OUR SKY - News

A new nova in Cygnus

Astronomy on 04/24/2008 at 12:55pm (UTC)

Amateur astronomers Koichi Nishiyama and Fujio Kabashima in Japan discovered a bright nova in the constellation Cygnus the Swan April 10. Astronomers initially catalog such events as variable stars. This one received the label V2491 Cygni. Recent estimates place the object's brightness at magnitude 7.6.

The magnitude scale provides a way to compare the brightnesses of celestial objects. The brightest stars have magnitudes of 0 and 1, and the faintest stars visible to the unaided eye from a dark site typically have a magnitude around 6.5. The nova, therefore, lies just below the naked-eye visibility limit. This means you can spot V2491 Cygni easily through binoculars.

To spot the nova, use the finder chart on this page. Cygnus rises in the northeast and is fully visible just after 11 P.M. local time. It continues to climb higher in the sky until dawn. The Moon, a few days after First Quarter, lies across the sky in the constellation Leo the Lion. Moonset occurs around 3:30 A.M. local time. The nova shines brightly enough that moonlight will not interfere with the view.

Amateur astronomers may want to sketch or photograph this region each night over the next week or so. Such images will show how the star brightens or fades, and are important in the study of novae. You can submit your images to the American Association of Variable Star Observers at www.aavso.org.

A nova is an explosion resulting when hydrogen from one star of a binary system falls onto the surface of the second star, which is a white dwarf. White dwarfs represent the last stage in the lives of Sun-like stars. In such cases, the star shines like the Sun from a few billion to about 20 billion years. Energy production exhausts the nuclear fuel in its core, and the core shrinks. This heats up the core, causing the star's outer layers to expand. As the core cools, it shrinks to form a white dwarf star.

Nishiyama, 70, is from Kurume, Fukuoka-Ken, and Kabashima, 68, from Miyaki-cho, Saga-ken. Both are well-known supernova hunters. Nishiyama takes images with the duo's 16-inch (0.4 meter) reflector using a charge-coupled device (CCD) camera in their Miyaki Argenteus Observatory. Kabashima then analyzes the images with a personal computer.

Planet-harboring star drifted from the Hyades

Astronomy on 04/24/2008 at 12:51pm (UTC)

By studying in great detail the 'ringing' of a planet-harboring star, a team of astronomers using ESO's 3.6-m telescope have shown that it must have drifted away from the metal-rich Hyades cluster. This discovery has implications for theories of star and planet formation, and for the dynamics of our Milky Way.

The yellow-orange star Iota Horologii, located 56 light-years away towards the southern Horologium (The Clock) constellation, belongs to the so-called Hyades stream, a large number of stars that move in the same direction.

Previously, astronomers using an ESO telescope had shown that the star harbors a planet, more than 2 times as large as Jupiter and orbiting in 320 days.

But until now, all studies were unable to pinpoint the exact characteristics of the star, and hence to understand its origin. A team of astronomers, led by Sylvie Vauclair from the University of Toulouse, France, therefore decided to use the technique of asteroseismology to unlock the star's secrets.

"In the same way as geologists monitor how seismic waves generated by earthquakes propagate through the Earth and learn about the inner structure of our planet, it is possible to study sound waves running through a star, which forms a sort of large, spherical bell," says Vauclair.

The ringing from this giant musical instrument provides astronomers with plenty of information about the physical conditions in the star's interior.

And to 'listen to the music,' the astronomers used one of the best instruments available. The observations were conducted in November 2006 during eight consecutive nights with the state-of-the-art HARPS spectrograph mounted on the ESO 3.6-m telescope at La Silla.

Up to 25 different notes could be identified in the unique dataset, most of them corresponding to waves having a period of about 6.5 minutes.

These observations allowed the astronomers to obtain a very precise portrait of Iota Horologii: its temperature is 6150 K, its mass is 1.25 times that of the Sun, and its age is 625 million years. Moreover, the star is found to be more metal-rich than the Sun by about 50 percent.

"These results show the power of asteroseismology when using a very precise instrument such as HARPS," says Vauclair. "It also shows that Iota Horologii has the same metal abundance and age as the Hyades cluster and this cannot be a coincidence."

The Hyades is an ensemble of stars that is seen with the unaided eye in the Northern constellation Taurus (The Bull). This open cluster, located 151 light-years away, contains stars that were formed together 625 million years ago.

