Circumstellar Dust Takes Flight in 'The Moth'

Credit: NASA, D. Hines (Space Science Institute, Corrales, New Mexico),
and G. Schneider (University of Arizona)

What superficially resembles a giant moth floating in space is giving astronomers new insight into the formation and evolution of planetary systems.

This is not your typical flying insect. It has a wingspan of about 22 billion miles. The wing- like structure is actually a dust disk encircling the nearby, young star HD 61005, dubbed "The Moth." Its shape is produced by starlight scattering off dust.

Dust disks around roughly 100-million-year-old stars like HD 61005 are typically flat, pancake-shaped structures where planets can form. But images taken with NASA's Hubble Space Telescope of "The Moth" are showing that some disks sport surprising shapes.

"It is completely unexpected to find a dust disk with this unusual shape," said senior research scientist Dean Hines of the Space Science Institute in Corrales, New Mexico, and a member of the Hubble team that discovered the disk. "We think HD 61005 is plowing through a local patch of higher-density gas in the interstellar medium, causing material within HD 61005's disk to be swept behind the star. What effect this might have on the disk, and any planets forming within it, is unknown."

Hines called this possible collision "unusual, because we don't expect very much interstellar material to be in the solar neighborhood. That's because the area through which our Sun is moving was evacuated within the past few million years by at least one supernova, the explosion of a massive star. Yet, here's evidence of dense material that's very close, only 100 light-years away."

Astronomers have found evidence that the environment in which a star forms influences its prospects for planet formation. Hubble has actually seen that young planet-forming disks can be affected directly by their environment. The harsh stellar radiation from the Trapezium stars in the Orion Nebula has altered some disks. It is unclear, however, what effect passage through a cloud similar to the one in which HD 61005 finds itself would have on planet formation. Researchers have speculated that passage through dense regions of the interstellar medium could impact the atmospheres of evolving planets.

The Moth is part of a survey of Sun-like stars that Hines and collaborators observed with Hubble's Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) and NASA's Spitzer Space Telescope to study the formation and evolution of planetary systems. Under the lead of Michael Meyer of the University of Arizona in Tucson, the team initially used Spitzer to look for heat radiation—the tell-tale sign of dust warmed by the star—to identify interesting star systems.

Hines then teamed with Glenn Schneider of the University of Arizona to use Hubble's high- contrast imaging capability of the NICMOS coronagraph to image these disks and reveal where the dust detected by Spitzer resides. The NICMOS coronagraph blocked out the starlight so that astronomers could see details in the surrounding disk.

"These symbiotic capabilities, uniquely implemented in NASA's Great Observatories, provide astronomers with the powerful observational tools to study the circumstellar environments of potentially planet-forming systems," Schneider said.

Added Meyer: "Combining observations from these two spacecraft gives us information about the composition of the dust grains, whether they're icy or sandy, or whether they're like the sooty smoke particles rising from a chimney. The composition and sizes of the dust can tell us a lot about the dynamics and evolution of a solar system. In our solar system, for example, astronomers have evidence of rocks smashing into each other and generating dust, as in the asteroid and Kuiper belts. We're seeing these same processes unfold in other planetary systems."

Hines and his collaborators will report their finding on Jan. 10 at the 211th meeting of the American Astronomical Society in Austin, Texas. The result also appeared in the December 20 issue of the Astrophysical Journal Letters.

Hubble Maps Dark Matter Web in a Large Galaxy Cluster

Credit for the Hubble Images: NASA, ESA, C. Heymans
(University of British Columbia, Vancouver),

M. Gray (University of Nottingham, U.K.), M. Barden (Innsbruck),
and the STAGES collaboration

These images reveal the distribution of dark matter in the supercluster Abell 901/902, composed of hundreds of galaxies.

The image in the center shows the entire supercluster. Astronomers assembled this photo by combining a visible-light image of the supercluster taken with the MPG/ESO 2.2-meter telescope in La Silla, Chile, with a dark matter map derived from observations with NASA's Hubble Space Telescope.

The magenta-tinted clumps represent a map of the dark matter in the cluster. Dark matter is an invisible form of matter that accounts for most of the universe's mass. The image shows that the supercluster galaxies lie within the clumps of dark matter.

Hubble cannot see the dark matter directly. Astronomers inferred its location by analyzing the effect of so-called weak gravitational lensing, where light from more than 60,000 galaxies behind Abell 901/902 is distorted by intervening matter within the cluster. Researchers used the observed, subtle distortion of the galaxies' shapes to reconstruct the dark matter distribution in the supercluster. The image was assembled by combining a visible-light image of the supercluster with a map of the dark matter distribution.

The Hubble study pinpointed four main areas in the supercluster where dark matter has pooled into dense clumps. These areas match the location of hundreds of galaxies that have experienced a violent history in their passage from the outskirts of the supercluster into these dense regions. The four close-up images flanking the central photo are Hubble views of the four dense clumps of matter. To make this image, astronomers superimposed the dark matter map over a Hubble visible-light image of the supercluster galaxies.

The images are part of the Space Telescope Abell 901/902 Galaxy Evolution Survey (STAGES), which covers one of the largest patches of sky ever observed by the Hubble telescope. The area surveyed is so wide that it took 80 Hubble images to cover the entire field.

