Category: Astronomy

The surface patterns for different torsional modes. Image credit: Max Planck. Click to enlarge
A massive explosion on the surface of a neutron star gave astronomers an opportunity to peer inside its surface, similar to how geologists understand the structure of the Earth beneath our feet. The explosion jolted the neutron star, and set it ringing like a bell. The vibrations then passed through layers of different density – slushy or solid – changing the X-rays streaming off. Astronomers calculated that it has a thicker crust approximately 1.6 km (1 mile) deep, matching theoretical estimates.

A US-German team of scientists from the Max Planck Institute for Astrophysics and NASA have used NASA’s Rossi X-ray Timing Explorer to estimate the depth of the crust on a neutron star, the densest object known in the universe. The crust, they say, is approximately 1.6 kilometres deep and so tightly packed that a teaspoon of this material would weigh about 10 million tonnes on Earth.

This measurement, the first of its kind, came courtesy of a massive explosion on a neutron star in December 2004. Vibrations from the explosion revealed details about the star’s composition. The technique is analogous to seismology, the study of seismic waves from earthquakes and explosions, which reveal the structure of the Earth’s crust and interior.

This new seismology technique provides a way to probe a neutron star’s interior, a place of great mystery and speculation. Pressure and density are so intense here that the core might harbour exotic particles thought to have existed only at the moment of the Big Bang.

Dr Anna Watts, of the Max Planck Institute for Astrophysics in Garching, carried out this research in collaboration with Dr. Tod Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

“We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell,” said Strohmayer. “The vibrations created in the explosion, although faint, provide very specific clues about what these bizarre objects are made of. Just like a bell, a neutron star’s ring depends on how waves pass through layers of differing density, either slushy or solid.”

A neutron star is the core remains of a star once several times more massive than the sun. A neutron star contains about 1.4 solar masses of material crammed into a sphere only about 20 kilometres across. The two scientists examined a neutron star named SGR 1806-20, which is situated about 40,000 light years from Earth in the constellation Sagittarius. The object is in a subclass of highly magnetic neutron stars called magnetars.

On December 27, 2004, the surface of SGR 1806-20 experienced an unprecedented explosion, the brightest event ever seen from beyond our solar system. The explosion, called a hyperflare, was caused by a sudden change in the star’s powerful magnetic field that cracked the crust, likely producing a massive starquake. The event was detected by many space observatories, including the Rossi Explorer, which observed the X-ray light emitted.

Strohmayer and Watts think that the oscillations are evidence of global torsional vibrations within the star’s crust. These vibrations are analogous to the S-waves observed during terrestrial earthquakes, like a wave moving through a rope. Their study, building on observations of vibrations from this source by Dr. GianLuca Israel of Italy’s National Institute of Astrophysics, found several new frequencies during the hyperflare.

Watts and Strohmayer subsequently confirmed their measurements using NASA’s Ramaty High Energy Solar Spectroscopic Imager, a solar observatory that also recorded the hyperflare, and found the first evidence for a high-frequency oscillation at 625 Hz, indicative of waves traversing the crust vertically.

The abundance of frequencies – similar to a chord, as opposed to a single note – enabled the scientists to estimate the depth of the neutron star crust. This is based on a comparison of frequencies from waves travelling around the star’s crust and from those travelling radially through it. The diameter of a neutron star is uncertain, but based on the estimate of about 20 kilometres across, the crust would be about 1.6 kilometres deep. This figure, based on the observed frequencies, is in line with theoretical estimates.

Starquake seismology holds great promise for determining many neutron star properties. Strohmayer and Watts have analyzed archived Rossi data from a dimmer 1998 magnetar hyperflare (from SGR 1900+14) and found telltale oscillations here too, although not strong enough to determine the crust thickness.

A larger neutron star explosion detected in X-rays might reveal deeper secrets, such as the nature of matter at the star’s core. One exciting possibility is that the core might contain free quarks. Quarks are the building blocks of protons and neutrons, and under normal conditions are always tightly bound together. Finding evidence for free quarks would aid in understanding the true nature of matter and energy. Laboratories on Earth, including massive particle accelerators, cannot generate the energies needed to reveal free quarks.

“Neutron stars are great laboratories for the study of extreme physics,” said Watts. “We’d love to be able to crack one open, but since that’s probably not going to happen, observing the effects of a magnetar hyperflare on a neutron star is perhaps the next best thing.”

Original Source: Max Planck Society

The cores of the two galaxies NGC 2207 and IC 2163. Image credit: NASA. Click to enlarge
The two “eyes” in this photograph are actually the cores of two merging galaxies; as viewed by NASA’s Spitzer Space Telescope. The galaxies are called NGC 2207 and IC 2163, and the surrounding material is their twisted spiral arms. Dotted along these arms are knotted clusters of newborn stars, created when the two galaxies smashed into each other. The pair is located 140 million light-years away in the Canis Major constellation, and they’ll eventually become a single galaxy in another 500 million years.

