Friday, June 27, 2008

Gemini Sees Twin Galaxies in Embrace

The Gemini South captured this image of NGC 5426-27 (Arp 271) using its Multi-Object Spectrograph. Credit: Gemini Observatory

In what appears to be a masterful illusion, astronomers at Gemini Observatory have imaged two nearly identical spiral galaxies in Virgo, 90 million light-years distant, in the early stages of a gentle gravitational embrace. The new image was obtained at the Gemini South telescope in Chile using GMOS, the Gemini Multi-Object Spectrograph.

Like two skaters grabbing hands while passing, NGC 5427 (the nearly open-faced spiral galaxy at lower left) and its southern twin NGC 5426 (the more oblique galaxy at upper right), are in the throes of a slow but disturbing interaction-one that could take a hundred million years to complete.

At a glance, these twin galaxies-which have similar masses, structures, and shapes and are together known as Arp 271-appear undisturbed. But recent studies have shown that the mutual pull of gravity has already begun to alter and distort their visible features.

Typically, the first sign of a galaxy interaction is the formation of a bridge-like feature. Indeed, the two spiral arms on the western (upper) side of NGC 5426 appear as long appendages that connect with NGC 5427. This intergalactic bridge acts like a feeding tube, allowing the twins to share gas and dust with one other across the 60,000 light years (less than one galaxy diameter) of space separating them.

Colliding gases caused by the interaction may have also triggered bursts of star formation (starbursts) in each galaxy. Star-forming, or HII, regions appear as hot pink knots that trace out the spiral patterns in each galaxy. HII regions are common to many spiral systems, but the giant ones in NGC 5426 are curiously knotted and more abundant on the side of the galaxy closest to NGC 5427. Starburst activity can also be seen in the galaxy's connecting bridge.

Likewise, the giant HII regions in NGC 5427's disk are forming at a higher rate, and are more plentiful, than expected for a galaxy of this type.
One giant star-forming region at the tip of NGC 5427's western (top) spiral arm, looks especially large and disturbed, as does the arm itself, which is unusually straight, as if strong tidal forces have broken the arm in two, causing it to bleed starlight.

Despite their appearance in this two-dimensional image, NGC 5426's western (top) spiral arm is the one closest to us, as opposed to NGC 5427's southeastern (bottom) arm. NGC 5426 is also the closer of the two galaxies. Over millions of years, however, NGC 5427 will perform a parabolic traverse, moving it from behind NGC 5426 towards the foreground in the upper-right corner of the frame. Thus, an imaginary long-lived observer on a planet in NGC 5427 would see an almost perpendicular passage of the companion galaxy.

Once thought to be unusual and rare, gravitational interactions between galaxies are now known to be quite common (especially in densely populated galaxy clusters) and are considered to play an important role in galaxy evolution. Most galaxies have probably had at least one major, if not many minor, interactions with other galaxies since the advent of the Big Bang some 13 billion years ago. Our own Milky Way, a spiral galaxy like those in this image, is, in fact, performing its own stately dance. Both with the nearby dwarf galaxy, called the Large Magellanic Cloud and a future interaction with the large spiral galaxy M-31 or the Great Andromeda Galaxy, which is now located about 2.6 million light years away from the Milky Way. This new Gemini image is possibly a preview of things to come for our own galaxy. Ultimately the end result of these types of collisions is thought to result in a large elliptical galaxy.

SOHO discovers its 1500th comet

Credits: ESA/ NASA/ SOHO

The ESA/NASA SOHO spacecraft has just discovered its 1500th comet, making it more successful than all other comet discoverers throughout history put together. Not bad for a spacecraft that was designed as a solar physics mission.

SOHO’s record-breaking discovery was made on 25 June. The small and faint Kreutz-group comet was discovered by US-based veteran comet hunter and amateur astronomer Rob Matson.

Kreutz-group comets, or sungrazing comets have been observed for many hundreds of years. They travel very close to the Sun (if they were to hit it, they would become 'sunstrikers'), with perihelion distance less than 0.01 Astronomical Units (the mean distance between the Earth and the Sun), or some 1460000 km.

Credits: ESA/ NASA/ SOHO

When it comes to comet catching, the SOlar and Heliospheric Observatory has one big advantage over everybody else: its location. Situated between the Sun and Earth, it has a privileged view of a region of space that can rarely be seen from Earth. From the surface, we can see regions close to the Sun clearly only during an eclipse.

Roughly 85% of SOHO discoveries are fragments from a once-great comet that split apart in a death plunge around the Sun, probably many centuries ago. The fragments are known as the Kreutz group and now pass within 1.5 million km of the Sun’s surface when they return from deep space.

At this proximity, which is a near miss in celestial terms, most of the fragments are finally destroyed, evaporated by the Sun’s fearsome radiation – within sight of SOHO’s electronic eyes. The images are captured by the Large Angle and Spectrometric Coronograph (LASCO), one of 12 instruments on board.

Of course, LASCO itself does not make the detections; that task falls to an open group of highly-skilled volunteers who scan the data as soon as it is downloaded to Earth. Once SOHO transmits to Earth, the data can be on the Internet and ready for analysis within 15 minutes.

Enthusiasts from all over the world look at each individual image for a tiny moving speck that could be a comet. When someone believes they have found one, they submit their results to Karl Battams at the Naval Research Laboratory, Washington DC, who checks all of SOHO’s findings before submitting them to the Minor Planet Center, where the comet is catalogued and its orbit calculated.

The wealth of comet information has value beyond mere classification. “This is allowing us to see how comets die,” says Battams. When a comet constantly circles the Sun, it loses a little more ice each time, until it eventually falls to pieces, leaving a long trail of fragments. Thanks to SOHO, astronomers now have a plethora of images showing this process. “It’s a unique data set and could not have been achieved in any other way,” says Battams.

All this is on top of the extraordinary revelations that SOHO has provided over the 13 years it has been in space, observing the Sun and the near-Sun environment. “Catching the enormous total of comets has been an unplanned bonus,” says Bernhard Fleck, ESA SOHO Project Scientist.

Notes for Editors:

Anyone can help to search for SOHO’s comets by visiting the Sungrazing comets page.

The Minor Planet Center operates under the auspices of the International Astronomical Union, and is located in Cambridge, Massachusetts.

For more information:

Karl Battams, Naval Research Laboratory, USA
Email: Karl.Battams @

Bernhard Fleck, ESA SOHO Project Scientist
Email: Bfleck @

Wednesday, June 25, 2008

What is Hanny's Voorwerp?

