Friday, December 28, 2007

Asteroid may hit Mars Next Month

Will asteroid 2007 WD5 crash into Mars January 30? Odds it'll happen are now 1 in 75.
Astronomy: Roen Kelly

A space rock dubbed 2007 WD5 is taking aim on the Red Planet.
Francis Reddy

A small asteroid discovered November 20 may strike Mars next month.

Astronomers with NASA's Near Earth Object (NEO) Program at the Jet Propulsion Laboratory in Pasadena, California, calculate the odds of a January 30 collision at 1 in 75. While this is remote, it's less so than last week's estimated 1-in-350 chance.

NEO astronomer Steve Chesley, who's used to dealing with million-to-one odds, calls the event "extremely unusual," and, in something of a twist, NEO astronomers are rooting for an impact.

An armada of spacecraft orbiting the Red Planet — the European Space Agency's Mars Express and NASA's Mars Reconnaissance Orbiter and Mars Odyssey — would have ringside seats to view the strike and its after-effects. Even Earth-based telescopes could potentially observe the impact because Mars is near opposition and, therefore, unusually close.

Astronomers say asteroid 2007 WD5 is about 160 feet (50 meters) across. If it struck Mars, the energy would be similar to the 1908 Tunguska blast in Siberia, where a stony asteroid exploded above the taiga. The blast felled and scarred trees over 810 square miles (2,100 square km).

One difference: Tunguska was an air burst and left no crater, whereas 2007 WD5 likely would reach Mars' surface intact.

Saturday, December 22, 2007

Tyrrhenian Sea and Solstice Sky

Credit & Copyright: Danilo Pivato

Today the Solstice occurs at 0608 Universal Time, the Sun reaching its southernmost declination in planet Earth's sky. Of course, the December Solstice marks the beginning of winter in the northern hemisphere and summer in the south.

When viewed from northern latitudes, the Sun will make its lowest arc through the sky along the southern horizon.

So in the north, the Solstice day has the shortest length of time between sunrise and sunset and fewest hours of daylight.

This striking composite image follows the Sun's path through the December Solstice day of 2005 in a beautiful blue sky, looking down the Tyrrhenian Sea coast from Santa Severa toward Fiumicino, Italy. The view covers about 115 degrees in 43 separate, well-planned exposures from sunrise to sunset.

Friday, December 21, 2007

Earth's Protective Magnetic Field

On August 11, 2000, the Extreme Ultraviolet (EUV) instrument aboard the IMAGE spacecraft captured this view of Earth's magnetosphere from above the north pole.
Credit: NASA / IMAGE Science Team

New research shows that Earth's magnetic field could help protect astronauts while working on the Moon.

It has been 35 years since humans last walked on the Moon, but there has been much recent discussion about returning, either for exploration or to stage a mission to Mars. However, there are concerns about potential radiation danger for astronauts during long missions on the lunar surface.

A significant part of that danger results from solar storms, which can shoot particles from the Sun to Earth at nearly the speed of light and can heat oxygen in the Earth's ionosphere and send it in a hazardous stream toward the Moon.

Earth is largely protected by its magnetic field, or magnetosphere. Now, new University of Washington research shows that some parts of the Moon also are protected by the magnetosphere for 7 days during the 28-day orbit around Earth.

"We found that there were areas of the Moon that would be completely protected by the magnetosphere and other areas that are not protected at all," says Erika Harnett, a UW assistant research professor of Earth and space sciences.

Solar energetic particles, which are generated during solar storms, carry enough energy to disrupt communications on Earth or even kill satellites in Earth orbit. During those same storms, particles from Earth's ionosphere, primarily oxygen, also can become significantly energized. Though they are not as powerful as solar energetic particles, they still pose a significant threat to astronauts working on the moon, or even en route to Mars.

Using computers to model properties of the magnetosphere, Harnett found that while solar storms can increase the danger from ionosphere particles hitting the moon, they also trigger conditions in the magnetosphere that deflect many hazardous solar particles.

Particles with high enough energy can pass directly through a human without much damage, Harnett says, but particles packing slightly less oomph, though unfelt by a human, can lodge in a person. Typically it's not just one particle, but many, and the accompanying radiation can damage cells, she says.

In the longest missions of NASA's Apollo Program, astronauts spent just a few days on the Moon. The last mission, Apollo 17, was launched December 7, 1972, landed on the Moon on December 11 and arrived back on Earth on December 19.

"During Apollo, people were not on the Moon for very long so there wasn't the concern about the radiation hazard to humans as there is with longer missions," Harnett says.

Today there is much greater understanding of the danger posed by solar energetic particles, particularly because of the adverse effects they can have on satellite communications during periods of intense solar flare activity.

"The problem is that we can't predict when this activity is going to take place so we can't warn astronauts to take shelter, so they could be vulnerable when the Moon is outside the magnetosphere," Harnett says. "The particles travel near the speed of light, so when we see them generated on the Sun's surface they will arrive in a few minutes and there is little time to react."

The new research could help determine when it is safe for astronauts to work far from a lunar base, she says. But she adds that models used in the work suggest that energetic oxygen from Earth's ionosphere also poses a danger, even though it is less energetic than solar particles.

"It wouldn't kill someone instantly, but it definitely could increase the radiation exposure for an astronaut on the Moon," Harnett says.

However, she notes that the danger from energetic oxygen could be overstated because the models do not take into account the positive electrical charge on the daylight side of the Moon that likely would significantly slow the oxygen stream.

Provided by the University of Washington

Thursday, December 20, 2007

Nebula NGC 2170

This enigmatic region in the constellation of Monoceros displays a wonderful mix of nebula types. The bluish areas are reflection nebulas, so-named because they reflect the light of nearby stars. The dust particle size in these areas preferentially reflects blue light, similar to cigarette and other kinds of smoke. The red areas are emission nebulas, and shine by a different mechanism. Ultraviolet light from nearby stars excites hydrogren and other gas atoms in the nebula, which then emit light of their own in specific colors. Finally, what looks a bit like black ink spilled across the image constitutes a dark nebula, and is only seen because of the light that it blocks. In other words, the dark nebula is seen in silhouette.

Cosmic Ornament of Gas and Dust

Image credit: NASA/JPL-Caltech /O.Krause/
(Stewart Observatoru)

Astronomers have at last found definitive evidence that the universe's first dust - the celestial stuff that seeded future generations of stars and planets - was forged in the explosions of massive stars.

The findings, made with NASA's Spitzer Space Telescope, are the most significant clue yet in the longstanding mystery of where the dust in our very young universe came from. Scientists had suspected that exploding stars, or supernovae, were the primary source, but nobody had been able to demonstrate that they can create copious amounts of dust - until now. Spitzer's sensitive infrared detectors have found 10,000 Earth masses worth of dust in the blown-out remains of the well-known supernova remnant Cassiopeia A.

"Now we can say unambiguously that dust - and lots of it - was formed in the ejecta of the Cassiopeia A explosion. This finding was possible because Cassiopeia A is in our own galaxy, where it is close enough to study in detail," said Jeonghee Rho of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena. Rho is the lead author of a new report about the discovery appearing in the Jan. 20 issue of the Astrophysical Journal.

Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe. Developing stars need dust to cool down enough to collapse and ignite, while planets and living creatures consist of the powdery substance. In our nearby universe, dust is pumped out by dying stars like our sun. But back when the universe was young, sun-like stars hadn't been around long enough to die and leave dust.