A pair of dancing galaxies appears dressed for a cosmic masquerade in a new image from NASA’s Spitzer Space Telescope.

The infrared picture shows what looks like two icy blue eyes staring through an elaborate, swirling red mask. These “eyes” are actually the cores of two merging galaxies, called NGC 2207 and IC 2163, which recently met and began to twirl around each other.

The “mask” is made up of the galaxies’ twisted spiral arms. Dotted along the arms, like strings of decorative pearls, are dusty clusters of newborn stars. This is the first time that clusters of this type, called “beads on a string” by astronomers, have been seen in NGC 2207 and IC 2163.

“This is the most elaborate case of beading we’ve seen in galaxies,” said Dr. Debra Elmegreen of Vassar College in Poughkeepsie, N.Y. “They are evenly spaced and sized along the arms of both galaxies.”

Elmegreen is lead author of a paper describing the Spitzer observations in the May 1 issue of the Astrophysical Journal.

Astronomers say the beads were formed when the galactic duo first met. “The galaxies shook each other, causing gas and dust to move around and collect into pockets dense enough to collapse gravitationally,” said Dr. Kartik Sheth of NASA’s Spitzer Science Center at the California Institute of Technology in Pasadena. Once this material condensed into thick bead-like clouds, stars of various sizes began to pop up within them.

Spitzer’s infrared camera was able to see the dusty clouds for the first time because they glow with infrared light. The hot, young stars housed inside the clouds heat up the dust, which then radiates at infrared wavelengths. This dust is false-colored red in the image, while stars are represented in blue.

The Spitzer data also reveal an unusually bright bead adorning the left side of the “mask.” This dazzling orb is so packed full of dusty materials that it accounts for five percent of the total infrared light coming from both galaxies. Elmegreen’s team thinks the central stars in this dense cluster might have merged to become a black hole.

Visible-light images of the galaxies show stars located inside the beads, but the beads themselves are invisible. In those pictures, the galaxies look more like a set of owl-like eyes with “feathers” of scattered stars.

NGC 2207 and IC 2163 are located 140 million light-years away in the Canis Major constellation. The two galaxies will meld into one in about 500 million years, bringing their masquerade days to an end.

Other authors of this research include Bruce Elmegreen of IBM Watson Research Center, Yorktown Heights, N.Y., Michele Kaufman of Ohio State University, Columbus; Curt Struck of Iowa State, Ames; Magnus Thomasson of Onsala Space Observatory, Sweden; and Elias Brinks of the University of Hertfordshire, United Kingdom.

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 array camera was built by NASA’s Goddard Space Flight Center, Greenbelt, Md. The instrument’s principal investigator is Dr. Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.

Original Source: Spitzer Space Telescope

Fossil galaxy cluster as observed by XMM-Newton. Image credit: ESA. Click to enlarge
Galaxies start small, but grow over time as they merge with other galaxies. After a while, however, the nearby space runs out of galaxies to merge with. All that’s left is one large galaxy called a fossil group, which sits inside an even larger halo of dark matter. Astronomers are puzzled at how these fossil groups are able to form rapidly – some shouldn’t be able to do it in the lifetime of the Universe. New observations from Chandra and ESA’s XMM-Newton observatories have provided new clues about how these clusters collapse and form.

Taking advantage of the high sensitivity of ESA’s XMM-Newton and the sharp vision of NASA’s Chandra X-Ray space observatories, astronomers have studied the behaviour of massive fossil galaxy clusters, trying to find out how they find the time to form.

Many galaxies reside in galaxy groups, where they experience close encounters with their neighbours and interact gravitationally with the dark matter – mass which permeates the whole intergalactic space but is not directly visible because it doesn’t emit radiation.

These interactions cause large galaxies to spiral slowly towards the centre of the group, where they can merge to form a single giant central galaxy, which progressively swallows all its neighbours.

If this process runs to completion, and no new galaxies fall into the group, then the result is an object dubbed a ‘fossil group’, in which almost all the stars are collected into a single giant galaxy, which sits at the centre of a group-sized dark matter halo. The presence of this halo can be inferred from the presence of extensive hot gas, which fills the gravitational potential wells of many groups and emits X-rays.

A group of international astronomers studied in detail the physical features of the most massive and hot known fossil group, with the main aim to solve a puzzle and understand the formation of massive fossils. In fact, according to simple theoretical models, they simply could not have formed in the time available to them!

The fossil group investigated, called ‘RX J1416.4+2315’, is dominated by a single elliptical galaxy located one and a half thousand million light years away from us, and it is 500 thousand million times more luminous than the Sun.

The XMM-Newton and Chandra X-ray observations, combined with optical and infrared analyses, revealed that group sits within a hot gas halo extending over three million light years and heated to a temperature of 50 million degrees, mainly due to shock heating as a result of gravitational collapse.