Credit: Galaxy Zoo Project, ING

Explanation: What is that green thing? A volunteer sky enthusiast surfing through online Galaxy Zoo images has discovered something really strange. The mystery object is unusually green, not of any clear galaxy type, and situated below relatively normal looking spiral galaxy IC 2497. Dutch schoolteacher Hanny van Arkel, discovered the strange green "voorwerp" (Dutch for "object") last year. The Galaxy Zoo project encourages sky enthusiasts to browse through SDSS images and classify galaxy types. Now known popularly as Hanny's Voorwerp, subsequent observations have shown that the mysterious green blob has the same distance as neighboring galaxy IC 2497. Research is ongoing, but one leading hypothesis holds that Hanny's Voorwerp is a small galaxy that acts like a large reflection nebula, showing the reflected light of a bright quasar event that happened in the center of IC 2497 about 100,000 years ago. Pictured above, Hanny's Voorwerp was imaged recently by the 4.2-meter William Hershel Telescope in the Canary Islands by Matt Jarvis, Kevin Schawinski, and William Keel.

Astronomy Picture of the Day

Tuesday, June 24, 2008

Radio Telescopes Reveal Unseen Galactic Cannibalism

Artist's Conception of Interacting Galaxies
CREDIT: Kuo et al.


Radio-telescope images have revealed previously-unseen galactic cannibalism -- a triggering event that leads to feeding frenzies by gigantic black holes at the cores of galaxies. Astronomers have long suspected that the extra-bright cores of spiral galaxies called Seyfert galaxies are powered by supermassive black holes consuming material. However, they could not see how the material is started on its journey toward the black hole.

One leading theory said that Seyfert galaxies have been disturbed by close encounters with neighboring galaxies, thus stirring up their gas and bringing more of it within the gravitational reach of the black hole. However, when astronomers looked at Seyferts with visible-light telescopes, only a small fraction showed any evidence of such an encounter. Now, new images of hydrogen gas in Seyferts made using the National Science Foundation's Very Large Array (VLA) radio telescope show the majority of them are, in fact, disturbed by ongoing encounters with neighbor galaxies.

"The VLA lifted the veil on what's really happening with these galaxies," said Cheng-Yu Kuo, a graduate student at the University of Virginia. "Looking at the gas in these galaxies clearly showed that they are snacking on their neighbors. This is a dramatic contrast with their appearance in visible starlight," he added.

The effect of the galactic encounters is to send gas and dust toward the black hole and produce energy as the material ultimately is consumed. Black holes, concentrations of matter so dense that not even light can escape their gravitational pull, reside at the cores of many galaxies. Depending on how rapidly the black hole is eating, the galaxy can show a wide range of energetic activity. Seyfert galaxies have the mildest version of this activity, while quasars and blazars are hundreds of times more powerful.

The astronomers picked a number of relatively nearby Seyfert galaxies that had previously been observed with visible-light telescopes. They then carefully studied the Seyferts with the VLA, specifically looking for radio waves emitted by hydrogen atoms. The VLA images showed the vast majority of the Seyferts were disturbed by encounters with neighbor galaxies.

By comparison, similar VLA images of inactive galaxies showed that very few were disturbed. "This comparison clearly shows a connection between close galactic encounters and the black-hole-powered activity in the cores," said Ya-Wen Tang, who began this work at the Institute of Astronomy & Astrophysics, Academia Sinica (ASIAA), in Taiwan and now is a graduate student at the National Taiwan University.

"This is the best evidence yet for the fueling of Seyfert galaxies. Other mechanisms have been proposed, but they have shown little if any difference between Seyferts and inactive galaxies," Tang added.

"Our results show that images of the hydrogen gas are a powerful tool for revealing otherwise-invisible gravitational interactions among galaxies," said Jeremy Lim, also of ASIAA. "This is a welcome advance in our understanding of these objects, made possible by the best and most extensive survey ever made of hydrogen in Seyferts," Lim said.

Kuo, Tang and Lim worked with Paul Ho, of ASIAA and the Harvard-Smithsonian Center for Astrophysics. The scientists reported their findings in the Astrophysical Journal.

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

Dave Finley, Public Information Officer
Socorro, NM
(575) 835-7302

Thursday, June 19, 2008

Ultraviolet gives view inside real ‘death star’

Images courtesy: NASA/HST/COSMOS/GALEX

Scientists have, for the first time, observed a flash of ultraviolet light from within a dying star giving vital evidence of how stars turn into supernovae.

An international team, including nine scientists from Oxford University, combined data from ground-bound telescopes observing visible light from supernovae with data from a space telescope looking for an earlier peak in ultraviolet light from an associated dying star. They were able to spot telltale signs of the shockwave that forms within a star before it explodes into a supernova. A report of the work appears in this week’s Science.

‘Supernovae are huge stellar explosions that light up galaxies but often we have no idea which star has exploded,’ said Dr Kevin Schawinski of Oxford University’s Department of Physics. ‘The nature of such an explosion is that we can’t look inside it and it destroys almost all evidence of the original star – scientists have been trying to catch such an event happening for decades.’

Previously, scientists have observed stars nearing the end of their lives and supernova explosions and their afterglow, but have had little firm evidence of what happens in between. The new observations give a first glimpse of what happens inside a star during its final hours of life.

‘Out of all the supernovae we looked at we found one that was preceded by a dramatic ‘flash’ of ultraviolet light given off by a red super-giant star in a galaxy around a billion light years away. This flash occurred about two weeks before it was detected as a normal supernova,’ said Dr Stephen Justham of Oxford University’s Department of Physics.

‘We believe that this light, emanating from deep within the star, was generated after its core collapsed and compressed the gas surrounding it to around one million degrees Kelvin.’ Around four hours after this light was observed a shockwave from the collapsed core, travelling at 50 million kilometres an hour, would have hit the surface of the star and blown it apart. However, it was almost two weeks before the resulting fireball was spotted by supernova hunters using telescopes in Hawaii.

‘With this observation we have managed to peer inside one of the hundred billion stars in a galaxy and see what it is like at the very moment that it dies,’ said Dr Christian Wolf of Oxford University’s Department of Physics. ‘We’ve been extremely fortunate to capture this moment but this is just one event and, of course, we’d love to capture other similar events with different stars which could deliver many more surprises.’

The team conducting the research included Dr Kevin Schawinski, Dr Stephen Justham, Dr Christian Wolf, Professor Philipp Podsiadlowski, Dr Mark Sullivan, Tony Bell, Emma Walker, Dr Isobel Hook from Oxford University’s Department of Physics and Dr Katrien Steenbrugge from St John’s College Research Centre, University of Oxford, as well as scientists from Germany, France, Canada and Korea.

