Universe Today

Image credit: ESA

Gamma ray bursts (or GRBs) are the most powerful known explosions in the Universe. Although astronomers aren’t exactly sure what causes them, they’re somehow linked to supernovae explosions – it could be the formation of a black hole after the supernova explodes. When a GRB goes off, it funnels a tremendous amount of energy into two lighthouse-like beams that would probably vaporize anything out to 200 light-years away. Fortunately there aren’t any stars in our galactic neighborhood that has the potential to explode as a supernova, so we’re probably safe from such an event, but astronomers will keep looking? just to be sure.

For a few seconds every day, Earth is bombarded by gamma rays created by cataclysmic explosions in distant galaxies. Such explosions, similar to supernovae, are known as ?gamma-ray bursts? or GRBs.

Astronomers using ESA?s X-ray observatory, XMM-Newton, are trying to understand the cause of these extraordinary explosions from the X-rays given out for a day or two after the initial burst.

Danger to life?
However, the violence of the process begs the question, what happens to the space surrounding a GRB? A few years ago, some astronomers thought that a GRB might wipe out all life in its host galaxy.

That now seems to be a pessimistic view because the latest evidence shows that GRBs focus their energy along two narrow beams, like a lighthouse might do on Earth, rather than exploding in all directions like a bomb.

That does not mean that GRBs are not dangerous. Some theories suggest that anything caught in the beam, out to a distance of around 200 light years, will be vaporised.

Have there been GRBs in our own galaxy?
Although none of the recently detected GRBs seem powerful enough, events in the distant past are another question. ?There are a lot of supernova remnants in our galaxy, so I suspect that most probably there have been several GRBs as well,? says ESA astronomer Norbert Schartel.

While astronomers have yet to detect a really close GRB, they may already have picked up the most distant ones. ESA?s gamma-ray observatory, Integral, continues to collect invaluable data about GRBs on a daily basis, but last year XMM-Newton recorded the fading afterglow of X-rays that accompanied one GRB.

When Schartel and collaborators analysed the results, they found that the X-rays contained the ?fingerprints? of gas that was glowing like the X-ray equivalent of a ?neon? strip light.

Link between GRBs and exploding stars
This was the first piece of hard evidence that GRBs were linked to exploding stars, similar to supernovae. Now, XMM-Newton has captured another X-ray afterglow that shows similar features, strengthening the link.

Using these data and the discovery of visible explosions of some GRBs by NASA/ESA?s Hubble Space Telescope, astronomers have pieced together a picture of what happens.

It seems that the explosion of the star is just the first stage. The GRB itself is generated sometime later but whether that is hours, days or even weeks afterwards, no one yet knows. The GRB occurs when the centre of the exploding star turns into a ?black hole? and the X-rays are released as the GRB shock wave collides with the gas thrown off in the star?s original explosion.

Are we at risk from GRBs?
Another question still remains: could we be vaporised by a nearby GRB? The answer is no, even though there are GRBs detected almost everyday, scattered randomly throughout the Universe, it is highly unlikely. There are no stars within 200 light years of our Solar System that are of the type destined to explode as a GRB, so we do not expect to witness such an event at close range!

However, we do know that ESA?s scientific study of these fascinating ? and frightening ? cosmic events will continue for many years to come.

Original Source: ESA News Release

Image credit: PPARC

Astronomers have long believed that galaxy formation in the early Universe was a spectacular event, with smaller groups smashing together to form larger elliptical galaxies, and star formation would have been everywhere. New data gathered by the SCUBA telescope help support this theory. A team of UK astronomers have captured images of galaxy formation 12 billion years ago, at the very limits of today’s astronomy. Their data will help astronomers understand how simple elliptical galaxies formed to help build models that could eventually help to explain how more complex spiral galaxies (like our own Milky Way) could have formed.

