Category: Astronomy

Artist’s impression of a pulsar ‘eating’ a companion star. Image credit: ESA Click to enlarge
ESA’s Integral space observatory, together with NASA’s Rossi X-ray Timing Explorer spacecraft, has found a fast-spinning pulsar in the process of devouring its companion.

This finding supports the theory that the fastest-spinning isolated pulsars get that fast by cannibalising a nearby star. Gas ripped from the companion fuels the pulsar’s acceleration. This is the sixth pulsar known in such an arrangement, and it represents a ‘stepping stone’ in the evolution of slower-spinning binary pulsars into faster-spinning isolated pulsars.
“We’re getting to the point where we can look at any fast-spinning, isolated pulsar and say, ‘That guy used to have a companion’,” said Dr Maurizio Falanga, who led the Integral observations, at the Commissariat ? l’Energie Atomique (CEA) in Saclay, France.

‘Pulsars’ are rotating neutron stars, which are created in stellar explosions. They are the remnants of stars that were once at least eight times more massive than the Sun. These stars still contain about the mass of our Sun compactified into a sphere of only about 20 kilometres across.

This pulsar, called IGR J00291+5934, belongs to a category of ‘X-ray millisecond pulsars’, which pulse with the X-ray light several hundred times a second, one of the fastest known. It has a period of 1.67 milliseconds which is much smaller that most other pulsars that rotate once every few seconds.

Neutron stars are born rapidly spinning in collapses of massive stars. They gradually slow down after a few hundred thousand years. Neutron stars in binary star systems, however, can reverse this trend and speed up with the help from the companion star.

For the first time ever, this speeding-up has been observed in the act. “We now have direct evidence for the star spinning faster whilst cannibalising its companion, something which no one had ever seen before for such a system,” said Dr Lucien Kuiper from the Netherlands Institute for Space Research (SRON), in Utrecht.

A neutron star can remove gas from its companion star in a process called ‘accretion’. The flow of gas onto the neutron star makes the star spin faster and faster. Both the flow of gas and its crashing upon the neutron star surface releases much energy in the form of X-ray and gamma radiation.

Neutron stars have such a strong gravitational field that light passing by the star changes its direction by almost 100 degrees (in comparison light passing by the Sun is deflected by an angle which is 200 thousands times smaller). “This ‘gravitational bending’ allows us to see the back side of the star,” points out Prof. Juri Poutanen from the University of Oulu, Finland.

“This object was about ten times more energetic than what is usually observed for similar sources,” said Falanga. “Only some kind of monster emits at these energies, which corresponds to a temperature of almost a billion degrees.”

From a previous Integral result, scientists deduced that because the neutron star has a strong magnetic field, charged particles from its companion are channeled along the magnetic field lines until they slam into the neutron star surface at one of its magnetic poles, forming ‘hot spots’. The very high temperatures seen by Integral arise from this very hot plasma over the accretion spots.

IGR J00291+5934 was discovered by Integral during a routine scan of the sky on 2 December 2004, in the outer reaches of our Milky Way galaxy, when it suddenly flared. On the day after, scientists accurately clocked the neutron star with the Rossi X-ray Timing Explorer.

Rossi observations revealed that the companion is already a fraction the size of our Sun, perhaps as small as 40 Jupiter masses. The binary orbit is 2.5 hours long (as opposed to the year long Earth-Sun orbit). The full system is very tight; both stars are so close that they will fit into the radius of the Sun. These details support the theory that the two stars are close enough for accretion to take place and that the companion star is being cannibalised.

“Accretion is expected to cease after a billion of years or so,” said Dr Duncan Galloway of the Massachusetts Institute of Technology, USA, responsible for the Rossi observations. “This Integral-Rossi discovery provides more evidence of how pulsars evolve from one phase to another – from an initially slowly spinning binary neutron star emitting high energies, to a rapidly spinning isolated pulsar emitting in radio wavelengths.”

The discovery is the first of its kind for Integral (four of the first five rapidly spinning X-ray pulsars were discovered by Rossi). This bodes well in the combined search for these rare objects. Integrals’s sensitive detectors can identify relatively dim and distant sources and so, knowing where to look, Rossi can provide timing information through a dedicated observation extending over the entire two-week period of the typical outburst.

