Saturn Has a 'Giant Sponge'

This is a false-color image of jets (blue areas) in the southern hemisphere of Enceladus
taken with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005.
It has been processed to reveal the individual jets that comprise the plume.
Image Credit: NASA/JPL/Space Science Institute

One of Saturn's rings does housecleaning, soaking up material gushing from the fountains on Saturn's tiny ice moon Enceladus, according to new observations from the Cassini spacecraft.

"Saturn's A-ring and Enceladus are separated by 100,000 kilometers (62,000 miles), yet there's a physical connection between the two," says William Farrell of NASA's Goddard Space Flight Center in Greenbelt, Md. "Prior to Cassini, it was believed that the two bodies were separate and distinct entities, but Cassini's unique observations indicate that Enceladus is actually delivering a portion of its mass directly to the outer edge of the A-ring." Farrell is lead author of a paper on this discovery that appeared in Geophysical Research Letters January 23.

This is the latest surprising phenomenon associated with the ice geysers of Enceladus to be discovered or confirmed by Cassini scientists. Earlier, the geysers were found to be responsible for the content of the E-ring. Next, the whole magnetic environment of Saturn was found to be weighed down by the material spewing from Enceladus, which becomes plasma -- a gas of electrically charged particles. Now, Cassini scientists confirm that the plasma, which creates a donut-shaped cloud around Saturn, is being snatched by Saturn's A-ring, which acts like a giant sponge where the plasma is absorbed.

Shot from Enceladus' interior, the gas particles become electrically charged (ionized) by sunlight and collisions with other atoms and electrons. Once electrically charged, the particles feel magnetic force and are swept into the space around Saturn dominated by the planet's powerful magnetic field. There, they are trapped by Saturn's magnetic field lines, bouncing back and forth from pole to pole. The fun ends, however, if their bouncing path carries them inward toward Saturn to the A-ring. There they stick, in essence becoming part of the ring. "Once they get to the outer A-ring, they are stuck," says Farrell.

"This is an example of how Saturn's rings mitigate the overall radiation environment around the planet, sponging up low- and high-energy particles," says Farrell. By contrast, Jupiter has no dense rings to soak up high-energy particles, so that planet's extremely high radiation environment persists.

The Cassini observations confirm a prediction by John Richardson and Slobodan Jurac of the Massachusetts Institute of Technology. In the early 1990's, Hubble Space Telescope observations revealed the presence of a large body of water-related molecules in orbit about 240,000 kilometers (almost 150,000 miles) from Saturn. Richardson and Jurac modeled this water cloud and demonstrated it could migrate inward to the A-ring. "We relied on their predictions to help us interpret our data," said Farrell. "They predicted it, and we were seeing it."

At the time of their prediction, the source of the water cloud was unknown. The source was not identified until 2005 when Cassini discovered the stunning geysers emitted from Enceladus.

Data for the discovery that Saturn's A-ring acts like a sponge were collected in July 2004 when Cassini arrived in orbit around Saturn, making its closest flyby over the A-ring. "We skimmed over the top of that ring fairly close," said Farrell.

Enceladus Afar
Enceladus is seen here as a white disk across the unilluminated side of Saturn's rings
(black and white stripes across the bottom of the image).
This image was taken with the Cassini spacecraft narrow-angle camera on Oct. 27, 2007.
Image Credit: NASA/JPL/Space Science Institute

Hot spots on the inside wall of the plasma donut -- the part colliding with the A-ring -- were emitting radio signals. These signals behaved as a sort of natural radio beacon, indicating the local plasma density at the inner edge of the donut. The signals were detected by Cassini's Radio and Plasma Wave instrument. The team used these signals to monitor the density of the plasma (the higher the frequency, the greater the density) and hence witness the change in gas density with time.

"As we approached the A-ring, the frequency dropped, implying that the plasma density was going down because it was being absorbed by the ring," said Farrell. "What really drove this home was what happened to the signal when we passed over a gap in the rings, called the Cassini division. There, the frequency went higher, implying that the plasma density was going up because plasma was leaking through the gap."

The research was funded by NASA through the Cassini-Huygens project. Cassini-Huygens is an international collaboration among NASA, the European Space Agency, and the Italian Space Agency. The Cassini orbiter was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

For more information:
Written by: Bill Steigerwald, Goddard Space Flight Center.
Media Relations Contact: Carolina Martinez (JPL) 818-354-9382 and Bill Steigerwald (Goddard) 301-286-5017

NGC 4013 and the Tidal Stream

Image Credit & Copyright: R Jay Gabany (Blackbird Observatory) - collaboration;
D.Martínez-Delgado(IAC, MPIA),
M.Pohlen (Cardiff), S.Majewski (U.Virginia), J.Peñarrubia (U.Victoria), C.Palma (Penn State)

Nearly 50 million light-years away in the constellation Ursa Major, NGC 4013 was long considered an isolated island universe.

Seen edge-on, the gorgeous spiral galaxy was known for its flattened disk and central bulge of stars, cut by silhouetted dust lanes. But this deep color image of the region reveals a previously unknown feature associated with NGC 4013, an enormous, faint looping structure extending (above and toward the left) over 80 thousand light-years from the galaxy's center.

A detailed exploration of the remarkable structure reveals it to be a stream of stars originally belonging to another galaxy, likely a smaller galaxy torn apart by gravitational tides as it merged with the larger spiral.

Astronomers argue that the newly discovered tidal stream also explains a warped distribution of neutral hydrogen gas seen in radio images of NGC 4013 and offers parallels to the formation of our own Milky Way galaxy.

Site Astronomy Picture of the Day

Cosmic Finger Taps Our Galaxy's Shoulder

The leading arm of gas streaming from the

Magellanic Clouds is piercing the disk of the Milky Way.

Credit: John Rowe Animations

As if reaching out with a come-hither motion, a giant gas finger emanating from two neighboring galaxies has hooked into the starry disk of the Milky Way and is pulling all three galaxies closer.

This extremity of hydrogen gas is actually the pointy end of the so-called Leading Arm of gas that streams ahead of two irregular galaxies called the Large and Small Magellanic Clouds.

The fate of these nearby galaxies, which are impacted by the Milky Way's gravity, has been somewhat of a mystery. The new finger findings suggest that the Magellanic Clouds will eventually merge with the Milky Way rather than zooming past.

Located about 160,000 light-years from Earth, the Large Magellanic Cloud (LMC) is only one-twentieth the diameter of our galaxy and contains one-tenth as many stars. The Small Magellanic Cloud resides 200,000 light-years from Earth and is about 100 times smaller than the Milky Way.