Revealing images produced by one of the world’s most sophisticated telescopes are enabling a team of Edinburgh astronomers to see clearly for the first time how distant galaxies were formed 12 billion years ago. Scientists from the UK Astronomy Technology Centre (UK ATC) and the University of Edinburgh have been targeting the biggest and most distant galaxies in the Universe with the world’s most sensitive submillimetre camera, SCUBA. The camera, built in Edinburgh, is operated on the James Clerk Maxwell Telescope in Hawaii. The images, published in Nature tomorrow (18 September), reveal prodigious amounts of dust-enshrouded star formation which could ultimately tell scientists more about the formation of our own galaxy.

It is thought these distant galaxies in the early Universe will evolve into the most massive elliptical galaxies seen at the present day. These giant galaxies consist of 1000 billion stars like our Sun and are found in large groups or clusters.

Dr Jason Stevens, astronomer at the UK ATC in Edinburgh explained why understanding the evolution of these galaxies is so important. “The distant, youthful Universe was a very different place to the one we inhabit today. Billions of years ago, massive galaxies are thought to have formed in spectacular bursts of star formation. These massive elliptical galaxies have relatively simple properties. We hope that by understanding how simple galaxies form we will be one step closer to understanding how our own, spiral, Milky Way galaxy formed”.

Prof. Jim Dunlop, Head of the University of Edinburgh’s Institute for Astronomy said: “For a long time astronomers have anticipated that the formation of the most massive galaxies should have been a spectacular event, but failed to find any observational evidence of massive galaxy formation from optical images. Now we have discovered that it is indeed spectacular, but because of the effects of interstellar dust, the spectacle is only revealed at submillimetre wavelengths.” The dust absorbs the bright blue light emitted by young stars. The energy from the light heats the dust and makes it glow. It is this glow that is detected by the SCUBA camera.

Dr Stevens and his colleagues suspected that these massive galaxies would form in particularly dense regions of space so they chose regions of very distant space that are known to be very dense because they contain massive radio galaxies – galaxies which emit high levels of radio waves. They found that many of the radio galaxies have near-by companion objects that had not previously been detected at any wavelength. Dr Rob Ivison, also at the UK ATC, described what they found. “The companion objects are located in the densest parts of the intergalactic medium, strung out like beads of water on a spider’s web due to the filamentary structure of the Universe”.

The SCUBA images support a popular current model of galaxy formation in which today’s massive elliptical galaxies were assembled in the early Universe in dense regions of space through the rapid merging of smaller building blocks.

Original Source: PPARC News Release

Image credit: Chandra

Although it’s usually peering into deep space, Chandra looked a little closer to home and inspected the Moon in the X-ray spectrum. Although the Moon doesn’t produce X-rays of its own, it does reflect the radiation of the Sun; various atoms such as oxygen, magnesium, aluminum and silicon fluoresce when the Sun’s X-rays bombard the Moon’s surface. Measuring the quantity and location of these elements will help test the theory that the Moon was formed when a Mars-sized object slammed into the Earth 4.5 billion years ago.

The Chandra observations of the bright portion of the Moon detected X-rays from oxygen, magnesium, aluminum and silicon atoms. The X-rays are produced by fluorescence when solar X-rays bombard the Moon’s surface.

According to the currently popular “giant impact” theory for the formation of the Moon, a body about the size of Mars collided with the Earth about 4.5 billion years ago. This impact flung molten debris from the mantle of both the Earth and the impactor into orbit around the Earth. Over the course of tens of millions of years, the debris stuck together to form the Moon. Measuring the amount and distribution of aluminum and other elements over a wide area of the Moon will help to test the giant impact theory.

Chandra’s observations have also solved a decade-long mystery about X-rays detected by ROSAT that were thought to be coming from the dark portion of the Moon. It turns out that these X-rays only appear to come from the Moon. Chandra shows that the X-rays from the dark moon can be explained by radiation from Earth’s geocorona (extended outer atmosphere) through which orbiting spacecraft move.