Original Source: ESA Portal

Station with active crossed dipole. Image credit: Haystack Observatory Click to enlarge
If you want to hear a little bit of the Big Bang, you’re going to have to turn down your stereo.

That’s what neighbors of MIT’s Haystack Observatory found out. They were asked to make a little accommodation for science, and now the results are in: Scientists at Haystack have made the first radio detection of deuterium, an atom that is key to understanding the beginning of the universe. The findings are being reported in an article in the Sept. 1 issue of Astrophysical Journal Letters.

The team of scientists and engineers, led by Alan E.E. Rogers, made the detection using a radio telescope array designed and built at the MIT research facility in Westford, Mass. Rogers is currently a senior research scientist and associate director of the Haystack Observatory.

After gathering data for almost one year, a solid detection was obtained on May 30.

The detection of deuterium is of interest because the amount of deuterium can be related to the amount of dark matter in the universe, but accurate measurements have been elusive. Because of the way deuterium was created in the Big Bang, an accurate measurement of deuterium would allow scientists to set constraints on models of the Big Bang.

Also, an accurate measurement of deuterium would be an indicator of the density of cosmic baryons, and that density of baryons would indicate whether ordinary matter is dark and found in regions such as black holes, gas clouds or brown dwarfs, or is luminous and can be found in stars. This information helps scientists who are trying to understand the very beginning of our universe.

Until now the deuterium atom has been extremely difficult to detect with instruments on Earth. Emission from the deuterium atom is weak since it is not very abundant in space-there is approximately one deuterium atom for every 100,000 hydrogen atoms, thus the distribution of the deuterium atom is diffuse. Also, at optical wavelengths the hydrogen line is very close to the deuterium line, which makes it subject to confusion with hydrogen; but at radio wavelengths, deuterium is well separated from hydrogen and measurements can provide more consistent results.

In addition, our modern lifestyle, filled with gadgets that use radio waves, presented quite a challenge to the team trying to detect the weak deuterium radio signal. Radio frequency interference bombarded the site from cell phones, power lines, pagers, fluorescent lights, TV, and in one case from a telephone equipment cabinet where the doors had been left off. To locate the interference, a circle of yagi antennas was used to indicate the direction of spurious signals, and a systematic search for the RFI sources began.

At times, Rogers asked for help from Haystack’s neighbors, and in several instances replaced a certain brand of answering machine that was sending out a radio signal with one that did not interfere with the experiment. The interference caused by one person’s stereo system was solved by having a part on the sound card replaced by the factory.

The other members of the team working with Rogers are Kevin Dudevoir, Joe Carter, Brian Fanous and Eric Kratzenberg (all of Haystack Observatory) and Tom Bania of Boston University.

The Deuterium Array at Haystack is a soccer-field size installation conceived and built at the Haystack facility with support from the National Science Foundation, MIT and TruePosition Inc.

Original Source: MIT News Release

XMM-Newton image of galaxy cluster. Image credit: ESA Click to enlarge
ESA?s X-ray observatory, XMM-Newton, has for the first time allowed scientists to study in detail the formation history of galaxy clusters, not only with single arbitrarily selected objects, but with a complete representative sample of clusters.

Knowing how these massive objects formed is a key to understanding the past and future of the Universe.
Scientists currently base their well-founded picture of cosmic evolution on a model of structure formation where small structures form first and these then make up larger astronomical objects.

Galaxy clusters are the largest and most recently formed objects in the known Universe, and they have many properties that make them great astrophysical ?laboratories?. For example, they are important witnesses of the structure formation process and important ?probes? to test cosmological models.

To successfully test such cosmological models, we must have a good observational understanding of the dynamical structure of the individual galaxy clusters from representative cluster samples.

For example, we need to know how many clusters are well evolved. We also need to know which clusters have experienced a recent substantial gravitational accretion of mass, and which clusters are in a stage of collision and merging. In addition, a precise cluster mass measurement, performed with the same XMM-Newton data, is also a necessary prerequisite for quantitative cosmological studies.

The most easily visible part of galaxy clusters, i.e. the stars in all the galaxies, make up only a small fraction of the total of what makes up the cluster. Most of the observable matter of the cluster is composed of a hot gas (10-100 million degrees) trapped by the gravitational potential force of the cluster. This gas is completely invisible to human eyes, but because of its temperature, it is visible by its X-ray emission.

