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.

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.

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.

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.

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

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.

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.

Planets Found Forming in Pleiades Star Cluster

UCLA NEWS RELEASE
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.

UCLA is California's largest university, with an enrollment of nearly 37,000 undergraduate and graduate students. The UCLA College of Letters and Science and the university's 11 professional schools feature renowned faculty and offer more than 300 degree programs and majors. UCLA is a national and international leader in the breadth and quality of its academic, research, health care, cultural, continuing education and athletic programs. Four alumni and five faculty have been awarded the Nobel Prize.

Hubble Zooms In on Heart of Mystery Comet

Credit for Hubble Image: NASA, ESA, and H. Weaver

(The Johns Hopkins University Applied Physics Laboratory)

NASA's Hubble Space Telescope has probed the bright core of Comet 17P/Holmes, which, to the delight of sky watchers, mysteriously brightened by nearly a millionfold in a 24-hour period beginning Oct. 23, 2007.

Astronomers used Hubble's powerful resolution to study Comet Holmes' core for clues about how the comet brightened. The orbiting observatory's Wide Field Planetary Camera 2 (WFPC2) monitored the comet for several days, snapping images on Oct. 29, Oct. 31, and Nov. 4. Hubble's crisp "eye" can see objects as small as 33 miles (54 kilometers) across, providing the sharpest view yet of the source of the spectacular brightening.

The Hubble image at right, taken Nov. 4, shows the heart of the comet. The central portion of the image has been specially processed to highlight variations in the dust distribution near the nucleus. About twice as much dust lies along the east-west direction (the horizontal direction) as along the north-south direction (the vertical direction), giving the comet a "bow tie" appearance.

The composite color image at left, taken Nov. 1 by an amateur astronomer, shows the complex structure of the entire coma, consisting of concentric shells of dust and a faint tail emanating from the comet's right side.

The nucleus-the small solid body that is the ultimate source of all the comet's activity- is still swaddled in bright dust, even 12 days after the spectacular outburst. "Most of what Hubble sees is sunlight scattered from microscopic particles," explained Hal Weaver of The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., who led the Hubble investigation. "But we may finally be starting to detect the emergence of the nucleus itself in this final Hubble image."

Hubble first observed Comet 17P/Holmes on June 15, 1999, when there was virtually no dusty shroud around the nucleus. Although Hubble cannot resolve the nucleus, astronomers inferred its size by measuring its brightness. Astronomers deduced that the nucleus' diameter was approximately 2.1 miles (3.4 kilometers), about the length of New York City's Central Park. They hope to use the new Hubble images to determine the size of the comet's nucleus to see how much of it was blasted away during the outburst.

Hubble's two earlier snapshots of Comet Holmes also showed some interesting features. On Oct. 29, the telescope spied three "spurs" of dust emanating from the nucleus, while the Hubble images taken on Oct. 31 revealed an outburst of dust just west of the nucleus.

The Hubble images, however, do not show any large fragments near the nucleus of Comet Holmes, unlike the case of Comet 73P/Schwassmann-Wachmann 3 (SW3). In the spring of 2006 Hubble observations revealed a multitude of "mini-comets" ejected by SW3 after the comet increased dramatically in brightness.

Ground-based images of Comet Holmes show a large, spherically symmetrical cloud of dust that is offset from the nucleus, suggesting that a large fragment broke off and subsequently disintegrated into tiny dust particles after moving away from the main nucleus.

Unfortunately, the huge amount of dust near the comet's nucleus and the comet's relatively large distance from Earth (149 million miles, or 1.6 astronomical units, for Holmes versus 9 million, or 0.1 astronomical unit for SW3), make detecting fragments near Holmes nearly impossible right now, unless the fragments are nearly as large as the nucleus itself.

Watching Galaxies Grow Old Gracefully

In the early 1900s, Edwin Hubble made the startling discovery that our Milky Way galaxy is not alone. It is just one of many galaxies, or "island universes," as Hubble dubbed them, swimming in the sea of space.