"We're thrilled because we can determine exactly where this gas is plowing into the Milky Way," said research team leader Naomi McClure-Griffiths of CSIRO's Australia Telescope National Facility.

Called HVC306-2+230, the gas finger is gouging into our galaxy's starry disk about 70,000 light-years away from Earth. In the night sky, the contact point would be nearest the Southern Cross.

Until last year, astronomers thought the Magellanic Clouds had orbited our galaxy many times. This scenario held a gloomy outlook for the clouds, which were said to be doomed to be ripped apart and swallowed by the gravitational goliath.

But then new Hubble Space Telescope measurements revealed the clouds are paying our galaxy a one-time visit rather than being its lunch.

McClure-Griffiths' results, however, are more in line with the previous tale pegging the Milky Way and the Magellanic Clouds as long-time companions. McClure-Griffiths remarks that this isn't the final word and that both theories are still on the table.

By pointing out the spot of contact between the Leading Arm and our galactic disk, the recent study will help astronomers to predict where the clouds themselves will travel in the future.

"We think the Leading Arm is a tidal feature, gas pulled out of the Magellanic Clouds by the Milky Way's gravity," McClure-Griffiths said. "Where this gas goes, we'd expect the clouds to follow, at least approximately."

In the distant future, the three galaxies could become one.

By Jeanna Bryner
Staff Writer
www.space.com

Isolated Galaxy or Corporate Merger? Hubble Spies NGC 1132

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

NGC 1132 is dubbed a "fossil group" because it contains enormous concentrations of dark matter, comparable to the dark matter found in an entire group of galaxies. NGC 1132 also has a strong X-ray glow from an abundant amount of hot gas that is normally only found in galaxy groups.

In visible light, however, it appears as a single, isolated, large elliptical galaxy. The origin of fossil-group systems remains a puzzle. They may be the end-products of complete merging of galaxies within once-normal groups. Or, they may be very rare objects that formed in a region or period of time where the growth of moderate-sized galaxies was somehow suppressed, and only one large galaxy formed.

Elliptical galaxies are smooth and featureless. Containing hundreds of millions to trillions of stars, they range from nearly spherical to very elongated shapes. Their overall yellowish color comes from the aging stars. Because ellipticals do not contain much cool gas, they no longer can make new stars.

This image of NGC 1132 was taken with Hubble's Advanced Camera for Surveys. Data obtained in 2005 and 2006 through green and near-infrared filters were used in the composite. In this Hubble image, NGC 1132 is seen among a number of smaller dwarf galaxies of similar color. In the background, there is a stunning tapestry of numerous galaxies that are much larger but much farther away.

NGC 1132 is located approximately 318 million light-years away in the constellation Eridanus, the River.

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

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 Hawai‘i), 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!

Three Month Composite of Comet Holmes

Credit & Copyright: John Pane

How has Comet Holmes changed? Since brightening unexpectedly by nearly one million fold in late October, the last three months have found the coma of Comet 17P/Holmes both expanding and fading. This spectacular composite image shows how the coma and tail of Comet Holmes have changed.

Due to Earth's changing vantage point, Comet Holmes, out beyond the orbit of Mars, was seen in November nearly head-on, but in recent months is seen more from the side. Additionally, the comet's motion, when combined with Earth's changing perspective, has caused the comet to have shifted relative to the background stars.

The curved path of Comet Holmes shows it to be undergoing apparent retrograde motion as the Earth orbits quickly in front of it. The extent of the coma currently makes Comet Holmes over five times the physical size of our Sun.

Anecdotal evidence holds that the comet is hard to see without long photographic exposures, but on such exposures the comet may still be an impressive sight.

Astronomy Picture of the Day

A Theory of Everything

Astronomers propose an astronomical test of string theory.
Provided by the U of I at Urbana-Champaign

This image shows the how the Big Bang formed our universe.
Astronomy: Roen Kelly

Ancient light absorbed by neutral hydrogen atoms could be used to test certain predictions of string theory, say cosmologists at the University of Illinois. Making the measurements, however, would require a gigantic array of radio telescopes to be built on Earth, in space or on the Moon.

String theory, a theory whose fundamental building blocks are tiny one-dimensional filaments called strings, is the leading contender for a "theory of everything." Such a theory would unify all four fundamental forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity). But finding ways to test string theory has been difficult.

Now, cosmologists at the U of I say absorption features in the 21-centimeter spectrum of neutral hydrogen atoms could be used for such a test.

"High-redshift, 21-centimeter observations provide a rare observational window in which to test string theory, constrain its parameters and show whether or not it makes sense to embed a type of inflation, called brane inflation, into string theory," says Benjamin Wandelt, a professor of physics and of astronomy at the U of I.

"If we embed brane inflation into string theory, a network of cosmic strings is predicted to form," Wandelt says. "We can test this prediction by looking for the impact this cosmic string network would have on the density of neutral hydrogen in the universe."

About 400,000 years after the Big Bang, the universe consisted of a thick shell of neutral hydrogen atoms (each composed of a single proton orbited by a single electron) illuminated by what became known as the cosmic microwave background.

Because neutral hydrogen atoms readily absorb electromagnetic radiation with a wavelength of 21 centimeters, the cosmic microwave background carries a signature of density perturbations in the hydrogen shell, which should be observable today, Wandelt says.

Cosmic strings are filaments of infinite length. Their composition can be loosely compared to the boundaries of ice crystals in frozen water. When water in a bowl begins to freeze, ice crystals will grow at different points in the bowl, with random orientations. When the ice crystals meet, they usually will not be aligned to one another. The boundary between two such misaligned crystals is called a discontinuity or a defect.

Cosmic strings are defects in space. A network of strings is predicted by string theory (and also by other supersymmetric theories known as Grand Unified Theories, which aspire to unify all known forces of nature except gravity) to have been produced in the early universe, but has not been detected so far.

Cosmic strings produce characteristic fluctuations in the gas density through which they move, a signature of which will be imprinted on the 21-centimeter radiation.

The cosmic string network predicted to occur with brane inflation could be tested by looking for the corresponding fluctuations in the 21-centimeter radiation. Like the cosmic microwave background, the cosmological 21-centimeter radiation has been stretched as the universe has expanded. Today, this relic radiation has a wavelength closer to 21 meters, putting it in the long-wavelength radio portion of the electromagnetic spectrum.

To precisely measure perturbations in the spectra would require an array of radio telescopes with a collective area of more than 1,000 square kilometers. Such an array could be built using current technology, Wandelt says, but would be prohibitively expensive.

