Survey Confirms Dark Energy Theories

Image credit: Hubble

Recent evidence seems to indicate that the expansion of the Universe is actually accelerating – some kind of “dark energy” is pushing it apart. And a new redshift survey of galactic clusters seems to support this. Astronomers using data gathered by the Chandra X-Ray Observatory have determined that there is insufficient matter (both regular and dark matter) in various galactic clusters to account for their shape and position, so something else must be having an effect.

The universe appears to be permeated with an invisible force ? dark energy ? that is pushing it apart faster and faster. By conducting redshift surveys of galaxy clusters, astronomers hope to learn more about this mysterious force, and about the structure and geometry of the universe.

“Galaxy clusters consist of thousands of galaxies gravitationally bound into huge structures,” said Joseph Mohr, a professor of astronomy at the University of Illinois. “Because of the expansion of the universe, the clusters appear denser at larger redshifts, when the universe was younger and denser.”

Galaxy cluster surveys that probe the high-redshift universe can potentially provide a wealth of information about the amount and nature of both dark matter and dark energy, said Mohr, who will present the results of an ongoing study of galaxy clusters at a meeting of the American Physical Society, to be held in Albuquerque, N.M., April 20-23.

“Till now, galaxy clusters have only been used to study the dark matter component of the universe,” Mohr said. “We would measure the total mass in a galaxy cluster, and then determine the fraction of mass that was ordinary, baryonic matter.”

Those measurements have shown there is insufficient baryonic and dark matter to account for the geometry of the universe. Astronomers now believe the universe is expanding at ever-increasing speed, and is dominated by a mysterious dark energy that must be doing the pushing.

“The next step is to try to figure out some of the specifics of the dark energy, such as its equation of state,” Mohr said. “By mapping the redshift distribution of galaxy clusters, we should be able to measure the equation of state of dark energy, which would provide some important clues to what it is and how it came to be.”

Mohr is using data collected by NASA’s Chandra X-ray Observatory to study scaling relations ? such as the relationship between mass and luminosity or size ? of galaxy clusters and how they change with redshift. “These scaling relations are expected to evolve with redshift, reflecting the increasing density of the universe at earlier times,” Mohr said.

In particular, Mohr ? in collaboration with John Carlstrom at the University of Chicago and scientists at the University of California and Harvard Smithsonian Center for Astrophysics ? is studying the effect that hot electrons within galaxy clusters have on the cosmic microwave background, the afterglow of the big bang.

Galaxy clusters are filled with dark matter, galaxies and hot gas. Electrons in the gas scatter off the protons and produce X-rays. The emission of X-rays diminishes with higher redshift, because of the larger distances involved.

“There also is a tendency for the electrons to give some of their energy to the photons of the cosmic microwave background, which causes the blackbody spectrum to shift slightly,” Mohr said. “The resulting distortion ? called the Sunyaev-Zeldovich effect ? appears as a cold spot on the cosmic microwave background at certain frequencies. Because this is a distortion in the spectrum, however, it doesn’t dim with distance like X-rays.”

By comparing the X-ray emission and the Sunyaev-Zeldovich effect, Mohr can study even faint, high-redshift galaxy clusters that are currently inaccessible by other means. Such measurements, correlating galaxy cluster redshift distribution, structure and spatial distribution, should determine the equation of state of dark energy and, therefore, help define the essence of dark energy.

“Within the context of our standard structure formation scenario, galaxy surveys provide measurements of the geometry of the universe and the nature of the dark matter and dark energy,” Mohr said. “But, to properly interpret these surveys, we must first understand how the structure of galaxy clusters are changing as we look backward in time.”

Original Source: UIUC News Release

Star Formation Exposed

Image credit: Chandra

A new photograph taken by the Chandra X-Ray Observatory shows a close up view of the dynamics of star formation in the Tarantula Nebula (aka 30 Doradus). This region, located 160,000 light years away is one of the most active star forming regions in our local group of galaxies and provides a lot of clues to astronomers. In this region, astronomers have identified at least 11 extremely massive stars with ages of only 2 million years with many more young stars packed together so tightly individual stars can’t be resolved.

The Chandra image of the Tarantula Nebula gives scientists a close-up view of the drama of star formation and evolution. The Tarantula, also known as 30 Doradus, is in one of the most active star-forming regions in our Local Group of galaxies. Massive stars are producing intense radiation and searing winds of multimillion-degree gas that carve out gigantic super-bubbles in the surrounding gas. Other massive stars have raced through their evolution and exploded catastrophically as supernovas, leaving behind pulsars and expanding remnants that trigger the collapse of giant clouds of dust and gas to form new generations of stars.