Now, a century later, NASA's Galaxy Evolution Explorer is helping piece together the evolution of these cosmic species. Since its launch in 2003, the mission has surveyed tens of thousands of galaxies in ultraviolet light across nine billion years of time. The results provide new, comprehensive evidence for the "nurture" theory of galaxy evolution, which holds that the galaxies first described by Hubble – the elegant spirals and blob-like ellipticals -- are evolutionarily linked.

According to this "nurture" theory, a typical young galaxy begins life as a spiral that is actively churning out stars. Over time, the spiral might merge with another spiral or perhaps an irregular-shaped galaxy, before kicking out a few more bursts of newly minted stars. Eventually, the galaxy slows down its production of stars and settles into later life as an elliptical.

"Our data confirm that all galaxies begin life forming stars," said Chris Martin, the principal investigator for the Galaxy Evolution Explorer at the California Institute of Technology in Pasadena, Calif. "Then through a combination of mergers, fuel exhaustion and perhaps suppression by black holes, the galaxies eventually stop producing stars."

When astronomers talk about galaxies today, they tend to refer to them by their color, either blue or red, instead of by their shape. Most blue galaxies are smaller spirals or irregulars, and most red galaxies are larger ellipticals, though there are some exceptions.

Why color-code the galaxies? Their color indicates how actively they are making new stars. Younger stars shine in ultraviolet or blue light, so galaxies that appear blue are busily producing stars. Older stars emit infrared or red light, so galaxies that look red have shut down their star-making factories. Roughly half of all galaxies are blue and half are red.

Scientists have long postulated that blue galaxies grow up to become red. They proposed that something happens to the blue galaxies to cause them to run out of star-making material, or gas, and mature into the passive red ones. For this "nurture" theory to be true, there should be a population of "teenage" galaxies in the process of transitioning from blue to red, or young to old. But such a cosmic metamorphosis should take billions of years. How can astronomers, with a significantly shorter lifespan, study a process that takes that long?

One solution is to look at lots and lots of galaxies. Imagine a hypothetical alien trying to figure out how and if humans age from only a handful of snapshots showing people of different ages. The aliens might assume that little people grow into big ones, but they could better piece together the life of a typical human if they could look through boxes and boxes of photographs.

The Galaxy Evolution Explorer was designed to provide astronomers with just such a massive portfolio of galaxies. Its troves of data have allowed scientists to find a significant number of teenage galaxies – and thus proof that youthful spiral, or blue, galaxies will eventually grow up to become the elderly elliptical, or red, galaxies.

"The nurture theory of galaxy evolution predicted that there would be galaxies in transition," said Martin. "Finding these galaxies required ultraviolet light, because they really stand out at this wavelength. And because they are rare, we had to look at many. The Galaxy Evolution Explorer allowed us to do this."

Visible-light data from the Sloan Digital Sky Survey also helped to establish the age of the teenage galaxies and the rates at which they are running out of star-making fuel. These findings suggest that some of the young galaxies are ripening into old age quickly, while others are leisurely strolling into their golden years.

Evidence for the "nurture" theory of galaxy evolution can be found in a report in the Astrophysical Journal. Martin is the lead author.

New evidence from NASA's Galaxy Evolution Explorer supports the long-held notion that many galaxies begin life as smaller spirals before transforming into larger, elliptical-shaped galaxies.

Examples of young, teenage and adult galaxies are shown here from left to right. The data making up these photos come from both the Galaxy Evolution Explorer and visible-light telescopes. Long-wavelength ultraviolet light is blue; short-wavelength ultraviolet light is green; and visible red light is red.

The galaxy on the left is NGC 300, a spiral located about seven million light-years away in the constellation Sculptor. Younger galaxies like this one tend to form more stars, and since new stars give off more ultraviolet and blue light, the galaxies appear blue.

The galaxy on the right is NGC 1316, located about 62 million light-years away in the constellation Fornax. It is an older elliptical. Older stars emit more red light, so this galaxy appears red.

The galaxies in the middle of the diagram represent the teenagers, which are on their way from becoming blue to red. The relatively small patches of ultraviolet light in these transitional galaxies indicate that star formation is winding down. The galaxy at center left is NGC 4569, located about four million light-years away in the constellation Virgo. The galaxy at center right is NGC 1291, located about 33 million light-years away in the constellation Eradinus.