The star Iota Horologii must have thus formed together with the stars of the Hyades cluster but must have slowly drifted away, being presently more than 130 light-years away from its original birthplace. This is an important result to understand how stars move on the galactic highways of the Milky Way.

This also means that the amount of metals present in the star is due to the original cloud from which it formed and not because it engulfed planetary material. "The chicken and egg question of whether the star got planets because it is metal-rich, or whether it is metal-rich because it made planets that were swallowed up is at least answered in one case," says Vauclair.

Cassini receives extension

Astronomy on 04/24/2008 at 12:49pm (UTC)

NASA is extending the international Cassini-Huygens mission by 2 years. The historic spacecraft's stunning discoveries and images have revolutionized our knowledge of Saturn and its moons.

Cassini's mission originally had been scheduled to end in July 2008. The newly-announced 2-year extension will include 60 additional orbits of Saturn and more flybys of its exotic moons.

These will include 26 flybys of Titan, seven of Enceladus, and one each of Dione, Rhea and Helene. The extension also includes studies of Saturn's rings, its complex magnetosphere, and the planet.

"This extension is not only exciting for the science community, but for the world to continue to share in unlocking Saturn's secrets," says Jim Green, director of the Planetary Science Division at NASA Headquarters in Washington, D.C.

"New discoveries are the hallmarks of its success, along with the breathtaking images beamed back to Earth that are simply mesmerizing," Green says.

"The spacecraft is performing exceptionally well and the team is highly motivated, so we're excited at the prospect of another 2 years," says Bob Mitchell, Cassini program manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

Based on findings from Cassini, scientists think liquid water may be just beneath the surface of Saturn's moon, Enceladus. That's why the small moon, only one-tenth the size of Titan and one-seventh the size of Earth's Moon, is one of the highest-priority targets for the extended mission.

Cassini discovered geysers of water-ice jetting from the Enceladus' surface. The geysers, which shoot out at a distance three times the diameter of Enceladus, feed particles into Saturn's most expansive ring. In the extended mission, the spacecraft may come as close as 15 miles from the moon's surface.

Cassini's observations of Saturn's largest moon, Titan, have given scientists a glimpse of what Earth might have been like before life evolved. They now believe Titan possesses many parallels to Earth, including lakes, rivers, channels, dunes, rain, snow, clouds, mountains and possibly volcanoes.

"When we designed the original tour, we really did not know what we would find, especially at Enceladus and Titan," says Dennis Matson, the JPL Cassini project scientist. "This extended tour is responding to these new discoveries and giving us a chance to look for more."

Unlike Earth, Titan's lakes, rivers and rain are composed of methane and ethane, and temperatures reach a chilly minus 290° Fahrenheit (143° Celsius). Although Titan's dense atmosphere limits viewing the surface, Cassini's high-resolution radar coverage and imaging by the infrared spectrometer have given scientists a better look.

Other activities for Cassini scientists will include monitoring seasons on Titan and Saturn, observing unique ring events, such as the 2009 equinox when the Sun will be in the plane of the rings, and exploring new places within Saturn's magnetosphere.

Cassini has returned a daily stream of data from Saturn's system for almost 4 years. Its travel scrapbook includes nearly 140,000 images and information gathered during 62 revolutions around Saturn, 43 flybys of Titan and 12 close flybys of the icy moons.

More than 10 years after launch and almost 4 years after entering into orbit around Saturn, Cassini is a healthy and robust spacecraft. Three of its science instruments have minor ailments, but the impact on science-gathering is minimal. The spacecraft will have enough propellant left after the extended mission to potentially allow a third phase of operations. Data from the extended mission could lay the groundwork for possible new missions to Titan and Enceladus.

Cassini launched October 15, 1997, from Cape Canaveral, Florida, on a 7-year journey to Saturn, traversing 2.2 billion miles (3.5 billion kilometers). It is one of the most scientifically capable spacecraft ever launched, with a record 12 instruments on the orbiter and six more instruments on the European Space Agency's Huygens probe, which piggybacked a ride to Titan on Cassini.

Cassini receives electrical power from three radioisotope thermoelectric generators, which generate electricity from heat produced by the natural decay of plutonium. The spacecraft was captured into Saturn orbit in June 2004 and immediately began returning data to Earth.