Such a high temperature, about as twice as the previously estimated values, is usually characteristic of galaxy clusters. Another interesting feature of the whole cluster system is its large mass, reaching over 300 trillion solar masses. Only about two percent of it in the form of stars in galaxies, and 15 percent in the form of hot gas emitting X-rays. The major contributor to the mass of the system is the invisible dark matter, which gravitationally binds the other components.

According to calculations, a fossil cluster as massive as RX J1416.4+2315 would have not had the time to form during the whole age of the universe. The key process in the formation of such fossil groups is the process known as ‘dynamical friction’, whereby a large galaxy loses its orbital energy to the surrounding dark matter. This process is less effective when galaxies are moving more quickly, which they do in massive ‘clusters’ of galaxies.

This, in principle, sets an upper limit to the size and mass of fossil groups. The exact limits are, however, still unknown since the geometry and mass distribution of groups may differ from that assumed in simple theoretical models.

“Simple models to describe the dynamical friction assume that the merging galaxies move along circular orbits around the centre of the cluster mass”, says Habib Khosroshahi from the University of Birmingham (UK), first author of the results. “Instead, if we assume that galaxies fall towards the centre of the developing cluster in an asymmetric way, such as along a filament, the dynamic friction and so the cluster formation process may occur in a shorter time scale,” he continues. Such a hypothesis is supported by the highly elongated X-ray emission we observed in RX J1416.4+2315, to sustain the idea of a collapse along a dominant filament.”

The optical brightness of the central dominant galaxy in this fossil is similar to that of brightest galaxies in large clusters (called ‘BCGs’). According to the astronomers, this implies that such galaxies could have originated in fossil groups around which the cluster builds up later. This offers an alternative mechanism for the formation of BCGs compared to the existing scenarios in which BCGs form within clusters during or after the cluster collapse.

“The study of massive fossil groups such as RX J1416.4+2315 is important to test our understanding of the formation of structure in the universe,” adds Khosroshahi. “Cosmological simulations are underway which attempt to reproduce the properties we observe, in order to understand how these extreme systems develop,” he concludes.

Original Source: ESA News Release

A scientifically accurate model of Beta Pictoris and its disk. Image credit: NAOJ. Click to enlarge
The disks of gas and dust that surround newborn stars are known as proto-planetary disks; which are thought to be regions where planets will eventually form. These disks disappear as the stars mature, but some stars can still be seen with a cloud of material around them called debris disks. One of the most famous of these is the disk surrounding Beta Pictoris, located only 60 light years away.

Planets form in disks of gas and dust that surround new born stars. Such disks are called proto-planetary disks. The dust in these disks become rocky planets like Earth and the inner cores of giant gas planets like Saturn. This dust is also a repository of elements that form the basis of life.

Proto-planetary disks disappear as stars mature, but many stars have what are called debris disks. Astronomers hypothesize that once objects such as asteroids and comets are born from the proto-planetary disk, collisions among them can produce a secondary dust disk.

The most well-known example of such dust disks is the one surrounding the second brightest star in the constellation Pictor, meaning “painter’s easel”. This star, known as Beta Pictoris or Beta Pic, is a very close neighbor of the Sun, only sixty light years away, and therefore easy to study in great detail.

Beta Pic is twice as bright as the Sun, but the light from the disk is much fainter. Astronomers Smith and Terrile were the first to detect this faint light in 1984, by blocking the light from the star itself using a technique called coronagraphy. Since then, many astronomers have observed the Beta Pic disk using ever better instruments and ground and space-based telescopes to understand in detail the birth place of planets, and hence life.

A team of astronomers from the National Astronomical Observatory of Japan, Nagoya University and Hokkaido University combined several technologies for the first time to obtain an infrared polarization image of the Beta Pic disk with better resolution and higher contrast than ever before: a large aperture telescope (the Subaru telescope, with its large 8.2 meter primary mirror), adaptive optics technology, and a coronagraphic imager capable of taking images of light with different polarizations (Subaru’s Coronagraphic Imager with Adaptive Optics,CIAO).

A large aperture telescope, especially with Subaru’s great imaging quality, allows faint light to be seen at high resolution. Adaptive optics technology reduces Earth’s atmosphere’s distorting effects on light, allowing higher resolution observations. Coronagraphy is a technique for blocking light from a bright object such as a star, to see fainter objects near it, such as planets and dust surrounding a star. By observing polarized light, reflected light can be distinguished from light coming directly from its original source. Polarization also contains information about the size, shape, and alignment of dust reflecting light.

With this combination of technologies, the team succeeded in observing Beta Pic in infrared light two micrometers in wavelength at a resolution of a fifth of an arcsecond. This resolution corresponds to being able to see an individual grain of rice from one mile away or a mustard seed from a kilometer away. Achieving this resolution represents a huge improvement over comparable previous polarimetric observations from the 1990’s that had only resolutions of about one and a half arcseconds.

The new results strongly suggest that Beta Pic’s disk contains planetesimals, asteroid or comet-like objects, that collide to generate dust that reflects starlight.