Before the Galaxy Evolution Explorer launched more than four years ago, there weren't a lot of examples of transitional galaxies, which made it difficult to demonstrate that galaxies mature from blue to red. The Galaxy Evolution Explorer allowed astronomers to find good examples of these elusive teenagers through its extensive catalogue of tens of thousands of galaxies photographed in ultraviolet light.

NGC 300 image credit: NASA/JPL-Caltech/Las Campanas Observatory
NGC 4569 image credit: NASA/JPL-Caltech/Palomar 1.5m
NGC 1291 image credit: NASA/JPL/CTIO
NGC 1316 image credit: NASA/JPL-CTIO

Cosmic Rays from Galactic Centers

Illustration Credit: Pierre Auger Observatory Team

Where do cosmic rays come from? A major step toward answering this century old question may have just come in from the Auger Observatory project, the world's premier cosmic ray observatory. That high energy fundamental particles are barreling through the universe has been known for about a century.

Because ultra high energy cosmic rays are so rare and because their extrapolated directions are so imprecise, no progenitor objects have ever been unambiguously implied. New results from Auger, however, indicate that 12 of 15 ultra high energy cosmic rays have sky directions statistically consistent with the positions of nearby active galactic nuclei.

These galactic centers are already known to emit great amounts of light and are likely powered by large black holes.

The Auger results also indicate that the highest energy cosmic rays are protons, since the electric charge of higher energy nuclei would force the Milky Way Galaxy's magnetic field to deflect and effectively erase progenitor source direction. Pictured above, an artist illustrates a cosmic ray striking the Earth's atmosphere and creating a shower of secondary particles detectable on the surface. The image of Centaurus A digitally superposed near the top signifies one such active galaxy from which cosmic rays might originate.

A Beautiful Blue Ion Tail - Comet Holmes

Credit & Copyright: Ivan Eder and (inset) Paolo Berardi

A beautiful blue ion tail has become visible in deep telescopic images of Comet Holmes. Pointing generally away from the Sun and also planet Earth, the comet's ion tail is seriously foreshortened by our extreme viewing angle. Still, enthusiastic comet watchers have remarked that on the whole, the compact but tentacled appearance suggests a jellyfish or even a cosmic calamari.

This stunning view of the comet's greenish coma and blue tail was recorded on November 4 in clear skies near Budapest, Hungary.

The colors are caused by molecules in the tenuous gas, like C2 (green) and CO+ (blue), fluorescing in sunlight. In a more recent development, the dramatic inset is a deep image from L'Aquila, Italy on November 8, showing the ion tail disconnecting from the comet.

Spitzer Spies a Stellar Bubble Blower

Credit: NASA/JPL-Caltech/T. Velusamy (Jet Propulsion Laboratory)

In this processed Spitzer Space Telescope image, baby star HH 46/47 can be seen blowing two massive "bubbles." The star is 1,140 light-years away from Earth.

The infant star can be seen as a white spot toward the center of the Spitzer image. The two bubbles are shown as hollow elliptical shells of bluish-green material extending from the star. Wisps of green in the image reveal warm molecular hydrogen gas, while the bluish tints are formed by starlight scattered by surrounding dust.

These bubbles formed when powerful jets of gas, traveling at 200 to 300 kilometers per second, or about 120 to 190 miles per second, smashed into the cosmic cloud of gas and dust that surrounds HH 46/47. The red specks at the end of each bubble show the presence of hot sulfur and iron gas where the star's narrow jets are currently crashing head-on into the cosmic cloud's gas and dust material.

Whenever astronomers observe a star, or snap a stellar portrait, through the lens of any telescope, they know that what they are seeing is slightly blurred. To clear up the blurring in Spitzer images, astronomers at the Jet Propulsion Laboratory developed an image processing technique for Spitzer called Hi-Res deconvolution.

This process reduces blurring and makes the image sharper and cleaner, enabling astronomers to see the emissions around forming stars in greater detail. When scientists applied this image processing technique to the Spitzer image of HH 46/47, they were able to see winds from the star and jets of gas that are carving the celestial bubbles.

This infrared image is a three-color composite, with data at 3.6 microns represented in blue, 4.5 and 5.8 microns shown in green, and 24 microns represented as red.