This is where XMM-Newton comes in. With its unprecedented photon-collecting power and capability of spatially resolved spectroscopy, XMM-Newton has enabled scientists to perform these studies so effectively that not only single objects, but also whole representative samples can be studied routinely.

XMM-Newton produces a combination of X-ray images (in different X-ray energy bands, which can be thought of as different X-ray ?colours?), and makes spectroscopic measurements of different regions in the cluster.

While the image brightness gives information on the gas density in the cluster, the colours and spectra provide an indication of the cluster?s internal gas temperature. From the temperature and density distribution, the physically very important parameters of pressure and ?entropy? can be also derived. Entropy is a measure of the heating and cooling history of a physical system.

The accompanying three images illustrate the use of entropy distribution in the ?X-ray luminous? gas as a way of identifying various physical processes. Entropy has the unique property of decreasing with radiative cooling, increasing due to heating processes, but staying constant with compression or expansion under energy conservation.

The latter ensures that a ?fossil record? of any heating or cooling is kept even if the gas subsequently changes its pressure adiabatically (under energy conservation).

These examples are drawn from the REFLEX-DXL sample, a statistically complete sample of some of the most X-ray luminous clusters found in the ROSAT All-Sky Survey. ROSAT was an X-ray observatory developed in the 1990s in co-operation between Germany, USA and UK.

The images provide views of the entropy distribution coded in colour where the values increase from blue, green, yellow to red and white.

Original Source: ESA Portal

Pulsar path over about 2.5 million years. Image credit: Bill Saxton, NRAO/AUI/NSF Click to enlarge
A speeding, superdense neutron star somehow got a powerful “kick” that is propelling it completely out of our Milky Way Galaxy into the cold vastness of intergalactic space. Its discovery is puzzling astronomers who used the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope to directly measure the fastest speed yet found in a neutron star.

The neutron star is the remnant of a massive star born in the constellation Cygnus that exploded about two and a half million years ago in a titanic explosion known as a supernova. Ultra-precise VLBA measurements of its distance and motion show that it is on course to inevitably leave our Galaxy.

“We know that supernova explosions can give a kick to the resulting neutron star, but the tremendous speed of this object pushes the limits of our current understanding,” said Shami Chatterjee, of the National Radio Astronomy Observatory (NRAO) and the Harvard-Smithsonian Center for Astrophysics. “This discovery is very difficult for the latest models of supernova core collapse to explain,” he added.

Chatterjee and his colleagues used the VLBA to study the pulsar B1508+55, about 7700 light-years from Earth. With the ultrasharp radio “vision” of the continent-wide VLBA, they were able to precisely measure both the distance and the speed of the pulsar, a spinning neutron star emitting powerful beams of radio waves. Plotting its motion backward pointed to a birthplace among groups of giant stars in the constellation Cygnus — stars so massive that they inevitably explode as supernovae.

“This is the first direct measurement of a neutron star’s speed that exceeds 1,000 kilometers per second,” said Walter Brisken, an NRAO astronomer. “Most earlier estimates of neutron-star speeds depended on educated guesses about their distances. With this one, we have a precise, direct measurement of the distance, so we can measure the speed directly,” Brisken said. The VLBA measurements show the pulsar moving at nearly 1100 kilometers (more than 670 miles) per second — about 150 times faster than an orbiting Space Shuttle. At this speed, it could travel from London to New York in five seconds.

In order to measure the pulsar’s distance, the astronomers had to detect a “wobble” in its position caused by the Earth’s motion around the Sun. That “wobble” was roughly the length of a baseball bat as seen from the Moon. Then, with the distance determined, the scientists could calculate the pulsar’s speed by measuring its motion across the sky.

“The motion we measured with the VLBA was about equal to watching a home run ball in Boston’s Fenway Park from a seat on the Moon,” Chatterjee explained. “However, the pulsar took nearly 22 months to show that much apparent motion. The VLBA is the best possible telescope for tracking such tiny apparent motions.”

The star’s presumed birthplace among giant stars in the constellation Cygnus lies within the plane of the Milky Way, a spiral galaxy. The new VLBA observations indicate that the neutron star now is headed away from the Milky Way’s plane with enough speed to take it completely out of the Galaxy. Since the supernova explosion nearly 2 and a half million years ago, the pulsar has moved across about a third of the night sky as seen from Earth.