The polarization of the light reflected from the disk can reveal the physical properties of the disk such as composition, size, and distribution. An image of all the two micrometer wavelength light shows the long thin structure of the disk seen nearly edge on. The polarization of the light shows that ten percent of the two micrometer light is polarized. The pattern of polarization indicates that the light is a reflection of light that originated from the central star.

An analysis of how the brightness of the disk changes with distance from the central shows a gradual decrease in brightness with a small oscillation. The slight oscillation in brightness corresponds to variations in the density of the disk. The most likely explanation is that denser regions correspond to where planetesimals are colliding. Similar structures have been seen closer to the star in earlier observations at longer wavelengths using Subaru’s COoled Mid-Infrared Camera and Spectrograph (COMICS) and other instruments.

A similar analysis of how the amount of polarization changes with distance from the star shows a decrease in polarization at a distance of one hundred astronomical units (an astronomical unit is the distance between Earth and the Sun). This corresponds to a location where the brightness also decreases, suggesting that at this distance from the star there are fewer planetesimals.

As the team investigated models of the Beta Pic disk that can explain both the new and old observations, they found that the dust in Beta Pic’s disk is more than ten times larger than typical grains of interstellar dust. Beta Pics dust disk is probably made of micrometer sized loose clumps of dust and ice like miniscule bacteria-size dust bunnies.

Together, these results provide very strong evidence that the disk surrounding Beta Pic is generated by the formation and collision of planetesimals. The level of detail of this new information solidifies our understanding of the environment in which planets form and develop.

Motohide Tamura who leads the team says “few people have been able to study the birth place of planets by observing polarized light with a large telescope. Our results show that this is a very rewarding approach. We plan on extending our research to other disks, to get a comprehensive picture of how dust transforms into planets.”

These results were published in the April 20, 2006, edition of the Astrophysical Journal.

Team Members: Motohide Tamura, Hiroshi Suto, Lyu Abe (NAOJ), Misato Fukagawa (Nagoya University, California Institute of Technology), Hiroshi Kimura, Tetsuo Yamamoto (Hokkaido University)

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan through a Grant-in-Aid for Scientific Research on Priority Areas for the “Development of Extra-solar Planetary Science.”

Original Source: NAOJ News Release

Massive star forming region as seen by ISO. Image credit: ESA. Click to enlarge
Astronomers have used ESA’s Infrared Space Observatory to peer at the biggest stars being born. These stars form from the collapse of enormous clouds of gas, and can shine more than 100,000 times as bright as our own Sun. The images were captured as a bonus, taken while the space observatory was slowly turning from one target to another. A team of astronomers built up a vast web of images comprised from 10,000 of these telescope repositionings, and then identified potential star forming regions from the data.

Scientists have secured their first look at the birth of monstrous stars that shine 100 000 times more brightly than the Sun, thanks to ESA’s Infrared Space Observatory (ISO).

The discovery allows astronomers to begin investigating why only some regions of space promote the growth of these massive stars.

Space is littered with giant clouds of gas. Occasionally, regions within these clouds collapse to form stars. “One of the major questions in the field of study is why do some clouds produce high- and low-mass stars, whilst others form only low-mass stars?” asks Oliver Krause, Max-Planck-Institut fur Astronomie, Heidelberg and Steward Observatory, Arizona.

The conditions necessary to form high-mass stars are difficult to deduce because such stellar monsters form far away and are shrouded behind curtains of dust. Only long wavelengths of infrared radiation can escape from these obscuring cocoons and reveal the low temperature dust cores that mark the sites of star formation. This radiation is exactly what ISO’s ISOPHOT far-infrared camera has collected.

Stephan Birkmann, Oliver Krause and Dietrich Lemke, all of the Max-Planck-Institut f?r Astronomie, Heidelberg, used ISOPHOT’s data to zero-in on two intensely cold and dense cores, each containing enough matter to form at least one massive star. “This opens up a new era for the observations of the early details of high-mass star formation,” says Krause.

The data was collected in the ISOPHOT Serendipity Survey (ISOSS), a clever study pioneered by Lemke. He realised that when ISO was turning from one celestial object to another, valuable observing time was being lost. He organised for ISOPHOT’s far-infrared camera to continuously record during such slews and beam this data to Earth.

During the ISO mission, which lasted for two and a half years during 1995?98, the spacecraft made around 10 000 slews, providing a web of data across the sky for the previously unexplored window of infrared emission at 170 micrometres. This wavelength is 310 times longer than optical radiation and reveals cold dust down to just 10K (-263 degrees Celsius). A catalogue was produced of the cold sites in the survey.

Birkmann and his colleagues investigated this catalogue and found fifty potential places of high-mass stellar birth. A campaign of follow-up observations using ground-based telescopes revealed that object ISOSS J18364-0221 was in fact two cold dense cores that looked suspiciously like those associated with the birth of low-mass stars, but containing much more mass.

The first core is at 16.5 Kelvin (?256.5 degrees Celsius). It contains seventy-five times the mass of the Sun and shows signs of gravitational collapse. The second one is around 12K (?261 degrees Celsius) and contains 280 solar masses. The team are currently studying the other potential sites.