Comet Holmes Grows a Tail

Credit & Copyright: Vicent Peris and José Luis Lamadrid (astrofoto.es)

Comet Holmes continues to be an impressive sight to the unaided eye. The comet has diminished in brightness only slightly, and now clearly appears to have a larger angular extent than stars and planets. Astrophotographers have also noted a distinctly green appearance to the comet's coma over the past week.

Pictured above over Spain in three digitally combined exposures, Comet 17P/Holmes now clearly sports a tail. The blue ion tail is created by the solar wind impacting ions in the coma of Comet Holmes and pushing them away from the Sun.

Comet Holmes underwent an unexpected and dramatic increase in brightness starting only two weeks ago. The detail visible in Comet Holmes' tail indicates that the explosion of dust and gas that created this dramatic brightness increase is in an ongoing and complex event. Comet Holmes will move only slightly on the sky over during the next month.

Comet Holmes' Coma Expands

Credit & Copyright: Eric Allen Observatoire du Cégep de Trois-Rivières

Go outside tonight and see Comet Holmes. No binoculars or telescopes are needed -- just curiosity and a sky map. Last week, Comet 17P/Holmes underwent an unusual outburst that vaulted it unexpectedly from obscurity into one of the brightest comets in recent years. Sky enthusiasts from the northern hemisphere have been following the comet's progress closely.

Pictured above Quebec, Canada, the coma of Comet Holmes has been noticeably expanding over the past few days. In the above picture, an image of Jupiter has been placed artificially nearby to allow for a comparison of angular sizes. Jupiter has been scaled to the size it would appear at the current location of Comet Holmes. How Comet Holmes will further evolve is unknown, with one possibility being that the expanding gas cloud that started from its recent outburst will slowly disperse and fade.

Comet Holmes in Outburst ( 2007 October 26)

Credit & Copyright: Babak Tafreshi and (inset) Alan Friedman

Comet 17P/Holmes stunned comet watchers across planet Earth earlier this week. On October 24, it increased in brightness over half a million times in a matter of hours. The outburst transformed it from an obscure and faint comet quietly orbiting the Sun with a period of about 7 years to a naked-eye comet rivaling the brighter stars in the constellation Perseus. Recorded on that date, this view from Tehran, Iran highlights the comet's (enhanced and circled) dramatic new visibility in urban skies.

The inset (left) is a telescopic image from a backyard in Buffalo, New York showing the comet's greatly expanded coma, but apparent lack of a tail. Holmes' outburst could be due to a sudden exposure of fresh cometary ice or even the breakup of the comet nucleus. The comet may well remain bright in the coming days.

Interacting Galaxy Pair Arp 87

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

Two galaxies perform an intricate dance in this new Hubble Space Telescope image. The galaxies, containing a vast number of stars, swing past each other in a graceful performance choreographed by gravity.

The pair, known collectively as Arp 87, is one of hundreds of interacting and merging galaxies known in our nearby universe. Arp 87 was originally discovered and cataloged by astronomer Halton Arp in the 1970s. Arp's Atlas of Peculiar Galaxies is a compilation of astronomical photographs using the Palomar 200-inch Hale and the 48-inch Samuel Oschin telescopes.

The resolution in the Hubble image shows exquisite detail and fine structure that was not observable when Arp 87 was first discovered in the 1970's.

The two main players comprising Arp 87 are NGC 3808 on the right (the larger of the two galaxies) and its companion NGC 3808A on the left. NGC 3808 is a nearly face-on spiral galaxy with a bright ring of star formation and several prominent dust arms. Stars, gas, and dust flow from NGC 3808, forming an enveloping arm around its companion. NGC 3808A is a spiral galaxy seen edge-on and is surrounded by a rotating ring that contains stars and interstellar gas clouds. The ring is situated perpendicular to the plane of the host galaxy disk and is called a "polar ring."

As seen in other mergers similar to Arp 87, the corkscrew shape of the tidal material or bridge of shared matter between the two galaxies suggests that some stars and gas drawn from the larger galaxy have been caught in the gravitational pull of the smaller one. The shapes of both galaxies have been distorted by their gravitational interaction with one another.