Hubble's Advanced Camera for Surveys made the observations in June and July 2005 and in January 2006.

Source: HubbleSite

Hubble Finds Double Einstein Ring

Credit: NASA, ESA, and R. Gavazzi and T. Treu (University of California, Santa Barbara),
and the SLACS team

NASA's Hubble Space Telescope has revealed a never-before-seen optical alignment in space: a pair of glowing rings, one nestled inside the other like a bull's-eye pattern. The double-ring pattern is caused by the complex bending of light from two distant galaxies strung directly behind a foreground massive galaxy, like three beads on a string.

More than just a novelty, this very rare phenomenon can offer insight into dark matter, dark energy, the nature of distant galaxies, and even the curvature of the universe.

The ring was found by an international team of astronomers led by Raphael Gavazzi and Tommaso Treu of the University of California, Santa Barbara. The discovery is part of the ongoing Sloan Lens Advanced Camera for Surveys (SLACS) program. The team is reporting its results at the 211th meeting of the American Astronomical Society in Austin, Texas. A paper has been submitted to The Astrophysical Journal.

The phenomenon, called gravitational lensing, occurs when a massive galaxy in the foreground bends the light rays from a distant galaxy behind it, in much the same way as a magnifying glass would. When both galaxies are exactly lined up, the light forms a circle, called an "Einstein ring," around the foreground galaxy. If another background galaxy lies precisely on the same sightline, a second, larger ring will appear.

Because the odds of seeing such a special alignment are estimated to be 1 in 10,000, Tommaso says that they "hit the jackpot." The odds of seeing this phenomenon are less than winning two consecutive bets on a single number at Roulette.

"Such stunning cosmic coincidences reveal so much about nature. Dark matter is not hidden to lensing," added Leonidas Moustakas of the Jet Propulsion Laboratory in Pasadena, Calif. "The elegance of this lens is trumped only by the secrets of nature that it reveals."

The massive foreground galaxy is almost perfectly aligned in the sky with two background galaxies at different distances. The foreground galaxy is 3 billion light-years away. The inner ring and outer ring are comprised of multiple images of two galaxies at a distance of 6 billion and approximately 11 billion light-years.

SLACS team member Adam Bolton of the University of Hawaii's Institute for Astronomy in Honolulu first identified the lens in the Sloan Digital Sky Survey (SDSS). "The original signature that led us to this discovery was a mere 500 photons (particles of light) hidden among 500,000 other photons in the SDSS spectrum of the foreground galaxy," commented Bolton.

"The twin rings were clearly visible in the Hubble image, added Tommaso. "When I first saw it I said 'wow, this is insane!' I could not believe it!"

The distribution of dark matter in the foreground galaxies that is warping space to create the gravitational lens can be precisely mapped. Tommaso finds that the fall-off in density of the dark matter is similar to what is seen in spiral galaxies (as measured by the speed of a galaxy's rotation, which yields a value for the amount of dark matter pulling on it), though he emphasizes there is no physical reason to explain this relationship.

In addition, the geometry of the two Einstein rings allowed the team to measure the mass of the middle galaxy precisely to be a value of 1 billion solar masses. The team reports that this is the first measurement of the mass of a dwarf galaxy at cosmological distance (redshift of z=0.6).

A sample of several dozen double rings such as this one would offer a purely independent measure. The comparative radius of the rings could also be used to provide an independent measure of the curvature of space by gravity. This would help in determining the matter content of the universe and the properties of dark energy.

Observations of the cosmic microwave background (a relic from the Big Bang) favor flat geometry. A sample of 50 suitable double Einstein rings would be sufficient to measure the dark matter content of the universe and the equation of state of the dark energy (a measure of its pressure) to 10 percent precision. Other double Einstein rings could be found with wide-field space telescope sky surveys that are being proposed for the Joint Dark Energy Mission (JDEM) and recently recommended by the National Research Council.

Spiral Galaxy Winds Backwards

Credit: NASA and The Hubble Heritage Team (STScI/AURA)

New discoveries in a strange spiral galaxy show it has a pair of arms winding backward compared to the typical direction for most galaxies.

"While the existence of a galaxy with a pair of 'backward' arms may seem like an inconvenient truth to many, our latest analysis indicates it is, nonetheless, a reality," said Gene Byrd, a University of Alabama astronomer.

Most spiral arms observed so far tend to trail in the wake of their galaxy's spin, meaning they wind in the direction opposite the rotation. The strange galaxy, known as NGC4622, lies 200 million light years away and has a large outer arm pair that winds clockwise.

Byrd and his colleagues analyzed a 2001 Hubble Space Telescope image of the galaxy and found a previously hidden inner pair of arms winding counter-clockwise. Whichever way the galaxy happens to rotate, one pair of arms ends up turning in the unusual direction.

"Contrary to conventional wisdom, with both an inner counter-clockwise pair and an outer clockwise pair of spiral arms, NGC4622 must have a pair of leading arms," Byrd said. "With two pairs of arms winding in opposite directions, one pair must lead and one pair must trail."

The team also found a single outer clockwise arm and a single inner counterclockwise arm, which again points to the galaxy's strange characteristic.