That's where supernovae come in. These violent explosions occur when the most massive stars in the universe die. Because massive stars don't live very long, theorists reasoned that the very first exploding massive stars could be the suppliers of the unaccounted-for dust. These first stars, called Population III, are the only stars that formed without any dust.

Other objects in addition to supernovae might also contribute to the universe's first dust. Spitzer recently found evidence that highly energetic black holes, called quasars, could, together with supernovae, manufacture some dust in their winds ( .

Rho and her colleagues analyzed the Cassopeia A supernova remnant, located about 11,000 light-years away. Though this remnant is not from the early universe, its proximity to us makes it easier to address the question of whether supernovae have the ability to synthesize significant amounts of dust. The astronomers analyzed the infrared light coming from Cassiopeia A using Spitzer's infrared spectrograph, which spreads light apart to reveal the signatures of different elements and molecules. "Because Spitzer is extremely sensitive to dust, we were able to make high-resolution maps of dust in the entire structure," said Rho.

The map reveals the quantity, location and composition of the supernova remnant's dust, which includes proto-silicates, silicon dioxide, iron oxide, pyroxene, carbon, aluminium oxide and other compounds. One of the first things the astronomers noticed was that the dust matches up perfectly with the gas, or ejecta, known to have been expelled in the explosion. This is the smoking gun indicating the dust was freshly made in the ejecta from the stellar blast. "Dust forms a few to several hundred days after these energetic explosions, when the temperature of gas in the ejecta cools down," said Takashi Kozasa, a co-author at the Hokkaido University in Japan.

The team was surprised to find freshly-made dust deeper inside the remnant as well. This cooler dust, mixed in with gas referred to as the unshocked ejecta, had never been seen before.

All the dust around the remnant, both warm and cold, adds up to about three percent of the mass of the sun, or 10,000 Earths. This is just enough to explain where a large fraction, but not all, of the universe's early dust came from. "Perhaps at least some of the unexplained portion is much colder dust, which could be observed with upcoming telescopes, such as Herschel," said Haley Gomez, a co-author at University of Wales, Cardiff. The Herschel Space Observatory, scheduled to launch in 2008, is a European Space Agency mission with significant NASA participation.

Rho also said that more studies of other supernovae from near to far are needed to put this issue to rest. She notes that the rate at which dust is destroyed - a factor in determining how much dust is needed to explain the dusty early universe - is still poorly understood.

The principal investigator of the research program, and a co-author of the paper, is Lawrence Rudnick of the University of Minnesota, Twin Cities. Other co-authors include W.T. Reach of the Spitzer Science Center; J. D. Smith of the Steward Observatory, Tucson, Ariz.; T. Delaney of the Massachusetts Institute of Technology, Cambridge; J.A. Ennis of the University of Minnesota; and A. Tappe of the Spitzer Science Center and the Harvard Smithsonian Center for Astrophysics, Cambridge, Mass.

Monday, December 17, 2007

Lifestyles of the Galaxies Next Door

Credit:NASA/JPL-Caltech/K. Gordon (Space Telescope Science Institute) and SINGS Team
High-Resolution (4200x3600) : JPEG (9.4 MB)

The "lifestyles" of 75 neighboring galaxies are illuminated in this poster from NASA's Spitzer Space Telescope. Scientists say this fresh perspective of our cosmic neighborhood provides valuable insights into growth process of galaxies at a glance.

Over the past four years, Spitzer snapped infrared portraits of some of our most fascinating galactic neighbors as part of the Spitzer Infrared Nearby Galaxy Survey (SINGS) Legacy project. By understanding the mechanisms that fuel and hinder star production in these nearby galaxies, SINGS astronomers hope to solve the mystery of where galaxies come from, and how they've developed throughout the universe's history.

"Once the SINGS observations were done, I began to wonder how to look at all of the galaxies and make sense of the big picture. The SINGS sample of 75 galaxies was just too many to display at once on a computer screen and still be able to appreciate the spatial details present in the images," said Dr. Karl Gordon, of the Space Science Telescope Institute, in Baltimore, Md., who is a member of the SINGS team.

Eventually, Gordon decided to create a poster with the 75 galaxies organized by shape -- using the classification system that astronomer Edwin Hubble created in 1925, soon after the physical nature of galaxies was discovered. The grouping system is called "Hubble's Tuning-Fork" because its overarching shape resembles a musical tuning-fork.

In this structure, elliptical galaxies sit on the left side of the poster, creating the tuning fork's handle. They are designated by the letter "E", and given a number from zero to seven. An "E0" galaxy looks round, while an E7 galaxy is very long and thin.

Spiral galaxies are located to the right side of the poster creating the fork's two prongs. The top prong is made up of regular spiral galaxies, and identified by the letter "S." Barred spiral galaxies make up the bottom prong, and are branded "SB." Meanwhile, letters -- "a", "b", and "c" -- indicate how tightly the spiral arms are wound. An "Sa" galaxy's arms are wound very tightly, while an "Sc" galaxy's spiral arms are very loosely wound.

"Irregular galaxies were not represented in Hubble's original diagram, so we organized them on the bottom-left side of the poster," says Gordon.

In this poster, blue colors reveal light from an older population of stars. Tints of green represent organic molecules called polycyclic aromatic hydrocarbons, while red lumps show clouds of warm dust and gas heated by radiation from newborn stars.

"One of the most striking things about putting these galaxies into the tuning-fork pattern is that you see right away, elliptical galaxies are bluer, which means that they are made up of primarily older stars. The spiral galaxies on the other hand have wisps of green and red, indicating the presence of warm dust and star formation," says Dr. Robert Kennicutt, of the Institute of Astronomy at the University of Cambridge, United Kingdom, and leader of the SINGS team.

According to Kennicutt, astronomers can infer that spiral galaxies on the right-hand side of the tuning fork are younger because most are rich in dust and actively forming stars. Stars form like raindrops in space, when dense cosmic clouds of dust and gas condense, and nuclear fusion is ignited.

He also notes that the elliptical galaxies on the left largely lack dust, indicating that they are not forming many new stars. The rich-blue color of elliptical galaxies also reveals the presence of a primarily older stellar population. In contrast to the spirals, the elliptical galaxies exhausted their gas and dust supplies billions of years ago.

"When I see this poster, I am just so amazed by Spitzer's sensitivity. This infrared view really gives you a sense of the 'lifestyles' of these nearby galaxies," says Kennicutt.

"You get a sense that galaxy classification is not a simple black and white process -- tints of red in galaxies like NGC 3265, show that not all ellipticals are void of dust and star formation, and extremely blue spiral galaxies like NGC 4826, show that some spirals do have a large population of old stars. Like people, galaxies are unique individuals."

The images in this poster are three-color composites where blue depicts the galaxies at a light wavelength of 3.6 microns, while 8.0 microns is green, and 24 microns is red.

Written by Linda Vu, Spitzer Science Center

Spitzer Studies Struggle of Galactic Teenagers

Credit: An artist concept of galaxies colliding
NASA/JPL-Caltech/T. Pyle (SSC)

Billions of years ago, small galaxies across the universe regularly collided -- forcing the gas, dust, stars, and black holes within them to unite. The clashing of galactic gases was so powerful it ignited star formation, while fusing central black holes developed an insatiable appetite for gas and dust.