30 Doradus is located about 180,000 light years from Earth in the Large Magellanic Cloud, a satellite galaxy of our Milky Way Galaxy. It allows astronomers to study the details of starbursts – episodes of extremely prolific star formation that play an important role in the evolution of galaxies.

At least 11 extremely massive stars with ages of about 2 million years are detected in the bright star cluster in the center of the primary image (left panel). This crowded region contains many more stars whose X-ray emission is unresolved. The brightest source in this region known as Melnick 34, a 130 solar-mass star located slightly to the lower left of center. On the lower right of this panel is the supernova remnant N157B, with its central pulsar.

Two off-axis ACIS-S chips (right panel) were used to expand the field of view. They show SNR N157C, possibly a large shell-like supernova remnant or a wind-blown bubble created by OB stars. Supernova 1987A is also visible just above and to the right of the Honeycomb Nebula at the bottom center.

In the image, lower energy X-rays appear red, medium energy green and high-energy are blue.

Original Source: Chandra News Release

Older Quasars a Source of Cosmic Rays

Image credit: NASA

NASA astronomers believe that retired quasars may be a source of rare, high-energy cosmic rays. They’ve identified four elliptical galaxies relatively nearby that contain massive black holes. If these black holes are spinning, they could be a source of ultra high-energy cosmic rays. The source of cosmic rays is a mystery, but astronomers have calculated that they must come from objects within 200 million light years from the Earth – these “retired quasars” could be the source.

They are old but not forgotten. Nearby “retired” quasar galaxies, billions of years past their glory days as the brightest beacons in the Universe, may be the current source of rare, high-energy cosmic rays, the fastest-moving bits of matter known and whose origin has been a long-standing mystery, according to scientists at NASA and Princeton University.

The scientists have identified four elliptical galaxies that may have started this second career of cosmic-ray production, all located above the handle of the Big Dipper and visible with backyard telescopes. Each contains a central black hole of at least 100 million solar masses that, if spinning, could form a colossal battery sending atomic particles, like sparks, shooting off towards Earth at near light speed.

These findings are discussed today in a press conference at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, N.M. The team includes Dr. Diego Torres of Princeton University and Drs. Elihu Boldt, Timothy Hamilton and Michael Loewenstein of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Quasar galaxies are thousands of times brighter than ordinary galaxies, fueled by a central black hole swallowing copious amounts of interstellar gas. In galaxies with so-called quasar remnants, the black hole nucleus is no longer a strong source of radiation.

“Some quasar remnants might not be so lifeless after all, keeping busy in their later years,” said Torres. “For the first time, we see the hint of a possible connection between the arrival directions of ultra-high energy cosmic rays and locations on the sky of nearby dormant galaxies hosting supermassive black holes.”

Ultra high-energy cosmic rays represent one of astrophysics’ greatest mysteries. Each cosmic ray — essentially a single sub-atomic particle such as a proton traveling just shy of light speed — packs as much energy as a major league baseball pitch, over 40 million trillion electron volts. (The rest energy of a proton is about a billion electron volts.) The particles’ source must be within 200 million light years of Earth, for cosmic rays from beyond this distance would lose energy as they traveled through the murk of the cosmic microwave radiation pervading the Universe. There is considerable uncertainty, however, over what kinds of objects within 200 million light years could generate such energetic particles.

“The very fact that these four giant elliptical galaxies are apparently inactive makes them viable candidates for generating ultra high-energy cosmic rays,” said Boldt. Drenching radiation from an active quasar would dampen cosmic-ray acceleration, sapping most of their energy, Boldt said.

The team concedes it cannot determine if the black holes in these galaxies are spinning, a basic requirement for a compact dynamo to accelerate ultra-high energy cosmic rays. Yet scientists have confirmed the existence of at least one spinning supermassive black hole, announced in October 2001. The prevailing theory is that supermassive black holes spin up as they accrete matter, absorbing orbital energy from the infalling matter.

Ultra-high-energy cosmic rays are detected by ground-based observatories, such as the Akeno Giant Air Shower Array near Yamanashi, Japan. They are extremely rare, striking the Earth’s atmosphere at a rate about one per square kilometer per decade. Construction is underway for the Auger Observatory, which will cover 3,000 square kilometers (1,160 square miles) on an elevated plain in western Argentina. A proposed NASA mission called OWL (Orbiting Wide-angle Light-collectors) would detect the highest-energy cosmic rays by looking down on the atmosphere from space.

