A Young Erupting Pre-main Sequence Star Takes a (Long) Nap Print

Figure 1: Two images of V1647 Orionis and McNeil’s Nebula. The image on the left is an optical color composite taken about four years ago with GMOS-North on UT 2004 February 14. The image on the right is also an optical color image taken about one year ago on UT 2007 February 22.

Figure 2: Expanded view of the 2.12-2.35 micron region of the near infrared spectroscopy of V1647.

Figure 3: Plot of the 8-13 micron silicate absorption band optical depth extracted from the mid infrared spectrum.

Figure 4: Optical spectroscopy of V1647 Orionis from GMOS-North obtained on UT 2007 February 22.

A “new” star appeared in the constellation of Orion in late 2003 when the young pre-main sequence star V1647 Orionis went into outburst. The eruption and huge increase in brightness of the object resulted in the appearance of a reflection nebula called “McNeil’s Nebula,” named after the amateur astronomer, Jay McNeil, who discovered the object and alerted the world.

During the outburst the star and nebula remained bright for approximately 18 months before fading rapidly over a six month period. By early 2006 the star and its environment were very similar to their pre-burst stage. The event was monitored and observed with many ground- and space-based facilities and Gemini Observatory played a key role in monitoring the event during its eruptive and quiescent phases. A team led by Colin Aspin (IfA/University of Hawaii), Tracy Beck (STScI) and Bo Reipurth (IfA/University of Hawai‘i) spearheaded the monitoring campaign of this unique event.

The eruption of V1647 Orionis is most likely associated with a mass dumping of the inner regions of a heated circumstellar disk onto the young stellar photosphere. The spectacular flaring in brightness of the object is due to a significant increase in accretion luminosity and the clearing or destroying of surrounding dust by an energetic wind that made the star visible. These eruptions are thought to be repetitive and indicative of periods when a significant fraction of the final star’s mass is accreted.

The authors describe three phases for the V1647 Orionis latest eruption:

1. Before November 2004 is the pre-outburst phase
2. From November 2004 to February 2006 is the outburst phase
3. From February 2006 is the quiescent phase

The Gemini observing campaign led by Aspin has revealed some interesting results, particularly for the quiescent period. These include:

* McNeil’s Nebula is faintly visible in these GMOS-N images (Figure 1 right) indicating that the nebular material is still weakly illuminated by the star V1647 Orionis. At the time of acquisition of the GMOS-N imaging and spectroscopic data , V1647 Orionis had an r’ magnitude of 23.3.
* NIRI spectroscopy has revealed for the first time in this type of object the presence of molecular overtone absorption from CO and other key diagnostic atoms like Na and Ca (possibly betraying the photosphere of the star), see Figure 2. The 2um spectroscopy shown in the paper is from IRTF not NIRI. We did publish NIRI spectroscopy but from just after the outburst, not in quiescence.
* The star has a mass of about 0.8 solar mass and its age is about half a million years or less.
* V1647 Orionis in this pre-main sequence phase is about five times more luminous than the Sun.
* Material is falling onto the star at a rate of about one millionth of a solar mass per year.
* Mid infrared observation with MICHELLE/Gemini show evidence of silicate dust evolution over the outburst-to-quiescence period, see Figure 3.

In a previous article on V1647 Orionis, Aspin studied a previous outburst of the star which occurred in 1966. It seems that perhaps V1647 Orionis ‘wakes up’ every 37 years but soon (after 1 to 2 years) tires and takes another long nap!

For more details, read the article "V1647 Orionis: One year into quiescence", by C. Aspin, T. Beck and B. Reipurth in The Astronomical Journal, January 2008, pp. 423-440.

For more details on the 1966 outburst of V1647 Orionis, read the article "The 1966-1967 Outburst of V1647 Orionis and the Appearance of McNeil's Nebula", by C. Aspin and others in The Astronomical Journal, Volume 132, Issue 3, pp. 1298-1306.