If such an enormous array were eventually constructed, measurements of perturbations in the density of neutral hydrogen atoms could also reveal the value of string tension, a fundamental parameter in string theory, Wandelt says. "And that would tell us about the energy scale at which quantum gravity begins to become important."

www.astronomy.com

Old galaxies discovered

Astronomers uncover ancestors of Milky Way-type galaxies.
Provided by Rutgers University

Galaxy M74 is representative of spiral galaxies that evolved from
recently discovered galaxies in the early universe.
Credit: odd Boroson/NOAO/AURA/NSF

Astronomers at Rutgers and Penn State universities have discovered galaxies in the distant universe that are ancestors of spiral galaxies like our Milky Way.

These ancient objects, some of the first galaxies ever to form, are being observed as they looked when the universe was a mere 2 billion years old. Today, scientists peg the universe's age at 13.7 billion years, so light from these galaxies traveled almost 12 billion years to reach Earth.

The newly discovered galaxies are quite small, one-tenth the size and one-twentieth the mass of our Milky Way. They also have fewer stars, only one-fortieth as many as are in the Milky Way. From ground-based telescopes, they look like individual stars in size. Recent images made by the Hubble Space Telescope, however, reveal them as regions of active star formation.

"Finding these objects and discovering that they are a step in the evolution of our galaxy is akin to finding a key fossil in the path of human evolution," says Eric Gawiser, assistant professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences.

This image shows an extended, chain-like Lyman alpha emitter.
Credit: NASA/ESA/C. Gronwall/Penn State

The researchers determined that these galaxies were fertile breeding grounds for new stars, which burned hot and bright. These stars ionized the hydrogen atoms around them, stripping them of their electrons and causing them to emit a tell-tale sharp band of ultraviolet light known as Lyman alpha.

The researchers also noted that several of these galaxies, sometimes 10 or more, pulled together over the ensuing few billion years to form a single spiral galaxy.

"The Hubble Space Telescope has delivered striking images of these early galaxies, with 10 times the resolution of ground-based telescopes," says Caryl Gronwall, senior research associate in Penn State's Department of Astronomy & Astrophysics. "They come in a variety of shapes, round, oblong, and even somewhat linear, and we are starting to make precise measurements of their sizes."

The astronomers discovered these galaxies as part of a 5-year-old census of galaxies in the early universe, a project called MUSYC (Multi-Wavelength Survey by Yale and Chile). Gawiser, while a National Science Foundation (NSF) astronomy and astrophysics postdoctoral fellow at Yale, initiated a search for several types of galaxies that could be precursors of Milky Way-type spirals; Gronwall led an investigation into the luminosity, density and distribution of the distinctive Lyman alpha emitters. Their statistical analyses and computer simulations of how galaxies cluster led them to the conclusion they first reported in December 2007: Lyman alpha emitters are the ancestors of spiral galaxies.

This image shows an extended Lyman alpha emitter which shows evidence of merging.
Credit: NASA/ESA/C. Gronwall/Penn State

"We knew by our understanding of cosmological theory that spiral galaxies had to evolve from low-mass galaxies such as these," Gawiser says. "The challenge was to actually find them. We'd seen other early universe galaxies, but they were bigger and destined to evolve into elliptical galaxies, not spirals."

The astronomers undertook four types of observations to find and characterize the objects they were seeking. They performed the first step, actually finding the Lyman alpha-emitting galaxies amid all the visible objects of deep space, using the Blanco 4-meter telescope at the NSF Cerro Tololo Inter-American Observatory in Chile. To measure their distance, they used the Magellan Telescope at Las Campanas Observatory, also in Chile, to measure redshift, an effect that shows how fast an object is receding from view due to a rapidly expanding universe. (The redshift at which they studied these galaxies is 3.1.) To determine how many stars are in the galaxies, they used the NASA Spitzer Space Telescope's Infrared Array Camera. And to determine how big the galaxies are, they used the NASA Hubble Space Telescope's Advanced Camera for Surveys.

"Astronomy has long used a model where big surveys are followed by detailed studies of the interesting objects they find," says Nigel Sharp, program officer in NSF's Division of Astronomical Sciences. "This work nicely couples the large area, wide-field view of our ground-based telescope with the sharp focus of the Hubble, to probe to the faintest light levels. This team has come the closest yet to finding young galaxies that resemble our own Milky Way in its infancy."

www.astronomy.com

New Light on Dark Energy

Probing the cosmic Web of the Universe

Astronomers have used ESO's Very Large Telescope to measure the distribution and motions of thousands of galaxies in the distant Universe. This opens fascinating perspectives to better understand what drives the acceleration of the cosmic expansion and sheds new light on the mysterious dark energy that is thought to permeate the Universe.

ESO PR Photo 04a/08
Large-scale structures
(Artist's Impression)

"Explaining why the expansion of the Universe is currently accelerating is certainly the most fascinating question in modern cosmology," says Luigi Guzzo, lead author of a paper in this week's issue of Nature, in which the new results are presented. "We have been able to show that large surveys that measure the positions and velocities of distant galaxies provide us with a new powerful way to solve this mystery."

Ten years ago, astronomers made the stunning discovery that the Universe is expanding at a faster pace today than it did in the past.

"This implies that one of two very different possibilities must hold true," explains Enzo Branchini, member of the team. "Either the Universe is filled with a mysterious dark energy which produces a repulsive force that fights the gravitational brake from all the matter present in the Universe, or, our current theory of gravitation is not correct and needs to be modified, for example by adding extra dimensions to space."

Current observations of the expansion rate of the Universe cannot distinguish between these two options, but the international team of 51 scientists from 24 institutions found a way that could help in tackling this problem. The technique is based on a well-known phenomenon, namely the fact that the apparent motion of distant galaxies results from two effects: the global expansion of the Universe that pushes the galaxies away from each other and the gravitational attraction of matter present in the galaxies' neighbourhood that pulls them together, creating the cosmic web of large-scale structures.

ESO PR Photo 04b/08
A Cone in the Universe

"By measuring the apparent velocities of large samples of galaxies over the last thirty years, astronomers have been able to reconstruct a three-dimensional map of the distribution of galaxies over large volumes of the Universe. This map revealed large-scale structures such as clusters of galaxies and filamentary superclusters," says Olivier Le Fèvre, member of the team. "But the measured velocities also contain information about the local motions of galaxies; these introduce small but significant distortions in the reconstructed maps of the Universe. We have shown that measuring this distortion at different epochs of the Universe's history is a way to test the nature of dark energy."