Loewenstein joins NASA Goddard’s Laboratory for High Energy Astrophysics as a research associate with the University of Maryland, College Park. Hamilton, also a member of the Lab, is a National Research Council fellow.

Original Source: NASA News Release

Planets Line Up in Spectacular Show

Image credit: Harvard

During late April and May you’ll get an opportunity to see the five brightest planets lined up in a single evening. Look West in the early evening sky and you’ll be able to see Mercury, Venus, Mars, Jupiter, and Saturn grouped up. The grouping is fairly rare and won’t be seen again until 2040.

Comet Hale-Bopp dazzled us for weeks. The Perseid meteor shower thrilled us for one night. But the world hasn’t seen anything like the planetary traffic jam that’s going to occur the last week of April and the first two weeks in May!

Inching across the sky like bumper-to-bumper commuters on their way to work, a rare planetary alignment will allow sky observers to see every planet in our solar system in a single evening! “There will be other opportunities in the future to see the planets in different configurations,” says Philip Sadler, Director of the Science Education Department at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, ” but it won’t be anything like this for at least another 70 years. This is truly a once-in-a-lifetime experience.”

In the past, many different configurations of planetary alignments have been seen from Earth. They occur due to the random positions of the planets in their eccentric orbits around the Sun. In the early 1980s and in May of 2000, the planets stacked up directly behind the Sun. Many people thought the combined gravitational pull might create havoc here on Earth resulting in giant earthquakes, sweeping tidal waves or erupting volcanoes. But, the collective gravitational pull was so insignificant, nothing happened. What was the reason? The other planets are simply too small or too far away in space to affect us back on Earth. To see just how insignificant the gravitational pull of the planets can be, let’s do what many good, red-blooded Americans like to do. Let’s go shopping!

Imagine if we stood in the produce section of a grocery store and held up a big yellow grapefruit representing the Sun. The planet Mercury would be the size of a small grain of salt orbiting around it 18 feet away. Venus would be somewhat larger, like a grain of sugar you get in those little brown packets at the coffee shops, 34 feet away. Earth, also a grain of sugar, would be located 50 feet away. Mars also would be the size of a grain of salt 75 feet away. As for the rest of the planets: Jupiter, a cherry-sized tomato, would be found at 240 feet; Saturn, the size of a green grape, at 420 feet; Uranus, a frozen green pea, at 300 yards; Neptune, also the size of a frozen pea, at 470 yards; and Pluto, represented by a speck of dust, would orbit our grapefruit-sized Sun at a distance of 475-600 yards. As you’ve probably guessed, not much gravitational pull is exerted on the Earth by these grocery store lightweights!

In early May, when the planets line up, they will not be arranged behind one another or the Sun. Instead, they will present a beautiful line across the sky from horizon to near zenith. For a period of a little more than three weeks, anyone looking west at sunset will be able to see the planets Mercury, Venus, Mars, Saturn and Jupiter. A few hours later at 4 A.M., armed with a large-size amateur telescope, they can continue their grand tour by observing Uranus, Neptune, and Pluto. By quickly glancing down at the ground, they will have completed their grand tour of the solar system.

Looking at the planets spread out across the sky during this alignment also demonstrates, better than any book, how our solar system formed 4 billion years ago; something astronomers just recently have begun seeing around other distant stars in space. “Our solar system condensed out of a nebular dust cloud that flattened down into a giant disk that resembled a big pizza pan,” says CfA astrophysicist David Wilner. “Utilizing instruments like the Hubble Space Telescope and data from the Infrared Astronomical Satellite, we are now witnessing the formation of new solar systems spread out into flattened discs of gas and dust. We are even detecting large lumps of material in the dust disks that may be the signatures of planets in formation. Astronomers are now assembling snapshots of our own past frozen in time billions of years ago.”

This pathway of planets, or the ecliptic as astronomers call it, is what remains after our dust cloud coalesced into planets. Tracing the path of this ancient dust ring across the sky is easy. Stand sideways facing south with your right hand extended and pointing to where the Sun recently set along the western horizon. Now, extend your arm up to point at the Moon or a bright planet overhead. Connecting these two points together, continue to sweep your arm in an arc until it reaches the opposite horizon. Bingo! You have just traced out the ecliptic. All the planets will be found along this line and nowhere else. And this is where the traffic jam will occur.