Spinning Black Holes Survey - Chandra Data Reveal Rapidly Whirling Black Holes

Credit: NASA/CXC/UFRGS/R.Nemmen et al.;
Illustration: NASA/CXC/M.Weiss

Results from NASA's Chandra X-ray Observatory, combined with new theoretical calculations, provide one of the best pieces of evidence yet that many supermassive black holes are spinning extremely rapidly. The images on the left show 4 out of the 9 large galaxies included in the Chandra study, each containing a supermassive black hole in its center.

The Chandra images show pairs of huge bubbles, or cavities, in the hot gaseous atmospheres of the galaxies, created in each case by jets produced by a central supermassive black hole. Studying these cavities allows the power output of the jets to be calculated. This sets constraints on the spin of the black holes when combined with theoretical models.
Click for large jpgIllustration of Black Hole Engine

The Chandra images were also used to estimate how much fuel is available for each supermassive black hole, using a simple model for the way matter falls towards such an object. The artist's impression on the right side of the main graphic shows gas within a "sphere of influence" falling straight inwards towards a black hole before joining a rapidly spinning disk of matter near the center. Most of the material in this disk is swallowed by the black hole, but some of it is swept outwards in jets (colored blue) by quickly spinning magnetic fields close to the black hole.

Previous work with these Chandra data showed that the higher the rate at which matter falls towards these supermassive black holes, the higher their power output is in jets. However, without detailed theory the implications of this result for black hole behavior were unclear. The new study uses these Chandra results combined with leading theoretical models for the production of jets, plus general relativity, to show that the supermassive black holes in these galaxies must be spinning at close to the maximum rate. If black holes are spinning at this limit, material can be dragged around them at close to the speed of light, the speed limit from Einstein's theory of relativity.

(Credit: NASA/CXC/M.Weiss)

Illustration of Black Hole Engine

The first artist's illustration shows a close-up view of a supermassive black hole in a galaxy's center. Gas becomes hotter as it approaches the black hole, turning from red to yellow to white. Most of the gas is swallowed by the black hole, but some is launched in jets away from the black hole at almost the speed of light. The next illustration shows a larger area where gas is first attracted to the black hole, a region about a million times larger than the black hole's event horizon. The final illustration shows enormous cavities -- a hundred times larger -- that have been created in the galaxy's hot gas by jets from the black hole.

Source of Mysterious Antimatter Found

Looping eruptions on the Sun, like this one on July 24, 1999,
create antimatter. Earth is shown for size comparison.

Antimatter, which annihilates matter upon contact, seems to be rare in the universe. Still, for decades, scientists had clues that a vast cloud of antimatter lurked in space, but they did not know where it came from.

The mysterious source of this antimatter has now been discovered — stars getting ripped apart by neutron stars and black holes.

While antimatter propulsion systems are so far the stuff of science fiction, antimatter is very real.

What it is

All elementary particles, such as protons and electrons, have antimatter counterparts with the same mass but the opposite charge. For instance, the antimatter opposite of an electron, known as a positron, is positively charged.

When a particle meets its antiparticle, they destroy each other, releasing a burst of energy such as gamma rays. In 1978, gamma ray detectors flown on balloons detected a type of gamma ray emerging from space that is known to be emitted when electrons collide with positrons — meaning there was antimatter in space.

"It was quite a surprise back then to discover part of the universe was made of antimatter," researcher Gerry Skinner, an astrophysicist at Goddard Space Flight Center in Greenbelt, Md., told SPACE.com.

These gamma rays apparently came from a cloud of antimatter roughly 10,000 light-years across surrounding our galaxy's core. This giant cloud shines brightly with gamma rays, with about the energy of 10,000 suns.

What exactly generated the antimatter was a mystery for the following decades. Suspects have included everything from exploding stars to dark matter.

Now, an international research team looking over four years of data from the European Space Agency's International Gamma Ray Astrophysics Laboratory (INTEGRAL) satellite has pinpointed the apparent culprits. Their new findings suggest these positrons originate mainly from stars getting devoured by black holes and neutron stars.

As a black hole or neutron star destroys a star, tremendous amounts of radiation are released. Just as electrons and positrons emit the tell-tale gamma rays upon annihilation, so too can gamma rays combine to form electrons and positrons, providing the mechanism for the creation of the antimatter cloud, scientists think.