Interacting galaxies often exhibit high rates of star formation. Many lines of evidence - colors of their starlight, intensity of emission lines from interstellar gas, far-infrared output from heated interstellar dust - support this fact. Some merging galaxies have the highest levels of star formation we can find anywhere in the nearby universe.

A major aspect of this excess star formation could be properly revealed only when Hubble turned its imaging capabilities toward colliding galaxies. Among the observatory's first discoveries was that galaxies with very active star formation contain large numbers of super star clusters - clusters more compact and richer in young stars than astronomers were accustomed to seeing in our galactic neighborhood.

Arp 87 is in the constellation Leo, the Lion, approximately 300 million light-years away from Earth. These observations were taken in February 2007 with the Wide Field Planetary Camera 2. Light from isolated blue, green, red, and infrared ranges was composited together to form this color image.

Hubble Spies Shells of Sparkling Stars Around Quasar

Credit: NASA, ESA, and G. Canalizo (University of California, Riverside)

These sharp images taken with NASA's Hubble Space Telescope reveal at least five shells of stars surrounding a brilliant quasar at the heart of a giant elliptical galaxy. The image at left shows the quasar, known as MC2 1635+119, and its host galaxy [center] against a backdrop of distant galaxies. In the image at top,right, the shells can barely be seen because of the bright light from the central quasar.

The image at bottom, right was enhanced to reveal details of the faint shells. In both right-hand images, the objects below and to the left of the shells are background galaxies. A foreground star resides at top, left. The shells have never been seen before in this galaxy, located about 2 billion light-years away. They are evidence that the giant galaxy clashed with another galaxy in the relatively recent past. The shells are similar to ripples forming in a pond when a stone is tossed in. They sparkle with stars that were swept up from the encounter. The interaction may be providing enough fuel to power the quasar, which dominates the galaxy's center. This observation supports the idea that quasars are born from mergers between galaxies.

The images were taken June 28 and July 4, 2005 with Hubble's Advanced Camera for Surveys.

The observation team consists of Gabriela Canalizo and Nicola Bennert of the University of California, Riverside; Bruno Jungwiert of the University of California, Riverside/Astronomical Institute, Academy of Sciences of the Czech Republic, Prague; Alan Stockton of the University of Hawaii, Honolulu; Francois Schweizer of the Carnegie Observatories, Pasadena; Mark Lacy of the California Institute of Technology, Pasadena; and Chien Peng of the Space Telescope Science Institute, Baltimore.

Rainbow on the Rings

Credit: NASA/JPL/Space Science Institute

The opposition effect, a brightness surge that is visible on Saturn's rings when the sun is directly behind the spacecraft, is captured here as a colorful halo of light moving across Saturn's sunlit rings.

The rainbow of color seen here is actually an artifact and a by-product of the spot's movement and the way the color image was produced. Cassini acquires color images by taking sequential exposures using red, green and blue spectral filters, which are then composited together to form a color view. The bright patch traveled across the rings between exposures taken for this view, creating a series of three colorful spots showing its position at three separate moments.

This view looks toward the sunlit side of the rings from about 9 degrees below the ringplane.

The images in this view were acquired with the Cassini spacecraft wide-angle camera on June 12, 2007, at a distance of approximately 523,000 kilometers (325,000 miles) from Saturn. Image scale is 31 kilometers (19 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 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 and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

Wanted: Galactic Thief Who Steals Gas

NASA/JPL-Caltech/P. Ogle (Spitzer Science Center)

A big galaxy is stealing gas right off the "back" of its smaller companion in this new image from NASA's Spitzer Space Telescope. The stolen gas is hot, but it might eventually cool down to make new stars and planets.

The robber galaxy, called 3C 326 North, and its victim, 3C 326 South, are located about a billion light-years away from Earth in the Serpens constellation. They are both called radio galaxies, because the relativistic jets streaming out of their centers give off a great deal of radio waves. Other dots in the picture are foreground stars and background galaxies.

When astronomers first collected data on the 3C 326 galaxies with Spitzer's infrared spectrometer, they were surprised to find that 3C 326 North is loaded with an enormous amount of hot gas, called molecular hydrogen gas, which is fuel for stars and planets. They then studied this archived picture taken with Spitzer's infrared array camera and noticed a tail of stars connecting 3C 326 North to 3C 326 South. This tail revealed that the galactic pair are gravitationally tangled and might eventually merge – and that 3C 326 North must be hoisting gas from its smaller companion.