“Coincidentally,” says Sadler,” have you ever wondered why the zodiac sign were chosen? Why someone you know wasn’t born under the sign of Hercules or Orion?”

To the Greeks and Romans, the ecliptic was the Highway of the Gods or the path the planets and Moon moved across at night and the Sun traveled during the daytime. “Located directly behind this highway were the twelve special constellations the Gods passed by as they moved across the sky. They constituted the signs of the zodiac. This was the basis for astrology – religious beliefs and basic sky observations mixed together. It should not be confused with the science of astronomy that emerged centuries later,” says Sadler. Today, it is widely held by many historians and planetarium directors that a conjunction of the planets, similar to the one on May 5, accounts for the Star of Bethlehem that sent the Magi on their way to seek the Christ child. Certainly the timing was right. An almost identical triangular alignment of Saturn, Mars and Venus did take place on April 1, 2 B.C. And the planets Jupiter, Saturn and Mars also formed a triangular conjunction in 6 B.C., in the constellation Pisces, the sign of the Christians. However, renowned astronomical historian Prof. Owen Gingerich of the CfA disagrees. “The very, very short duration of a grouping of planets was not the Star of Bethlehem,” he states. “A conjugation like this would have meant nothing to the Magi. It was not part of their astrological tradition. It really wasn’t until Kepler became fascinated with the harmony of the planets in the 16th century that the idea of a planetary conjunction came about to try to attach a scientific explanation to this event. In fact, Kepler even went so far as to add an imaginary supernova to the conjunction of planets in 6 B.C. to try to make it even more spectacular to catch the Magi’s attention. ”

Will this event be religiously significant or just an astronomical oddity? Is it the most dramatic way to visualize how our solar system formed? Or, is it an exciting challenge for amateur astronomers to conduct their only whirlwind tour of the solar system in just one evening? Answering yes to any or all of the above makes the alignment of late April and early May something not to be missed. Nothing like it will occur again in our lifetime. At the very least, it presents a wonderful opportunity for friends and family to come together and share an experience beyond the daily routine. It also may be an opportunity to ponder our fragile existence on this tiny blue world racing around an ordinary yellow star with eight other planetary companions and maybe help us, just a little bit, bring our own world back into perspective.

Headquartered in Cambridge, Massachusetts, 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 seven research divisions study the origin, evolution, and ultimate fate of the universe.

Original Source: CfA News Release

Supernovas May Cause Gamma Ray Bursts

Gamma ray bursts are the most powerful explosions ever detected in the Universe, but astronomers have been uncertain what causes them. There are two theories: collisions between neutron stars, or supernova explosions from very massive stars. New data gathered by the European Space Agency’s XMM-Newton X-ray observatory have helped rule out the first, and maybe confirm the second. By analyzing the afterglow of a recent burst, astronomers were able to detect chemical elements which are found in supernovae.

Gemini Builds Animation of Galactic Core

Image credit: Gemini

The Gemini Observatory located on top of Hawaii’s Mauna Kea has been used to create an animation of the action going on in galaxy NGC 1068. Using a tool called the Integral Field Unit, astronomers have been able to create a 3-dimensional animation of the tremendous jet emanating from the supermassive black hole as it slams into the galactic gas disk.

Astronomers observing with the Gemini North Telescope on Hawaii’s Mauna Kea have a powerful new tool to probe mysterious cosmic caldrons like those at the cores of galaxies and stellar nurseries.

Using the recently commissioned Integral Field Unit (IFU) on the Gemini Multi-Object Spectrograph (GMOS), astronomers at the observatory have recently obtained a complete multi-dimensional picture of the dynamic flow of gas and stars at the core of an active galaxy named NGC 1068 in a single snap-shot. The resulting windfall of data has been transformed into an animation that dramatically reveals the internal gyrations of the galaxy – including the interactions of a pair of galactic-scale jets that spew material for thousands of light years away from the suspected black hole at the galaxy’s core.

“The Gemini data of NGC 1068 reveal one of the lesser know features of galaxy jets,” explains Gemini North Associate Director Dr. Jean-Ren? Roy. “For the first time we were able to clearly see the jet’s expanding lobe as its hypersonic bow shock slams directly into the underlying gas disk of the galaxy. It’s like a huge wave smashing onto a galactic shoreline.”