Billions and billions

The researchers calculate that a relatively ordinary star getting torn apart by a black hole or neutron star orbiting around it — a so-called "low mass X-ray binary" — could spew on the order of one hundred thousand billion billion billion billion positrons (a 1 followed by 41 zeroes) per second. These could account for a great deal of the antimatter that scientists have inferred, reducing or potentially eliminating the need for exotic explanations such as ones involving dark matter.

"Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries," said researcher Georg Weidenspointner of the Max Planck Institute for Extraterrestrial Physics in Germany.

Now that they have witnessed the death of antimatter, the scientists hope to see its birth.

"It would be interesting if black holes produced more matter than neutron stars, or vice versa, although it's too early to say one way or the other right now," Skinner explained. "It can be surprisingly hard to tell the difference between an X-ray binaries that hold black holes and neutron stars."

Weidenspointner, Skinner and their colleagues, detailed their findings in the Jan. 10 issue of the journal Nature.

By Charles Q. Choi Special to SPACE.com

Jupiter's Rings Revealed

Image Credit: NASA, JPL, Galileo Project, (NOAO), J. Burns (Cornell) et al.

Why does Jupiter have rings? Jupiter's rings were discovered in 1979 by the passing Voyager 1 spacecraft, but their origin was a mystery. Data from the Galileo spacecraft that orbited Jupiter from 1995 to 2003 later confirmed that these rings were created by meteoroid impacts on small nearby moons.

As a small meteoroid strikes tiny Adrastea, for example, it will bore into the moon, vaporize and explode dirt and dust off into a Jovian orbit. Pictured above is an eclipse of the sun by Jupiter, as viewed from Galileo. Small dust particles high in Jupiter's atmosphere, as well as the dust particles that compose the rings, can be seen by reflected sunlight.

Image catches Pickering's Triangle

This image of Pickering’s Triangle was taken with the National Science Foundation’s Mayall 4-meter telescope at Kitt Peak National Observatory. T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF

A new wide-field image of Pickering's Triangle is being released today in Austin, Texas, at the 211th meeting of the American Astronomical Society. The image was taken with the National Science Foundation's Mayall 4-meter telescope at Kitt Peak National Observatory.

Pickering's Triangle is part of the Cygnus Loop supernova remnant, which includes the famous Veil Nebula. It is located about 1,500 light-years from Earth, in the constellation Cygnus, the Swan. Astronomers estimate that the supernova explosion that produced the nebula occurred between 5,000 to 10,000 years ago; the entire shell stretches more than six full Moons in width across the sky.

This new image was obtained September 2007 by Travis Rector and Heidi Schweiker by combining two full pointings of the 64-megapixel NOAO Mosaic-1 imager, mounted on the historic Mayall telescope.

www.astronomy.com

Dark Matter Discovered in Accretion Disks

Previous Concept

New Concept

An artist's concept of the accretion disk around the binary star system WZ Sge, with the previous concept above and the revised concept using new data from Kitt Peak National Observatory and the Spitzer Space Telescope below, which includes an asymmetric outer disk of dark matter.

Observations of the interacting binary star using telescopes at Kitt Peak National Observatory and NASA’s Spitzer Space Telescope suggest that the disks of hot gas that accumulate around a wide variety of astronomical objects—from degenerate stars in energetic binary systems to supermassive black holes at the hearts of active galaxies—are likely to be much larger than previously believed.

The target of this specific investigation, named WZ Sagittae (WZ Sge), is an interacting binary star located in the constellation Sagitta, the arrow of the archer Sagittarius. As part of a program called the Spitzer-NOAO Observing Program for Teachers and Students, Steve B. Howell and a team of astronomers and educators imaged WZ Sge using the National Science Foundation’s 2.1-meter telescope and the WIYN 0.9-meter telescope, both located at Kitt Peak, and the Infrared Array Camera (IRAC) on Spitzer.

“We were very surprised to see the contrasting results obtained with the optical telescopes on the ground and the infrared telescope in space,” says Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) and leader of the team who made the discovery being reported today in Austin, TX, at the 211th meeting of the American Astronomical Society (AAS). “The much larger size of the infrared-emitting portion of the accretion disk around WZ Sge was immediately obvious in the data. Our observations strongly imply the presence of dark matter in these structures, which are ubiquitous throughout the Universe.”

Interacting binary stars such as WZ Sge contain a white dwarf star (a compact star about the size of the Earth, but with a mass near that of the Sun) and a larger, but less massive and much cooler companion star. The companion, usually a low-mass star or a brown dwarf, has material ripped off its surface by the stronger gravity of the white dwarf. This material flows toward the more massive star and, in the process, forms a disk surrounding the white dwarf, known as an accretion disk.

Stars such as WZ Sge are called cataclysmic variables due to their rapid and often large changes in brightness, all caused by variations in the accretion disk. The two stars in such systems orbit about each other at a similar distance to that between Earth and the Moon, but with tremendous angular momentum that results in orbital periods ranging from a few hours down to as short as tens of minutes (the period of WZ Sge is 81 minutes).

Whether they form in cataclysmic variable systems or they surround the massive black hole hearts of active galaxies, accretion disks have been well observed and modeled using measurements obtained across much of the electromagnetic spectrum, from X-rays to the near-infrared. The derived picture of the “standard accretion disk” model is a geometrically thin disk of gaseous material surrounding the white dwarf or black hole. Accretion disk models, bolstered by observation, are generally composed of hot gas having a temperature distribution within them, being hottest near the center and falling off in temperature toward the outer edge.

In order to confirm the general accretion disk models and extend them into the mid-infrared portion of the spectrum, Howell’s team obtained the first time series observations of an accretion disk system at 4.5 and 8 microns with the Spitzer Space Telescope. At nearly the same time, they obtained optical observations of WZ Sge at Kitt Peak. The optical observations confirmed the standard view of the accretion disk size and temperature, values known for over a decade.

The mid-infrared observations, however, were completely unexpected and revealed that a larger, thicker disk of cool dusty material surrounds much of the gaseous accretion disk. This outer dust disk likely contains as much mass as a medium-sized asteroid. The newly discovered outer disk extends about 20 times the radius of the gaseous disk.

“This discovery suggests that our current model for accretion disks of all kinds is wrong,” says team member Donald Hoard of the Spitzer Science Center. “We will need to rethink and recast these models for accretion disks, not only in interacting binary stars but also in distant, highly luminous active galaxies.”

The implications from such a discovery are far reaching, affecting not only the theoretical models (since the formation and evolution of the disks are modeled based on their size, temperature, and composition—all quantities that now need to be revised), but also nearly all previous observations of systems containing accretion disks.

In addition, the dust disk (which is thicker than the known gaseous disk) blocks infrared light emitted by the compact central object and the inner hot regions of the gaseous disk. Not knowing that some mid to far infrared light is blocked by the newly discovered outer dust ring can lead observers to significantly underestimate the total luminosity of the central object. “The amount of this underestimation is not yet accurately known from our initial discovery, but may be as large as 50 percent,” Howell says.

An artist’s concept comparing the previous view and the new view of the accretion disk around WZ Sge is available above.

The observational program making this discovery was a joint effort between research scientists Howell, Hoard, and Carolyn Brinkworth of Spitzer Science Center, and high school teacher Beth Thomas and student Kimmerlee Johnson (Great Falls Public Schools, Great Falls, MT), teacher Jeff Adkins and student John Michael Santiago (Deer Valley High School, Antioch, CA), and teacher Tim Spuck and student Matt Walentosky (Oil City High School, Oil City, PA).

The work was funded by Spitzer Science Center as part of a joint project with NOAO to expand and extend the national observatory’s Research Based Science Education (RBSE) teacher professional development program to include observations with the Spitzer Space Telescope. RBSE has been training groups of 20 teachers in the research process (including regular observations at Kitt Peak National Observatory) every year for more than a decade, using funding support from NSF.

Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, based in Tucson, AZ, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF.