Byrd and his colleagues first published the idea about the backward arms in 2002, but met with skepticism from astronomers who thought that the galaxy's slight tilt and clumpy dust clouds could be misleading.

This time the team used a new Fourier component method that takes advantage of the tilt to analyze the galaxy and ignores the effects of dust.

The more complicated analysis of the image revealed that the strong outer clockwise pair of arms winds in the same direction as the galaxy's spin, making it the unusual leading arm pair. The full results appear in the January issue of Astronomical Journal.

Questions still remain about what led to the galaxy's strange behavior. The Hubble image revealed a dark dust lane in the galaxy center, suggesting NGC4622 may have devoured a smaller galaxy.

Credit: NASA and The Hubble Heritage Team (STScI/AURA)

By Jeremy Hsu
Staff Writer

Space.com

Centaurus A - Jet Power and Black Hole Assortment Revealed in New Chandra Image

Credit: NASA/CXC/CfA/R.Kraft et al

A dramatic new Chandra image of the nearby galaxy Centaurus A provides one of the best views to date of the effects of an active supermassive black hole. Opposing jets of high-energy particles can be seen extending to the outer reaches of the galaxy, and numerous smaller black holes in binary star systems are also visible.

Multi-panel Images of Cen A Multi-panel Images of Centaurus A

Credit: NASA/CXC/CfA/R.Kraft et al;Radio: NSF/VLA/Univ.Hertfordshire/M.Hardcastle;
Optical: ESO/VLT/ISAAC/M.Rejkuba et al.

The image was made from an ultra-deep look at the galaxy Centaurus A, equivalent to more than seven days of continuous observations. Centaurus A is the nearest galaxy to Earth that contains a supermassive black hole actively powering a jet.

A prominent X-ray jet extending for 13,000 light years points to the upper left in the image, with a shorter "counterjet" aimed in the opposite direction. Astronomers think that such jets are important vehicles for transporting energy from the black hole to the much larger dimensions of a galaxy, and affecting the rate at which stars form there.

High-energy electrons spiraling around magnetic field lines produce the X-ray emission from the jet and counterjet. This emission quickly saps the energy from the electrons, so they must be continually reaccelerated or the X-rays will fade out. Knot-like features in the jets detected in the Chandra image show where the acceleration of particles to high energies is currently occurring, and provides important clues to understanding the process that accelerates the electrons to near-light speeds.

The inner part of the X-ray jet close to the black hole is dominated by these knots of X-ray emission, which probably come from shock waves -- akin to sonic booms -- caused by the jet. Farther from the black hole there is more diffuse X-ray emission in the jet. The cause of particle acceleration in this part of the jet is unknown.

Hundreds of point-like sources are also seen in the Chandra image. Many of these are X-ray binaries that contain a stellar-mass black hole and a companion star in orbit around one another. Determining the population and properties of these black holes should help scientists better understand the evolution of massive stars and the formation of black holes.

Credit: NASA/CXC/CfA/R.Kraft et al

Another surprise was the detection of two particularly bright X-ray binaries. These sources may contain stellar mass black holes that are unusually massive, and this Chandra observation might have caught them gobbling up material at a high rate.

In this image, low-energy X-rays are colored red, intermediate-energy X-rays are green, and the highest-energy X-rays detected by Chandra are blue. The dark green and blue bands running almost perpendicular to the jet are dust lanes that absorb X-rays. This dust lane was created when Centaurus A merged with another galaxy perhaps 100 million years ago.

Hubble Finds that "Blue Blobs" in Space Are Orphaned Clusters of Stars

Credit: NASA, ESA, and D. de Mello (Catholic University of America/GSFC)

Finding blue blobs in space sounds like an encounter with an alien out of a science fiction movie. But the Hubble Space Telescope's powerful vision has resolved strange objects nicknamed "blobs" and found them to be brilliant blue clusters of stars born in the swirls and eddies of a galactic smashup 200 million years ago.

The findings are being reported by Duilia de Mello of the Catholic University of America, Washington, D.C. and NASA's Goddard Space Flight Center, Greenbelt, Md. and her colleagues at the 211th meeting of the American Astronomical Society in Austin, Texas.

Such "blue blobs"-weighing tens of thousands of solar masses-have never been seen in detail before in such sparse locations, say researchers. They are more massive than most open clusters found inside galaxies but a fraction of the mass of globular star clusters that orbit a galaxy.

Because the orphan stars don't belong to any particular galaxy, the heavier elements produced in their fusion furnaces may easily be expelled back into intergalactic space. This may offer clues as to how the early universe was "polluted" with heavier elements early in its history, say researchers.

The mystery is that the "blue blobs" are found along a wispy bridge of gas strung among three colliding galaxies, M81, M82, and NGC 3077, residing approximately 12 million light-years from Earth. This is not the place astronomers expect to find star clusters: in the "abyssal plain" of intergalactic space. "We could not believe it, the stars were in the middle of nowhere," says de Mello.

The "blue blobs" are clumped together in a structure called Arp's Loop, along the tenuous gas bridge. The gas filaments were considered too thin to accumulate enough material to actually build these many stars, says de Mello. But Hubble reveals the "blue blobs" contain the equivalent of five Orion Nebulae.