A report of the research, entitled ‘Supernova shock breakout from a red supergiant’ was published in Science Express on 12 June 2008.

Feeding Your Black Hole is Easy

Spiral galaxy M81. Image Credit: X-ray: NASA/CXC/Wisconsin/D.Pooley and CfA/A.Zezas; Optical: NASA/ESA/CfA/A.Zezas; UV: NASA/JPL-Caltech/CfA/J.Huchra et al.;
IR: NASA/JPL-Caltech/CfA

Worried about how you're going to feed your black hole once it grows up and gets big? Have no fear. New data from the Chandra X-ray Observatory indicates that even the biggest black holes may feed just like the smallest ones. Using new observations and a detailed theoretical model, a research team compared the properties of the black hole of the spiral galaxy M81 with those of smaller, stellar mass black holes. The results show that big or little, black holes appear to eat similarly to each other, and produce a similar distribution of X-rays, optical and radio light. This discovery supports the implication of Einstein's relativity theory that black holes of all sizes have similar properties.

M81 is about 12 million light years from Earth. In the center of M81 is a black hole that is about 70 million times more massive than the Sun, and generates energy and radiation as it pulls gas in the central region of the galaxy inwards at high speed.

In contrast, so-called stellar mass black holes, which have about 10 times more mass than the Sun, have a different source of food. These smaller black holes acquire new material by pulling gas from an orbiting companion star. Because the bigger and smaller black holes are found in different environments with different sources of material to feed from, a question has remained about whether they feed in the same way.

"When we look at the data, it turns out that our model works just as well for the giant black hole in M81 as it does for the smaller guys," said Michael Nowak, from the Massachusetts Institute of Technology. "Everything around this huge black hole looks just the same except it's almost 10 million times bigger."

One of the implications of Einstein's theory of General Relativity is that black holes are simple objects and only their masses and spins determine their effect on space-time. The latest research indicates that this simplicity manifests itself in spite of complicated environmental effects.

The model that Markoff and her colleagues used to study the black holes includes a faint disk of material spinning around the black hole. This structure would mainly produce X-rays and optical light. A region of hot gas around the black hole would be seen largely in ultraviolet and X-ray light. A large contribution to both the radio and X-ray light comes from jets generated by the black hole. Multi-wavelength data is needed to disentangle these overlapping sources of light.

Among actively feeding black holes the one in M81 is one of the dimmest, presumably because it is "underfed". It is, however, one of the brightest as seen from Earth because of its relative proximity, allowing high quality observations to be made.

"It seems like the underfed black holes are the simplest in practice, perhaps because we can see closer to the black hole," said Andrew Young of the University of Bristol in England. "They don't seem to care too much where they get their food from."

This work should be useful for predicting the properties of a third, unconfirmed class called intermediate mass black holes, with masses lying between those of stellar and supermassive black holes. Some possible members of this class have been identified, but the evidence is controversial, so specific predictions for the properties of these black holes should be very helpful.

In addition to Chandra, three radio arrays (the Giant Meterwave Radio Telescope, the Very Large Array and the Very Long Baseline Array), two millimeter telescopes (the Plateau de Bure Interferometer and the Submillimeter Array), and Lick Observatory in the optical were used to monitor M81.
The results of this study will appear in an upcoming issue of The Astrophysical Journal.

Identical Twin Stars Not So Identical

I'm lucky enough to have twin sons. They aren't identical (one looks like me, the other looks like my husband – which is about as different as things get) but they have a lot of similarities. One of my favorite stories about having twins is the time we took the whole family out to a restaurant shortly after the twins were born. The waitress commented that our babies looked the same size, and we said, "Yes, they're twins." And she replied, "Oh really? How far apart in age are they?"

I used to think that waitress was a real ditz, but after seeing a press release today from Vanderbilt University, I'm wondering if the waitress was on to something, and maybe she was even an astronomer.

Astronomers recently found a very young pair of identical binary stars that have surprising differences in brightness, surface temperature and size. They also believe one of the stars formed significantly earlier than its twin. Astrophysicists have assumed that binary stars form simultaneously, and so this discovery forces theorists back to the drawing board to determine if their models can produce binaries with stars that form at different times.

The identical twins were discovered in the Orion Nebula, a well-known stellar nursery, 1,500 light years from Earth. The newly formed stars are about 1 million years old. With a full lifespan of about 50 billion years, that makes them equivalent to one-day-old human babies.

"Very young eclipsing binaries like this are the Rosetta stones that tell us about the life history of newly formed stars," says Keivan Stassun, associate professor of astronomy at Vanderbilt University. He and Robert D. Mathieu from the University of Wisconsin-Madison headed up the project.

The astronomers calculated that these twin stars have nearly identical masses, about 41 percent that of the sun. According to current theories, mass and composition are the two factors that determine a star's physical characteristics and dictate its entire life cycle. Because the two stars condensed from the same cloud of gas and dust they should have the same composition. And with identical mass and composition, they should be identical in every way. So the astronomers were surprised when they discovered that the twins exhibited significant differences in brightness, surface temperature and possibly size.

"The easiest way to explain these differences is if one star was formed about 500,000 years before its twin," says Stassun. "That is equivalent to a human birth-order difference of about half of a day."

Now, I have heard stories of twins being born several hours apart and even in different years (one late on Dec. 31, and the other early on Jan. 1) so, maybe this difference in star formation isn't such a big deal, and it happens all the time. However, further study is needed.

But this new discovery may cause astronomers to readjust their estimates of the masses and ages of thousands of young stars less than a few million years old, as current estimates are based on models that presumed binary stars formed simultaneously.

Just like having twins causes you to readjust your entire life. But it’s a good readjustment.

Original News Source: Vanderbilt University (this link includes a nice multimedia presentation about the discovery)

Sunday, June 15, 2008

The Little Man and the Cosmic Cauldron

On the occasion of the 10th anniversary of the Very Large Telescope's First Light, ESO is releasing two stunning images of different kinds of nebulae, located towards the Carina constellation. The first one, Eta Carinae, has the shape of a 'little man' and surrounds a star doomed to explode within the next 100 000 years. The second image features a much larger nebula, whose internal turmoil is created by a cluster of young, massive stars.

ESO PR Photo 17a/08
The Homunculus (NACO/VLT)

Being brighter than one million Suns, Eta Carinae is the most luminous star known in the Galaxy. It is the closest example of a luminous blue variable, the last phase in the life of a very massive star before it explodes in a fiery supernova.

Eta Carinae is surrounded by an expanding bipolar cloud of dust and gas known as the Homunculus ('little man' in Latin), which astronomers believe was expelled from the star during a great outburst seen in 1843 [1].