Guzzo and his collaborators have been able to measure this effect by using the VIMOS spectrograph on Melipal, one of the four 8.2-m telescopes that is part of ESO's VLT. As part of the VIMOS-VLT Deep Survey (VVDS), of which Le Fèvre is the Principal Investigator, spectra of several thousands of galaxies in a 4-square-degree field (or 20 times the size of the full Moon) at epochs corresponding to about half the current age of the Universe (about 7 billion years ago) were obtained and analysed.

ESO PR Video 04/08
Journey through galaxies

"This is the largest field ever covered homogeneously by means of spectroscopy to this depth," declares Le Fèvre. "We have now collected more than 13,000 spectra in this field and the total volume sampled by the survey is more than 25 million cubic light-years."

The astronomers compared their result with that of the 2dFGRS survey that probed the local Universe, i.e. measures the distortion at the present time.

Within current uncertainties, the measurement of this effect provides an independent indication of the need for an unknown extra energy ingredient in the 'cosmic soup', supporting the simplest form of dark energy, the so-called cosmological constant, introduced originally by Albert Einstein. The large uncertainties do not yet exclude the other scenarios, though.

"We have also shown that by extending our measurements over volumes about ten times larger than the VVDS, this technique should be able to tell us whether cosmic acceleration originates from a dark energy component of exotic origin or requires a modification of the laws of gravity," explains Guzzo.

"VIMOS on the VLT would certainly be a wonderful tool to perform this future survey and help us answer this fundamental question. This strongly encourages scientists to proceed with even more ambitious surveys of the distant Universe," concludes Le Fèvre.

Cosmic Interactions

VLT images triplet of dancing galaxies

An image based on data taken with ESO's Very Large Telescope reveals a triplet of galaxies intertwined in a cosmic dance.

Eso PR Photo 02/08

The three galaxies, catalogued as NGC 7173 (top), 7174 (bottom right) and 7176 (bottom left), are located 106 million light-years away towards the constellation of Piscis Austrinus (the 'Southern Fish').

NGC 7173 and 7176 are elliptical galaxies, while NGC 7174 is a spiral galaxy with quite disturbed dust lanes and a long, twisted tail. This seems to indicate that the two bottom galaxies - whose combined shape bears some resemblance to that of a sleeping baby - are currently interacting, with NGC 7176 providing fresh material to NGC 7174. Matter present in great quantity around the triplet's members also points to the fact that NGC 7176 and NGC 7173 have interacted in the past.

Astronomers have suggested that the three galaxies will finally merge into a giant 'island universe', tens to hundreds of times as massive as our own Milky Way.

Eso PR Photo 02b/08
NGC 7173, 7174, and 7176

The triplet is part of a so-called 'Compact Group', as compiled by Canadian astronomer Paul Hickson in the early 1980s. The group, which is the 90th entry in the catalogue and is therefore known as HCG 90, actually contains four major members. One of them - NGC 7192 - lies above the trio, outside of this image, and is another peculiar spiral galaxy.

Compact groups are small, relatively isolated, systems of typically four to ten galaxies in close proximity to one another. Another striking example is Robert's Quartet. Compact groups are excellent laboratories for the study of galaxy interactions and their effects, in particular the formation of stars.

As the striking image reveals, there are many other galaxies in the field. Some are distant ones, while others seem to be part of the family. Studies made with other telescopes have indeed revealed that the HCG 90 group contains 16 members, most of them much smaller in size than the four members with an entry in the NGC catalogue.

Notes
A list of all the work about this group of galaxies can be for example obtained through the Simbad astronomical database.

Elliptical galaxies are smooth and featureless entities, often composed of older, low mass stars, which produce very few new stars. Spiral galaxies consist of a flat, rotating disc of stars, gas and dust, and a central concentration of stars known as the bulge. The spiral arms are sites of ongoing star formation. Our own Milky Way is a spiral galaxy.

The Growing-up of a Star

VLT decodes the innermost surroundings of a star in the maturing

Using ESO's Very Large Telescope Interferometer, astronomers have probed the inner parts of the disc of material surrounding a young stellar object, witnessing how it gains its mass before becoming an adult.

ESO PR Photo 03a/08
The disc around MWC 147
(Artist's Impression)

The astronomers had a close look at the object known as MWC 147, lying about 2,600 light years away towards the constellation of Monoceros ('the Unicorn'). MWC 147 belongs to the family of Herbig Ae/Be objects. These have a few times the mass of our Sun and are still forming, increasing in mass by swallowing material present in a surrounding disc.

MWC 147 is less than half a million years old. If one associated the middle-aged, 4.6 billion year old Sun with a person in his early forties, MWC 147 would be a 1-day-old baby [1].

The morphology of the inner environment of these young stars is however a matter of debate and knowledge of it is important to better understand how stars and their cortège of planets form.

The astronomers Stefan Kraus, Thomas Preibisch, and Keiichi Ohnaka have used the four 8.2-m Unit Telescopes of ESO's Very Large Telescope to this purpose, combining the light from two or three telescopes with the MIDI and AMBER instruments.

"With our VLTI/MIDI and VLTI/AMBER observations of MWC147, we combine, for the first time, near- and mid-infrared interferometric observations of a Herbig Ae/Be star, providing a measurement of the disc size over a wide wavelength range [2]," said Stefan Kraus, lead-author of the paper reporting the results. "Different wavelength regimes trace different temperatures, allowing us to probe the disc's geometry on the smaller scale, but also to constrain how the temperature changes with the distance from the star."

The near-infrared observations probe hot material with temperatures of up to a few thousand degrees in the innermost disc regions, while the mid-infrared observations trace cooler dust further out in the disc.

The observations show that the temperature changes with radius are much steeper than predicted by the currently favoured models, indicating that most of the near-infrared emission emerges from hot material located very close to the star, that is, within one or two times the Earth-Sun distance (1-2 AU). This also implies that dust cannot exist so close to the star, since the strong energy radiated by the star heats and ultimately destroys the dust grains.

ESO PR Photo 03b/08
The Region Around MWC 147

"We have performed detailed numerical simulations to understand these observations and reached the conclusion that we observe not only the outer dust disc, but also measure strong emission from a hot inner gaseous disc. This suggests that the disc is not a passive one, simply reprocessing the light from the star," explained Kraus. "Instead, the disc is active, and we see the material, which is just transported from the outer disc parts towards the forming star."

ESO PR Photo 03c/08
Close-up on MWC 147

The best-fit model is that of a disc extending out to 100 AU, with the star increasing in mass at a rate of seven millionths of a solar mass per year.