With stellar nurseries and black holes hungry both for galactic gas, a struggle ensued.

Astronomers have long suspected that these merging structures would eventually grow into some of the most massive galaxies in our universe. Now, NASA's Spitzer Space Telescope has finally identified several of these transitional, or "teenage," galaxies for further study.

"We believe that the most massive galaxies formed through mergers of spiral galaxies like our own Milky Way. Such events were much more common a few billion years after the big bang," says Dr. Anna Sajina, of the Spitzer Science Center in Pasadena, Calif. "This is the epoch we need to look at in order to study the galactic collisions."

Space is like a time machine; the farther away an object is, the further back in time astronomers peer to capture a glimpse of it. Using Spitzer, Sajina looked back to a few billion years after the big bang and spotted galaxies that are nearing the end of the merging process. She notes that these galaxies share a very unique characteristic -- all have massive central black holes that are smothered in dust and producing radio jets.

"What we essentially see in these galaxies is a competition for limited resources. Two processes, star formation and black hole accretion are competing for gas," says Sajina. "At the beginning of the collision, most of the gas will go towards forming stars. Towards the end of the merger, black holes will consume more gas."

According to Sajina, this struggle for resources is relatively short-lived, lasting only ten to 100 million years. Eventually, much of the gas will be pushed out of the galaxy by the powerful winds of newborn stars, stars going supernovae (dying in a cataclysmic explosion), or radio jets shooting out of central supermassive black holes. The removal of gas will stunt the growth of black holes by "starving'' them, and quench star formation.

"The exact process of quenching the extreme star-formation and black hole growth following such merger events is still poorly understood. What we need is to discover sources in the brief transition period after the radio jets have been turned-on, but while the galaxy and its central black holes are still embedded in their dusty cocoon," says Sajina. "The presence of copious amounts of dust in conjunction with strong radio jets in these newly discovered galaxies, makes them prime candidates for being such transition objects."

With Spitzer's supersensitive infrared spectrometer instrument, Sajina's team was able to determine the distance of these galaxies, pinpoint the epoch they live in, and see that they are extremely dusty. After sifting through an astronomical archive called the Sptizer First Look Survey, Sajina also noticed that some of these galaxies also had radio jets.

"The discovery of these transitional objects provides a new avenue for studying the co-evolution [development] of black holes and their host galaxies," she adds.

Sajina's paper was published in the September 2007 issue of Astrophysical Journal. Drs. Lin Yan, Mark Lacy, and Minh Huynh, all of the Spitzer Science Center, were co-authors of the paper.

Two other teams also studied systems like these. A recent study from the Spitzer Space Telescope's Great Observatories Origins Deep Survey (GOODS) also found hundreds of black holes producing X-ray jets, hiding deep inside dusty galaxies billions of light-years away. Their paper was published in the November 10, 2007 issue of Astrophysical Journal.

Results consistent with the GOODS study were also obtained by Fabrizio Fiore of the Osservatorio Astronomico di Roma, Italy, and his team. Their results appear in the Jan. 1, 2008, issue of Astrophysical Journal.

Written by Linda Vu, Spitzer Science Center

"Death Star" Galaxy Black Hole Fires at Neighboring Galaxy

Credit: NASA, ESA, D. Evans (Harvard-Smithsonian Center for Astrophysics),
[X-ray: NASA/CXC/CfA/D.Evans et al.;
UV: NASA/STScI; Radio: NSF/VLA/CfA/D.Evans et al.,

This composite image shows the jet from a black hole at the center of a galaxy striking the edge of another galaxy, the first time such an interaction has been found. In the image, data from several wavelengths have been combined. X-rays from Chandra (colored purple), optical and ultraviolet (UV) data from Hubble (red and orange), and radio emission from the Very Large Array (VLA) and MERLIN (blue) show how the jet from the main galaxy on the lower left is striking its companion galaxy to the upper right. The jet impacts the companion galaxy at its edge and is then disrupted and deflected, much like how a stream of water from a hose will splay out after hitting a wall at an angle.

Each wavelength shows a different aspect of this system, known as 3C321. The Chandra X-ray image provides evidence that each galaxy contains a rapidly growing supermassive black hole at its center. Hubble's optical light images (orange) show the glow from the stars in each galaxy. A bright spot in the VLA and MERLIN radio image shows where the jet has struck the side of the galaxy - about 20,000 light-years from the main galaxy - dissipating some of its energy. An even larger "hotspot" of radio emission detected by VLA (seen in an image with a much larger field-of-view) reveals that the jet terminates much farther away from the galaxy, at a distance of about 850,000 light-years away. The Hubble UV image shows large quantities of warm and hot gas in the vicinity of the galaxies, indicating the supermassive black holes in both galaxies have had a violent past. Faint emission from Chandra, Hubble and Spitzer, not shown in this image, indicate that the galaxies are orbiting in a clockwise direction, implying that the companion galaxy is swinging into the path of the jet.

Since the Chandra data shows that particle acceleration is still occurring in this hotspot, the jet must have struck the companion galaxy relatively recently, less than about a million years ago (i.e. less than the light travel time to the hotspot). This relatively short cosmic time frame makes this event a very rare phenomenon. This "death star galaxy" will produce large amounts of high-energy radiation, which may cause severe damage to the atmospheres of any planets in the companion galaxy that lie in the path of the jet. From the Earth we look down the barrel of jets from supermassive black holes, however these so-called "blazars" are at much safer distances of millions or billions of light-years.

Friday, December 14, 2007

XMM-Newton Unveils Hidden Cosmic Giant

Credits: ESA/ XMM/ EPIC/ SRON (N. Werner et al.)
X-ray image of the area around the cluster Abell 3128 taken with XMM Newton. The bright spot on the left is hot gas in the recently discovered distant cluster, the spot on the right is hot gas in the cluster Abell 3128.

Astronomers working with XMM-Newton have discovered a new cluster of galaxies, hidden behind a previously identified cluster of galaxies. The recently exposed cosmic giant is apparently just as bright as the first group, but is six times further away.

The discovery was made by an international team using ESA’s orbiting X-ray observatory. Being fooled by a cosmic giant is no laughing matter for an astronomer. For years, astronomers racked their brains over the relation between two regions equally bright and large in X-rays, located in the galaxy cluster known as Abell 3128. “That is the charm of science”, says Norbert Werner, PhD student at SRON Netherlands institute for Space Research. “You always find things that you did not expect.”
Galaxy clusters are the largest structures in the universe. They consist of tens to hundreds of massive galaxies, of which each in turn consists of hundreds of billions of stars. Gravity is the binding factor. The hot cluster gas, at temperatures of tens of millions of degrees Celsius, emits X-rays, which renders the cluster visible for space telescopes such as XMM-Newton. Detailed analyses of these X-rays tell astronomers more about the composition of the gas and accordingly, its origin.

Cosmic Web

Credits: Springel et al., Virgo Consortium
This is a model of the cosmic web.
Clusters of galaxies are expected to develop at the intersections of the web.

What was so intriguing about the two X-ray spots in cluster Abell 3128 was the fact that although they had the same size and brightness, the gas clouds seemed to have completely different compositions.

Werner says, “While one spot was clearly caused by a hot gas cloud rich in metals released by supernova explosions in the galaxies, the other spot seemed to contain a much lower amount of metals than any other cluster previously observed. What we observed completely contradicted the current theories about how large structures in the universe arise.”

The observations with XMM-Newton made the surprise complete. The gas cloud behind the puzzling X-ray spot was found to be 4.6 thousand million light years away, at least six times further than Abell 3128. “We were therefore looking at two completely different objects, which from our perspective were in exactly the same line of sight,” said Werner.

Foam bath

Credits: Werner et al.2007
Image of the area in visible light made by the 6.5-metre Magellan Telescope in Chile. Visible in the centre of this image is the light arc around the very massive galaxy in the centre of the newly found distant cluster. The light arc is caused by the gravity field of the galaxy that works as a lens magnifying an object that lies even much farther away, behind the cluster.

“The research into this large cluster of galaxies mainly centres on the question as to how the large structures of the universe have been formed’, explains project leader Jelle Kaastra. According to current belief, material is spread throughout the universe as a web of thread-like structures of rarefied hot gas - the cosmic web. Between these threads are cavities that are becoming increasingly large as the universe expands. “Compare it to bubbles in a bubble bath”, says the astronomer. The density of material is highest at the intersections in the web. Therefore that is where galaxy clusters develop.

Due to their enormous mass and gravitational attraction, the clusters have their own dynamics. Kaastra says, “They attract each other, collide and fly through each other; a whole host of things happen that we can study with X-ray telescopes such as the XMM-Newton.”

Notes for editors:

SRON Netherlands Institute for Space Research built the Reflection Grating Spectrometer (RGS), capable of analysing the X-rays in detail for ESA’s orbiting X-ray observatory, XMM-Newton. The satellite was launched in 1999 from French Guyana and still functions superbly. The operation of the satellite has recently been extended for five more years, until December 2012.

The results from the research of Norbert Werner and Jelle Kaastra were recently published in the scientific journal Astronomy & Astrophysics. The article ‘Complex X-ray morphology of Abell 3128: a distant cluster behind a disturbed cluster’ is by N. Werner, E. Churazov, A. Finoguenov, M. Markevitch, R. Burenin, J. Kaastra, and H. Böhringer.

Thursday, December 13, 2007

Cassini Captures Best View Yet Of Saturn’s Ring Currents

Credit: (All Images)
NASA/Jet Propulsion Laboratory/
Johns Hopkins University Applied Physics Laboratory (NASA/JPL/JHUAPL)

Particle Population in Saturn's Magnetosphere

This is an artist’s concept of the Saturnian plasma sheet based on data from the Cassini Magnetospheric Imaging Instrument. It shows Saturn's embedded “ring current,” an invisible ring of energetic ions trapped in the planet’s magnetic field.

Saturn is at the center, with the red “donut” representing the distribution of dense neutral gas outside Saturn's icy rings. Beyond this region, energetic ions populate the plasma sheet to the dayside magnetopause filling the faintly sketched magnetic flux tubes to higher latitudes and contributing to the ring current. The plasma sheet thins gradually toward the nightside. The view is from above Saturn’s equatorial plane, which is represented by grid lines. The moon Titan’s location is shown for scale. The location of the bow shock is marked, as is the flow of the deflected solar wind in the magnetosheath.

Saturn's ‘Ring Current’

Like Earth, Saturn has an invisible ring of energetic ions trapped in its magnetic field. This feature is known as a “ring current.” This ring current has been imaged with a special, APL-designed camera on Cassini sensitive to energetic neutral atoms.

This is a false color map of the intensity of the energetic neutral atoms emitted from the ring current through a processed called charge exchange. In this process a trapped energetic ion steals and electron from cold gas atoms and becomes neutral and escapes the magnetic field.

The Cassini Magnetospheric Imaging Instrument’s ion and neutral camera records the intensity of the escaping particles, which provides a map of the ring current. In this image, the colors represent the intensity of the neutral emission, which is a reflection of the trapped ions. This “ring” is much farther from Saturn (roughly five times farther) then Saturn’s famous icy rings. Red in the image represents the higher intensity of the particles, while blue is less intense.

Saturn's ring current had not been mapped before on a global scale, only "snippets" or areas were mapped previously but not in this detail. This instrument allows scientists to produce movies that show how this ring changes over time. These movies reveal a dynamic system, which is usually not as uniform as depicted in this image. The ring current is doughnut shaped but in some instances appears as if someone took a bite out of it.

This image was obtained on March 19, 2007, at a latitude of about 54.5 degrees and radial distance of 1.5 million kilometers (920,000 miles). Saturn is at the center, and the dotted circles represent the orbits of the moons Rhea and Titan. The Z axis points parallel to Saturn’s spin axis, the X axis points roughly sunward in the sun–spin axis plane, and the Y axis completes the system, pointing roughly toward dusk. The ion and neutral camera’s field of view is marked by the white line and accounts for the cutoff of the image on the left. The image is an average of the activity over a (roughly) 3-hour period.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Magnetospheric Imaging Instrument was designed, built and is operated by an international team lead by the Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

‘Ring Current’ Rotation

This series of Magnetospheric Imaging Instrument images shows the energetic neutral atom emission from Saturn's ring current. The sun is to lower left (X axis), and the orbits of the moons Titan, Rhea and Dione, and Saturn’s rings, are shown. The pronounced asymmetry (bright emission in the upper quadrant, located between midnight and dawn) rotates with the planet, and the bright spot rotates through 360 degrees over one Saturn rotation (about 10 hours and 40 minutes).

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Magnetospheric Imaging Instrument was designed, built and is operated by an international team lead by the Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

Wednesday, December 12, 2007

Spiral Galaxy M74

Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)
ESA/Hubble Collaboration

Acknowledgment: R. Chandar (University of Toledo) and J. Miller
(University of Michigan

Resembling festive lights on a holiday wreath, this NASA/ESA Hubble Space Telescope image of the nearby spiral galaxy M74 is an iconic reminder of the impending season. Bright knots of glowing gas light up the spiral arms, indicating a rich environment of star formation.

Messier 74, also called NGC 628, is a stunning example of a "grand-design" spiral galaxy that is viewed by Earth observers nearly face-on. Its perfectly symmetrical spiral arms emanate from the central nucleus and are dotted with clusters of young blue stars and glowing pink regions of ionized hydrogen (hydrogen atoms that have lost their electrons). These regions of star formation show an excess of light at ultraviolet wavelengths. Tracing along the spiral arms are winding dust lanes that also begin very near the galaxy's nucleus and follow along the length of the spiral arms.

M74 is located roughly 32 million light-years away in the direction of the constellation Pisces, the Fish. It is the dominant member of a small group of about half a dozen galaxies, the M74 galaxy group. In its entirety, it is estimated that M74 is home to about 100 billion stars, making it slightly smaller than our Milky Way.

The spiral galaxy was first discovered by the French astronomer, Pierre Méchain, in 1780. Weeks later it was added to Charles Messier's famous catalog of deep-sky objects.

This Hubble image of M74 is a composite of Advanced Camera for Surveys' data taken in 2003 and 2005. The filters used to create the color image isolate light from blue, visible, and infrared portions of the spectrum, as well as emission from ionized hydrogen (known as HII regions).

A small segment of this image used data from the Canada-France-Hawaii Telescope and the Gemini Observatory to fill in a region that Hubble did not image.

M51 - A Classic Beauty

NASA/CXC/Wesleyan Univ./R.Kilgard et al &
NASA/JPL-Caltech &
NASA/ESA/S. Beckwith &
Hubble Heritage Team (STScI/AURA) &

NASA/JPL-Caltech/ Univ. of AZ/R. Kennicutt

M51, whose name comes from being the 51st entry in Charles Messier's catalog, is considered to be one of the classic examples of a spiral galaxy. At a distance of about 30 million light years from Earth, it is also one of the brightest spirals in the night sky. A composite image of M51, also known as the Whirlpool Galaxy, shows the majesty of its structure in a dramatic new way through several of NASA's orbiting observatories. X-ray data from NASA's Chandra X-ray Observatory reveals point-like sources (purple) that are black holes and neutron stars in binary star systems. Chandra also detects a diffuse glow of hot gas that permeates the space between the stars. Optical data from the Hubble Space Telescope (green) and infrared emission from the Spitzer Space Telescope (red) both highlight long lanes in the spiral arms that consist of stars and gas laced with dust. A view of M51 with the GALEX telescope shows hot, young stars that produce lots of ultraviolet energy (blue).

The textbook spiral structure is thought be the result of an interaction M51 is experiencing with its close galactic neighbor, NGC 5195, which is seen just above. Some simulations suggest M51's sharp spiral shape was partially caused when NGC 5195 passed through its main disk about 500 million years ago. This gravitational tug of war may also have triggered an increased level of star formation in M51. The companion galaxy's pull would be inducing extra starbirth by compressing gas, jump-starting the process by which stars form.

Tuesday, December 11, 2007

Hubble Finds that Extrasolar Planet Has a Hazy Sunset

Image Credit: NASA, ESA, and G. Bacon (STScI)
This is an artist's concept of HD 189733b and its parent star

A team of astronomers, led by Frederic Pont from the Geneva University Observatory in Switzerland, has detected for the first time strong evidence of hazes in the atmosphere of a planet orbiting a distant star. The new Hubble Space Telescope observations were made as the extrasolar planet, dubbed HD 189733b, passed in front of its parent star in an eclipse. As the light from the star briefly passes through the exoplanet's atmosphere, the gases in the atmosphere stamp their unique spectral fingerprints on the starlight.

Where the scientists had expected to see the fingerprints of sodium and potassium, there were none; implying that high-level hazes (with an altitude of nearly 2,000 miles) are responsible for blocking the light from these elements.

The Universe Nearby

Credit & Copyright: 2MASS, T. H. Jarrett, J. Carpenter, & R. Hurt

What does the universe nearby look like? This plot shows over one and a half million of the brightest stars and galaxies in the nearby universe detected by the Two Micron All Sky Survey (2MASS) in infrared light. The resulting image is an incredible tapestry of stars and galaxies that provides limits on how the universe formed and evolved.

Across the center are stars that lie in the plane of our own Milky Way Galaxy. Away from the Galactic plane, vast majority of the dots are galaxies, color coded to indicate distance, with blue dots representing the nearest galaxies in the 2Mass survey, and red dots indicating the most distant survey galaxies that lie at a redshift near 0.1.

Named structures are annotated. Many galaxies are gravitationally bound together to form clusters, which themselves are loosely bound into superclusters, which in turn are sometimes seen to align over even larger scale structures.

Friday, December 07, 2007

NGC 281 - A Bustling Hub of Star Formation

Credit X-ray: NASA/CXC/CfA/S.Wolk et al; Optical:
NSF/AURA/WIYN/Univ. of Alaska/T.A.Rector

NGC 281 is a bustling hub of star formation about 10,000 light years away. This composite image of optical and X-ray emission includes regions where new stars are forming and older regions containing stars about 3 million years old.

The optical data (seen in red, orange, and yellow) show a small open cluster of stars, large lanes of obscuring gas and dust, and dense knots where stars may still be forming. The X-ray data (purple), based on a Chandra observation lasting more than a day, shows a different view. More than 300 individual X-ray sources are seen, most of them associated with IC 1590, the central cluster. The edge-on aspect of NGC 281 allows scientists to study the effects of powerful X-rays on the gas in the region, the raw material for star formation.

Credit: NASA/CXC/CfA/S.Wolk et al

A second group of X-ray sources is seen on either side of a dense molecular cloud, known as NGC 281 West, a cool cloud of dust grains and gas, much of which is in the form of molecules. The bulk of the sources around the molecular cloud are coincident with emission from polycyclic aromatic hydrocarbons, a family of organic molecules containing carbon and hydrogen. There also appears to be cool diffuse gas associated with IC 1590 that extends toward NGC 281 West. The X-ray spectrum of this region shows that the gas is a few million degrees and contains significant amounts of magnesium, sulfur and silicon. The presence of these elements suggests that supernova recently went off in that area.

Tuesday, December 04, 2007

SOHO - keeping an eye on the Sun for 12 years

The Solar and Heliospheric Observatory (SOHO) celebrated its twelfth launch anniversary on 2 December 2007. The satellite has witnessed the Sun change through almost a complete solar cycle - from quiet to stormy, and back again.

The solar cycle normally lasts about 11 years. In late 1996, shortly after its launch, SOHO was able to observe the last minimum of the 11-year activity cycle. The minimum was followed by a rapid rise in solar activity, peaking 2001 and 2002.

One way of measuring the solar cycle, is to observe sunspots on the Sun. Sunspots are areas of very high magnetic fields on the Sun’s surface, their numbers vary with the cycle. The sunspot cycles measured since the mid-18th century vary in length from 9.0 to 13.5 years.

While a team of experts has attempted to predict when the next solar minimum will be, we won't really know until we get there. In fact, the experts were sharply divided about the time of the next minimum and the intensity of the next maximum, which should arrive at about 2012 or 2013.

Whenever the next cycle begins, SOHO will be there to observe it.

Activity levels have slowly declined since then, but we haven't reached solar minimum yet, despite passing 11.1 years since the last minimum - the average length of a solar cycle.

Credits: SOHO/EIT (ESA & NASA) -All images

This is a composite of several images taken by the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO, taken at a wavelength of 30.4 nanometres, shown in orange. It shows plasma at a temperature of about 60 000 – 80 000 Kelvin. The images were taken over an entire solar cycle and illustrate the changes in solar activity.

This is a composite of several images taken by the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO, taken at a wavelength of 17.1 nanometres, shown in blue. It shows plasma at a temperature of about 1 million Kelvin. The images were shot over an entire solar cycle and illustrate the changes in solar activity.

This is a composite of several images taken by the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO, taken at a wavelength of 19.5 nanometres, shown in green. It shows plasma at a temperature of about 1.5 million Kelvin. The images were taken over an entire solar cycle and illustrate the changes in solar activity.

This is a composite of several images taken by the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO, taken at a wavelength of 28.4 nanometres, shown in yellow. It shows plasma at a temperature of about 2 million Kelvin. The images were taken over an entire solar cycle and illustrate the changes in solar activity.

How White Dwarfs Get Their 'Kicks'

Credit for Hubble Images: NASA, ESA, and H. Richer
(University of British Columbia)

NASA's Hubble Space Telescope is providing strong evidence that white dwarfs, the burned out relics of stars, are given a "kick" when they form.