Eta Carinae was one of the first objects to be imaged during First Light with ESO's VLT, 10 years ago. At the time, the image obtained with a test camera already showed the unique capabilities of the European flagship telescope for ground-based optical and infrared astronomy, as well as of its unique location on the mountain of Paranal. The image had a resolution of 0.38 arcseconds.

The new, recently obtained image reveals even more, with a resolution a factor of 6 to 7 times better. It was obtained with the NACO near-infrared instrument on Yepun, Unit Telescope 4 of the VLT. NACO is an adaptive optics instrument, which means that it can correct for the blurring effect of the atmosphere. And looking at the image, the power of adaptive optics is clear. The image quality is as though the whole 8.2-m telescope had been launched into space [2].

When viewed through the eyepiece of a small telescope, the Homunculus may indeed resemble a little man, but the astounding NACO image clearly shows a bipolar structure. Also very well resolved is the fine structure of the jets coming out from the central star.

Last year, the Very Large Telescope Interferometer also studied Eta Carinae in great detail and provided invaluable information about the stellar wind of Eta Carinae (see ESO 06/07).

ESO PR Photo 17b/08
NGC 3576

The second image was obtained with the ISAAC infrared imager on Antu, Unit Telescope 1.

Located 9 000 light-years away, i.e. farther away than Eta Carinae, NGC 3576 is also in the direction of the southern Carina constellation. NGC 3576 is about 100 light-years across, that is, 25 times larger than the distance between the Sun and its closest neighbouring star.

This intriguing nebula is a gigantic region of glowing gas, where stars are currently forming. The intense radiation and winds from the massive stars are shredding the clouds from which they form, creating dramatic scenery. It is estimated that the nebula is about 1.5 million year old, the blink of an eye on cosmological timescales.

Astronomers from the University of Cologne [3], Germany, have studied this region with ESO's Very Large Telescope and ISAAC to determine the proportion of stars still having a protoplanetary disc from which planets form. Looking at young regions of different ages, the astronomers hope to estimate the lifetime of protoplanetary discs and thereby better understand how planets form. In particular, the scientists are interested in looking at the effect of the strong radiation of the stars, as well as of stellar encounters in these dense regions, on the survival of the discs.

[1]: In fact, since the distance to Eta Carinae is about 7500 light-years, the eruption must have taken place about 7700 years ago.

[2]: Given the large size of each Unit Telescope of the VLT, the resolution achievable when using adaptive optics (the 'diffraction limit') is as good in the longer near-infrared wavelengths, where NACO observes, as what the HST can achieve in the visible. The resolution is indeed close to 0.05 arcseconds, ten times better than what one can typically obtain without adaptive optics on an excellent site. A resolution of 0.05 arcseconds corresponds to being able to read a book 10 km away.

[3]: The astronomers are C. Olczak, R. Schödel, S. Pfalzner, and A. Eckart.

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A New Type of Comet Dust Mineral

Comets contain pristine samples of the original building 
blocks of the Solar System. 
Image: Observatoire de Haute, Provence, France.

NASA researchers have found a new mineral in a material that likely originated from comet 26P/Grigg-Skjellerup, which orbits the Sun once every five years.

"When I saw this mineral for the first time, I immediately knew this was something no one had seen before," said Nakamura-Messenger, a space scientist at NASA's Johnson Space Centre in Houston. "But it took several more months to obtain conclusive data because these mineral grains were only 1/10,000 of an inch (0.00025 centimetres) in size."

The mineral has been named ‘brownleeite’ after Donald Brownlee, a University of Washington astronomer who founded the field of interplanetary dust particle (IDP) research and is also the principal investigator of NASA's Stardust mission. The understanding of the early Solar System established from IDP studies would not exist without his efforts.

"This really did surprise me because I know it took a lot of effort to get this mineral approved," says Brownlee. "I've always been very intrigued by minerals, so it's great to be one. I never dreamed I'd have a mineral named after me.”

NASA researchers have found a new mineral in a material that likely originated from comet 26P/Grigg-Skjellerup, which orbits the Sun once every five years.

"When I saw this mineral for the first time, I immediately knew this was something no one had seen before," said Nakamura-Messenger, a space scientist at NASA's Johnson Space Centre in Houston. "But it took several more months to obtain conclusive data because these mineral grains were only 1/10,000 of an inch (0.00025 centimetres) in size."

The mineral has been named ‘brownleeite’ after Donald Brownlee, a University of Washington astronomer who founded the field of interplanetary dust particle (IDP) research and is also the principal investigator of NASA's Stardust mission. The understanding of the early Solar System established from IDP studies would not exist without his efforts.

"This really did surprise me because I know it took a lot of effort to get this mineral approved," says Brownlee. "I've always been very intrigued by minerals, so it's great to be one. I never dreamed I'd have a mineral named after me.”

Comets contain pristine samples of the original building blocks of the Solar System. Image: Observatoire de Haute, Provence, France.

The Earth collects an astounding 40,000 tons of dust particles every year from disintegrated comets and asteroids, equivalent to one particle per square metre of planet every day which therefore makes them very hard to find. This ‘gold-dust’ is extremely important because it is made of the original building blocks of the Solar System.

NASA has routinely collected cosmic and interplanetary dust with high-altitude research aircraft since 1982, but this new particle was captured after an innovative method of collection was suggested by Johnson space scientist Scott Messenger, who predicted comet 26P/Grigg-Skjellerup was a source of dust grains that could be captured in Earth's stratosphere at a specific time of the year. The aircraft collected IDPs from this particular comet stream in April 2003 and the new mineral was found in one of the particles.

"Because of their exceedingly tiny size, we had to use
state-of-the-art nano-analysis techniques in the microscope to
measure the chemical composition and crystal structure of the new mineral," said Lindsay Keller, co-discoverer of the mineral. "This is a highly unusual material that has not previously been predicted either to be a cometary component or to have formed by condensation in the solar nebula."

The mineral contains a combination of manganese and silicon (and is therefore known as a manganese silicide) and was surrounded by multiple layers of other minerals that also have been reported only in extraterrestrial rocks. It’s official name of brownleeite joins a list of over 4,300 other minerals.

By Dr Emilly Baldwin

Saturday, June 14, 2008

Thinking About Time Before the Big Bang

Written by Nancy Atkinson

What happened before the Big Bang? The conventional answer to that question is usually, "There is no such thing as 'before the Big Bang.'" That's the event that started it all. But the right answer, says physicist Sean Carroll, is, "We just don't know." Carroll, as well as many other physicists and cosmologists have begun to consider the possibility of time before the Big Bang, as well as alternative theories of how our universe came to be. Carroll discussed this type of "speculative research" during a talk at the American Astronomical Society Meeting last week in St. Louis, Missouri.