"Our study demonstrates the power of ESO's VLTI to probe the inner structure of discs around young stars and to reveal how stars reach their final mass," said Stefan Kraus.

More Information

The authors report their results in a paper in the Astrophysical Journal ("Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147", by Stefan Kraus, Thomas Preibisch, Keichii Ohnaka").

Notes

[1] Being 6.6 times more massive than the Sun, however, MWC 147 will only live for about 35 million years, or to draw again the comparison with a person, about 100 days, instead of the 80 year equivalent of our Sun.

[2] MIDI is the mid-infrared instrument of the VLT interferometer. It operates between 8 and 13 microns. AMBER observes in the near-infrared, from 1 to 2.4 microns.

Contacts

Stefan Kraus
Max-Planck-Institute for Radio Astronomy, Bonn, Germany
E-mail: skraus@mpifr-bonn.mpg.de

Phone: +49 (0)228 525-395

Dust Strangely Vaporized by Stellar Explosion

Artist concept of the RS Ophiuchi binary system shortly after a white dwarf (right) has exploded as a nova. Scientists have detected dust in the system, depicted here as spiral dust lanes.

Credit: Casey Reed

Explosions of small stars, long thought to create stellar dust, actually sweep dust away, scientists discovered.

For years, researchers have observed swirling dust clouds around systems called recurring novas, which periodically explode. New images of a distant nova have now overturned astronomers' long-standing assumption that the dust originates in the blasts.

Scientists recently observed the RS Ophiuchi system, where a small white dwarf star and large red giant orbit each other. Over time, the giant sheds its outer layer of gas, which the dwarf sweeps up. The little star's mass grows gradually, eventually reaching a tipping point, when the top layer ignites in a thermonuclear explosion and expels the surface into space. The process then starts over — astronomers have already seen this system "go nova" in 1898, 1933, 1958, 1967 and 1985.

When RS Ophiuchi blew again in February 2006, researchers took advantage of a new instrument, called the Keck Nuller, at the W. M. Keck Observatory in Mauna Kea, Hawaii, to watch the event in action. The Nuller used two giant telescopes to block out the overwhelming light from the explosion so scientists could study its fainter surroundings.

They were surprised to see no dust in the bright zone around the star and only to see dust farther away, where the blast wave had not yet reached. The researchers surmised that the detonation had vaporized nearby dust particles, and that the outer dust must have been created before the bang.

"This flies in the face of what we expected," said Richard Barry, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md., who headed up the observations. "Astronomers had previously thought that nova explosions actually create dust."

The team suspects the dust is really produced when the white dwarf plows through the red giant's trail of debris, creating patches of gas where atoms are cool and dense enough to clump together into dust particles.

The findings will be detailed in the May 1 issue of the Astrophysical Journal.

By Clara Moskowitz
Staff Writer
www.space.com

Westerlund 2 - A Stellar Sight

Credit: NASA/CXC/Univ. de Liège/Y. Naze et al

This Chandra X-ray Observatory image shows Westerlund 2, a young star cluster with an estimated age of about one or two million years. Until recently little was known about this cluster because it is heavily obscured by dust and gas. However, using infrared and X-ray observations to overcome this obscuration, Westerlund 2 has become regarded as one of the most interesting star clusters in the Milky Way galaxy. It contains some of the hottest, brightest and most massive stars known.

This Chandra image of Westerlund 2 shows low energy X-rays in red, intermediate energy X-rays in green and high energy X-rays in blue. The image shows a very high density of massive stars that are bright in X-rays, plus diffuse X-ray emission.

An incredibly massive double star system called WR20a is visible as the bright yellow point just below and to the right of the cluster's center. This system contains stars with masses of 82 and 83 times that of the Sun. The dense streams of matter steadily ejected by these two massive stars, called stellar winds, collide with each other and produce copious amounts of X-ray emission. This collision is seen at different angles as the stars orbit around each other every 3.7 days. Several other bright X-ray sources may also show evidence for collisions between winds in massive binary systems.

X-ray & Infrared
(Credit: X-ray: NASA/CXC/Univ. de Liege/Y. Naze et al;
IR: NASA/JPL-Caltech/Univ. of Wisconsin/E. Churchwell)

This colored Chandra X-ray Observatory image (inset) shows Westerlund 2 in context with the Spitzer infrared observation (black & white). Westerlund 2 is a young star cluster with an estimated age of about one or two million years. Until recently little was known about this cluster because it is heavily obscured by dust and gas. However, using infrared and X-ray observations to overcome this obscuration, Westerlund 2 has become regarded as one of the most interesting star clusters in the Milky Way galaxy. It contains some of the hottest, brightest and most massive stars known.

Cosmic mid-life crisis

One of the many interacting galaxy pairs seen by the GEMS

survey with the Hubble Space Telescope. S. Jogee/UT-Austin/
GEMS Collaboration/STScI/NASA

A new study of 'GEMS' from Hubble and Spitzer reveals cosmic fireworks fizzled out when the universe reached middle age.

We all start to party less around middle age, and new studies by a team led by University of Texas at Austin astronomer Shardha Jogee now finds that the universe, as a whole, is no exception.

According to the current models of galaxy formation, dubbed "hierarchical lambda cold dark matter" models, galaxies built up to their current masses, shapes, and sizes through the successive mergers of less massive protogalaxies made of gas, stars, and dark matter. In the first quarter of the universe's lifespan, the cosmic landscape was dominated by violent galaxy mergers, which could radically transform the shape of a galaxy and convert its gas into stars at an extreme rate. More than half of bright galaxies were indulging in such violent "partying."

New research is showing that all changed when the universe hit middle age. "Our study finds that over the last 7 billion years, after the universe hit its mid-forties, so to speak, it transitioned from a violent merger-driven mode into a quieter mode," Jogee says.

She and her team find that over each billion-year interval, only 10 percent of galaxies are typically involved in strong interactions and mergers.

Jogee's team has analyzed more than 5,000 galaxies imaged by Hubble Space Telescope as part of GEMS, one of the largest-area surveys conducted with Hubble in two filters.

This Hubble image shows another interacting galaxy pair. Credit:
S. Jogee/UT-Austin/GEMS Collaboration/STScI/NASA

"With Hubble's spectacular resolution," says Kyle Penner, member of the international GEMS collaboration. "We could discern amazing tell-tale clues of the mergers and interactions, huge tails, warps, ripples, double nuclei, in galaxies billions of light-years away."