“We’ve thought for some time that supernova explosions can give a kick to the resulting neutron star, but the latest computer models of this process have not produced speeds anywhere near what we see in this object,” Chatterjee said. “This means that the models need to be checked, and possibly corrected, to account for our observations,” he said.

“There also are some other processes that may be able to add to the speed produced by the supernova kick, but we’ll have to investigate more thoroughly to draw any firm conclusions,” said Wouter Vlemmings of the Jodrell Bank Observatory in the UK and Cornell University in the U.S.

The observations of B1508+55 were part of a larger project to use the VLBA to measure the distances and motions of numerous pulsars. “This is the first result of this long-term project, and it’s pretty exciting to have something so spectacular come this early,” Brisken said. The VLBA observations were made at radio frequencies between 1.4 and 1.7 GigaHertz.

Chatterjee, Vlemmings and Brisken worked with Joseph Lazio of the Naval Research Laboratory, James Cordes of Cornell University, Miller Goss of NRAO, Stephen Thorsett of the University of California, Santa Cruz, Edward Fomalont of NRAO, Andrew Lyne and Michael Kramer, both of Jodrell Bank Observatory. The scientists presented their findings in the September 1 issue of the Astrophysical Journal Letters.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreeement by Associated Universities, Inc.

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

An artist’s illustration of a circumstellar disk around a massive star. Image credit: NAOJ Click to enlarge
An international group of astronomers has used the Coronagraphic Imager for Adaptive Optics (CIAO) on the Subaru telescope in Hawai’i to obtain very sharp near-infrared polarized-light images of the birthplace of a massive proto-star known as the Becklin-Neugebauer (BN) object at a distance of 1500 light years from the Sun. The group’s images led to the discovery of a disk surrounding this newly forming star. This finding, described in detail in the September 1 issue of Nature, deepens our understanding of how massive stars form.

The research group, which includes astronomers from the Purple Mountain Observatory, China, National Astronomical Observatories of Japan, and University of Hertfordshire, UK, explored the region close to the Becklin-Neugebauer object and analyzed how infrared light is affected by dust. To do this, they took a polarized-light image of the object at a wavelength of 1.6 micrometers (the H band of infrared light). Images of the brightness of the object just show a circular distribution of light. However, an image of the light’s polarization shows a butterfly shape that reveals details that are undetectable by looking at the brightness distribution alone. To understand the environment around the star and what the butterfly shape implies, the astronomers created a computer model for comparison, along with a schematic of star formation. These models show that the butterfly shape is the signature of a disk and an outflow structure near the newborn star.

This discovery is the most concrete evidence for a disk around a massive young star and shows that massive stars like the BN object (which is about seven times the mass of the Sun) form the same way as lower-mass stars like the Sun.

There are two main theories to explain the formation of massive stars. The first states that massive stars are the results of the mergers of several low-mass stars. The second says that they are formed through gravitational collapse and mass accretion within circumstellar disks. Lower-mass stars like the Sun are most likely to have formed through the second method. The collapse-accretion theory assumes that a system has a star associated with a bipolar outflow, a circumstellar disk and an envelope, while the merger theory does not. The presence or absence of such structures can distinguish between the two formation scenarios.

Until recently, there has been little direct observational evidence in support of either theory of massive star formation. This is because, unlike lower-mass stars, newly forming massive stars are so rare and so far away from us that they have been difficult to observe. Large telescopes and adaptive optics, which greatly improve image sharpness, now make it possible to observe these objects with unprecedented clarity. High-resolution infrared polarimetry is an especially powerful tool for probing the environment hidden behind the bright glow of a massive star.

Polarization-the direction that light waves oscillate in as they stream away from an object-is an important characteristic of radiation. Sun light doesn?t have a preferred direction of oscillation, but can become polarized when scattered by Earth?s atmosphere, or after reflecting off the surface of water. A similar action occurs in a circumstellar cloud around a newborn star. The star lights up its surroundings-the circumstellar disk, the envelope and the cavity walls formed by the outflow streams. The light can travel freely within the cavity and then reflect off its walls. This reflected light becomes highly polarized. By contrast, the disk and the envelope are relatively opaque to light. This reduces the polarization of light coming from those regions.