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

Even Thin Galaxies Can Grow Fat Black Holes

This artist's concept illustrates the two types of spiral galaxies
that populate our universe:
those with plump middles, or central bulges (upper left),
and those lacking the bulge (foreground).
Credit: NASA/JPL-Caltech

NASA's Spitzer Space Telescope has detected plump black holes where least expected -- skinny galaxies.

Like people, galaxies come in different shapes and sizes. There are thin spirals both with and without central bulges of stars, and more rotund ellipticals that are themselves like giant bulges. Scientists have long held that all galaxies except the slender, bulgeless spirals harbor supermassive black holes at their cores. Furthermore, bulges were thought to be required for black holes to grow.

The new Spitzer observations throw this theory into question. The infrared telescope surveyed 32 flat and bulgeless galaxies and detected monstrous black holes lurking in the bellies of seven of them. The results imply that galaxy bulges are not necessary for black hole growth; instead, a mysterious invisible substance in galaxies called dark matter could play a role.

"This finding challenges the current paradigm. The fact that galaxies without bulges have black holes means that the bulges cannot be the determining factor," said Shobita Satyapal of the George Mason University, Fairfax, Va. "It's possible that the dark matter that fills the halos around galaxies plays an important role in the early development of supermassive black holes."

Satyapal presented the findings today at the 211th meeting of the American Astronomical Society in Austin, Texas. A study from Satyapal and her team will be published in the April 10 issue of the Astrophysical Journal.

Our own Milky Way is an example of a spiral galaxy with a bulge; from the side, it would look like a plane seen head-on, with its wings out to the side. Its black hole, though dormant and not actively "feeding," is several million times the mass of our sun.

Previous observations had suggested that bulges and black holes flourished together like symbiotic species. For instance, supermassive black holes are almost always about 0.2 percent the mass of their galaxies' bulges. In other words, the more massive the bulge, the more massive the black hole. Said Satyapal, "Scientists reasoned that somehow the formation and growth of galaxy bulges and their central black holes are intimately connected."

But a wrinkle appeared in this theory in 2003, when astronomers at the University of California, Berkeley, and Observatories of the Carnegie Institution of Washington, Pasadena, Calif., discovered a relatively "lightweight" supermassive black hole in a galaxy lacking a bulge. Then, earlier this year, Satyapal and her team uncovered a second supermassive black hole in a similarly svelte galaxy.

In the latest study, Satyapal and her colleagues report the discovery of six more hefty black holes in thin galaxies with minimal bulges, further weakening the "bulge-black hole" theory. Why hadn't anybody seen these black holes before? According to the scientists, bulgeless galaxies tend to be very dusty, letting little visible light escape. But infrared light can penetrate dust, so the team was able to use Spitzer's infrared spectrograph to reveal the "fingerprints" of active black holes lurking in galaxies millions of light years away.

"A feeding black hole spits out high-energy light that ionizes much of the gas in the core of the galaxy," said Satyapal. "In this case, Spitzer identified the unique fingerprint of highly ionized neon -- only a feeding black hole has the energy needed to excite neon to this state." The precise masses of the newfound black holes are unknown.

If bulges aren't necessary ingredients for baking up supermassive black holes, then perhaps dark matter is. Dark matter is the enigmatic substance that permeates galaxies and their surrounding halos, accounting for up to 90 percent of a galaxy's mass. So-called normal matter makes up stars, planets, living creatures and everything we see around us, whereas dark matter can't be seen. Only its gravitational effects can be felt. According to Satyapal, dark matter might somehow determine the mass of a black hole early on in the development of a galaxy.

"Maybe the bulge was just serving as a proxy for the dark matter mass -- the real determining factor behind the existence and mass of a black hole in a galaxy's center," said Satyapal.

Other authors of this study include: D. Vega of the George Mason University; R.P. Dudik of the George Mason University and NASA Goddard Space Flight Center, Greenbelt, Md.; N.P. Abel of the University of Cincinnati, Ohio; and Tim Heckman of the Johns Hopkins University, Baltimore, Md.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell.

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