"It's been exciting to apply different complementary techniques in this large survey and to sift through the merger history of the universe during this elusive era," says Sarah Miller, member of the international GEMS collaboration.
In addition to estimating the frequency of mergers, Jogee and her colleagues found that contrary to what is commonly assumed, the average star formation rate in these interacting and merging galaxies is only enhanced by a modest factor of two to three compared to that in normal non-interacting galaxies.

"While extreme bursts of star formation, so-called cosmic fireworks, may happen in some galaxy mergers or interactions, they are not the norm in the vast majority of galaxy interactions taking place over the last 7 billion years," Jogee says.

The findings of Jogee's team result from a powerful synergy of data from NASA's Hubble and Spitzer space telescopes. "Mid-infrared observations from the Spitzer Space Telescope, taken by George Rieke of The University of Arizona, were key for tracing hidden star formation, obscured by dust," Jogee says. "The exquisite resolution of the GEMS Hubble data in turn allowed us to identify strongly interacting and merging galaxies at much earlier cosmological times than conventional ground-based telescopes," says team member Daniel McIntosh of the University of Massachusetts, Amherst.

Jogee and her team, in fact, find that only 20 percent of the total cosmic star formation that took place over the last 7 billion years appears to come from strongly interacting and merging galaxies. These results extend the similar trend found for a smaller sample of about 1,500 galaxies over a narrower time interval by fellow team members Christian Wolf from Oxford University and Eric Bell of the Max Planck Institute of Astronomy.

Furthermore, the results reported by Jogee and her team on the modest fraction (about 20 percent) of merger-induced star formation, and the frequency of galaxy mergers over the last 7 billion years, are in remarkably good agreement with prevailing theoretical cold dark matter models of galaxy evolution.

According to team member Rachel Somerville of the Max Planck Institute of Astronomy, "Mergers are thought to be a crucial process in transforming galaxies, causing bursts of star formation, and perhaps even feeding gas to the supermassive black holes lurking in the galaxy's nucleus.

"Although the frequency of mergers predicted by the models agrees quite well with the observed frequency," Somerville says, "these observations can also teach us much more about the effect these violent episodes have on galaxies."

In fact, Jogee says, "Our results raise many additional questions which can only be addressed with next generation facilities. For example, the cosmic star-formation rate is declining in normal galaxies, but it remains unclear what drives this decline. Are galaxies using up their internal cold gas supply, or is the accretion rate of gas from external filaments declining?"

This Hubble image shows one more interacting galaxy pair. 
Credit: S. Jogee/UT-Austin/GEMS Collaboration/STScI/NASA

Next-generation radio facilities, such as ALMA (the Atacama Large Millimeter/Sub-millimeter Array) will be critical for exploring how the cold gas content of galaxies changes over the last seven billion years, she said.

"Another key thing to note is that some of our results starkly disagree with prevailing hierarchical models of galaxy evolution," Jogee says. According to these models, the frequency of pure disk galaxies or so-called "bulgeless galaxies" is expected to be extremely low, because a past major merger in the life of every galaxy invariably builds a bulge.

Contrary to such predictions, postdoctoral fellow Fabio Barazza, formerly working with Jogee at The University of Texas and now at Geneva Observatory's Ecole Polytechnique Federale de Lausanne, found that about 20 percent of present-day spiral galaxies are bulgeless or disk-dominated, based on the analysis of about 1,000 galaxies from the Sloan Digital Sky Survey.

"We also see striking super-thin bulgeless galaxies in GEMS, at earlier epochs," Jogee says. "We yet have to characterize the frequency and origin of these enigmatic bulgeless galaxies at different epochs, but there is no denying their prevalence in the local universe."

All in all, "We have made important headway in piecing part of the cosmic puzzle of galaxy evolution, but daunting challenges loom ahead for both observers and theorists, " she says.

Provided by the McDonald Observatorywww.astronomy.com

Hubble finds saucer-shaped galaxies

The Hubble Space Telescope captured this massive saucer-shaped galaxy more than 9 billion light-years distant from us.

Its age implies it was formed when the universe was very young. Hubble

Some of the first massive galaxies in the universe formed when huge gas clouds rapidly collapsed, according to Elizabeth McGrath of the University of California, Santa Cruz, Alan Stockton of the University of Hawaii, and their collaborators. This discovery, which is based on new Hubble Space Telescope images, challenges the commonly held idea that all of the earliest massive galaxies formed when smaller galaxies merged.

The standard theory of galaxy formation predicts that the most massive galaxies in the universe take a long time to grow, accumulating mass through the coalescence of many smaller galaxies in a process that continues until relatively recent times. To test this theory, McGrath, Stockton, and their collaborators searched for the oldest, most massive galaxies they could find and used clues from their shape and structure to deduce how they may have formed. High-resolution images from the Hubble Space Telescope revealed galaxies more massive than our own Milky Way that existed when the universe was very young, only one-fifth its current age. It is believed that such galaxies are the distant ancestors of the most massive galaxies in the universe today.

"We expected these galaxies to look similar to the football-shaped elliptical galaxies that we see at the centers of dense groups of galaxies today, where mergers are common. We were quite surprised to find that many of them appear instead to be flattened, rotating disks of ordered material," says McGrath.

Disk galaxies are pancake or saucer-shaped, and their stars orbit in circles around the center of the galaxy, much like the planets in our solar system orbit around the Sun, or like a Frisbee spins as it moves through the air. This type of galaxy is more likely to have formed from a single massive cloud that collapses under its own gravity into a flattened disk rather than through violent collisions of previously formed galaxies. Computer simulations of the latter scenario predict that collisions would destroy disks and send stars from each merging galaxy into more chaotic, three-dimensional orbits, producing football-shaped, or elliptical galaxies. The most massive galaxies in the universe today all appear to be elliptical in shape, and therefore can be quite naturally explained through the merger hypothesis. The existence of massive disk galaxies in the early universe, however, challenges this perspective.

In total, McGrath and her collaborators observed seven of what are likely some of the first massive galaxies to form in the universe. Of these, four had shapes dominated by disk-like profiles. By age-dating the galaxies from studying properties of the stars within them, McGrath's team discovered that these disk structures have remained in pristine condition for over 1 billion years. Even so, it seems inevitable that eventually these galaxies will merge with others and be reformed into the more elliptical-shaped massive, old galaxies that are familiar to us in the nearby universe.

Provided by the University of Hawaii
www.astronomy.com

More to neutron stars and black holes

This image shows a massive neutron star. Casey Reed (Penn State)

Arecibo Observatory finds that neutron stars can be more massive, while black holes are more rare.

Neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to new research developed by using the Arecibo Observatory in Arecibo, Puerto Rico.