Dr. Gerald Cecil of the University of North Carolina, recently led an international team to study this particular galaxy using spectra taken with the Hubble Space Telescope and believes that the new Gemini spectra will clarify many patterns revealed by Hubble. “Large ground-based telescopes like Gemini are the perfect complement to Hubble because they can collect so much more light. But it’s critical to use all this light cunningly, and not throw most of it away as standard slit spectrographs do. The GMOS’s integral field capability now enables detailed studies of fundamental physical processes that were previously too time consuming to conduct on faint cosmic sources.” The Hubble findings by Dr. Cecil et al. will appear in the April 1, 2002 issue of the Astrophysical Journal.

“By using Integral Field Spectroscopy we add dimensions to the data and can essentially make a movie with one click of the shutter,” says Dr. Bryan Miller, the Gemini instrument scientist for IFUs. “When we play back our movie of the galaxy NGC1068, we see a 3-dimensional view of the core of this galaxy. It is striking how much easier it is to interpret features with this kind of data. With integral-field data we can determine the mass distributions, the true shapes, and the histories of galaxies more accurately than before.” The Integral Field Spectroscopy findings by Dr. Miller et al. will appear in the Conference Series of the Astronomical Society of the Pacific.

This technology is new to the world of 8-10 meter class telescopes and is especially powerful on new generation telescopes like Gemini that use the latest optical technologies to focus starlight to razor sharpness. “We are very excited by these results and the superb capabilities that the integral field unit has given the GMOS in Hawaii”, notes Dr. Jeremy Allington-Smith, the scientist from the University of Durham in the United Kingdom who managed the construction of the GMOS Integral Field Unit. “In effect we have added an extra dimension to the instrument so that it can map the motion of gas and stars at any point in the image of the object under study. The GMOS IFU will be a powerful new tool for studying the centers of active galaxies that may harbor black holes, as well as the dynamic internal motions of galaxies and star forming regions.” The GMOS IFU findings by Dr. Allington-Smith et al. will appear in the Conference Series of the Astronomical Society of the Pacific.

An Integral Field Unit (IFU) like the one used in the GMOS uses hundreds of tiny optical fibers (each thinner than an human hair) with tiny micro-lenses attached to guide light from the telescope’s 2-D image to a spectrograph. The spectrograph produces one individual spectrum for each fiber for a total of 1500 individual spectra that can each reveal details of the physical conditions and velocity of the gas, dust and stars it studies. This system was the first IFU to be installed on the new generation of 8 and 10m telescopes when it was commissioned on the Gemini-North telescope in 2001.

The Integral Field Spectroscopy capabilities of the Gemini Observatory are still developing. Within the next two years both telescopes will have optical and near-infrared integral field units. Some of these systems will work with adaptive optics to provide the highest spatial resolution images deliverable by the telescopes, including images in the infrared that will be sharper than can be produced by the Hubble Space Telescope at those wavelengths.

The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The telescopes are located at Mauna Kea, Hawaii (Gemini North) and Cerro Pach?n in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space. Gemini North began science operations in 2000 and Gemini South began scientific operations in late 2001.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country’s contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Particle Physics and Astronomy Research Council (PPARC), the Canadian National Research Council (NRC), the Chilean Comisi?n Nacional de Investigaci?n Cientifica y Tecnol?gica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Cient?ficas y T?cnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq). The Observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.

Original Source: Gemini News Release

New Evidence of the Universe’s Expansion

A team of UK and Australian astronomers have come up with independent evidence that the expansion of the universe is accelerating. Three years ago astronomers stunned the scientific community when they announced their evidence of an accelerating universe (they calculated the velocity of supernovas in distant galaxies). This team came to the same conclusion after measuring the position of 250,000 galaxies and plotted their movement compared it to the structure of the early universe.

Tightest Binary System Discovered

Image credit: ESO

Astronomers have discovered a pair of white dwarf stars that revolve around each other at a distance of only 80,000km (1/5th the distance between the Earth and the Moon) – the closest binary system ever discovered. The system, known as RX J0806.3+1527, was investigated with the European Southern Observatory’s Very Large Telescope (VLT), and observers noticed that the object dimmed once every five minutes suggesting a binary system.

Observations with ESO’s Very Large Telescope (VLT) in Chile and the Italian Telescopio Nazionale Galileo (TNG) on the Canary Islands during the past two years have enabled an international group of astronomers [1] to unravel the true nature of an exceptional binary stellar system.

This system, designated RX J0806.3+1527, was first discovered as an X-ray source of variable brightness – once every five minutes, it “switches off” for a short moment. The new observations have shown beyond doubt that this period reflects the orbital motion of two “white dwarf” stars that revolve around each other at a distance of only 80,000 km. Each of the stars is about as large as the Earth and this is the shortest orbital period known for any binary stellar system.