"This is an interesting time to be a cosmologist," Carroll said. "We are both blessed and cursed. It's a golden age, but the problem is that the model we have of the universe makes no sense."

First, there's an inventory problem, where 95% of the universe is unaccounted for. Cosmologists seemingly have solved that problem by concocting dark matter and dark energy. But because we have "created" matter to fit the data doesn't mean we understand the nature of the universe.

Another big surprise about our universe comes from actual data from the WMAP (Wilkinson Microwave Anisotropy Probe) spacecraft which has been studying the Cosmic Microwave Background (CMB) – the "echo" of the Big Bang.

"The WMAP snapshot of how the early universe looked shows it to be hot, dense and smooth [low entropy] over a wide region of space," said Carroll. "We don't understand why that is the case. That's an even bigger surprise than the inventory problem. Our universe just doesn’t look natural." Carroll said states of low-entropy are rare, plus of all the possible initial conditions that could have evolved into a universe like ours, the overwhelming majority have much higher entropy, not lower.

But the single most surprising phenomenon about the universe, said Carroll, is that things change. And it all happens in a consistent direction from past to future, throughout the universe.

"It's called the arrow of time," said Carroll. This arrow of time comes from the second law of thermodynamics, which invokes entropy. The law states that invariably, closed systems move from order to disorder over time. This law is fundamental to physics and astronomy.

One of the big questions about the initial conditions of the universe is why did entropy start out so low? "And low entropy near the Big Bang is responsible for everything about the arrow of time" said Carroll. "Life and death, memory, the flow of time." Events happen in order and can't be reversed.

"Every time you break an egg or spill a glass of water you're doing observational cosmology," Carroll said.

Therefore, in order to answer our questions about the universe and the arrow of time, we might need to consider what happened before the Big Bang.

Carroll insisted these are important issues to think about. "This is not just recreational theology," he said. "We want a story of the universe that makes sense. When we have things that seem surprising, we look for an underlying mechanism that makes what was a puzzle understandable. The low entropy universe is clue to something and we should work to find it."

Right now we don't have a good model of the universe, and current theories don't answer the questions. Classical general relativity predicts the universe began with a singularity, but it can't prove anything until after the Big Bang.

Inflation theory, which proposes a period of extremely rapid (exponential) expansion of the universe during its first few moments, is no help, Carroll said. "It just makes the entropy problem worse. Inflation requires a theory of initial conditions."

There are other models out there, too, but Carroll proposed, and seemed to favor the idea of multi-universes that keep creating "baby" universes. "Our observable universe might not be the whole story," he said. "If we are part of a bigger multiverse, there is no maximal-entropy equilibrium state and entropy is produced via creation of universes like our own."

Carroll also discussed new research he and a team of physicists have done, looking at, again, results from WMAP. Carroll and his team say the data shows the universe is "lopsided."

Measurements from WMAP show that the fluctuations in the microwave background are about 10% stronger on one side of the sky than on the other.

An explanation for this "heavy-on-one-side universe" would be if these fluctuations represented a structure left over from the universe that produced our universe.

Carroll said all of this would be helped by a better understanding of quantum gravity. "Quantum fluctuations can produce new universes. If thermal fluctuation in a quiet space can lead to baby universes, they would have their own entropy and could go on creating universes."

Granted, — and Carroll stressed this point – any research on these topics is generally considered speculation at this time. "None of this is firmly established stuff," he said. "I would bet even money that this is wrong. But hopefully I'll be able to come back in 10 years and tell you that we've figured it all out."

Admittedly, as writer, trying to encapsulate Carroll's talk and ideas into a short article surely doesn't do them justice. Check out Carroll's take on these notions and more at his blog, Cosmic Variance. Also, read a great summary of Carroll's talk, written by Chris Lintott for the BBC. I've been mulling over Carroll's talk for more than a week now, and contemplating the beginnings of time – and even that there might be time before time – has made for an interesting and captivating week. Whether that time has brought me forward or backward in my understanding remains to be seen….

Tuesday, June 10, 2008

Hubble's Sweeping View of the Coma Cluster of Galaxies

Coma Cluster, Abell 1656
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

NASA's Hubble Space Telescope captures the magnificent starry population of the Coma Cluster of galaxies, one of the densest known galaxy collections in the universe.

The Hubble's Advanced Camera for Surveys viewed a large portion of the cluster, spanning several million light-years across. The entire cluster contains thousands of galaxies in a spherical shape more than 20 million light-years in diameter.

Also known as Abell 1656, the Coma Cluster is over 300 million light-years away. The cluster, named after its parent constellation Coma Berenices, is near the Milky Way's north pole. This places the Coma Cluster in an area unobscured by dust and gas from the plane of the Milky Way, and easily visible by Earth viewers.

Most of the galaxies that inhabit the central portion of the Coma Cluster are ellipticals. These featureless "fuzz-balls" are pale goldish brown in color and contain populations of old stars. Both dwarf, as well as giant ellipticals, are found in abundance in the Coma Cluster.

Farther out from the center of the cluster are several spiral galaxies. These galaxies have clouds of cold gas that are giving birth to new stars. Spiral arms and dust lanes "accessorize" these bright bluish-white galaxies that show a distinctive disk structure.

In between the ellipticals and spirals is a morphological class of objects known as S0 (S-zero) galaxies. They are made up of older stars and show little evidence of recent star formation; however, they do show some assemblage of structure — perhaps a bar or a ring, which may give rise to a more disk-like feature.

This Hubble image consists of a section of the cluster that is roughly one-third of the way out from the center of the cluster. One bright spiral galaxy is visible in the upper left of the image. It is distinctly brighter and bluer than galaxies surrounding it. A series of dusty spiral arms appears reddish brown against the whiter disk of the galaxy, and suggests that this galaxy has been disturbed at some point in the past. The other galaxies in the image are either ellipticals, S0 galaxies, or background galaxies far beyond the Coma Cluster sphere.

The data of the Coma Cluster were taken as part of a survey of a nearby rich galaxy cluster. Collectively they will provide a key database for studies of galaxy formation and evolution. This survey will also help to compare galaxies in different environments, both crowded and isolated, as well as to compare relatively nearby galaxies to more distant ones (at higher redshifts).

For additional information, contact:
Ray Villard
Space Telescope Science Institute, Baltimore, Md.

Lars Lindberg Christensen
Hubble/ESA, Garching Germany

Detective astronomers unearth hidden celestial gem

XMM-Newton image of the young, luminous supernova remnant G350.1-0.3 (to the left) and its neutron star companion (to the right).
The image was taken by the spacecraft’s European Photon Imaging Cameras (EPIC), in the energy range of 0.5–10 keV.
Credits: ESA/ XMM-Newton/ EPIC (Gaensler et al.)

ESA’s orbiting X-ray observatory XMM-Newton has re-discovered an ignored celestial gem. The object in question is one of the youngest and brightest supernova remnants in the Milky Way, the corpse of a star that exploded around 1000 years ago.

Its shape, age and chemical composition will allow astronomers to better understand the violent ways in which stars end their lives.

Exploding stars seed the Universe with heavy chemical elements necessary to build planets and create life. The expanding cloud of debris that each explosion leaves behind, known as a supernova remnant (SNR), is a bright source of X-rays and radio waves. Generally, the debris is thought to appear as an expanding bubble or ring.

When astronomers took the first high-resolution radio images of a celestial object known as ‘G350.1-0.3’ in the 1980s, they saw an irregular knot of gases that did not seem to meet these expectations. So it was classified as a probable background galaxy and was quietly forgotten. 

Now Bryan Gaensler and Anant Tanna, both at the University of Sydney, have used the X-ray capabilities of XMM-Newton with their colleagues to prove that appearances can be deceptive. G350.1-0.3 is indeed the debris of an exploded star despite its misshapen configuration.

In fact, it turns out to be one of the youngest and brightest supernova remnants in the Milky Way.

To explain its shape, the team looked at radio surveys and discovered that G350.1-0.3 had exploded next to a dense cloud of gas about 15 000 light-years from Earth. The cloud prevented the blast from expanding evenly in all directions, resulting in an example of a rare kind of misshapen supernova remnant.

G350.1-0.3 is incredibly small and young in astronomical terms, only eight light years across and about 1000 years old. “Only a handful of such young supernova remnants are known. So even having one more is important,” says Tanna. That is because young supernova remnants are highly luminous, with the newly-formed chemical elements glowing brightly, making them easier to study.

“We're seeing these heavy elements fresh out of the oven,” says Gaensler. Young supernova remnants exhibit the newly created elements and also contain clues about the way the original star exploded. Such information is lost in most supernova remnants because, as they expand and age, they lose their initial characteristics. “After 20 000 years, all sorts of explosions look more or less the same,” says Gaensler.

Astronomers now recognise that stars explode in many different ways. Some might be just big enough for an explosion to occur, others might be much more massive. There are differences in the chemical composition of the exploding stars and some may have a companion star in orbit around them.

Gaensler and Tanna hope that further investigations of G350.1-0.3 will yield clues as to exactly what kind of star exploded. “It may turn out that many of the youngest supernova remnants have these strange shapes,” says Tanna, “The hunt to find more is now on.”

Despite the light from the supernova having reached Earth during the time of William the Conqueror, Gaensler thinks humans would not have seen it. “The X-ray data tell us that there's a lot of dust lying between it and Earth. Even if you'd been looking straight at it when it exploded, it would've been invisible to the naked eye,” he says.

Thankfully, XMM-Newton’s sensitivity and the detective work by Gaensler and Tanna mean that this important celestial object will never again be forgotten.

Notes for editors:
These findings will be published today in ‘The (re-)discovery of G350.1–0.3: A young, luminous supernova remnant and its neutron star’ by B. Gaensler, A. Tanna, P. Slane, C. Brogan, J. Gelfand, N. McClure-Griffiths, F. Camilo, C. Ng and J. Miller in The Astrophysical Journal Letters.

For more information:
Bryan Gaensler, School of Physics, University of Sydney
Email: Bgaensler @

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.Schartel @

Monday, June 09, 2008

Cassini sees collisions of moonlets on Saturn's ring

Saturn's F ring
Credit: NASA/JPL/Space Science Institute

A team of scientists led from the UK has discovered that the rapid changes in Saturn's F ring can be attributed to small moonlets causing perturbations. Their results are reported in Nature (5th June 2008).

Saturn's F ring has long been of interest to scientists as its features change on timescales from hours to years and it is probably the only location in the solar system where large scale collisions happen on a daily basis. Understanding these processes helps scientists understand the early stages of planet formation.

Prof Carl Murray of Queen Mary, University of London and member of the Cassini Imaging Team led the analysis. He says “Saturn’s F ring is perhaps the most unusual and dynamic ring in the solar system; it has multiple structures with features changing on a variety of timescales from hours to years.”

The team used images gathered by the NASA-ESA Cassini Huygens mission. Images snapped by Cassini in 2006 and 2007 show the formation and evolution of a series of structures (called "jets" in the paper) that are the result of collisions between small nearby moonlets and the core of the F ring.

A ~5km object discovered by Cassini in 2004 (called S/2004 S 6) is the best candidate to explain some of the largest jets seen in the images.

Prof Murray adds “Previous research has noted the features in the F ring and concluded that either another moon of radius about 100km must be present and scattering the particles in the ring, or a much smaller moonlet was colliding with its constituent particles. We can now say that the moonlet is the most likely explanation and even confirm the identity of one culprit.”

The F ring and all the nearby objects are being continually perturbed by encounters with the shepherding moon Prometheus and this allows the gravitational signature of the embedded objects to be detected, even when the objects themselves cannot be seen.

Dr Sébastien Charnoz of Université Paris 7 / CEA Saclay is a co-author on the paper. He says “Large scale collisions happen in Saturn’s F ring almost daily – making it a unique place to study. We can now say that these collisions are responsible for the changing features we observe there.”

The Cassini images also show new features (called "fans") which result from the gravitational effect of small (~1km) satellites orbiting close to the F ring core.

Prof Keith Mason, STFC Chief Executive Officer, which funds UK involvement in Cassini-Huygens said “This incredibly successful mission has taught us a great deal about the solar system and the processes at work in it. Understanding how small objects move within the dust rings around Saturn gives an insight into the processes that drive planetary formation, where the proto-planet collects material in its orbit through a dust plane and carves out similar grooves and tracks.”

Friday, June 06, 2008

W28 - A Mixed Bag

Credit: Chandra X-ray: NASA/CXC/HSC/J. Keohane et al.; ROSAT X-ray: NASA/ROSAT; Optical: NOAO/CTIO/P.F. Winkler et al.; Radio: NSF/NRAO/VLA/G. Dubner et al.