How is 3C 326 stealing the gas? The answer is gravity. The larger 3C 326 North, which is about the same mass as our Milky Way galaxy, has more gravity so the gas from 3C 326 South falls toward it in the same way that water rolls down hill on Earth. Even in space, it seems the bullies are bigger!

This image shows infrared light of three wavelengths: 8-micron light is red; 4.5 microns is green; 3.6 microns is blue.

Impact Crater Cut by Ganges Chasma (PSP_005543_1725)

Credit: NASA/JPL/University of Arizona

The parial circular or ringed structure in the middle of this scene is an impact crater, approximately 3.25 kilometers (approx. 2 miles) in diameter.

Since its formation, the crater has had its southern half cut away by the formation of the gorge, called Ganges Chasma. The resulting exposure of rocks along the rim of the cliff allows planetary geologists to study a cut-away, side-view of layered rocks. This view is particularly interesting here because the rock layers of the upper plains are visible in their original form outside of the crater, and in modified form within/beneath the crater, along with structures imparted by the impact.

The floor of the crater may have been filled by lavas or other material that is more resistant to erosion than the surrounding layers, since the floor of the crater sticks out into the chasma.

OBSERVATION TOOLBOX

Acquisition date: 02 October 2007 Local Mars time: 2:15 PM
Latitude (centered): -7.6 ° Longitude (East): 318.6 °
Range to target site: 263.8 km (164.9 miles)
Original image scale range: 26.4 cm/pixel
(with 1 x 1 binning) so objects ~79 cm across are resolved
Map projected scale: 25 cm/pixel and north is up
Map projection: EQUIRECTANGULAR
Emission angle: 2.6 ° Phase angle: 31.3 °
Solar incidence angle: 34 °, with the Sun about 56 ° above the horizon Solar longitude: 323.9 °, Northern Winter
For non-map projected products:
North azimuth: 97 ° Sub-solar azimuth: 350.4 °
For map projected products:

North azimuth: 270° Sub solar azimuth 165.263°

Hubble Finds 'Dorian Gray' Galaxy

Credit: NASA, ESA, and A. Aloisi
(Space Telescope Science Institute and European Space Agency, Baltimore, Md.)

NASA's Hubble Space Telescope quashed the possibility that what was previously believed to be a toddler galaxy in the nearby universe may actually be considered an adult. Called I Zwicky 18, this galaxy has a youthful appearance that resembles galaxies typically found only in the early universe. Hubble has now found faint, older stars within this galaxy, suggesting that the galaxy may have formed at the same time as most other galaxies.

I Zwicky 18 is classified as a dwarf irregular galaxy and is much smaller than our Milky Way Galaxy. The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions are bubbles of gas that have been blown away by stellar winds and supernovae explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a
new generation of hot, young stars.

A companion galaxy lies just above and to the left of I Zwicky 18. The companion may be interacting with I Zwicky 18 by gravitationally tugging on the galaxy. The interaction may have triggered the galaxy's recent star formation that is responsible for the youthful appearance. Besides the bluish-white young stars, white-reddish stars also are visible in both I Zwicky 18 and its companion. These stars may be as old as 10 billion years. The reddish extended objects surrounding I Zwicky 18 and its companion are ancient, fully formed galaxies of different shapes that are much farther away.

Hubble data also allowed astronomers for the first time to identify Cepheid variable stars in I Zwicky 18. These flashing stellar mile-markers were used to determine that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed.

The observations of I Zwicky 18 were taken in 2005 and 2006 with Hubble's Advanced Camera for Surveys. Astronomers made this image by combining observations taken with blue and red filters.

The science team consists of Alessandra Aloisi and Marco Sirianni of the Space Telescope Science Institute and the European Space Agency; Francesca Annibali, Jennifer Mack, and Roeland van der Marel of the Space Telescope Science Institute; Abhijit Saha of the National Optical Astronomy Observatories; and Gisella Clementini, Rodrigo Contreras, Giuliana Fiorentino, Marcella Marconi, Ilaria Musella, and Monica Tosi of the Italian National Astrophysics Institutes in Bologna and Naples.