Original Source: ESA Portal

Effects of the extragalactic background light on the gamma emission by a quasar before reaching Earth. Image credit: HESS Collaboration. Click to enlarge
When astronomers look into the sky, they see bright objects, but also a diffuse glow coming from objects across the Universe in many different wavelengths. This glow could serve like a fossil record, to help astronomers untangle the different stages the Universe went through from the beginning, to the current day. Research teams are using very high energy gamma rays, generated in the most violent objects in the Universe – quasars – as a probe to understand this background light.

The light emitted from all objects in the Universe during its entire history – stars, galaxies, quasars etc. forms a diffuse sea of photons that permeates intergalactic space, referred to as “diffuse extragalactic background light” (EBL). Scientists have long tried to measure this fossil record of the luminous activity in the Universe in their quest to decipher the history and evolution of the Cosmos, but its direct determination from the diffuse glow of the night sky is very difficult and uncertain.

Very high energy (VHE) gamma-rays, some 100,000,000,000 times more energetic than normal light, offer an alternative way to probe this background light, and UK researchers from Durham University in collaboration with international partners used the High Energy Stereoscopic System (HESS) gamma-ray telescopes in the Khomas Highlands of Namibia to observe several quasars (the most luminous VHE gamma-ray sources known) with this goal in mind. The results, to be published in the April 20 issue of Nature, turned out to be rather striking.

Gamma-rays, which are produced in the most violent objects in the Universe, are absorbed in their journey from distant objects to Earth if they happen to hit a photon of “normal” background light. This fog of light in which the Universe is bathed is a fossil record of all the light emitted in the Universe over its lifetime, from the glare of the first stars and galaxies up to the present time. So, using the distant quasars as a probe and studying the effect of the fossil light on the energy distribution of the initial gamma-rays, astrophysicists used HESS to derive a limit on the maximum amount of this ‘extragalactic background light’, which is remarkably lower than what previous estimates had suggested.

This result, published in the April 20 issue of Nature, has important consequences for our understanding of galaxy formation and evolution, and expands the horizon of the gamma-ray Universe which is clearly more transparent to gamma-rays than previously believed

Commenting in the findings, Dr Lowry McComb of Durham University, said, “HESS has in the last few years achieved a number of important discoveries concerning high-energy gamma-ray sources in our own Galaxy and has revolutionized high-energy gamma-ray astronomy. These new HESS results illustrate the power of the instrument for extragalactic astronomy and cosmology. The discovery of lower levels of intergalactic starlight has the interesting side effect that the Universe becomes more transparent to gamma rays and that the telescopes can look deeper into the cosmos, increasing their reach for further discoveries!”

Original Source: PPARC News Release

The afterglow of GRB 030329 (white dot in center of image). Image credit: ESA/NASA. Click to enlarge
Since gamma ray bursts release a torrent of radiation visible across the Universe, it goes without saying that we wouldn’t one to blow up near us. Well, don’t worry. According to researchers at Ohio State University, our Milky Way is the just wrong type of galaxy for potential bursts – they almost always happen within small, misshapen galaxies that lack heavy chemical elements. That’s good news, since a burst within 3,000 light years of the Earth would give us a lethal dose of radiation.

Are you losing sleep at night because you’re afraid that all life on Earth will suddenly be annihilated by a massive dose of gamma radiation from the cosmos?

Well, now you can rest easy.

Some scientists have wondered whether a deadly astronomical event called a gamma ray burst could happen in a galaxy like ours, but a group of astronomers at Ohio State University and their colleagues have determined that such an event would be nearly impossible.

Gamma ray bursts (GRBs) are high-energy beams of radiation that shoot out from the north and south magnetic poles of a particular kind of star during a supernova explosion, explained Krzysztof Stanek, associate professor of astronomy at Ohio State. Scientists suspect that if a GRB were to occur near our solar system, and one of the beams were to hit Earth, it could cause mass extinctions all over the planet.

The GRB would have to be less than 3,000 light years away to pose a danger, Stanek said. One light year is approximately 6 trillion miles, and our galaxy measures 100,000 light years across. So the event would not only have to occur in our galaxy, but relatively close by, as well.

In the new study, which Stanek and his coauthors submitted to the Astrophysical Journal, they found that GRBs tend to occur in small, misshapen galaxies that lack heavy chemical elements (astronomers often refer to all elements other than the very lightest ones — hydrogen, helium, and lithium — as metals). Even among metal-poor galaxies, the events are rare — astronomers only detect a GRB once every few years.

But the Milky Way is different from these GRB galaxies on all counts — it’s a large spiral galaxy with lots of heavy elements.

The astronomers did a statistical analysis of four GRBs that happened in nearby galaxies, explained Oleg Gnedin, a postdoctoral researcher at Ohio State. They compared the mass of the four host galaxies, the rate at which new stars were forming in them, and their metal content to other galaxies catalogued in the Sloan Digital Sky Survey.

Though four may sound like a small sample compared to the number of galaxies in the universe, these four were the best choice for the study because astronomers had data on their composition, Stanek said. All four were small galaxies with high rates of star formation and low metal content.

Of the four galaxies, the one with the most metals — the one most similar to ours — hosted the weakest GRB. The astronomers determined that the odds of a GRB occurring in a galaxy like that one to be approximately 0.15 percent.

And the Milky Way’s metal content is twice as high as that galaxy, so our odds of ever having a GRB would be even lower than 0.15 percent.

“We didn’t bother to compute the odds for our galaxy, because 0.15 percent seemed low enough,” Stanek said.

He figures that most people weren’t losing sleep over the possibility of an Earth-annihilating GRB. “I wouldn’t expect the stock market to go up as a result of this news, either,” he said. “But there are a lot of people who have wondered whether GRBs could be blamed for mass extinctions early in Earth’s history, and our work suggests that this is not the case.”

Astronomers have studied GRBs for more than 40 years, and only recently determined where they come from. In fact, Stanek led the team that tied GRBs to supernovae in 2003.

He and Gnedin explained that when a very massive, rapidly rotating star explodes in a supernova, its magnetic field directs gamma radiation to flow only out of the star’s north and south magnetic poles, forming high-intensity jets.

Scientists have measured the energies of these events and assumed — rightly so, Stanek said — that such high-intensity radiation could destroy life on a planet. That’s why some scientists have proposed that a GRB could have been responsible for a mass extinction that occurred on Earth 450 million years ago.

Now it seems that gamma ray bursts may not pose as much a danger to Earth or any other potential life in the universe, either, since they are unlikely to occur where life would develop.

Planets need metals to form, Stanek said, so a low-metal galaxy would probably have fewer planets, and fewer chances for life.

He added that he didn’t originally intend to address the question of mass extinctions. The study grew out of a group discussion during the Ohio State Department of Astronomy’s “morning coffee” — a daily half-hour where faculty and students review new astronomy journal articles that have been posted to Internet preprint servers overnight. In February, Stanek published a paper on a GRB he had observed, and during coffee someone asked whether he thought it was just a coincidence that these events seem to happen in small, metal-poor galaxies.

“My initial reaction was that it’s not a coincidence, and everyone just knows that GRBs happen in metal-poor galaxies. But then people asked, ‘Is it really that well known? Has anybody actually proven it to be true?’ And we realized that nobody had.”

As a result, the list of coauthors on the paper includes astronomers across a broad range of expertise, which Stanek said is somewhat unusual in these days of specialized research. The coauthors were among faculty gathered for coffee that day, plus a few friends they recruited to help them: Stanek and Gnedin; John Beacom, assistant professor of physics and astronomy; Jennifer Johnson, assistant professor of astronomy; Juna Kollmeier, a graduate student; Andrew Gould, Marc Pinsonneault, Richard Pogge, and David Weinberg, all professors of astronomy at Ohio State; and Maryam Modjaz, a graduate student at the Harvard-Smithsonian Center for Astrophysics.

This work was sponsored by the National Science Foundation.

Original Source: Ohio State University

Pulsar RX J0720.4-3125 captured by XMM-Newton. Image credit: ESA/MPE. Click to enlarge
ESA’s orbiting X-ray telescope, the XMM-Newton space observatory, has located a neutron star that’s out of control. Researchers found that its temperature rose steadily for more than four years, but now it’s starting to decrease again. The object’s overall temperature isn’t changing, it’s just tumbling, and slowly displaying different areas to observers here on Earth – like a wobbling top. These observations will help astronomers understand some of the internal processes that govern these kinds of objects.

Using data from ESA’s XMM-Newton X-ray observatory, an international group of astrophysicists discovered that one spinning neutron star doesn’t appear to be the stable rotator scientists would expect. These X-ray observations promise to give new insights into the thermal evolution and finally the interior structure of neutron stars.

Spinning neutron stars, also known as pulsars, are generally known to be highly stable rotators. Thanks to their periodic signals, emitted either in the radio or in the X-ray wavelength, they can serve as very accurate astronomical ‘clocks’.

The scientists found that over the past four and a half years the temperature of one enigmatic object, named RX J0720.4-3125, kept rising. However, very recent observations have shown that this trend reversed and the temperature is now decreasing.

According to the scientists this effect is not due to a real variation in temperature, but instead to a changing viewing geometry. RX J0720.4-3125 is most probably ‘precessing’, that is it is slowly tumbling and therefore, over time, it exposes to the observers different areas of the surface.

Neutron stars are one of the endpoints of stellar evolution. With a mass comparable to that of our Sun confined into a sphere of 20-40 km diameter, their density is even somewhat higher than that of an atomic nucleus – a billion tonnes per cubic centimetre. Soon after their birth in a supernova explosion their temperature is of the order of 1 000 000 degrees celsius and the bulk of their thermal emission falls in the X-ray band of the electromagnetic spectrum. Young isolated neutron stars are slowly cooling down and it takes a million years before they become too cold to be observable in X-rays.