The geocoronal X-rays are caused by collisions of heavy ions of carbon, oxygen and neon in the solar wind with hydrogen atoms located tens of thousands of miles above the surface of Earth. During the collisions, the solar ions capture electrons from hydrogen atoms. The solar ions then kick out X-rays as the captured electrons drop to lower energy states.

Original Source: Chandra News Release

Image credit: NASA

A team of astronomers believe they’ve figured out the explanation for an unusual object V838 Monocerotis – it’s a red giant star consuming its planets as it nears the end of its life. The object recently flared up to become the brightest cool supergiant in the Milky Way – 600,000 times more luminous than our own Sun. Detailed observations showed that the object flared up three times with similar peaks; they believe this is when the star consumed three gas giants in tight orbits – one after the other. This research could help astronomers find more subtle evidence of this happening to smaller planets in other star systems.

Astronomers from Sydney University have come forth with a solution to a mysterious new object recently discovered in our Milky Way.

In a letter soon to be published in the journal Monthly Notices of the Royal Astronomical Society, Dr Alon Retter and Dr Ariel Marom from the Department of Physics suggest that this phenomenon is an expanding giant star swallowing nearby planets, an event which may one day befall our own planet.

Their research provides data to support the theory that the multi-stage eruption of the “red giant” known as V838 Monocerotis observed last year was fuelled as it engulfed three near orbiting planets. This could be the first evidence for an event that had been predicted but not known to have been observed so far. The work identifies a new group of objects with stars that swallow planets.

Astronomers had previously been unable to explain a spectacular explosion that transformed a dim innocuous star into the brightest cool supergiant in the Milky Way. The event was originally discovered by Australian amateur astronomer, Nicholas Brown in January 2002, when V838 Monocerotis suddenly became 600,000 times more luminous than our Sun. In an ordinary nova explosion, the outer layers of a compact star are ejected into space, exposing the super hot core where nuclear fusion was taking place. By contrast, V838 Monocerotis increased enormously in diameter and its outer layers cooled and were very disrupted but still conceal the giant’s core. Beautiful images taken by the Hubble Space Telescope showed evidence of a previous eruption that ejected material from this object in the past. This too is very unusual.

The Sydney team suggests that the outburst of V838 Monocerotis took place as it swallowed three massive Jupiter-like planets in succession. Evidence for this is provided through study of the shape of the light curve and comparison between the observed properties of the star and several theoretical works. In their scenario, in addition to the gravitational energy generated by the process, there may also have been a rapid release of nuclear energy as “fresh” hydrogen was driven into the hydrogen burning shell of the post-main sequence star.

Interestingly past studies have also suggested that the inner planets in our solar system, Mercury, Venus and maybe even Earth, should be eventually swallowed by the Sun. Previous research has proposed that this is in fact a common characteristic and that many giant stars have consumed planets during their evolution. The current work suggests that the engulfment of a massive planet can cause an eruption of the host star.

Explaining the methods used during their study, Dr Retter said: “The careful inspection of the light curve of V838 Monocerotis showed that the three peaks have a similar structure, namely each maximum is followed by a decline and a very weak secondary peak. The shape of the light curve prompts us to argue that V838 Mon had three events of similar nature, but probably of different strengths. The obvious candidate for such behaviour is the swallowing of massive planets in close orbits around a parent star.”

According to this work, there should be more examples of expanding giants that swallow less and lighter planets thus showing weaker and less spectacular eruptions.

Original Source: University of Sydney News Release

Image credit: NASA

Until recently, astronomers thought that nearly two-thirds of gamma ray bursts – the most powerful known explosions in the Universe – don’t seem to leave an afterglow. It turns out, they just weren’t looking quickly enough. Gamma-ray bursts explode suddenly, last for only a few fractions of a second and then disappear. All that’s left is the afterglow, which astronomers can study to try to understand what caused the explosion. NASA’s HETE spacecraft has quickly determined the positions of 15 gamma-ray bursts and passed this info along to astronomers to follow up with optical telescopes. In this case, only one hasn’t had an afterglow. So, it appears afterglows are common, you just need to look quickly.

Astronomers have solved the mystery of why nearly two-thirds of all gamma-ray bursts, the most powerful explosions in the Universe, seem to leave no trace or afterglow: In some cases, they just weren’t looking fast enough.

New analysis from NASA’s speedy High Energy Transient Explorer (HETE), which locates bursts and directs other satellites and telescopes to the explosion within minutes (and sometimes seconds), reveals that most gamma-ray bursts likely have some afterglow after all.

Scientists announce these results today at a press conference at the 2003 Gamma Ray Burst Conference in Santa Fe, N.M., a culmination of a year’s worth of HETE data.

“For years, we thought of dark gamma-ray bursts as being more unsociable than the Cheshire Cat, not having the courtesy to leave a visible smile behind when they faded away,” said HETE Principal Investigator George Ricker of the Massachusetts Institute of Technology in Cambridge, Mass.

“Now we are finally seeing that smile. Bit by bit, burst by burst, the gamma-ray mystery is unfolding. This new HETE result implies that we now have a way to study most gamma-ray bursts, not just a meager one third.”

Gamma-ray bursts, likely announcing the birth of a black hole, last only for a few milliseconds to upwards of a minute and then fade forever. Scientists say that many bursts seem to emanate from the implosion of massive stars, over 30 times the mass of the Sun. They are random and can occur in any part of the sky at a rate of about one per day. The afterglow, lingering in lower-energy X-ray and optical light for hours or days, offers the primary means to study the explosion.

The lack of an afterglow in a whopping two thirds of all bursts had prompted scientists to speculate that the particular gamma-ray burst might be too far away (so the optical light is “redshifted” to wavelengths not detectable with optical telescopes) or the burst occurred in dusty star-forming regions (where the dust hides the afterglow).

More reasonably, Ricker said, most of the dark bursts are actually forming afterglows, but the afterglows may initially fade very quickly. An afterglow is produced when debris from the initial explosion rams into existing gas in the interstellar regions, creating shock waves and heating the gas until it shines. If the afterglow initially fades too quickly because the shock waves are too weak, or the gas is too tenuous, the optical signal may drop precipitously below the level at which astronomers can pick it up and track it. Later, the afterglow may slow down its rate of decline–but too late for optical astronomers to recover the signal.

HETE, an international mission assembled at and operated by MIT for NASA, determines a quick and accurate location for about two bursts per month. Over the past year, HETE’s tiny but powerful Soft X-ray Camera (SXC), one of three main instruments, accurately determined positions for 15 gamma-ray bursts. Surprisingly, only one out of the SXC’s fifteen bursts has proven to be dark, whereas ten would have been expected based on results from previous satellite.

An MIT-led team has concluded that the reason that afterglows are finally being found are twofold: The accurate, prompt SXC burst locations are being searched quickly and more thoroughly by optical astronomers; and the SXC bursts are somewhat brighter in X rays than the more run-of-the-mill gamma-ray bursts studied by most previous satellites, and thus the associated optical light is also brighter.

Thus, HETE seems to have accounted for all but about 15 percent of gamma-ray bursts, greatly reducing the severity of the “missing afterglow” problem. Studies planned by teams of optical astronomers over the next year should further reduce, and possibly even eliminate, the remaining discrepancy.

Gamma-ray hunters are challenged. Because of the nature of gamma-rays and X-rays, which cannot be focused like optical light, HETE locates bursts within only a few arcminutes by measuring the shadows cast by incident X-rays passing through an accurately calibrated mask within the SXC. (An arcminute is about the size of an eye of a needle held at arm’s length.) Most gamma-ray bursts are exceedingly far, so myriad stars and galaxies fill that tiny circle. Without prompt localization of a bright and fading afterglow, scientists have great difficulty locating the gamma-ray burst counterpart days or weeks later. HETE must continue to localize gamma-ray bursts to settle the discrepancy of the remaining dark bursts.

The HETE spacecraft, on an extended mission into 2004, is part of NASA’s Explorer Program. HETE is a collaboration among MIT; NASA; Los Alamos National Laboratory, New Mexico; France’s Centre National d’Etudes Spatiales (CNES), Centre d’Etude Spatiale des Rayonnements (CESR), and Ecole Nationale Superieure del’Aeronautique et de l’Espace (Sup’Aero); and Japan’s Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago, as well as from Brazil, India and Italy.

Original Source: NASA News Release

Image credit: NASA/JPL

Researchers from NASA and MIT have cooled sodium gas to the lowest temperature ever recorded – one-half billionth degree above absolute zero. At absolute zero temperature (-273 degrees Celsius), all molecular motion would stop completely since the cooling process has extracted all energy from the material. The gas needed to be confined in a magnetic field; otherwise it would stick to the walls of the container and be impossible to cool down. The researchers used a similar methodology that led to the Nobel Prize for Physics in 2001with the discovery of Bose-Einstein condesates (where the molecules move together in an orderly way at low temperatures).

NASA-funded researchers at the Massachusetts Institute of Technology (MIT), Cambridge, Mass., have cooled sodium gas to the lowest temperature ever recorded, one-half-billionth degree above absolute zero. This absolute temperature is the point, where no further cooling is possible.

This new temperature is six times lower than the previous record and marks the first time a gas was cooled below one nanokelvin (one billionth of a degree). At absolute zero (-273? Celsius or -460? Fahrenheit), all motion stops, except for tiny atomic vibrations, since the cooling process has extracted all energy from the particles.

By improving cooling methods, scientists have succeeded in getting closer to absolute zero. “To go below one nanokelvin is like running a mile below four minutes for the first time,” said Dr. Wolfgang Ketterle, a physics professor at MIT and co-leader of the research team.

“Ultra-low temperature gases could lead to vast improvements in precision measurements by allowing better atomic clocks and sensors for gravity and rotation,” said Dr. David E. Pritchard, MIT physics professor, pioneer in atom optics, atom interferometry, and co-leader of the team.

In 1995, a group at the University of Colorado, Boulder, Colo., and a MIT group led by Ketterle, cooled atomic gases to below one microkelvin (one millionth degree above absolute zero). In doing so, they discovered a new form of matter, the Bose-Einstein condensate, where the particles march in lockstep instead of flitting around independently. The discovery was recognized with the 2001 Nobel Prize in Physics, which Ketterle shared with his Boulder colleagues Drs. Eric Cornell and Carl Wieman.

Since the 1995 breakthrough, many groups have routinely reached nanokelvin temperatures; with three nanokelvin as the lowest temperature recorded. The new record set by the MIT group is 500 picokelvin or six times lower.

At such low temperatures, atoms cannot be kept in physical containers, because they would stick to the walls. Also, no known container can be cooled to such temperatures. To circumvent this problem, magnets surround the atoms, which keeps the gaseous cloud confined without touching it. To reach the record-low temperatures, the researchers invented a novel way of confining atoms, which they call a “gravito-magnetic trap.” The magnetic fields acted together with gravitational forces to keep the atoms trapped.

All the researchers are affiliated with the MIT physics department, the Research Laboratory of Electronics and the MIT-Harvard Center for Ultracold Atoms, funded by the National Science Foundation. Ketterle, Leanhardt and Pritchard co-authored the low-temperature paper, scheduled to appear in the September 12 issue of Science. NASA, National Science Foundation, the Office of Naval Research and the Army Research Office funded the research.

Ketterle conducts research under NASA’s Fundamental Physics in Physical Sciences Research Program, part of the agency’s Office of Biological and Physical Research, Washington. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., a division of the California Institute of Technology, Pasadena, manages the Fundamental Physics program.

Original Source: NASA News Release