In the cosmic continuum of dead, remnant stars, the Arecibo astronomers have increased the mass limit for when neutron stars turn into black holes.

"The matter at the center of a neutron star is highly incompressible. Our new measurements of the mass of neutron stars will help nuclear physicists understand the properties of super-dense matter," says Paulo Freire, an astronomer from the observatory. "It also means that to form a black hole, more mass is needed than previously thought. Thus, in our universe, black holes might be more rare and neutron stars slightly more abundant."

When the cores of massive stars run out of nuclear fuel, their enormous gravitation then causes their collapse then becomes a supernova. The core, typically with a mass 1.4 times larger than that of the sun is compressed into a neutron star. These extreme objects have a radius about 10 to 16 kilometers and a density on the order of a billion tons per cubic centimeter. Freire says that a neutron star is like one single, giant atomic nucleus with about 460,000 times the mass of the Earth.

Astronomers had thought the neutron stars needed a maximum mass between 1.6 and 2.5 Suns in order to collapse and become black holes. However, this new research shows that neutron stars remain neutron stars between the mass of 1.9 and up to possibly 2.7 Suns.

"The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don't know what it is made out of," says Freire. "For that reason, we have at present no idea of how large or how massive neutron stars can be."

From June 2001 to March 2007, Freire used Arecibo's "L-wide" receiver (sensitive to radio frequencies from 1100 to 1700 MHz) and the Wide-Band Arecibo Pulsar Processors, a very fast spectrometer on the Arecibo telescope, to examine a binary pulsar called M5 B, in the globular cluster M5, which is located in the constellation Serpens. Like a lighthouse emits light, a pulsar is a strongly magnetized neutron star that emits large amounts of electromagnetic radiation, usually from its magnetic pole. As in the case of a lighthouse, distant observers perceive a sequence of pulsations, which are caused by the rotation of the pulsar. In the case of M5 B, these radio pulsations arrive at the Earth every 7.95 milliseconds.

These radio pulsations were scanned by the wide-band spectrometers once every 64 microseconds for 256 spectral channels, and then recorded to a computer disk, with accurate timing information. The precise arrival time of the pulses were then used by the astronomers to accurately measure the orbital motion of M5 B about its companion. This allowed the astronomers to estimate the mass (1.9 solar masses) of the pulsar.

Provided by Cornell University www.astronomy.com

High-Energy X-Rays Detected at Galaxy Cluster

Shockwaves travel through hot gas (in red) as two galaxy clusters collide and merge
Credit: ESA (Image by Christophe Carreau)

A distant galaxy cluster has turned into a giant particle accelerator, spinning electrons over vast distances at high speeds.

Scientists discovered this phenomenon by observing highly energetic X-rays emanating from the Ophiuchus cluster of galaxies.

The European Space Agency's orbiting gamma-ray observatory Integral detected the X-rays, which are too energetic to originate from the inert gas in the cluster and must instead come from accelerated particles.

Previous observations have been able to detect only lower-energy radio waves released in other clusters-turned-particle accelerators.

"This is the first time we have detected significant high-energy X-ray radiation from a cluster," said Stephane Paltani, an astrophysicist at Geneva Observatory in Switzerland, who was involved in the finding. "Only now are we reaching the sensitivity that we need to detect this radiation."

The Ophiuchus cluster must have recently merged with a smaller galaxy cluster, Paltani said. The collision would have mixed the gases in each cluster, producing rippling shock waves. As electrons bounced back and forth in the chaotic merger, they likely picked up energy and accelerated.

This cosmic particle accelerator is 20 times more powerful than the largest man-made atom smasher, the Large Hadron Collider being constructed at CERN, the particle physics lab in Switzerland, Paltani said.

"Of course the Ophiuchus cluster is somewhat bigger," Paltani said. "While LHC is 27 kilometers [17 miles] across, the Ophiuchus galaxy cluster is over two million light-years in diameter."

The scientists don't know for sure why the sped-up electrons release X-rays, but there are two possibilities. Perhaps the electrons created synchrotron radiation, which is produced when charged particles fly though magnetic fields. Or maybe the electrons collided with the Cosmic Microwave Background radiation left over in the universe from the big bang. When the sped-up particles hit the radiation they would have given it an energy boost, pumping its frequency up to the X-ray range of the electromagnetic spectrum.

New observations will be needed to tell which scenario occurred, the scientists said.

"These findings will help us better understand the properties of these clusters," Paltani told LiveScience. "This has important consequences for the history of the cluster itself. We will be able to put constraints on when the particle acceleration takes place and understand better what happens when these clusters merge."

By Clara Moskowitz Staff Writer
www.space.com

Cosmic Suburbia is a Better Breeding Ground for Stars

Credit: NASA/JPL-Caltech/D.Fadda (SSC-Caltech)

New observations from NASA's Spitzer Space Telescope suggest that galaxies prefer to raise stars in cosmic suburbia rather than in "big cities."

Galaxies across the universe reside in cosmic communities, big and small. Large, densely populated galactic communities are called galaxy clusters. Like big cities on Earth, galaxy clusters are scattered throughout the universe, connected by a web of dusty "highways" called filaments. While thousands of galaxies live within the limits of a cluster, smaller galactic communities are sprinkled along filaments, creating celestial suburbs. Over time, astronomers suspect that all galactic suburbanites will make their way to a cluster by way of filaments.

For the first time, Spitzer's supersensitive eyes have caught an infrared glimpse of several galaxies traveling along two filamentary roads into a galaxy cluster called Abell 1763.

"This is the first time we've ever seen a filament leading into a cluster with an infrared telescope," says Dario Fadda, of the Herschel Science Center, which is located at the California Institute of Technology in Pasadena, Calif.

"Our observations show that the fraction of starburst galaxies in the filaments is more than double the number of starburst galaxies inside the cluster region," he adds.

According to Fadda, clusters and the filaments that connect them are among the largest structures in the cosmos. To see them, astronomers need instruments that can map large areas of sky and have the sensitivity to resolve individual galaxies.

Luckily, instruments aboard Spitzer can do both. Using the telescope's multiband imaging photometer, Fadda and his colleagues saw structures spanning 23 million light-years. They used the observatory's infrared array camera to collect a census of each galaxy's star formation and used a ground-based telescope at the Kitt Peak National Observatory near Tucson, Ariz. to determine which galaxies belonged to the cluster and surrounding filaments. Ultimately, Fadda found that galaxies in the filaments form stars at a higher rate than their cluster counterparts.

"The new Spitzer findings will provide valuable insights into how galaxies grow and change as they leave cosmic suburbia for the big cities," says Fadda.