Neutron stars are known to possess very strong magnetic fields, typically several trillion times stronger than that of the Earth. The magnetic field can be so strong that it influences the heat transport from the stellar interior through the crust leading to hot spots around the magnetic poles on the star surface.

It is the emission from these hotter polar caps which dominates the X-ray spectrum. There are only a few isolated neutron stars known from which we can directly observe the thermal emission from the surface of the star. One of them is RX J0720.4-3125, rotating with a period of about eight and a half seconds. “Given the long cooling time scale it was therefore highly unexpected to see its X-ray spectrum changing over a couple of years,” said Frank Haberl from the Max-Planck-Institute for Extraterrestrial Physics in Garching (Germany), who led the research group.

“It is very unlikely that the global temperature of the neutron star changes that quickly. We are rather seeing different areas of the stellar surface at different times. This is also observed during the rotation period of the neutron star when the hot spots are moving in and out of our line of sight, and so their contribution to the total emission changes,” Haberl continued.

A similar effect on a much longer time scale can be observed when the neutron star precesses (similarly to a spinning top). In that case the rotation axis itself moves around a cone leading to a slow change of the viewing geometry over the years. Free precession can be caused by a slight deformation of the star from a perfect sphere, which may have its origin in the very strong magnetic field.

During the first XMM-Newton observation of RX J0720.4-3125 in May 2000, the observed temperature was at minimum and the cooler, larger spot was predominantly visible. On the other hand, four years later (May 2004) the precession brought into view mostly the second, hotter and smaller spot, that made the observed temperature increase. This likely explains the observed variation in temperature and emitting areas, and their anti-correlation.

In their work Haberl and colleagues developed a model for RX J0720.4-3125 which can explain many of the peculiar characteristics which have been a challenge to explain so far. In this model the long-term change in temperature is produced by the different fractions of the two hot polar caps which enter into view as the star precesses with a period of about seven to eight years.

In order for such a model to work, the two emitting polar regions need to have different temperatures and sizes, as it has been recently proposed in the case of another member of the same class of isolated neutron stars.

According to the team, RX J0720.4-3125 is probably the best case to study precession of a neutron star via its X-ray emission directly visible from the stellar surface. Precession may be a powerful tool to probe the neutron star interior and learn about the state of matter under conditions which we can not produce in the laboratory.

Additional XMM-Newton observations are planned to further monitor this intriguing object. “We are continuing the theoretical modelling from which we hope to learn more about the thermal evolution, the magnetic field geometry of this particular star and the interior structure of neutron stars in general,” Haberl concluded.

Original Source: ESA Portal

As part of his general theory of relativity, Einstein predicted that mass should emit gravity waves. They’ll be weak, though, so it would take very massive objects to produce waves detectable here on Earth. One experiment working towards their detection is the Laser Interferometer Gravitational-Wave Observatory (or LIGO). It should be able to detect the most powerful gravity waves as they pass through the Earth. And a space-based observatory planned for launch in 2015 called LISA should be stronger still.

Scientists are close to actually see gravitational waves. Image credit: NASA
Gravity is a familiar force. It’s the reason for fear of heights. It holds the moon to the Earth, the Earth to the sun. It keeps beer from floating out of our glasses.

But how? Is the Earth sending secret messages to the moon?

Well, yes — sort of.

Eanna Flanagan, Cornell associate professor of physics and astronomy, has devoted his life to understanding gravity since he was a student at University College Dublin in his native Ireland. Now, nearly two decades after leaving Ireland to study for his doctorate under the famous relativist Kip Thorne at the California Institute of Technology, his work focuses on predicting the size and shape of gravitational waves — an elusive phenomenon forecast by Einstein’s 1916 Theory of General Relativity but which have never been directly detected.

In 1974, Princeton University astronomers Russell Hulse and Joseph H. Taylor Jr. indirectly measured the influence of gravity waves on co-orbiting neutron stars, a discovery that earned them the 1993 Nobel Prize in physics. Thanks to the recent work of Flanagan and his colleagues, scientists are now on the verge of seeing the first gravity waves directly.

Sound cannot exist in a vacuum. It requires a medium, such as air or water, through which to deliver its message. Similarly, gravity cannot exist in nothingness. It, too, needs a medium through which to deliver its message. Einstein theorized that that medium is space and time, or the “spacetime fabric.”

Changes in pressure — a thump on a drum, a vibrating vocal cord — produce sound waves, ripples in air. According to Einstein’s theory, changes in mass — the collision of two stars, dust landing on a bookshelf — produce gravity waves, ripples in spacetime.

Because most everyday objects have mass, gravity waves should be all around us. So why can’t we find any?

“The strongest gravity waves will cause measurable disturbances on Earth 1,000 times smaller than an atomic nucleus,” explained Flanagan. “Detecting them is a huge technical challenge.”