The VLT spectrum displays lines of ionized helium, indicating that the presence of an exceedingly hot area on one of the stars – a “hot spot” with a temperature of approx. 250,000 degrees. The system is currently in a rarely seen, transitory evolutionary state.

An amazing stellar binary system
One year is the time it takes the Earth to move once around the Sun, our central star. This may seem quite fast when measured on the scale of the Universe, but this is a snail’s motion compared to the the speed of two recently discovered stars. They revolve around each other 100,000 times faster; one full revolution takes only 321 seconds, or a little more than 5 minutes! It is the shortest period ever observed in a binary stellar system.

This is the surprising conclusion reached by an international team of astronomers led by GianLuca Israel of the Astronomical Observatory of Rome [1], and based on detailed observations of the faint light from these two stars with some of the world’s most advanced telescopes. The record-holding binary stellar system bears the prosaic name RX J0806.3+1527 and it is located north of the celestial equator in the constellation Cancer (The Crab).

The scientists also find that the two partners in this hectic dance are most likely a dying white dwarf star, trapped in the strong gravitational grip of another, somewhat heavier star of the same exotic type. The two Earth-size stars are separated by only 80,000 kilometers, a little more than twice the altitude of the TV-broadcasting satellites in orbit around the Earth, or just one fifth of the distance to the Moon.

The orbital motion is very fast indeed – over 1,000 km/sec, and the lighter star apparently always turns the same hemisphere towards its companion, just as the Moon in its orbit around Earth. Thus, that star also makes one full turn around its axis in only 5 minutes, i.e. its “day” is exactly as long as its “year”.

The discovery of RX J0806.3+1527
The visible light emitted by this unusual system is very faint, but it radiates comparatively strong X-rays. It was due to this emission that it was first detected as a celestial X-ray source of unknown origin by the German ROSAT space observatory in 1994. Later it was found to be a periodically variable source [2]. Once every 5 minutes, the X-ray radiation disappears for a couple of minutes. It was recently studied in greater detail by the NASA Chandra observatory.

The position of the X-ray source in the sky was localised with sufficient accuracy to reveal a very faint visible-light emitting object in the same direction, over one million times weaker than the faintest star that can be seen by unaided eye (V-magnitude 21.1). Follow-up observations were carried out with several world class telescopes, including the ESO Very Large Telescope (VLT) at the Paranal Observatory in Chile, and also the Telescopio Nazionale Galileo (TNG), the Italian 4-m class observatory at the Roche de Muchachos Observatory on La Palma in the Canary Islands.

The nature of RX J0806.3+1527
The observations in visible light also showed the same effect: RX J0806.3+1527 was getting dimmer once every 5 minutes, while no other periodic modulation was seen. By observing the spectrum of this faint object with the FORS1 multi-mode instrument on the 8.2-m VLT ANTU telescope, the astronomers were able to determine the composition of RX J0806.3+1527. It was found to contain large amounts of helium; this is unlike most other stars, which are mainly made up of hydrogen.

“At the outset, we thought that this was just another of the usual binary systems that emit X-rays”, says Gianluca Israel. “None of us could imagine the real nature of this object. We finally solved the puzzle by eliminating all other possibilities one by one, while we kept collecting more data. As the famous detective said: when you have eliminated the impossible, whatever remains, however improbable, must be the truth!”.

Current theory predicts that the two stars, which are bound together by gravity in this tight system, produce X rays when one of them acts as a giant “vacuum cleaner”, drawing gas off its companion. That star has already lost a significant fraction of its mass during this process.

The incoming matter impacts at high speed on the surface of the other star and the corresponding area – a “hot spot” – is heated to some 250,000 ?C, whereby X rays are emitted. This radiation disappears for a short time during each orbital revolution when this area is on the far side of the accreting star, as seen from the Earth.

A very rare class of stars
Our Sun is a normal star of comparatively low mass and it will eventually develop into a white dwarf star. Contrary to the violent demise of heavier stars in a glorious supernova explosion, this is a comparatively “quiet” process during which the star slowly cools while losing energy. It shrinks until it finally becomes as small as the Earth.

The Sun is a single star. However when a solar-like star is a member of a binary system, the evolution of its component stars is more complicated. During an initial phase, one star continues to move along an orbit that is actually inside the outer, very tenuous atmospheric layers of its companion. Then the system rids itself of this matter and develops into a binary system with two orbiting white dwarf stars, like RX J0806.3+1527.

Systems in which the orbital period is very short (less than 1 hour) are referred to as AM Canis Venaticorum (AM CVn) systems, after first known binary star of this rare class. It is likely that such systems, after having reached a minimum orbital period of a few minutes, then begin to evolve towards longer orbital periods. This indicates that RX J0806.3+1527 is now at the very beginning of the “AM CVn phase”.

Gravitational waves
With its extremely short orbital period, RX J0806.3+1527 is also a prime candidate for the detection of the elusive gravitational waves, predicted by Einstein’s General Theory of Relativity. They have never been measured directly, but their existence has been revealed indirectly in binary neutron star systems.

A planned gravitational wave space experiment, the European Space Agency’s Laser Interferometer Space Antenna (LISA) that will be launched in about 10 years’ time, will be sufficiently sensitive to be able to reveal this radiation from RX J0806.3+1527 with a high degree of confidence. Such an observational feat would open an entirely new window on the universe.

Original Source: ESO News Release

Young Pulsar Defies Theories

Astronomers working with the National Science Foundation’s Very Large Array have found a pulsar that is much younger than previously thought. The team tracked the movement of a pulsar, located 8,000 light years from Earth, against the remains of the supernova that created it. By calculating the distance it had moved, they were able to calculate the point at which they were at the same place – 64,000 years ago. Using a different method of calculating age, astronomers had previously pegged the pulsar as 107,000 years old. (source: NSF)

Oops, the Universe is Beige

Image credit: JHU

Astronomers from John Hopkins University announced several weeks back that if you averaged out the colour of all stars in the universe, the result would be an aquamarine colour. Well, it turns out they had a bug in their software that mixed the colours together incorrectly. Once they squished the bug, and reran their calculations, the average colour of the entire universe became beige.

What is the color of the Universe? This seemingly simple question has never really been answered by astronomers. It is difficult to take an accurate and complete census of all the light in the Universe.

However using the 2dF Galaxy Redshift Survey – a new survey of more than 200,000 galaxies which measures the light from a large volume of the Universe – we have recently been able to try and answer this question. We have constructed what we call “The Cosmic Spectrum”, which represents all the sum of all the energy in the local volume of the universe emitted at different optical wavelengths of light. This is what the cosmic spectrum looks like:

This is a graph of the energy emitted in the Universe for different wavelengths of light (data here). Ultraviolet and blue light is on the left and red light is on the right. This is constructed by adding together all the individual spectra of the separate galaxies in the 2dF survey. The sum represents the light of all the stars. We believe that because the 2dF survey is so large (reaching out several billion light years) that this spectrum is truly representative. We can also show the cosmic spectrum this way:

Here we have put in the approximate color the eye would see at each wavelength of light (though we cannot really see much light below about 4000 Angstroms, the near ultraviolet; and strictly, monitors cannot accurately display monochromatic colors, the colors of the rainbow).

You can think of this as what the eye would see if we put all the light in the Universe through a prism to produce a rainbow. The intensity of the color is in proportion to it’s intensity in the Universe.

So what is the average color? i.e. the color an observer would see if they had the Universe in a box, and could see all the light at once (and it wasn’t moving, for a real observer on earth, the further away a galaxy from us the more it is redshifted. We have de-redshifted all our light before combining).

To answer this question we must compute the average response of the human eye to these colors. How do we express this color? The most objective way to is quote the CIE x,y values which specify the color’s location in the CIE chromaticity diagram and hence the stimulus the eye would see. Any spectrum with the same x,y must give the same perceived color. These numbers are (0.345,0.345) and they are robust, we have calculated them for different sub-samples of the 2dF survey and they vary insignificantly. We have even computed them for the Sloan Digital Sky Survey spectroscopic survey (which will overtake 2dFGRS as the biggest redshift survey sometime in 2002) and they are essentially the same.

But what is the actual color? Well to do this we have to make some assumptions about human vision and the degree of general illumination. We also need to know what monitor you, the reader, are using! Of course this is impossible, but we can make an average guess. So here are the colors:

What are all these colors? They represent the color of the universe for different white points, which represent the adaptation of the human eye to different kinds of illumination. We will perceive different colors under different circumstances, and the kind of spectrum that appears ‘white’ will vary. A common standard is ‘D65’, which is close to setting daylight (in a slightly overcast sky) as white, and compared to which the universe appears reddish. ‘Illuminant E’ (equal energy white point) is perhaps what you would see for white when dark adapted. ‘Illuminant A’ represents indoor lighting, compared to which the Universe (and daylight) is very blue. We also show the color with and without a gamma correction of 2.2, which is the best thing to do for display on typical monitors. We provide the linear file, so you can apply your own gamma if you wish.

Almost certainly you need to look at the color patches labeled ‘gamma’, but not all displays are the same so your mileage may vary.

So what happened to “turquoise” ?
We found a bug in our code! In our original calculation, which you may have read in the press, we used (in good faith) software with a non-standard white point. Rather it was supposed to use a D65 white point, but did not apply it. The result was an effective white point somewhat redder than Illuminant E (as if some red neon lights were around) at 0.365,0.335. Although the x,y values of the Universe are unchanged from our original calculation the shift in the white point made the universe appear ‘turquoise’. (i.e. x,y, remains the same, but the corresponding effective RGB values shift).

Needless to say since that first calculation we have had a lot of correspondence with color scientists, and have now written our own software to obtain a more accurate color value. We admit the color of the Universe was something of a gimmick, to try and make our story on spectra more accessible. Nevertheless it is an actual calculable thing so we believe it is important to get it right.

We would like to point out that our original intention was merely an amusing footnote in our paper, the original press story blew up beyond our wildest expectations! The mistake took some time to realize and track down. Only a handful of color scientists had the expertise to spot the error. One moral of this story is we should have paid more attention to the ‘color science’ aspect and had that refereed as well.

Enough talk. So what color is the Universe?
Really the answer is so close to white, it is difficult to say. That is why such a small error had such a large effect. The most common choice for white is D65. However if one were to introduce a beam of cosmic spectrum into a room strongly illuminated by light bulbs only (Illuminant A) it would appear very blue, as shown above. Overall, probably Illuminant E is the most correct, for looking at the Universe from afar in dark conditions. So our new best guess is:

BEIGE

Although it’s arguable that it might look more pinkish (like D65 above). Good luck if you can see the difference between this color and white! You should be able to just see it, however if we had made the page background black, it would be very difficult! We have had numerous suggestions for this color emailled to us. We have a top ten, and deem the winner to be “Cosmic Latte” being caffeine biased!

A simulation of the Universe
Because of all these complexities we have decided to see for ourselves. Mark Fairchild at Munsell Color Laboratories in Rochester, NY is working with us to make a simulation of the cosmic spectrum, they can control light sources to give exactly the same red/green/blue eye stimulation as you would see from the cosmic spectrum. We will then be able to view this under a variety of lighting conditions, perhaps simulating deep space, and see for ourselves the true color of the Universe.

The real science story
Of course, our real motive for calculating the cosmic spectrum was really a lot more than producing these pretty color pictures. The color is interesting but in fact the cosmic spectrum is rich in detail and tells us a lot more about the history of star formation in the Universe. You may have noticed above that the cosmic spectrum contains dark lines and bright bands, these correspond to the characteristic emission and absorption of different elements:

These may remind you of Fraunhofer lines in the Solar Spectrum. Exactly the same process of atomic absorption is at work. The strength of the dark lines is determined by the temperatures of the stars contributing to the cosmic spectrum. Older stars have cooler atmospheres and produce a different set of lines to hot young stars. By analyzing the spectrum we can work out the relative proportions of these and try and infer what the star-formation rate was in past ages of the Universe. The gory details of this analysis are given in Baldry, Glazebrook, et al. 2002. A simple picture of our inferred most likely histories of star formation in the Universe is shown here:

All these models give the correct cosmic spectrum in the 2dF survey and all of them say that the majority of stars in the Universe today formed more than 5 billion years ago. This of course implies that the color of the Universe would have been different in the past when there were more hot young blue stars. In fact we can calculate what this would be from our best fitting model. The evolution of the color from 13 billion years ago to 7 billion years in the future looks like this under our various assumptions:

The universe started out young and blue, and grew gradually redder as the population of evolved ‘red’ giant stars built up. The rate of formation of new stars has declined precipitously in the last 6 billion years due to the decline in reserves of interstellar gas for forming new stars. As the star-formation rate continues to decline and more stars become red giants the color of the Universe will become redder and redder. Eventually all stars will disappear and nothing will be left but black holes. These too will eventually evaporate via the Hawking process and nothing will be left except for old light, which will itself redden as the Universe expands forever (in the current cosmological model).

Original Source: JHU News Release