The sharp vision of Hubble's Advanced Camera for Surveys uncovered the speedy white dwarfs in the ancient globular star cluster NGC 6397, a dense swarm of hundreds of thousands of stars.

Before the stars burned-out as white dwarfs, they were among the most massive stars in NGC 6397. Because massive stars are thought to gather at a globular cluster's core, astronomers assumed that most newly minted white dwarfs dwelled near the center.

Hubble, however, discovered young white dwarfs residing at the edge of NGC 6397, which is about 11.5 billion years old.

"The distribution of young white dwarfs is the exact opposite of what we expected," said astronomer Harvey Richer of the University of British Columbia in Vancouver. "Our idea is that as aging stars evolve into white dwarfs, they are given a kick of 7,000 to 11,000 miles an hour (3 to 5 kilometers a second), which rockets them to the outer reaches of the cluster."

Richer suggested that white dwarfs propel themselves by ejecting mass, like rockets do. Before stars evolve into white dwarfs, they swell up and become red giants. Red giant stars lose about half their mass by shedding it into space. If more of this mass is ejected in one direction, it could propel the emerging white dwarf through space, just as exhaust from a rocket engine thrusts the rocket from the launch pad, Richer proposed.

Observations of some planetary nebulae display similarly directed outflows. (Planetary nebulae are the glowing material ejected by red giant stars.) The jets in those planetary nebulae are shown to flow in opposite directions. If they are not perfectly balanced, Richer reasoned, the stronger jet could accelerate the white dwarf in the opposite direction.

The idea that young white dwarfs are born with a kick was suggested 30 years ago to explain why there were so few of them in open star clusters. In 2003 Michael Fellhauer of the University of California at Santa Cruz and colleagues calculated that if white dwarfs were given a small boost, they could be expelled from open clusters. It is easier, however, for white dwarfs to escape the weak gravitational clutches of open clusters than to rocket out of globular clusters, which are as much as 100 times more massive than open clusters.

Richer and his team, therefore, decided to test the acceleration theory in a globular cluster. The astronomers chose NGC 6397 because, at 8,500 light-years away, it is one of the closest globular star clusters to Earth. About 150 globular clusters exist in the Milky Way, each containing up to a million stars.

The team studied 22 young white dwarfs less than 800 million years old and 62 older white dwarfs between 1.4 and 3.5 billion years old. The astronomers distinguished the younger from the older white dwarfs based on their color and brightness. The younger white dwarfs are hotter and therefore bluer and brighter than the older ones.

Globular clusters sort out stars according to their mass, governed by a gravitational pinball game between stars. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its outskirts. Richer's team found that the older white dwarfs were behaving as expected: They were scattered throughout the cluster according to weight.

The young white dwarfs, however, were found unexpectedly at the edge of the cluster, puzzling Richer and his team.

Their expected neighborhood is near the center because their progenitor stars were the heaviest stars present in the cluster. These fledgling white dwarfs are so young that they have not had enough encounters with other stars to spread them across the cluster, suggesting that some other mechanism (a kick) is at work.

"The first time we plotted up the distribution and found a difference, we thought, 'My goodness, what is happening?'" said team member Saul Davis, a graduate student at the University of British Columbia in Vancouver. "For a long time, we thought we had made a mistake. But no matter what we did, it didn't go away."

The team considered other explanations for the young white dwarfs' location. They could have been part of binary systems and gotten kicked out by their partners. Or perhaps they were given a boost after encountering heavier stars. The team, however, ruled out those explanations through computer simulations.

Richer hopes to study other globular clusters for runaway white dwarfs. The results will appear in the January 2008 issue of the Monthly Notices of Royal Astronomical Society Letters.

Friday, November 30, 2007

An X-Ray Santa Claus in Orion

(The images show the Orion nebula with its hot gas cloud)
Credits: Panel A: XMM-Newton EPIC (Guedel et al.),
Panel B: AAAS/Science
(ESA XMM-Newton and NASA Spitzer data)

Right in time for the festive season, ESA's XMM-Newton X-ray observatory has discovered a huge cloud of high-temperature gas resting in a spectacular nearby star-forming region, shaped somewhat like the silhouette of Santa Claus.

An early present for astronomers, the cloud suggests that hot gas from many star-forming regions leaks into the interstellar medium.

The Orion nebula is the nearest dense star-forming region to Earth that contains stars much more massive than the Sun. XMM-Newton’s newly-discovered gas cloud is composed of winds blowing from these high-mass stars that are heated to millions of degrees as they slam into the surrounding gas.

“There is one star in particular that dominates the nebula,” says Manuel Güdel, Paul Scherrer Institut, Switzerland, who led the team that discovered the gas. The star in question is theta1 Orionis C, a giant star around 40 times mass of the Sun, with a surface temperature of 40,000°C. Güdel and his colleagues think that the violent collision between the wind from this star and the surrounding dense gas is largely responsible for the newly-discovered hot gas cloud.

(This is a Spitzer image of the Orion nebula in the infrared overlaid with XMM-Newton X-ray data in blue)
Credits: AAAS/Science (ESA XMM-Newton and NASA Spitzer data)

The high-temperature gas fills a region of the nebula that appears to be a huge cavity in optical and infrared images. The new observations, taken with XMM-Newton’s European Photon Imaging Camera (EPIC) camera, suggest that astronomers are seeing only a particular portion of the gas. The X-rays from this portion escape absorption by patches of cold gas covering much of the front of the Orion nebula.

The surrounding pattern of absorbing clouds gives the detected gas its Santa Claus shape, with his prominent hat outlined by the northern gas bubble. In its entirety, the hot gas probably fills the whole nebula.

The team discovered it whilst conducting a survey of the young stars in the region. In the background of many of those images was a faint glow of X-rays. “The diffuse signal came up time and time again. Finally, we realized that it was something real,” says Güdel.

(This is a multi-wavelength image of the Orion nebula)
Panel A: AAAS/Science (ESA XMM-Newton and NASA Spitzer data)
B: NASA/ Spitzer,
C: © Anglo-Australian Observatory/David Malin Images,
D: © Anglo-Australian Observatory/David Malin Images, VLA

The presence of the hot gas in a fairly common nebula like Orion is surprising. Although theory has predicted such hot gas clouds, previous observations suggested that a large number of massive stars shedding winds, or supernova explosions are required. These are found in some regions of vigorous high-mass star formation, which are scattered only rarely throughout the galaxy. The new observations show that much smaller collections of high mass stars can produce hot gas as well.
There are many star-forming regions similar to the Orion nebula throughout the galaxy, so there should be a network of channels and bubbles being filled up by the hot gas leaking from these various regions. “This is another possible way to enrich the interstellar medium. You don’t have to wait for a sudden supernova to explode. You can do it with just one or two massive stars over millions of years,” says Güdel.

Credits: © Anglo-Australian Observatory/David Malin Images

The team now plans to obtain new observations to determine how the gas flows out of the Orion nebula. In particular, they want to see whether it connects with a giant bubble created by supernova explosions from previous generations of massive stars.

Thursday, November 29, 2007

Baby Picture of our Solar System

Credit: NASA/JPL-Caltech/UIUC & Caltech/AURA

A rare, infrared view of a developing star and its flaring jets taken by NASA's Spitzer Space Telescope shows us what our own solar system might have looked like billions of years ago. In visible light, this star and its surrounding regions are completely hidden in darkness.