The group?s success in detecting evidence for a disk and outflow around the BN object through high-resolution infrared polarimetry suggests that the same technique can be applied to other forming stars. This would allow astronomers to obtain a comprehensive observational description of the formation of massive stars greater than ten times the mass of the Sun.

Original Source: NAOJ News Release

Galaxy cluster Abell 3266. Image credit: NOAO Click to enlarge
A comprehensive survey of more than 4,000 elliptical and lenticular galaxies in 93 nearby galaxy clusters has found a curious case of galactic ?downsizing.?

Contrary to expectations, the largest, brightest galaxies in the census consist almost exclusively of very old stars, with much of their stellar populations having formed as long ago as 13 billion years. There appears to be very little recent star formation in these galaxies, nor is there strong evidence for recent ingestion of smaller, younger galaxies.

By contrast, the smaller, fainter galaxies studied by the NOAO Fundamental Plane Survey are significantly younger?their stars were formed as little as four billion years ago, according to new results from the survey team to be published in the September 10, 2005, Astrophysical Journal.

These findings are based on a sample more than five times larger than previous efforts. The results of the survey contrast sharply with conventional hierarchical model of galaxy formation and evolution, where large elliptical galaxies in the nearby universe formed by swallowing smaller galaxies with young stars; this theory predicts that, on average, the stars in the largest elliptical galaxies should be no older than those in the smallest ones.

?This sample probes the largest and richest galaxy clusters in the nearby universe, out to a distance of about a billion light-years from Earth,? says Jenica Nelan, lead author of the study. ?Our analysis shows that there is a clear relationship between mass and age in these red galaxies, meaning that the stars in the biggest, oldest galaxies that we studied formed early in the history of the Universe. On average, the smaller galaxies have one-tenth the mass of the larger ones, and are only about half their age.?

?The term ?downsizing? essentially means that when the Universe was young, the star formation activity occurred in large galaxies, but as the Universe aged, the ?action? stopped in the larger galaxies, even as it continued in smaller galaxies,? says Michael Hudson of the University of Waterloo, Ontario, Canada, principal investigator for the NOAO Fundamental Plane Survey.

The new study is based on thousands of spectra obtained by the Fundamental Plane Survey team over dozens of nights at the WIYN 3.5-meter telescope at Kitt Peak National Observatory, southwest of Tucson, AZ, and the National Science Foundation?s Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, east of La Serena, Chile. With some painstaking work, these spectra can reveal the average age of the stars that make up a galaxy.

?Although we cannot directly see these galaxies as they were in the past, their stars are a kind of ?fossil record? that can be used to unearth their histories,? Hudson explains. ?It appears that the older galaxies are much less of a ?melting pot? than had been thought, and that their star formation activity turned off somehow while they were being put together.?

The evolutionary history of elliptical galaxies and lenticular galaxies (which have a central bulge and a disk, but no evidence of spiral arms) is not well understood. Their colors appear to be ?redder? than typical spiral galaxies. The largest ellipticals are the reddest of all, but until this work it has not been clear whether this property results primarily from being older in age, as the survey found, or from having a higher proportion of heavy chemical elements (metallicity content).

?These so-called red galaxies contain the bulk of the stellar mass in the nearby universe, but we know little about their formation and evolution,? says co-author Russell Smith of the University of Waterloo. ?It was thought that all of the red galaxies were made of stars that formed very early, and are now quite old. Our results show that while this is true for the large galaxies, the smaller ones formed their stars comparatively recently in the history of the Universe. We predict that as new surveys look deeper and hence further into the past, they should see fewer faint red galaxies?

An image of galaxy cluster Abell 3266 taken by survey team members at the Gemini South telescope as part of their follow-up work is available above.

Lead author Jenica Nelan completed this work while earning her doctorate at Dartmouth College; she is now an astronomer at Yale University.

Co-authors of this paper include Hudson and Smith of the University of Waterloo; Gary Wegner of Dartmouth College; John R. Lucey, Stephen A. W. Moore, and Stephen J. Quinney of the University of Durham, and Nicholas B. Suntzeff of NOAO?s Cerro Tololo Inter-American Observatory.