When some stars die, they explode as supernovas and their debris fields (aka, "supernova remnants") expand into the surrounding environments. There are several different types, or categories, of supernova remnants. One of these is known as a mixed-morphology supernova remnant. This type gets its name because it shares several characteristics from other types of supernova remnants. More specifically, particles that have been superheated are seen in X-rays in the center of the remnant. This inner region is enclosed by shell structure detected in radio emission.

This composite shows a classic example of mixed-morphology supernova remnant known as W28. Each wavelength shows detailed structure of how the supernova shock wave is interacting, or has interacted, with the complex cloudy environment which surrounded its parent star. In this image, the stars and fine structure in the background are seen in optical light (grey and white) by the Cerro Tololo Inter-American Observatory in Chile. The radio (orange) data were obtained by the Very Large Array in New Mexico, while the blue in the wide-field view comes from the ROSAT satellite (X-rays). Data from NASA’s Chandra X-ray Observatory give new detail into the heart of W28 as seen in the inset. In this close-up view of the center, low-energy X-rays are colored red, the medium are green, and the highest found by Chandra are blue. The Chandra data show the shape and extent of the high-energy emission in the central region. By studying W28 and others like it, astronomers hope to better understand the complexities involved when a star explodes in a crowded neighborhood.

Involved in this W28 study were Jonathan Keohane (Hampden-Sydney College), Jeonghee Rho (Spitzer Science Center), Thomas Pannuti (Morehead State University), Kazik Borkowski (North Carolina State University) and Frank Winkler (Middlebury College).

Super-luminous Supernovae

According to observations by NASA's Chandra X-ray Observatory and ground-based optical telescopes, the supernova SN 2006gy is the brightest and most energetic stellar explosion ever recorded and may be a long-sought new type of explosion. Here is an artist's illustration that shows what SN 2006gy may have looked like if viewed at a close distance. 

Astronomers are announcing today that they have found that the explosive conversion of a neutron star into a quark star (namely, a Quark-Nova) has the right properties to explain the super-luminous supernovae SN2006gy, SN2005gj and SN2005ap. Denis Leahy and Rachid Ouyed of the University of Calgary in Canada are presenting their results today at the American Astronomical Society meeting in St. Louis, Missouri. Their results are of special interest for two reasons: astronomers previously did not have a satisfactory explanation for super-luminous supernovae; and this provides supporting evidence for the existence of quark stars — a manifestation of a new state of matter.

The objects of study are the three most luminous supernova ever observed. SN2006gy was in the galaxy NGC1260 at a distance from earth of 240 million light-years; SN2005gj and SN2005ap occurred in more distant galaxies. They were observed at Lick Observatory for SN2006gy (Smith et al, 2008), at Mount Palomar for SN2005gj (Aldering et al 2006), and at McDonald Observatory for SN2005ap (Quimby et al 2008). They produced 100 times more light energy than normal supernova and are a challenge to explain.

We study the properties of quark stars, which have been proposed to exist but are not yet been confirmed. The most compact solid objects in the universe known to exist are neutron stars: 16 miles across and about 1.5 times as massive as our Sun. Neutron stars are made up of neutrons tightly packed together and produced by collapse of the core of a massive star at the end of its life, which also produces a supernova explosion. Quark stars are even more dense: the same mass but only 12 miles across. Quark stars may be produced when the density inside a neutron star becomes high enough. In that case the neutrons dissolve into quarks, and also release much energy- enough to power an explosion similar to the original explosion that formed the neutron star.

Super-luminous supernova can be the result of the second explosion (the Quark-Nova), which converts the neutron star into a quark star. The first explosion that made the neutron star, would be not noticed since the known super-luminous supernovae have occurred so far away from Earth. The shock wave from the second explosion takes a few weeks to heat up the gas ejected by the first explosion. As a result, the gas is very large (a hundred times the Sun-Earth distance) when it is heated and produces a bright long-lived supernovae.

There are alternate models for such super-luminous supernovae, so observations of more of these events are needed to confirm our model.

Provided by the AAS

Thursday, June 05, 2008

Two of the Milky Way's spiral arms may be 'demoted

Two major and two minor arms wind outwards from the centre of our galaxy in this artist's impression llustration: NASA/JPL-Caltech

Kicking Pluto out of the planet club was nothing compared to this. An astronomer is calling for demoting two entire arms of our galaxy, after they failed to turn up in a sensitive new map of the Milky Way's stars.

Astronomers have long believed that our galaxy possesses four spiral arms, since radio observations show concentrations of gas that trace such a spiral structure.

But now, two of the Milky Way's arms have failed to turn up in a sensitive new survey that used the Spitzer Space Telescope to map the distribution of millions of stars. Spitzer is well-suited to mapping the galaxy's stars because its infrared vision can pierce through the dust that obscures stars at optical wavelengths of light.

Astronomer Robert Benjamin of the University of Wisconsin in Whitewater, US, says these two arms, called Sagittarius and Norma, may be mostly concentrations of gas, perhaps sprinkled with pockets of young stars.

By contrast, the other two arms, called Scutum-Centaurus and Perseus, appear rich not only in gas, but in stars both young and old. "These major arms . . . could be the things that would really stand out if you were looking at the Milky Way galaxy from Andromeda [a nearby galaxy]," Benjamin says.

Small stub

Thomas Dame of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Masssachusetts, US, who is not a member of Benjamin's team, says the major-minor arm idea is interesting. "I think it could be right, but I think we have a lot of work to do to shore this up," he told New Scientist.

Benjamin admits that much is still unclear about the structure of our galaxy. "Trying to create a picture of the Milky Way is about 40% hard science and 60% imagination," he says.

In addition to the four large arms, the Milky Way has some smaller, partial arms. The Sun is located in one such stub called the Orion Spur, which is wedged between the Sagittarius and Perseus arms.

The findings were presented on Tuesday at a meeting of the American Astronomical Society in St Louis, Missouri, US.

Another team of astronomers unveiled a vast mosaic image of the Milky Way - the most sensitive ever made in infrared light - created from Spitzer Space Telescope observations. The team, led by Sean Carey of Caltech in Pasadena, California, US, displayed a 55-metre-long poster version of the image at the meeting. news service
David Shiga

Tuesday, June 03, 2008

Spitzer Captures Stellar Coming of Age in Our Galaxy

Spitzer Finds Clarity in the Inner Milky Way
Credit: NASA/JPL-Caltech/Univ. of Wisconsin

Inner Milky Way Raging With Star Formation
Credit: NASA/JPL-Caltech/Univ. of Wisconsin

More than 800,000 snapshots from NASA's Spitzer Space Telescope have been stitched together to create a new "coming of age" portrait of stars in our inner Milky Way galaxy.

The image depicts an area of sky 120 degrees wide by two degrees tall. It was unveiled today at the 212th meeting of the American Astronomical Society in St. Louis, Mo.

