernova. Other bursts last for only a fraction of a second. Astronomers theorize that short GRBs originate from the collision of two neutrons stars, or a neutron star and a black hole.

Most double neutron star systems result from the evolution of two massive stars already orbiting each other. The natural aging process will cause both to become neutron stars (if they start with a given mass), which then spiral together over millions or billions of years until they merge and release a gamma-ray burst.

Grindlay’s research points to another potential source of short GRBs – globular clusters. Globular clusters contain some of the oldest stars in the universe crammed into a tight space only a few light-years across. Such tight quarters provoke many close stellar encounters, some of which lead to star swaps. If a neutron star with a stellar companion (such as a white dwarf or main-sequence star) exchanges its partner with another neutron star, the resulting pair of neutron stars will eventually spiral together and collide explosively, creating a gamma-ray burst.

“We see these precursor systems, containing one neutron star in the form of a millisecond pulsar, all over the place in globular clusters,” stated Grindlay. “Plus, globular clusters are so closely packed that you have a lot of interactions. It’s a natural way to make double neutron-star systems.”

The astronomers performed about 3 million computer simulations to calculate the frequency with which double neutron-star systems can form in globular clusters. Knowing how many have formed over the galaxy’s history, and approximately how long it takes for a system to merge, they then determined the frequency of short gamma-ray bursts expected from globular cluster binaries. They estimate that between 10 and 30 percent of all short gamma-ray bursts that we observe may result from such systems.

This estimate takes into account a curious trend uncovered by recent GRB observations. Mergers and thus bursts from so-called “disk” neutron-star binaries – systems created from two massive stars that formed together and died together – are estimated to occur 100 times more frequently than bursts from globular cluster binaries. Yet the handful of short GRBs that have been precisely located tend to come from galactic halos and very old stars, as expected for globular clusters.

“There’s a big bookkeeping problem here,” said Grindlay.

To explain the discrepancy, Grindlay suggests that bursts from disk binaries are likely to be harder to spot because they tend to emit radiation in narrower blasts visible from fewer directions. Narrower “beaming” might result from colliding stars whose spins are aligned with their orbit, as expected for binaries that have been together from the moment of their birth. Newly joined stars, with their random orientations, might emit wider bursts when they merge.

“More short GRBs probably come from disk systems – we just don’t see them all,” explained Grindlay.

Only about a half dozen short GRBs have been precisely located by gamma-ray satellites recently, making thorough studies difficult. As more examples are gathered, the sources of short GRBs should become much better understood.

The paper announcing this finding was published in the January 29 online issue of Nature Physics. It is available online at http://www.nature.com/nphys/index.html and in preprint form at http://arxiv.org/abs/astro-ph/0512654.

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

Saturn’s moon Dione in a false colour view. Image credit: NASA/JPL/SSI Click to enlarge
The leading hemisphere of Dione displays subtle variations in color across its surface in this false color view.

To create this view, ultraviolet, green and infrared images were combined into a single black and white picture that isolates and maps regional color differences.

This “color map” was then superposed over a clear-filter image. The origin of the color differences is not yet understood, but may be caused by subtle differences in the surface composition or the sizes of grains making up the icy soil.

Terrain visible here is on the moon’s leading hemisphere. North on Dione (1,126 kilometers, or 700 miles across) is up and rotated 17 degrees to the right.

See Detail on Dione (Monochrome) for a similar monochrome view.

All images were acquired with the Cassini spacecraft narrow-angle camera on Dec. 24, 2005 at a distance of approximately 597,000 kilometers (371,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 21 degrees. Image scale is 4 kilometers (2 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

An artist’s conception of an exiled star speeding out of the milkyway. Image credit: Ruth Bazinet, CfA Click to enlarge
TV reality show contestants aren’t the only ones under threat of exile. Astronomers using the MMT Observatory in Arizona have discovered two stars exiled from the Milky Way galaxy. Those stars are racing out of the Galaxy at speeds of more than 1 million miles per hour – so fast that they will never return.

“These stars literally are castaways,” said Smithsonian astronomer Warren Brown (Harvard-Smithsonian Center for Astrophysics). “They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space.”

Brown and his colleagues spotted the first stellar exile in 2005. European groups identified two more, one of which may have originated in a neighboring galaxy known as the Large Magellanic Cloud. The latest discovery brings the total number of known exiles to five.

“These stars form a new class of astronomical objects – exiled stars leaving the Galaxy,” said Brown.

Astronomers suspect that about 1,000 exile stars exist within the Galaxy. By comparison, the Milky Way contains about 100,000,000,000 (100 billion) stars, making the search for exiles much more difficult than finding the proverbial “needle in a haystack.” The Smithsonian team improved their odds by preselecting stars with locations and characteristics typical of known exiles. They sifted through dozens of candidates spread over an area of sky almost 8000 times larger than the full moon to spot their quarry.

“Discovering these two new exiled stars was neither lucky nor random,” said astronomer Margaret Geller (Smithsonian Astrophysical Observatory), a co-author on the paper. “We made a targeted search for them. By understanding their origin, we knew where to find them.”

Theory predicts that the exiled stars were thrown from the galactic center millions of years ago. Each star once was part of a binary star system. When a binary swings too close to the black hole at the galaxy’s center, the intense gravity can yank the binary apart, capturing one star while violently flinging the other outward at tremendous speed (hence their technical designation of hypervelocity stars).

The two recently discovered exiles both are short-lived stars about four times more massive than the sun. Many similar stars exist within the galactic center, supporting the theory of how exiles are created. Moreover, detailed studies of the Milky Way’s center previously found stars orbiting the black hole on very elongated, elliptical orbits – the sort of orbits that would be expected for former companions of hypervelocity stars.

“Computer models show that hypervelocity stars are naturally made near the galactic center,” said theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics. “We know that binaries exist. We know the galactic center holds a supermassive black hole. So, exiled stars inevitably will be produced when binaries pass too close to the black hole.”

Astronomers estimate that a star is thrown from the galactic center every 100,000 years on average. Chances of seeing one at the moment of ejection are slim. Therefore, the hunt must continue to find more examples of stellar exiles in order to understand the extreme environment of the galactic center and how those extremes lead to the formation of hypervelocity stars.

The characteristics of exiled stars give clues to their origin. For example, if a large cluster of stars spiraled into the Milky Way’s central black hole, many stars might be thrown out at nearly the same time. Every known hypervelocity star left the galactic center at a different time, therefore there is no evidence for a “burst” of exiles.

Hypervelocity stars also offer a unique probe of galactic structure. “During their lifetime, these stars travel across most of the Galaxy,” said Geller. “If we could measure their motions across the sky, we could learn about the shape of the Milky Way and about the way the mysterious dark matter is distributed.”

The first newfound exile, in the direction of the constellation Ursa Major, is designated SDSS J091301.0+305120. It is traveling out of the galaxy at a speed of about 1.25 million miles per hour and currently is located at a distance of about 240,000 light-years from the earth. The second exile, in the direction of the constellation Cancer, is designated SDSS J091759.5+672238. It is moving outward at 1.43 million miles per hour and currently is located about 180,000 light-years from the earth.

Both stars, although traveling at tremendous speeds through space, are located so far from the earth that their motion cannot be detected except with sophisticated astronomical instruments.

This research has been submitted to The Astrophysical Journal Letters for publication and will be available online at http://arxiv.org/abs/astro-ph/0601580. Authors on the paper are Brown, Geller, Scott Kenyon and Michael Kurtz (Smithsonian Astrophysical 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