After finding that these "blobs" were resolved into stars, the team used the Hubble image to measure an age for the clusters of less than 200 million years with many stars as young and even younger than 10 million years. Not coincidentally, 200 million years is the estimated age of the galactic collision that created the tidal gas streamers, pulled between the galaxies like taffy.

De Mello and her team propose that the star clusters in this diffuse structure might have formed from gas collisions and subsequent turbulence, which enhanced locally the density of the gas streams. Galaxy collisions were much more frequent in the early universe, so "blue blobs" should have been common. After the stars burned out or exploded, the heavier elements forged in their nuclear furnaces would have been ejected to enrich intergalactic space.

Radio observations with the Very Large Array of radio telescopes in Socorro, New Mexico, gave a detailed map of the intergalactic bridge that revealed knots of denser gas. Studies with the 3.5-meter WIYN telescope on Kitt Peak in Arizona mapped the optical light glow of hydrogen along the bridge. Observations with NASA's Galaxy Evolution Explorer (GALEX) ultraviolet space telescope revealed an ultraviolet glow at the knots, and that earned them the nickname "blue blobs." But GALEX did not have the resolution to see individual stars or clusters. Only Hubble's Advanced Camera for Surveys at last revealed the point sources of the ultraviolet radiation.

New Risk to Earth Found in Supernova Explosions

Eta Carinae is drawing closer to its ultimate explosive demise. When Eta Carinae explodes, it will be a spectacular fireworks display seen from Earth, perhaps rivaling the moon in brilliance. Its fate has been foreshadowed by the recent discovery of SN2006gy, a supernova in a nearby galaxy that was the brightest stellar explosion ever seen. This composite image shows optical light (blue) and X-ray light (orange and yellow).

Credit: NASA/CXC/GSFC/STScI

An explosive star within our galaxy is showing signs of an impending eruption, at least in a cosmic time frame, and has for quite some time. From 1838 to 1858, the star called Eta Carinae brightened to rival the light of Sirius, the brightest star in the sky, and then faded to a dim star. Since 1940 it has been brightening again, and scientists think Eta Carinae will detonate in 10,000 to 20,000 years.

Fortunately, Eta Carinae is far away, at least 7,500 light-years from Earth. If it explodes, most of its energy will be scattered or absorbed in the vast emptiness of space. It also happens to be tilted about 45 degrees from the line of sight to Earth, so any type of gamma-ray burst, a high-energy outburst expected with this star's eventual eruption, would miss the Earth. Cosmic rays would be diffused by magnetic fields, and most of the damaging light would not affect life on Earth.

In general, threats to life on Earth from supernovae are extremely small, for all except the nearest explosions — those 30 light-years away or closer.

But what if a supernova were 100 times brighter than usual? Would there be any risk to life on Earth then?

Astronomers found such a record-breaking supernova last year, SN 2006gy.

Brightest ever

SN 2006gy was the brightest supernova ever recorded until an even brighter one was discovered in November.

Astronomers now know the progenitor of SN 2006gy was remarkably similar to Eta Carinae. They warn a superluminous supernova might explode right in our own galaxy.

Brian Thomas at Washburn University has been studying the effects of astronomical explosions at the Goddard Space Flight Center. He decided to investigate what would happen to Earth's protective ozone layer if Eta Carinae explodes with the brilliance of SN 2006gy.

It turns out that even though SN 2006gy was one of the brightest supernovae ever recorded, it did not generate a large amount of X-rays. Thomas and his team found most of the light, including damaging X-rays and cosmic rays, would scatter into space before ever reaching Earth.

So would there be any damage to Earth from such a spectacular event? Though Thomas found X-rays and cosmic rays would cause little damage, he also looked at optical light, particularly short-wavelength blue light (400 nanometers), where the spectrum of SN 2006gy peaked. No one had ever considered the effects of this light before, either from supernovae or any other type of event.

"The visible light could be significant," Thomas says, "But this depends a little bit on your definition of significant."

Brighter than Venus

If Eta Carina were to explode like SN 2006gy, it would quickly become the brightest object in the sky other than the sun and the moon.

For those living where Eta Carina is always above the horizon (Antarctica, New Zealand and extreme southern regions of Australia and South America) the light would vastly outshine Venus, visible even during the day. The radiation would illuminate the evening sky with a bluish glow nearly strong enough to read by, and the effect would likely last for months — perhaps six or more.

The cumulative effects of long-duration exposure to blue-enhanced light would begin to interfere with life on Earth.

Those who study chronobiology, or the effects of biological timing, have found that low levels of blue light can strongly affect the endocrine systems of mammals by causing physiological and alerting responses. Blue-enhanced light is associated with reduced levels of melatonin production and affects circadian rhythms. For these reasons, it is sometimes prescribed to counteract seasonal affective disorder (SAD) or winter depression.

"This is not going to be an 'everything dies immediately' kind of event," Thomas said. "But with the risk factors associated with higher levels of this kind of light it's certainly something that could be important in the longer run."

New threshold

In a paper about to be published in the journal Astrobiology, Thomas explains that even short exposures to blue light can increase insomnia, reduce resistance to infection and is being studied as a possible risk of cancer.

Yet, in the case of Eta Carinae, the effect of these optical photons would be minimal. The scattering of photons by dust and gas is greatest at blue wavelengths (thus giving Earth its blue sky) and the sheer distance of Eta Carinae diminishes the optical intensity by about 20 percent.

But while damage from optical light is not a factor for Eta Carinae, the effects of this light should be considered to any risk assessment of supernovae. Based on his results, Thomas now estimates the biological threshold for supernovae to be about 100 light-years away. At that distance, life on Earth can expect some sort of supernova radiation about once every 20 million years.

By Laura Kinoshita
Special to LiveScience
http://www.space.com

White dwarf pulses like a pulsar

Provided by NASA's Goddard Space Flight Center

New observations from Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and NASA X-ray observatory, have challenged scientists' conventional understanding of white dwarfs. Observers had believed white dwarfs were inert stellar corpses that slowly cool and fade away, but the new data tell a completely different story.

At least one white dwarf, known as AE Aquarii, emits pulses of high-energy (hard) X-rays as it whirls around on its axis. "We're seeing behavior like the pulsar in the Crab Nebula, but we're seeing it in a white dwarf," says Koji Mukai of NASA Goddard Space Flight Center in Greenbelt, Maryland. The Crab Nebula is the shattered remnant of a massive star that ended its life in a supernova explosion. "This is the first time such pulsar-like behavior has ever been observed in a white dwarf."

White dwarfs and pulsars represent distinct classes of compact objects that are born in the wake of stellar death. A white dwarf forms when a star similar in mass to the Sun runs out of nuclear fuel. As the outer layers puff off into space, the core gravitationally contracts into a sphere about the size of Earth, but with roughly the mass of the Sun. The white dwarf starts off scorching hot from the star's residual heat. But with nothing to sustain nuclear reactions, it slowly cools over billions of years, eventually fading to near invisibility as a black dwarf.

A pulsar is a type of neutron star, a collapsed core of an extremely massive star that exploded in a supernova. Whereas white dwarfs have incredibly high densities by earthly standards, neutron stars are even denser, cramming roughly 1.3 solar masses into a city-sized sphere. Pulsars give off radio and X-ray pulsations in lighthouse-like beams.
An artist depicts the Suzaku X-ray observatory in Earth's orbit. JAXA [View Larger Image]
The discovery team, led by Yukikatsu Terada of the Institute of Physical and Chemical Research (RIKEN) in Wako, Japan, was not expecting to find a white dwarf mimicking a pulsar. Instead, the astronomers were hoping to find out if white dwarfs could accelerate charged subatomic particles to near-light speed, meaning they could be responsible for many of the cosmic rays that zip through our galaxy and occasionally strike Earth.

Some white dwarfs, including AE Aquarii, spin very rapidly and have magnetic fields millions of times stronger than Earth's. These characteristics give them the energy to generate cosmic rays.

To find out if this is happening, Terada and his colleagues targeted AE Aquarii with Suzaku in October 2005 and October 2006. The white dwarf resides in a binary system with a normal companion star. Gas from the star spirals toward the white dwarf and heats up, giving off a glow of low-energy (soft) X-rays. But Suzaku also detected sharp pulses of hard X-rays. After analyzing the data, the team realized that the hard X-ray pulses match the white dwarf's spin period of once every 33 seconds.

The hard X-ray pulsations are very similar to those of the pulsar in the center of the Crab Nebula. In both objects, the pulses appear to be radiated like a lighthouse beam, and a rotating magnetic field is thought to be controlling the beam. Astronomers think that the extremely powerful magnetic fields are trapping charged particles and then flinging them outward at near-light speed. When the particles interact with the magnetic field, they radiate X-rays.

"AE Aquarii seems to be a white dwarf equivalent of a pulsar," says Terada. "Since pulsars are known to be sources of cosmic rays, this means that white dwarfs should be quiet but numerous particle accelerators, contributing many of the low-energy cosmic rays in our galaxy."

Launched in 2005, Suzaku is the fifth in a series of Japanese satellites devoted to studying celestial X-ray sources. Managed by JAXA, this mission is a collaborative effort between Japanese universities and institutions and Goddard.

www.astronony.com

Old Comets for a New Year

Giampaolo Salvato photographed Comet Tuttle on Dec. 30, 2007 as it appeared near the spiral galaxy M33. The image was taken from northern Italy with a backyard telescope and a digital camera.

Credit: Giampaolo Salvato (astrosurf.com/eyesinthesky)

As we kick off the year 2008, Comet Tuttle is putting on a nice show for backyard skywatchers. It had not been seen since 1994, but you'll have an excellent opportunity to pick it up with binoculars or small telescopes during the next two weeks.

Tuttle can even be glimpsed by sharp-eyed observers under pristine skies without any optical aids, for it is one of the brightest of the short-period comets, those that orbit the sun often enough to be seen again and again from Earth and identified as such.

And speaking of short-period comets, Comet Holmes continues to delight observers more than two months after its stupendous explosion to naked-eye visibility.

Discovery

As we all know, Halley's was the first comet-to be recognized as periodic, but it had been seen on many previous returns before Edmund Halley announced that fact in the year 1705. Similarly, although Encke's comet was discovered in 1786, it was observed on three more returns before Johann Franz Encke determined that it had an orbital period of 3.3-years.

The object that we today call Comet Tuttle had a similar history.

On Jan. 9, 1790, the renowned Parisian comet hunter Pierre Méchain discovered a fairly bright telescopic comet in the western evening sky. His friend and rival Charles Messier described it on the following night as resembling an unresolved star cluster or nebula without a nucleus. It was followed for just over three weeks; just not enough time for a sufficient number of observations to determine an accurate orbit.

Astronomers assumed that the object was traveling in a parabolic orbit and would never be seen again, and entered the literature simply as "Comet 1790 II." It wasn't to be seen again for nearly 70-years.

Tuttle's turn

Horace P. Tuttle, an assistant at Harvard College Observatory, discovered three comets by telescope during the year 1858. Tuttle found the first of them on Jan. 4 in the constellation Andromeda. Still approaching the perihelion point of its orbit (its least distance from the sun), the comet was favorably placed relative to the Earth, and this made possible a long series of positional measurements.

Comet Tuttle was brightest during February at about magnitude 7, meaning it was just below the threshold of naked eye visibility, though a relatively easy object to see with binoculars or a small telescope.

Tuttle himself was among the first to suggest that his object was identical with Comet 1790 II. A 13.7-year period was proposed by several astronomers, and it soon became clear that Comet Tuttle of 1858 had been missed at four intervening apparitions. At three of those returns (1803, 1817, and 1844) it was too close to the sun in the sky to be seen, while conversely, in 1830 it should have been an easy object in the morning sky but was somehow missed.

Comet Tuttle became the eighth comet to be recognized as a periodic object hence it is now designated as 8P/Tuttle.

The perihelion distance of 8P/Tuttle places it just outside of the Earth's orbit at 95.5 million miles (153.6 million kilometers). Also, around Dec. 22 of each year the Earth passes through the dusty trail left behind by the comet from its previous visits. This encounter gives rise to an annual display of meteors known as the Ursids, which appear to diverge from near the bright star Kochab in the bowl of the Little Dipper.

Because 8P/Tuttle was observed at each return following its 1858 rediscovery except in 1953, this time around will go down in the record books as its 12th observed apparition. And as it turns out, this apparition will be among its very best.

Where and when to look

Comet Tuttle

On New Year's Day, 8P/Tuttle passed closest to Earth; a distance of 23.5 million miles (37.8 million kilometers). Although it is now slowly moving away from the Earth, it will continue to slowly approach the sun, passing closest to it on Jan. 27. Comets are most visible when they near the sun, which lights up material that boils off the comet.

So, during these next two weeks, the comet will hold nearly steady in brightness at around magnitude 6. For those blessed with clear, dark skies far from significant light pollution, the comet might be even glimpsed with the unaided eye. But good binoculars or a small telescope will easily bring 8P/Tuttle into view if you know where to train them; it should appear as a small fuzzy star possibly sporting a faint, narrow tail.

The comet will be situated against the rather dim stars that compose the so-called "watery region" of the sky, passing through eastern Pisces (the fishes) into Cetus (the whale) during the night of Jan. 6-7. On that night, it will lie not far to the west from one of the brightest stars in Pisces: fourth magnitude, Al Rischa, located at the point where the two fish are tied. In fact, the name comes from the Arabic word for "cord."

For the next couple of weeks both Pisces and Cetus can be conveniently found well up in the southern sky between 6 to 8 p.m. local standard time.

Comet 8P/Tuttle will appear to skid south in its orbit against the background stars of these two constellations. After moving through Cetus, 8P/Tuttle will pass into the dim, shapeless constellation of Fornax (the furnace) on Jan. 16. It will continue to plunge south thereafter, gradually becoming unfavorably placed for viewers in the Northern Hemisphere, although those living south of the equator will be able to follow the now fading comet right on into February.

A reminder about Comet Holmes!

Comet Holmes

While the spotlight is now on Comet Tuttle, we should not forget about our old friend, Comet Holmes which continues to be dimly visible to the unaided eye as a diffuse, circular cloud, roughly twice the apparent diameter of the moon against the stars of the constellation Perseus.

This comet was no brighter than magnitude 17 in mid-October — that's about 25,000 times fainter than the faintest star that can normally be seen without any optical aid. But late on Oct. 23, the comet's brightness suddenly rocketed all the way up to magnitude 2.5, brightening nearly one million times in less than 24 hours!

In attempting to explain why Comet Holmes exploded, comet expert, John Bortle suggested that this comet's nucleus consists of low-density material that, over time developed into a large region with a very tenuous structure, like a honeycomb. At some point, the highly fragile bonds connecting the honeycomb of material reached a failing point and a sudden crushing collapse occurred, expelling a gigantic volume of dust into space, making this dim comet suddenly appear impressively bright.

Back in 1892, Comet Holmes suffered two major outbursts separated by about 75 days. This leads to the question as to whether this comet will undergo a similar "cosmic aftershock" in the wake of its recent late October explosion.

Bortle thinks it's a possibility, based on the theory that there may be a large amount of residual instability which might lead to a second major collapse of material on the comet nucleus. If what happens now parallels what happened in 1892, another possible explosive outburst may be imminent, so it might be wise to keep a close watch on Comet Holmes in the coming days ahead.

By Joe Rao
SPACE.com Skywatching Columnist

Hot Cyclones Churn at Both Ends of Saturn

This image shows newly discovered "hot spot" on Saturn's north pole and the mysterious hexagon that encircles the pole. The "hot spot" appears to be related to Saturn's dynamic weather systems, rather than to seasonal changes in the amount of sunlight at the pole. Credit:NASA/JPL/GSFC/Oxford University

Despite more than a decade of winter darkness, Saturn's north pole is home to an unexpected hot spot remarkably similar to one at the planet's sunny south pole. The source of its heat is a mystery. Now, the first detailed views of the gas giant's high latitudes from the Cassini spacecraft reveal a matched set of hot cyclonic vortices, one at each pole.

While scientists already knew about the hot spot at Saturn's south pole from previous observations by the W. M. Keck Observatory in Hawaii, the north pole vortex was a surprise. The researchers report their findings in the Jan. 4 issue of Science.

"We had speculated that the south pole hot spot was connected to the southern, sunlit conditions," said Glenn Orton, a senior research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and co-investigator on Cassini's composite infrared spectrometer. "Since the north pole has been deprived of sunlight since the arrival of winter in 1995, we didn't expect to find a similar feature there."

The infrared data show that the shadowed north pole vortex shares much the same structure and temperature as the one at the sunny south pole. The cores of both show a depletion of phospine gas, an imbalance probably caused by air moving downward into the lowest part of Saturn's atmosphere, the troposphere. Both polar vortices appear to be long-lasting and intrinsic parts of Saturn and are not related to the amount of sunlight received by one pole or the other.

"The hot spots are the result of air moving polewards, being compressed and heated up as it descends over the poles into the depths of Saturn," said Leigh Fletcher, a planetary scientist from the University of Oxford, England, and the lead author of the Science paper. "The driving forces behind the motion, and indeed the global motion of Saturn's atmosphere, still need to be understood."

Though similar, the two polar regions differ in one striking way. At the north pole, the newly discovered vortex is framed by the distinctive, long-lived and still unexplained polar hexagon. This mysterious feature encompassing the entire north pole was first spotted in the 1980s by NASA's Voyager 1 and 2 spacecraft. Cassini's infrared cameras also detected the hexagon in deep atmospheric clouds early in 2007.

In their paper, Fletcher and his colleagues report that the bright, warm hexagon is much higher than previous studies had shown. "It extends right to the top of the troposphere," says Fletcher. "It is associated with downward motion in the troposphere, though the cause of the hexagonal structure requires further study."

Winter lasts about 15 years on Saturn. Researchers anticipate that when the seasons change in the coming years and Saturn's north pole is once again in sunlight, they will be able to see a swirling vortex with high eye walls and dark central clouds like the one now visible at the south pole. "But Saturn may surprise us again," says Fletcher.

"The fact that Neptune shows a similar south polar hot spot whets our appetite for the strange dynamics of the poles of the other gas giants," Fletcher says.

More information about Jupiter's poles will come from NASA's Juno mission, currently scheduled for launch in 2011 and arrival in 2016.

Running Rings Around the Galaxy

An artist concept showing the newly discovered streams of stars around our Milky Way galaxy.
NASA/JPL-Caltech/R. Hurt (SSC)

An astronomer at the Spitzer Science Center has discovered three giant stellar streams arcing high over the Milky Way. Remnants of cannibalized galaxies and star clusters, the streams are between 13,000 and 130,000 light-years distant from Earth and extend over much of the northern sky. The new results are being presented by Carl Grillmair at this week's meeting of the American Astronomical Society in Honolulu, Hawaii.

Two of the newly discovered streams are almost certainly the remains of ancient star clusters. Known to astronomers as globular clusters, these giant stellar cities contain between tens of thousands and millions of stars. Though only about 150 globular clusters orbit the Milky Way today, they may once have numbered in the thousands.

Over billions of years, the relentless gravitational stresses inflicted on them by our galaxy have slowly torn them apart, leaving behind long, thin streams of stars. Once crowded so closely together that they could sometimes actually collide, these stars are now separated by many light-years, trailing one another at half a million miles an hour through the dark and lonely reaches of the galactic halo.

Grillmair, an associate research scientist at Caltech's Spitzer Science Center, found the streams by analyzing data from the Sloan Digital Sky Survey. Conducted over several years at the Apache Point Observatory in New Mexico, this survey has yielded measurements for some 70 million stars spread over one quarter of the whole sky.

By comparing the colors and brightnesses of the stars to those in surviving globular clusters, Grillmair was able to isolate stars into groups of similar age and distance. As he examined how these groups are distributed across the sky, the giant stellar streams quickly became apparent.

The third stream discovered by Grillmair is spread over a much larger region of the sky, and is most likely the scattered remains of a dwarf galaxy. Such dwarf galaxies may contain up to 100 million stars, along with sometimes substantial amounts of mysterious "dark matter." While the Milky Way galaxy currently hosts a family of 20 or so known dwarf galaxies, scientists who study the growth of galaxies in the early universe have long been puzzled as to why we don't see hundreds of them.

The new dwarf galaxy stream is particularly interesting in that Grillmair may also have found the feebly struggling remains of the original galaxy.

The stripping and cannibalization of dwarf galaxies and star clusters can be quite episodic, resulting in streams with strong concentrations of stars separated by often substantial gaps, much like the typical distribution of cars on a highway. However, one of the stellar concentrations is many times stronger than the others, and Grillmair believes this may be the remnant of the galaxy that gave rise to the stream. "We'll need to bring out the big telescopes to examine these stars in detail and find out whether they're still gravitationally bound to one another," he says.

The new streams are among the largest features in the sky. The streams extend to the limits of the Sloan survey area, and Grillmair considers it quite likely that they wrap completely around the galaxy.

Grillmair is now planning follow-up observations of stars in these streams. While this discovery strengthens the notion that there may be thousands of such streams and that the outer reaches of our galaxy look more like a "ball of yarn," the real payoff may be in using these streams as very sensitive probes of galactic gravity.

By measuring the velocities of stars in the streams, astronomers can both map their orbits and determine how matter is distributed in our galaxy. "It's a bit like looking at aerial TV coverage of kayakers on a river," says Grillmair. "The location of the river immediately tells you where the bottom of the valley and the major rock outcroppings are, and the speed of the kayakers gives you some idea of how high and how steep the surrounding mountains must be."

The new discoveries bring to nine the number of known stellar streams around the galaxy. These streams are helping astronomers study our galaxy's underlying structure more accurately than ever before.

Overall, the streams appear to be consistent with a picture in which our galaxy is dominated by huge amounts of smoothly distributed dark matter. On the other hand, Grillmair and others have already found intriguing departures that may be showing up cracks in this model.

"This is a very exciting time for galactic archeology, and finding more of these ancient streams will really help us to piece together the structure of our galaxy and how it evolved over time."

Taken from a Caltech press release
May 30, 2007

Young Planet Orbits Sun-Like Star

This computer graphic shows the newly discovered young,
massive planet around the its host star TW Hydrae.
The planet orbits its active host star inside the inner hole of a dusty circumstellar disk.
Credit Johny Setiawan/MPIA

Astronomers say they have discovered the youngest planet to date circling a sun-like star, a find that will be a boon to the field of planet-formation theory.

The extrasolar planet is an estimated 8 million to 10 million years old, a mere toddler compared to Earth, which is 4.5 billion years old. Until now, the researchers say, no planet younger than 100 million years old has been detected circling a sun-like star.

"It means we're opening up a new field of trying to find planets around very young stars," said Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington. "So it's the very first example, and we hope there will be a lot more." Boss was not involved in the discovery.

The newly found world is so infantile that it resides in the star's "protoplanetary disk," a ring of gas and dust circling the star. It has been catalogued as TW Hya b.

"This demonstrates that planets can form within 10 million years, before the disk has been dissipated by stellar winds and radiation," the researchers write in the Jan. 3 issue of the journal Nature.

Weighing in at nearly 10 Jupiter masses, the planet circles at a distance of .04 Astronomical Units (AU) from its host star, TW Hydrae, in the constellation Hydra. One AU is the average distance between the Earth and sun.

The gassy "hot Jupiter" takes 3.56 days to orbit its star. The host star is located 180 light-years away from Earth.

Planets are thought to form within disks of dust and gas around newly born stars. Catching a planet in its childhood can give astronomers lots of information about how planets materialize.

"The discovery shows that what we always call as 'protoplanetary' disks are indeed protoplanetary; they form planets," study researcher Johny Setiawan of the Max-Planck Institute for Astronomy in Germany told SPACE.com. "There are many 'protoplanetary' disks detected around young stars, but no planets so far have been detected within such young systems."

Around some young star systems, however, astronomers have found signs of planets by noting clear lanes of dust within the disks. In these cases, it's presumed that young planets are forming and have scooped up the dust, but the planets themselves have not been detected.

Setiawan and colleagues discovered their new world by measuring a wobble in the host star due to the gravitational tug from the orbiting planet. This so-called radial-velocity method is great at detecting extrasolar planets, but it also can produce false positives — suggesting a planet is there when in fact the data owe to some other object or phenomenon.

That's particularly true in young star systems. For one, nascent stars are incredibly active and their changing outer atmospheres can at the very least make for background noise. In addition, if the star rotates about its axis, that can be problematic.

"There are lots of other things going on in these young stars that could give you a false positive, where you think you're seeing a planet but you're actually seeing some other stellar activity," Boss said in a telephone interview.

Boss thinks the discoverers ruled out these non-planet signals. "They've done a good job of trying to address those worries," he said.

By Jeanna Bryner
Staff Writer
www.space.com

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.

www.astronomy.com

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.

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

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 (http://www.spitzer.caltech.edu/Media/releases/ssc2007-16/index.shtml) .

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.

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.;
Optical/UV: NASA/STScI; Radio: NSF/VLA/CfA/D.Evans et al.,
STFC/JBO/MERLIN]

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.