"This is the highest-resolution, largest, most sensitive infrared picture ever taken of our Milky Way," said Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology, Pasadena, Calif. Carey is lead investigator for one of two teams responsible for the new picture. "Where previous surveys saw a single source of light, we now see a cluster of stars. With this data, we can learn how massive stars form, map galactic spiral arms and make a better estimate of our galaxy's star-formation rate," Carey explained.

"I suspect that Spitzer's view of the galaxy is the best that we'll have for the foreseeable future. There is currently no mission planned that has both a wide field of view and the sensitivity needed to probe the Milky Way at these infrared wavelengths," said Barbara Whitney of the Space Science Institute, Madison, Wis. Whitney is a member of the second astronomy team.

Because Earth sits inside our dusty, flat, disk-shaped Milky Way, we have an edge-on view of our galactic home. We see the Milky Way as a blurry, narrow band of light that stretches almost completely across the sky. With Spitzer's dust-piercing infrared eyes, astronomers peered 60,000 light-years away into this fuzzy band, called the galactic plane, and saw all the way to the other side of the galaxy.

The result is a cosmic tapestry depicting an epic coming-of-age tale for stars. Areas hosting stellar embryos are identified by swaths of green, which are organic molecules, called polycyclic aromatic hydrocarbons, illuminated by light from nearby newborn stars. On Earth, these molecules are found in automobile exhaust and charred barbeque grills, essentially anywhere carbon molecules are burned incompletely.

The regions where young stars reside are revealed as "bubbles," or curved ridges in the green clouds. These bubbles are carved by the winds from young starlets blowing away their natal dust. The starlets appear as yellow and red dots, and the wisps of red that fill most bubbles are composed of graphite dust particles, similar to very small pieces of pencil lead.

Blue specks sprinkled throughout the photograph are individual older Milky Way stars. The bluish-white haze that hovers heavily in the middle two panels is starlight from the galaxy's older stellar population. A deep, careful examination of the image also shows the dusty remnants of dying and dead stars as translucent orange spheres.

"With these Spitzer data, we've been able to catalogue more than 100 million stars," said Edward Churchwell of the University of Wisconsin, at Madison. Churchwell is principal investigator of one of the teams.

"This picture shows us that our Milky Way galaxy is a crowded and dynamic place. We have a lot to learn. I've definitely found a lot of things in this map that I didn't expect to see," said Carey.

This infrared composite incorporates observations from two Spitzer instruments. Data from the infrared array camera were collected and processed by The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire team, led by Churchwell. The Multiband Imaging Photometer for Spitzer Galactic Plane Survey Legacy team, led by Carey, processed observations from Spitzer's multiband imaging photometer. Blue represents 3.6-micron light, green shows light of 8 microns and red is 24-micron light.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

Whitney Clavin 818-354-4673/818-648-9734
Jet Propulsion Laboratory, Pasadena, Calif.

White Dwarf Lost in Planetary Nebula

Credit: NASA, NOAO, H. Bond and K. Exter (STScI/AURA)

Evolution of Triple Star System Planetary Nebula SuWt 2
Credit: NASA, ESA, and A. Feild (STScI)


1. Triple star system consists of two white A stars in a tight orbit, and a more massive, hotter star.
2. More massive star evolves quickly and expands into a red giant.
3. The tight binary pair is engulfed by the red giant.
4. The binary pair "stirs" the red giant's shell, forming a thick disk.
5. Bipolar lobes form above and below the disk to make a planetary nebula. The star's core shrinks down to a white dwarf.

Call it the case of the missing dwarf.

A team of stellar astronomers is engaged in an interstellar CSI (crime scene investigation). They have two suspects, traces of assault and battery, but no corpse.

The southern planetary nebula SuWt 2 is the scene of the crime, some 6,500 light-years from Earth in the direction of the constellation Centaurus.

SuWt 2 consists of a bright, nearly edge-on glowing ring of gas. Faint lobes extend perpendicularly to the ring, giving the faintest parts of the nebula an hourglass shape.

These glowing ejecta are suspected to have been energized by a star that has now burned out and collapsed to a white dwarf. But the white dwarf is nowhere to be found.

The mystery deepened when researchers obtained ultraviolet observations in the early 1990's with NASA's International Ultraviolet Explorer satellite, expecting to see signs of a faint but very hot star. But no ultraviolet radiation was detected.

Instead, at the center of the nebular ring are two suspicious characters: a pair of tightly bound stars that whirl around each other every five days, neither one of which is a white dwarf. These stars are hotter than our Sun (their spectral class is A) but they are still not hot enough to make the nebula glow. Only a flood of ultraviolet radiation, such as that from the missing white dwarf, could do that.

The study is being conducted by Katrina Exter and Howard Bond of the Space Telescope Science Institute in Baltimore, Md. and a team of British and American colleagues. Their extensive photometry and spectroscopy of the binary show that both stars are larger than main-sequence stars of their masses. This may imply that they have started to evolve toward becoming red giants. Both stars also appear to be rotating more slowly than expected; they would be expected to always be facing the same sides toward each other, but they do not.

The astronomers suggest a simple explanation for the facts at the scene: the stars at the center of SuWt 2 were born as a family of three, with the A stars circling each other tightly and a more massive star orbiting further out. This allowed room for the massive star to evolve to become a red giant, which only then engulfed the pair of A stars. Trapped inside the red giant in what astronomers call a "common envelope," the pair spiraled down toward the core, causing the envelope to spin faster. Eventually, the outer layers of the red giant were ejected in the plane of the orbit, producing the ring-shaped nebula seen today. The unusually slow spins of the two A stars may have been another consequence of their victimization by their massive sibling.

The ground-based observations were obtained with telescopes at the Cerro Tololo Inter- American Observatory, Chile; the New Technology Telescope at the European Southern Observatory, Chile; the Anglo-Australian Telescope, Australia; and the South African Astronomical Observatory.

Ultraviolet radiation from the exposed hot core of the red giant would have caused the nebula to glow. If the giant's core were of high enough mass, it would then shrink and cool off rapidly to a faint white dwarf, which might explain its current invisibility.

Their results are being presented today at the 212th meeting of the American Astronomical Society in St. Louis, Mo. Other members of the team are Keivan Stassun (Vanderbilt University, Tenn.), Pierre Maxted and Barry Smalley (Keele University, UK), and Don Pollacco (Queen's University, UK).


Ray Villard
Space Telescope Science Institute, Baltimore, Md.

Howard Bond/Katrina Exter
Space Telescope Science Institute, Baltimore, Md.