The Fundamental Plane Survey is one of 18 projects granted long-term access to observing nights at the telescope of the National Optical Astronomy Observatory (NOAO) under the NOAO Survey Program.

See here for more information:
www.noao.edu/gateway/surveys/programs.html and astro.uwaterloo.ca/~mjhudson/nfp

Original Source: NOAO News Release

The SuperNova/Acceleration Probe, SNAP. Image credit: Berkeley Lab Click to enlarge
What is the mysterious dark energy that’s causing the expansion of the universe to accelerate? Is it some form of Einstein’s famous cosmological constant, or is it an exotic repulsive force, dubbed “quintessence,” that could make up as much as three-quarters of the cosmos? Scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) and Dartmouth College believe there is a way to find out.

In a paper to be published in Physical Review Letters, physicists Eric Linder of Berkeley Lab and Robert Caldwell of Dartmouth show that physics models of dark energy can be separated into distinct scenarios, which could be used to rule out Einstein’s cosmological constant and explain the nature of dark energy. What’s more, scientists should be able to determine which of these scenarios is correct with the experiments being planned for the Joint Dark Energy Mission (JDEM) that has been proposed by NASA and the U.S. Department of Energy.

“Scientists have been arguing the question ‘how precisely do we need to measure dark energy in order to know what it is?'” says Linder. “What we have done in our paper is suggest precision limits for the measurements. Fortunately, these limits should be within the range of the JDEM experiments.”

Linder and Caldwell are both members of the DOE-NASA science definition team for JDEM, which has the responsibility for drawing up the mission’s scientific requirements. Linder is the leader of the theory group for SNAP ? the SuperNova/Acceleration Probe, one of the proposed vehicles for carrying out the JDEM mission. Caldwell, a professor of physics and astronomy at Dartmouth, is one of the originators of the quintessence concept.

In their paper in Physical Review Letters Linder and Caldwell describe two scenarios, one they call “thawing” and one they call “freezing,” which point toward distinctly different fates for our permanently expanding universe. Under the thawing scenario, the acceleration of the expansion will gradually decrease and eventually come to a stop, like a car when the driver eases up on the gas pedal. Expansion may continue more slowly, or the universe may even recollapse. Under the freezing scenario, acceleration continues indefinitely, like a car with the gas pedal pushed to the floor. The universe would become increasingly diffuse, until eventually our galaxy would find itself alone in space.

Either of these two scenarios rules out Einstein’s cosmological constant. In their paper Linder and Caldwell show, for the first time, how to cleanly separate Einstein’s idea from other possibilities. Under any scenario, however, dark energy is a force that must be reckoned with.

Says Linder, “Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe.”

In 1998, two research groups rocked the field of cosmology with their independent announcements that the expansion of the universe is accelerating. By measuring the redshift of light from Type Ia supernovae, deep-space stars that explode with a characteristic energy, teams from the Supernova Cosmology Project headquartered at Berkeley Lab and the High-Z Supernova Search Team centered in Australia determined that the expansion of the universe is actually accelerating, not decelerating. The unknown force behind this accelerated expansion was given the name “dark energy.”

Prior to the discovery of dark energy, conventional scientific wisdom held that the Big Bang had resulted in an expansion of the universe that would gradually be slowed down by gravity. If the matter content in the universe provided enough gravity, one day the expansion would stop altogether and the universe would fall back on itself in a Big Crunch. If the gravity from matter was insufficient to completely stop the expansion, the universe would continue floating apart forever.

“From the announcements in 1998 and subsequent measurements, we now know that the accelerated expansion of the universe did not start until sometime in the last 10 billion years,” Caldwell says.

Cosmologists are now scrambling to determine what exactly dark energy is. In 1917 Einstein amended his General Theory of Relativity with a cosmological constant, which, if the value was right, would allow the universe to exist in a perfectly balanced, static state. Although history’s most famous physicist would later call the addition of this constant his “greatest blunder,” the discovery of dark energy has revived the idea.

“The cosmological constant was a vacuum energy (the energy of empty space) that kept gravity from pulling the universe in on itself,” says Linder. “A problem with the cosmological constant is that it is constant, with the same energy density, pressure, and equation of state over time. Dark energy, however, had to be negligible in the universe’s earliest stages; otherwise the galaxies and all their stars would never have formed.”