Enceladus Ice Geysers

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Ice geysers erupt on Enceladus, bright and shiny inner moon of Saturn. Shown in this false-color image, a backlit view of the moon's southern limb, the majestic, icy plumes were discovered by instruments on the Cassini Spacecraft during close encounters with Enceladus in November of 2005.

Eight source locations for these geysers have now been identified along substantial surface fractures in the moon's south polar region. Researchers suspect the geysers arise from near-surface pockets of liquid water with temperatures near 273 kelvins (0 degrees C).

That's hot when compared to the distant moon's surface temperature of 73 kelvins (-200 degrees C). The cryovolcanism is a dramatic sign that tiny, 500km-diameter Enceladus is surprisingly active. Enceladus ice geysers also likely produce Saturn's faint but extended E ring.

Cassini Provides New Views of Titan's Land of Lakes and Seas

Credit: NASA/JPL/USGS

Titan's North Polar Region

This Cassini false-color mosaic shows all synthetic-aperture radar images to date of Titan's north polar region. Approximately 60 percent of Titan's north polar region, above 60 degrees north latitude, is now mapped with radar. About 14 percent of the mapped region is covered by what is interpreted as liquid hydrocarbon lakes.

Features appearing darkest to the radar, which are thought to be liquid, are shown in blue and black, and the radar-bright areas likely to be solid surface are tinted brown. The terrain in the top center of this mosaic is imaged at lower resolution than the remainder of the image.

Most of the many lakes and seas seen so far are contained in this image, including the largest known body of liquid on Titan. These seas are most likely filled with liquid ethane, methane and dissolved nitrogen.

Many bays, islands and presumed tributary networks are associated with the seas. The large feature in the upper right center of this image is at least 100,000 square kilometers (40,000 square miles) in area, greater in extent than Lake Superior (82,000 square kilometers or 32,000 square miles), one of Earth's largest lakes. This Titan feature covers a greater fraction of the surface, at least 0.12 percent, than the Black Sea, Earth's largest terrestrial inland sea, at 0.085 percent. Larger seas may exist, as it is probable that some of these bodies are connected, either in areas unmapped by radar or under the surface (See Exploring the Wetlands of Titan).

Of the 400 observed lakes and seas, 70 percent of their area is taken up by large "seas" greater than 26,000 square kilometers (10,000 square miles).

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 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. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries.

Radar Sees Lakes in Titan's Southern Hemisphere

Cassini's radar instrument finds lakes in the southern hemisphere of Titan during the most recent Titan flyby. This is the first confirmation of lakes in the southern hemisphere with the radar instrument. Hundreds of lakes have already been discovered and imaged by radar at Titan's north pole. This finding is important to scientists who are trying to understand how Titan's environment works.

Cassini completed its 37th flyby of Saturn's moon Titan on Oct. 2, 2007, allowing the Cassini Titan Radar Mapper to obtain this southernmost image to date.

Shown here is a portion of the image swath and an inset with details of a small portion in false color. Titan's south pole is at the bottom center. The nature and similarities between the northern and southern near-polar regions supports the idea that much of Titan's poles are climate-driven.

A few small dark patches - liquid-hydrocarbon-filled lakes - stand out, at about 70 degrees south, and are highlighted in the insert (lakes are colored blue). Other features in the scene include broad, steep-sided depressions adjoined to sinuous depressions, interpreted to be empty topographic basins or drained lakes fed by channels, and complex mottled terrain, akin to those at similar northern latitudes. Similarities in features between northern and southern hemispheres imply that the climatic conditions are also similar.

The image shown here is a 1.4-kilometer (0.87-mile) resolution, 2,250-kilometer (1,400 mile) subsection of a 4,500-kilometer (2,800-mile) long swath, which is 150 kilometers (93 miles) wide at the narrowest point. The insert is 90 by 90 kilometers (56 by 56 miles), centered at 70.5 degrees south and 113.9 degrees west. Future southern flybys will image closer to the pole and are expected to show more lakes.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 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. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries.