Universe Today

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