For Einstein’s cosmological constant to result in the universe we see today, the energy scale would have to be many orders of magnitude smaller than anything else in the universe. While this may be possible, Linder says, it does not seem likely. Enter the concept of “quintessence,” named after the fifth element of the ancient Greeks, in addition to air, earth, fire, and water; they believed it to be the force that held the moon and stars in place.

“Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion,” says Caldwell. “Whereas the cosmological constant is a very specific form of energy ? vacuum energy ? quintessence encompasses a wide class of possibilities.”

To limit the possibilities for quintessence and provide firm targets for basic tests that would also confirm its candidacy as the source of dark energy, Linder and Caldwell used a scalar field as their model. A scalar field possesses a measure of value but not direction for all points in space. With this approach, the authors were able to show quintessence as a scalar field relaxing its potential energy down to a minimum value. Think of a set of springs under tension and exerting a negative pressure that counteracts the positive pressure of gravity.

“A quintessence scalar field is like a field of springs covering every point in space, with each spring stretched to a different length,” Linder said. “For Einstein’s cosmological constant, each spring would be the same length and motionless.”

Under their thawing scenario, the potential energy of the quintessence field was “frozen” in place until the decreasing material density of an expanding universe gradually released it. In the freezing scenario, the quintessence field has been rolling towards its minimum potential since the universe underwent inflation, but as it comes to dominate the universe it gradually becomes a constant value.

The SNAP proposal is in research and development by physicists, astronomers, and engineers at Berkeley Lab, in collaboration with colleagues from the University of California at Berkeley and many other institutions; it calls for a three-mirror, 2-meter reflecting telescope in deep-space orbit that would be used to find and measure thousands of Type Ia supernovae each year. These measurements should provide enough information to clearly point towards either the thawing or freezing scenario ? or to something else entirely new and unknown.

Says Linder, “If the results from measurements such as those that could be made with SNAP lie outside the thawing or freezing scenarios, then we may have to look beyond quintessence, perhaps to even more exotic physics, such as a modification of Einstein’s General Theory of Relativity to explain dark energy.”

Original Source: Berkeley Lab News Release

Similar close encounter last November. Image credit: Babak A. Tafreshi Click to enlarge
Something nice is happening in the sunset sky. Venus and Jupiter, the two brightest planets, are converging, and they’re going to be beautifully close together for the next two weeks.

Step outside tonight when the sun goes down and look west. If there are no trees or buildings in the way, you can’t miss Jupiter and Venus. They look like airplanes, hovering near the horizon with their lights on full blast. (Venus is the brighter of the two.) You can see them even from brightly-lit cities.

Try catching the pair just after sundown and just before the first stars appear. Venus and Jupiter pop into view while the sky is still twilight-blue. The scene has a special beauty.

When the sky darkens completely, look to the left of Jupiter for Spica, the brightest star in the constellation Virgo. Although it’s a bright star, Spica is completely outclassed by the two planets.

Venus and Jupiter are converging at the noticeable rate of 1o per day, with closest approach coming on September 1st when the two will be a little more than 1o apart. (How much is 1o? Hold your pinky finger at arm’s length. The tip is about 1o wide.)

When planets are so close together, not only do you notice them, you’ll have a hard time taking your eyes off them. They’re spellbinding.

There’s a biological reason for this phenomenon: In the back of your eye, near the center of the retina, lies a small patch of tissue called “the fovea” where cones are extra-densely packed. “Whatever you see with the fovea, you see in high-definition,” explains Stuart Hiroyasu, O.D., of Bishop, California. “The fovea is critical to reading, driving, watching television; it has the brain’s attention.” The field of view of the fovea is 5o. When two objects converge to, say, 1o as Venus and Jupiter will do, they can beam into your fovea simultaneously, signaling your brain?attention, please!

After September 1st, the two planets separate, but the show’s not over. On September 6th, with Jupiter and Venus still pleasingly close together, the slender crescent Moon will leap up from the sun’s glare and join the two planets. Together, they’ll form a compact triangle that will simply knock your socks off.

Feel like staring? Do.

Original Source: NASA News Release

GALEX , one of the telescopes that will study AE Aqr. Image credit: NASA Click to enlarge
Amateur astronomers are being asked to help a constellation of observatories unravel the mysteries of a puzzling binary star system.