Stars form out of spinning clouds, or envelopes, of gas and dust. As the envelopes flatten and collapse, jets of gas stream outward and a swirling disk of planet-forming material takes shape around the forming star. Eventually, the envelope and jets disappear, leaving a newborn star with a suite of planets. This process takes millions of years.

The Spitzer image shows a developing sun-like star, called L1157, that is only thousands of years old (for comparison, our solar system is around 4.5 billion years old). Why is the young system only visible in infrared light? The answer has to do with the fact that stars are born in the darkest and dustiest corners of space, where little visible light can escape. But the heat, or infrared light, of an object can be detected through the dust.

In Spitzer's infrared view of L1157, the star itself is hidden but its envelope is visible in silhouette as a thick black bar. While Spitzer can peer through this region's dust, it cannot penetrate the envelope itself. Hence, the envelope appears black. The thickest part of the envelope can be seen as the black line crossing the giant jets. This L1157 portrait provides the first clear look at stellar envelope that has begun to flatten.

The color white shows the hottest parts of the jets, with temperatures around 100 degrees Celsius (212 degrees Fahrenheit). Most of the material in the jets, seen in orange, is roughly zero degrees on the Celsius and Fahrenheit scales.

The reddish haze all around the picture is dust. The white dots are other stars, mostly in the background.

L1157 is located 800 light-years away in the constellation Cepheus.

This image was taken by Spitzer's infrared array camera. Infrared light of 8 microns is colored red; 4.5-micron infrared light is green; and 3.6-micron infrared light is blue.

Youthful Star Sprouts Planets Early

An artist concept of UX Tau A
NASA/JPL-Caltech/T. Pyle (SSC)

A stellar prodigy has been spotted about 450 light-years away in a system called UX Tau A by NASA's Spitzer Space Telescope. Astronomers suspect this system's central Sun-like star, which is just one million years old, may already be surrounded by young planets. Scientists hope the finding will provide insight into when planets began to form in our own solar system.

"This result is exciting because we see a gap, potentially carved out by planets, around a dusty Sun-like star. In almost all other star systems of this age, we typically see a primordial disk -- a thick disk of dust, without any clearings," said Catherine Espaillat, a graduate student at the University of Michigan, Ann Arbor.

Prior to the Spitzer observations, Espaillat and her teammates knew that a Sun-like star sat at the center of UX Tau A. Now, using the telescope's infrared spectrometer instrument, they have discerned details about the dusty disk swirling around the central star.

Such dusty disks are where planets are thought to be born. Dust grains clump together like snowballs to form larger rocks, and then the bigger rocks collide to form the cores of planets. When rocks revolve around their central star, they act like cosmic vacuum cleaners, picking up all the gas and dust in their path and creating gaps.

Spitzer saw a gap in UX Tau A's disk that extends from 0.2 to 56 astronomical units (an astronomical unit is the distance between the sun and Earth). In our solar system, this gap would occupy the space between Mercury and Saturn. Espaillat notes that the formation of one or more planets could be responsible for carving out the gap.

Although gaps have been detected in disks swirling around young stars before, Espaillat notes that UX Tau A is special because the gap is sandwiched between two thick disks of dust. An inner thick dusty disk hugs the central star, then, moving outward, there is a gap, followed by another thick doughnut-shaped disk. Other systems with gaps contain very little to no dust near the central star. In other words, those gaps are more like big holes in the centers of disks.

Some scientists suspect that the holes could have been carved out by a process called photoevaporation. Photoevaporation occurs when radiation from the central star heats up the gas and dust around it to the point where it evaporates away. The fact that there is thick disk swirling extremely close to UX Tau A's central star rules out the photoevaporation scenario. If photoevaporation from the star played a role, then large amounts of dust would not be floating so close to the star.

"This finding definitely affects the way astronomers look at planet formation. Spitzer's infrared spectrometer was able to see a gap in this system, but future, more sensitive telescopes maybe able to search for Earth-like planets in UX Tau A," said Espaillat.

Her paper will be published in the December 2007 issue of Astrophysical Journal Letters. Other authors on the paper include Nuria Calvet, Jesus Hernández and Lee Hartmann, also from the University of Michigan; Paola D'Alessio of the Universidad Nacional Autónoma de México, Michoacán; Chunhua Qi of the Harvard-Smithsonian Institute for Astrophysics, Cambridge, Mass.; Elise Furlan of the NASA Astrobiology Institute at the University of California at Los Angeles; and Dan Watson of the University of Rochester, N.Y.

Written by Linda Vu, Spitzer Science Center
November 28, 2007

Wednesday, November 28, 2007

Chandra Discovers Cosmic Cannonball

Credit: Chandra: NASA/CXC/Middlebury College/F.Winkler et al.
ROSAT: NASA/GSFC/S.Snowden et al.
Optical: NOAO/CTIO/Middlebury College/F.Winkler et al.

Astronomers used Chandra to observe a neutron star, known as RX J0822-4300, over a period of about five years. During that span, three Chandra observations clearly show the neutron star moving away from the center of the Puppis A supernova remnant. This remnant is the stellar debris field created during the same explosion in which the neutron star was created about 3700 years ago.

By combining how far it has moved across the sky with its distance from Earth, astronomers determined the neutron star is moving at over 3 million miles per hour. At this rate, RX J0822-4300 is destined to escape from the Milky Way after millions of years, even though it has only traveled about 20 light years so far.

"This star is moving at 3 million miles an hour, but it's so far away that the apparent motion we see in five years is less than the height of the numerals in the date on a penny, seen from the length of a football field," said Frank Winkler of Middlebury College in Vermont. "It's remarkable, and a real testament to the power of Chandra, that such a tiny motion can be measured."

"Just after it was born, this neutron star got a one-way ticket out of the Galaxy," said co-author Robert Petre of NASA's Goddard Space Flight Center in Greenbelt, Md. "Astronomers have seen other stars being flung out of the Milky Way, but few as fast as this."

So-called hypervelocity stars have been previously discovered shooting out of the Milky Way with speeds around one million miles per hour. One key difference between RX J0822-4300 and these other reported galactic escapees is the source of their speed. The hypervelocity stars are thought to have been ejected by interactions with the supermassive black hole in the Galaxy's center.

This neutron star, by contrast, was flung into motion by the supernova that created Puppis A. The data suggest the explosion was lop-sided, kicking the neutron star in one direction and the debris from the explosion in the other.

The supernova was precipitated when the core of a massive star imploded to form a neutron star. Computer simulations show that the infall of the outer layers of the star onto a neutron star releases an enormous amount of energy. As this energy propagates outward, it can reverse the infall and eject the outer layers of the star at speeds of millions of miles per hour. Due to the complexity of the flow, the ejection is not symmetric, leading to a rocket effect that propels the neutron star in the opposite direction.

The breakneck speed of the Puppis A neutron star, plus an apparent lack of pulsations from it, is not easily explained by even the most sophisticated supernova explosion models.

"The problem with discovering this cosmic cannonball is we aren't sure how to make the cannon powerful enough." said Winkler. "The high speed might be explained by an unusually energetic explosion, but the models are complicated and hard to apply to real explosions."