The response to that challenge is LIGO, the Laser Interferometer Gravitational-Wave Observatory, a colossal experiment involving a collaboration of more than 300 scientists.

LIGO consists of two installations nearly 2,000 miles apart — one in Hanford, Wash., and one in Livingston, La. Each facility is shaped like a giant “L,” with two 2.5-mile-long arms made of 4-foot-diameter vacuum pipes encased in concrete. Ultra-stable laser beams traverse the pipes, bouncing between mirrors at the end of each arm. Scientists expect a passing gravity wave to stretch one arm and squeeze the other, causing the two lasers to travel slightly different distances.

The difference can then be measured by “interfering” the lasers where the arms intersect. It is comparable to two cars speeding perpendicularly toward a crossroads. If they travel the same speed and distance, they will always crash. But if the distances are different, they might miss. Flanagan and his colleagues are hoping for a miss.

Furthermore, exactly how much the lasers hit or miss will provide information about the characteristics and origin of the gravitational wave. Flanagan’s role is to predict these characteristics so that his colleagues at LIGO know what to look for.

Due to technological limits, LIGO is only capable of sensing gravitational waves of certain frequencies from powerful sources, including supernova explosions in the Milky Way and rapidly spinning or co-orbiting neutron stars in either the Milky Way or distant galaxies.

To expand potential sources, NASA and the European Space Agency are already planning LIGO’s successor, LISA, the Laser Interferometer Space Antenna. LISA is similar in concept to LIGO, except the lasers will bounce among three satellites 3 million miles apart trailing the Earth in orbit around the sun. As a result, LISA will be able to detect waves at lower frequencies than LIGO, such as those produced by the collision of a neutron star with a black hole or the collision of two black holes. LISA is scheduled for launch in 2015.

Flanagan and collaborators at the Massachusetts Institute of Technology recently deciphered the gravitational wave signature that results when a supermassive black hole swallows a sun-sized neutron star. It is a signature that will be important for LISA to recognize.

“When LISA flies we should see hundreds of these things,” noted Flanagan. “We will be able to measure how space and time are warped, and how space is supposed to be twisted around by a black hole. We see electromagnetic radiation, and we think it’s probably a black hole — but that’s about as far as we’ve got. It will be very exciting to finally see that relativity actually works.”

But, he warned, “It may not work. Astronomers observe that the expansion of the universe is accelerating. One explanation is that general relativity needs to be modified: Einstein was mostly right, but in some regimes things could work differently.”

Thomas Oberst is a science writer intern at the Cornell News Service.

Original Source: Cornell University

Star clusters in small magellanic cloud. Image credit: ESA/NASA. Click to enlarge
The Hubble Space Telescope has captured these stunning images of open star clusters NGC 265 and NGC 290 in the Small Magellanic Cloud. The two clusters are about 200,000 light years away, and are roughly 65 light-years across. Clusters like this contain young stars roughly the same age, and born from the same cloud of interstellar gas. These clusters will eventually be broken apart by the gravity of other stars, gas clouds and clusters.

NASA’s Hubble Space Telescope has captured the most detailed images to date of the open star clusters NGC 265 and NGC 290 in the Small Magellanic Cloud – two sparkling sets of gemstones in the southern sky.

These images, taken with Hubble’s Advanced Camera for Surveys, show a myriad of stars in crystal clear detail. The brilliant open star clusters are located about 200,000 light-years away and are roughly 65 light-years across.

Star clusters can be held together tightly by gravity, as is the case with densely packed crowds of hundreds of thousands of stars, called globular clusters. Or, they can be more loosely bound, irregularly shaped groupings of up to several thousands of stars, like the open clusters shown in this image.

The stars in these open clusters are all relatively young and were born from the same cloud of interstellar gas. Just as old school-friends drift apart after graduation, the stars in an open cluster will only remain together for a limited time and gradually disperse into space, pulled away by the gravitational tugs of other passing clusters and clouds of gas. Most open clusters dissolve within a few hundred million years, whereas the more tightly bound globular clusters can exist for many billions of years.

Open star clusters make excellent astronomical laboratories. The stars may have different masses, but all are at about the same distance, move in the same general direction, and have approximately the same age and chemical composition. They can be studied and compared to find out more about stellar evolution, the ages of such clusters, and much more.

The Small Magellanic Cloud, which hosts the two star clusters, is one of the small satellite galaxies of the Milky Way. It can be seen with the unaided eye as a hazy patch in the constellation Tucana (the Toucan) in the Southern Hemisphere. The Small Magellanic Cloud is rich in gas nebulae and star clusters. It is most likely that this irregular galaxy has been disrupted through repeated interactions with the Milky Way, resulting in the vigorous star-forming activity seen throughout the cloud. NGC 265 and NGC 290 may very well owe their existence to these close encounters with the Milky Way.

The images were taken in October and November 2004 through F435W, F555W, and F814W filters (shown in blue, green, and red, respectively).

Original Source: HubbleSite News Release