He notes that future infrared missions will be able to follow in Spitzer's footsteps and study how filaments and clusters affect the growth of galaxies in greater detail. One such mission is the European Space Agency's Herschel Space Telescope, which has significant NASA involvement.

His paper on this topic has been accepted for publication in Astrophysical Journal Letters. Co-authors on the paper include Andrea Biviano of the INAF/Osservatorio Astronomico di Trieste, Italy; Florence Durret of Institut d'Astrophysique de Paris, France; and Francine Marleau and Lisa Storrie-Lombardi of the Spitzer Science Center, Pasadena, Calif.

Written by Linda Vu, Spitzer Science Center

Near-Earth Asteroid 2007 TU24 to Pass Close to Earth on Jan. 29 - Should be Observable with Modest Sized Telescopes

The illustration below is courtesy of amateur astronomer Dr. Dale Ireland from Silverdale, WA. The illustration shows the asteroid's track on the sky for 3 days near the time of the close Earth approach as seen from the city of Philadelphia. Since the object's parallax will be a significant fraction of a degree, observers are encouraged to use our on-line Horizons ephemeris generation service for their specific locations.

Asteroid 2007 TU24, discovered by the Catalina Sky Survey on October 11, 2007 will closely approach the Earth to within 1.4 lunar distances (334,000 miles) on 2008 Jan. 29 08:33 UT. This object, between 150 and 600 meters in diameter, will reach an approximate apparent magnitude 10.3 on Jan. 29-30 before quickly becoming fainter as it moves further from Earth. For a brief time the asteroid will be observable in dark and clear skies with amateur telescopes of 3 inch apertures or larger.

Given the estimated number of near-Earth asteroids of this size (about 7,000 discovered and undiscovered objects), an object of this size would be expected to pass this close to Earth, on average, about every 5 years or so. The average interval between actual Earth impacts for an object of this size would be about 37,000 years. For the January 29th encounter, near Earth asteroid 2007 TU24 has no chance of hitting, or affecting, Earth.

2007 TU24 will be the closest currently known approach by a potentially hazardous asteroid of this size or larger until 2027. Plans have been made for the Goldstone planetary radar to observe this object Jan 23-24 and for the Arecibo radar to observe it Jan 27-28 and then Feb 1-4. High resolution radar imaging is expected, which may permit later 3-D shape reconstruction.

Don Yeomans
NASA/JPL Near-Earth Object Program Office
Release January 22, 2008

Internal Heat Drives Jupiter's Giant Storm Eruption

Credit: NASA, ESA, IRTF, and A. Sánchez-Lavega and R. Hueso (Universidad del País Vasco, Spain)

Detailed analysis of two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances. Understanding this outbreak could be the key to unlock the mysteries buried in the deep Jovian atmosphere, say astronomers.

Understanding these phenomena is important for Earth's meteorology where storms are present everywhere and jet streams dominate the atmospheric circulation. Jupiter is a natural laboratory where atmospheric scientists study the nature and interplay of the intense jets and severe atmospheric phenomena.

An international team coordinated by Agustin Sánchez-Lavega from the Universidad del País Vasco in Spain presents its findings about this event in the January 24 issue of the journal Nature.

The team monitored the new eruption of cloud activity and its evolution with an unprecedented resolution using NASA's Hubble Space Telescope, the NASA Infrared Telescope Facility in Hawaii, and telescopes in the Canary Islands (Spain). A network of smaller telescopes around the world also supported these observations.

According to the analysis, the bright plumes were storm systems triggered in Jupiter's deep water clouds that moved upward in the atmosphere vigorously and injected a fresh mixture of ammonia ice and water about 20 miles (30 kilometers) above the visible clouds. The storms moved in the peak of a jet stream in Jupiter's atmosphere at 375 miles per hour (600 kilometers per hour). Models of the disturbance indicate that the jet stream extends deep in the buried atmosphere of Jupiter, more than 60 miles (approximately100 kilometers) below the cloud tops where most sunlight is absorbed.

Dwarf Galaxy Holmberg IX

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: D. de Mello (Catholic University of America and GSFC)

This loose collection of stars is actually a dwarf irregular galaxy, called Holmberg IX. It resides just off the outer edge of M81, a large spiral galaxy in Ursa Major. This image was taken with Hubble's Advanced Camera for Surveys in early 2006. Holmberg IX is of the so-called Magellanic type of galaxy, as its size and irregularity in structure are similar to the Small Magellanic Cloud, a neighbor to our own Milky Way. Holmberg IX was first discovered by astronomer Sidney van den Bergh in 1959, and cataloged as DDO 66. The galaxy received its “Holmberg IX” naming when it was discussed in Eric Holmberg's study of groups of galaxies ten years later. It is suspected that the dwarf galaxy was created as a result of a galactic interaction between M81 and neighboring galaxy M82.

Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 – 200 million years. Due to the Advanced Camera for Surveys' resolution in this image, astronomers have noted that the old and the young stars have distinct spatial distributions which might be related to their origin.

Simulations predict that the triplet M81, M82, and nearby NGC 3077 had a close passage 200-300 million years ago. This close encounter may have triggered the newer star formation that has occurred in Holmberg IX.

The bluish-white fuzz in the space surrounding M81 and Holmberg IX is new star formation triggered by gravitational interactions between the two galaxies. There are many low mass galaxies that form stars in nearby space. While none of these are as dominated by recently produced stars as Holmberg IX, they might be related to the same family. By understanding how Holmberg IX was formed, scientists hope to understand their role as building blocks of large galaxies.

Orientation/Scale

Supernova Factory NGC 2770

Credit: A. de Ugarte Postigo (ESO) et al., Dark Cosmology Centre (NBI, KU),
Instituto de Astrofísica de Andalucía (CSIC), University of Hertfordshire

The stellar explosions known as supernovae are among the most powerful events in the universe. Triggered by the collapsing core of a massive star or the nuclear demise of a white dwarf, supernovae occur in average spiral galaxies only about once every century.

But the remarkable spiral galaxy NGC 2770 has lately produced more than its fair share. Two still bright supernovae and the location of a third, originally spotted in 1999 but now faded from view, are indicated in this image of the edge-on spiral.

All three supernovae are now thought to be of the core-collapse variety, but the most recent of the trio, SN2008D, was first detected by the Swift satellite at more extreme energies as an X-ray flash (XRF) or possibly a low-energy version of a gamma-ray burst on January 9th.

Located a mere 90 million light-years away in the northern constellation Lynx, NGC 2770 is now the closest galaxy known to host such a powerful supernova event.