Other recent work on RX J0822-4300 was published by C.Y. Hui and Wolfgang Becker, both from the Max Planck Institute for Extraterrestrial Physics in Munich, in the journal Astronomy and Astrophysics in late 2006. Using two of the three Chandra observations reported in the Winkler paper and a different analysis technique, the Hui group found a speed for RX J0822-4300 that is about two-thirds as fast, but with larger reported margins of error.

The research by Winkler and Petre was published in the November 20 issue of The Astrophysical Journal. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Friday, November 23, 2007

Evaporating Planet

Image Credit: NASA, European Space Agency, Alfred Vidal-Madjar
(Institut d'Astrophysique de Paris, CNRS)

Planet HD 209458b is evaporating. It is so close to its parent star that its heated atmosphere is simply expanding away into space. Some astronomers studying this distant planetary system now believe they have detected water vapor among the gases being liberated.

This controversial claim, if true, would mark the first instance of planetary water beyond our solar system, and indicate anew that life might be sustainable elsewhere in the universe. Although spectroscopic observations from the Hubble Space Telescope are the basis for the water detection claim, the planetary system is too small and faint to image. The image is an artist's concept of the HD 209458b system.

Monday, November 19, 2007

Planets Found Forming in Pleiades Star Cluster

Posted: November 15, 2007

Rocky terrestrial planets, perhaps like Earth, Mars or Venus, appear to be forming or to have recently formed around a star in the Pleiades ("seven sisters") star cluster, the result of "monster collisions" of planets or planetary embryos.

Astronomers using the Gemini Observatory in Hawaii and the Spitzer Space Telescope report their findings in an upcoming issue of the Astrophysical Journal, the premier journal in astronomy.
Color composite image of the Pleiades star cluster produced by Inseok Song of the Spitzer Science Center, using montage software developed by IPAC/California Institute of Technology. An artist's rendering of a collision in the Pleiades (inset), by Lynette R. Cook, for Gemini Observatory.

"This is the first clear evidence for planet formation in the Pleiades, and the results we are presenting may well be the first observational evidence that terrestrial planets like those in our solar system are quite common," said Joseph Rhee, a UCLA postdoctoral scholar in astronomy and lead author of the research.

The Pleiades star cluster, in the constellation Taurus, is well-known in many cultures. It is named for the seven daughters of Atlas and Pleione, who were placed by Zeus among the stars in Greek mythology and is cited in the Bible - "Can you bind the beautiful Pleiades? Can you loose the cords of Orion?" (Job 38:31). The automaker Subaru's name is the Japanese word for the Pleiades, Rhee said.

The Pleiades is probably the best known star cluster and the most striking to the naked eye. "You've seen it many times, and it's now easily visible in the evening sky," said research co-author Benjamin Zuckerman, UCLA professor of physics and astronomy.

Although referred to as the "seven sisters," "the cluster actually contains some 1,400 stars," said co-author Inseok Song, a staff scientist at NASA's Spitzer Science Center at the California Institute of Technology and a former astronomer with the Gemini Observatory.

Located about 400 light-years away, the Pleiades is one of the closest star clusters to Earth. One of the cluster's stars, known as HD 23514, which has a mass and luminosity a bit greater than those of the sun, is surrounded by an extraordinary number of hot dust particles - "hundreds of thousands of times as much dust as around our sun," Zuckerman said. "The dust must be the debris from a monster collision, a cosmic catastrophe."

The astronomers analyzed emissions from countless microscopic dust particles and concluded that the most likely explanation is that the particles are debris from the violent collision of planets or planetary embryos.

Song calls the dust particles the "building blocks of planets," which can accumulate into comets and small asteroid-size bodies and then clump together to form planetary embryos, eventually becoming full-fledged planets.

"In the process of creating rocky, terrestrial planets, some objects collide and grow into planets, while others shatter into dust," Song said. "We are seeing that dust."

HD 23514 is the second star around which Song and Zuckerman recently have found evidence of terrestrial planet formation. They and their colleagues reported in the journal Nature in July 2005 that a sun-like star known as BD +20 307, located 300 light-years from Earth in the constellation Aries, is surrounded by one million times more dust than is orbiting our sun.

In an effort to uncover comparably dusty stars after their 2005 research, Rhee, Song and Zuckerman began looking through thousands of publicly accessible, deep-infrared images obtained by the Spitzer Space Telescope and soon discovered HD 23514. The astronomers then used the Gemini North telescope, located on Hawaii's dormant volcano Mauna Kea, to measure the heat radiation coming from the dust; the heat emerges at infrared wavelengths, just as the heat from our bodies does, Song said.

"The Gemini and Spitzer data were crucial in identifying and establishing the amount and location of dust around the star," Song said.

While our sun is 4.5 billion years old, the Pleiades Aries stars are "adolescents," about 100 million and 400 million years old, respectively, Rhee said. Based on the age of the two stars and the dynamics of the orbiting dust particles, the astronomers deduce that most adolescent sun-like stars are likely to be building terrestrial-like planets through recurring violent collisions of massive objects. The cosmic debris from only a small percentage of such collisions can be seen at any one time - currently, only HD 23514 and BD +20 307 have visible debris.

"Our observations indicate that terrestrial planets similar to those in our solar system are probably quite common," Zuckerman said.

The astronomers calculate that terrestrial planets or planetary embryos in the Pleiades collided within the last few hundred thousand years - or perhaps much more recently - but they cannot rule out the possibility that multiple, somewhat smaller collisions occurred.

Many astronomers believe our moon was formed through the collision of two planetary embryos - the young Earth and a body about the size of Mars. That crash created tremendous debris, some of which condensed to form the moon and some of which went into orbit around the young sun, Zuckerman said.

By contrast, the collision of an asteroid with Earth 65 million years ago, the most favored explanation for the final demise of the dinosaurs, was a mere pipsqueak, he said.

"Collisions between comets or asteroids wouldn't produce anywhere near the amount of dust we are seeing," Song said.

HD 23514 and BD +20 307 are by far the dustiest not-so-young stars in the sky. "Nothing else is even close," Song said.

Very young stars - those 10 million years old or younger - may have a similar amount of dust around them as a result of the star-formation process. However, by the time a star is 100 million years old, this "primordial" dust has dissipated because the dust particles get blown away or dragged onto the star, or the particles clump together to form much larger objects.

"Unusually massive amounts of dust, as seen at the Pleiades and Aries stars, cannot be primordial but rather must be the second-generation debris generated by collisions of large objects," Song said.

The Pleiades have been considered important by many cultures throughout history.

"To the Vikings, the Pleiades was Freyja's hens," Rhee said. In Bronze Age Europe, the Celts and others associated the Pleiades with mourning and funerals because the cluster rose in the eastern night sky between the autumnal equinox and the winter solstice, which was a festival devoted to the remembrance of the dead. The ancient Aztecs of Mexico and Central America based their calendar on the Pleiades.

The astronomers' research results are based on mid- and far- infrared observations made with the Gemini 8-meter Frederick C. Gillett Telescope at Gemini North and the space-based infrared observatories Infrared Astronomical Satellite (IRAS), Infrared Space Observatory (ISO) and NASA's Spitzer Space Telescope.

The Gemini Observatory is an international collaboration utilizing two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawaii (Gemini North); the other is at Cerro Pach�n in central Chile (Gemini South). Together they provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

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