On August 30-August 31, 2005 two space-based and four professional ground-based observatories are scheduled to observe the cataclysmic variable star AE Aqr. Each of the observatories covers a different wavelength of light and amateur astronomers have been asked to help cover the visible-light portion.

“This observing campaign will take place over nearly a full day, and since no single ground-based observatory can observe AE Aqr for that long due to Earth’s rotation, amateur astronomers can make a unique and invaluable contribution to this campaign,” said Dr. Christopher Mauche of Lawrence Livermore National Laboratory, the principal investigator of the project.

Because they are spaced all across the globe, amateur astronomers can observe this star and other celestial objects unhindered by nightfall or weather.

The Chandra and GALEX space telescopes will be working with the HESS, MAGIC, VLT, and VLA ground-based telescopes. Combined, they will provide coverage of AE Aqr from high-energy gamma-rays to low-energy radio waves. Such simultaneous multiwavelength coverage is required to provide the clearest picture of the locations, mass motions, energetics, and inter-relationships of the various emission regions in the star.

AE Aqr is an intermediate polar, a type of cataclysmic variable star. It actually consists of two stars – a red dwarf and rapidly spinning magnetic white dwarf. Material drawn off the red dwarf falls toward the white dwarf, but instead of landing on the white dwarf surface, it is flung out of the system by the white dwarf’s rapidly spinning magnetic field. This mechanism, which is uncommon but not unique to AE Aqr, is referred to as a magnetic propeller.

“Amateurs astronomers have been observing AE Aqr since 1944. Since then, they have recorded over 28,815 measurements of the star, most of them made with just a telescope and their eyes. This type of historical data is immensely valuable in studying variable stars and only amateurs can provide it,” Dr. Arne Henden, Director of the American Association of Variable Star Observers (AAVSO), said.

Amateur astronomers are being asked to observe AE Aqr every night possible until September 3. Those with CCD cameras on their telescopes are requested to make scientific brightness measurements, known as photometry, of the system as well. For information on how to measure the brightness of AE Aqr and submit results to professionals, visit the AAVSO web site at http://www.aavso.org/alertnotice .

The AAVSO is the world’s preeminent professional-amateur astronomical association. Specializing in the study of variable stars, the AAVSO’s International Database has over 11 million observations of variable stars dating back over 100 years. Founded in 1911 as part of the Harvard College Observatory, the AAVSO became independent in 1954 and currently has over 3,000 members and observers in over 40 countries.

Original Source: AAVSO News Release

NGC 520. Image credit: Gemini Click to enlarge
In the constellation of Pisces, some 100 million light-years from Earth, two galaxies are seen to collide – providing an eerie insight into the ultimate fate of our own planet when the Milky Way fatally merges with our neighbouring galaxy of Andromeda.

The image of the intertwined galaxies was captured on the night of 13-14th July 2005 by the Gemini Multi-Object Spectrograph [GMOS] instrument fitted to the 8-metre class Gemini North Observatory, sited on Mauna Kea, Hawaii.

Prof. Ian Robson, Director of the UK Astronomy Technology Centre which built GMOS in collaboration with other partners said,” This is quite scary. Since GMOS was installed on the telescope back in 2001 it has taken some amazing astronomical images of very faint, distant galaxies and star forming regions, providing a wealth of scientific data, but this one sends shivers down my spine. Our saving grace is that we have about 5 billion years left before we get swallowed up by Andromeda. Nevertheless, it’s amazing to see so far in advance how planet Earth and our own galaxy will ultimately end. Glad to say I won’t be around when the fireball happens”.

The image of the combined galaxies, which are known as NGC 520, may be fairly early in their galactic dance of death and it is likely that the situation has changed dramatically in the time it has taken for their light to reach Earth*.

Prof. Robson added, “Hints of new star formation taking place can be seen in the faint red glowing areas above and beneath the middle of the image. Perhaps even now the galaxies have totally combined to form a whole new galaxy with a brand new set of stars and associated planets – and maybe new life on one of those planets!”

The unique shape of NGC 520 is the result of the two galaxies colliding. One galaxy’s dust lane can be seen easily in the foreground and a distant tail is visible at the bottom centre. These features are the result of the gravitational interactions that have robbed both galaxies of their original shapes.

Original Source: PPARC News Release