Milky Way's black hole wakes up

Astronomy on 04/24/2008 at 12:49pm (UTC)

Using NASA, Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that our galaxy's central black hole let loose a powerful flare 3 centuries ago.

The finding helps resolve a long-standing mystery: why is the Milky Way's black hole so quiescent? The black hole, known as Sagittarius A* (pronounced "A-star"), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."

The new study, which will appear in the Publications of the Astronomical Society of Japan, combines results from Japan's Suzaku and ASCA X-ray satellites, NASA's Chandra X-ray Observatory, and the European Space Agency's XMM-Newton X-ray Observatory.

The observations, collected between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-ray light as they responded to X-ray pulses emanating from just outside the black hole. When gas spirals inward toward the black hole, it heats up to millions of degrees and emits X-rays. As more and more matter piles up near the black hole, the greater the X-ray output.

An X-ray satellite imaged a small region in the gas cloud Sagittarius B2, and saw pockets brighten and fade over the course of nearly 12 years. These light echoes are caused by varying X-ray output from our galaxy’s central black hole. JAXA [View Larger Image]These X-ray pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier. When the X-rays reach the cloud, they collide with iron atoms, kicking out electrons that are close to the atomic nucleus. When electrons from farther out fill in these gaps, the iron atoms emit X-rays. But after the X-ray pulse passes through, the cloud fades to its normal brightness.

Amazingly, a region in Sagittarius B2 only 10 light-years across varied considerably in brightness in just 5 years. These brightenings are known as light echoes. By resolving the X-ray spectral line from iron, Suzaku's observations were crucial for eliminating the possibility that subatomic particles caused the light echoes.

"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago," says team member Katsuji Koyama of Kyoto University. "The black hole was a million times brighter 3 centuries ago. It must have unleashed an incredibly powerful flare."

This new study builds upon research by several groups who pioneered the light-echo technique. Last year, a team led by Michael Muno, who now works at the California Institute of Technology, used Chandra observations of X-ray light echoes to show that Sagittarius A* generated a powerful burst of X-rays about 50 years ago — about a dozen years before astronomers had satellites that could detect X-rays from outer space. "The outburst three centuries ago was 10 times brighter than the one we detected," says Muno.

The galactic center is about 26,000 light-years from Earth, meaning we see events as they occurred 26,000 years ago. Astronomers still lack a detailed understanding of why Sagittarius A* varies so much in its activity. One possibility, says Koyama, is that a supernova a few centuries ago plowed up gas and swept it into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber and produced the giant flare.

Light echo of a high-energy flash from a black hole has been observed in detail for the first time.

Astronomy on 04/24/2008 at 12:47pm (UTC)

A light echo occurs when interstellar gas is heated by radiation and reacts by emission of light. An international team led by Stefanie Komossa from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, has observed the light echo of an enormous X-ray flare, which was almost certainly produced when a single star was disrupted by a supermassive black hole. For the first time, the light echo of such a rare and highly dramatic event could be observed in great detail. The light echo not only revealed the stellar disruption process, but it also provides a powerful new method for mapping galactic nuclei.

When a star is disrupted by a black hole in the nucleus of a galaxy, its debris is inevitably attracted and absorbed by the black hole. This sudden increase in the accretion rate causes an abrupt burst of ultraviolet and X-ray light because the gas from the disrupted star becomes very hot. As the high-energy radiation travels through the core of the galaxy it illuminates surrounding matter and so makes it possible to probe regions of the galaxy that would otherwise be unobservable.

"To study the core of a normal galaxy is like looking at the New York skyline at night during a power failure: You can't learn much about the buildings, roads and parks," says Stefanie Komossa. "The situation changes, for example, during a fireworks display. It's exactly the same when a sudden burst of high-energy radiation illuminates a galaxy." However the astronomers had to hurry up and look through the telescope at the right moment, because X-ray bursts don't last very long.

From the strength, the degree of ionization and the deduced velocities of the rapidly varying emission lines, the physicists can tell in which part of the galaxy they are emitted. The emission lines represent the "fingerprints" of the atoms in the hot gases heated by the flare. The galaxy with catalog name SDSSJ0952+2143 which was detected in December 2007 by Komossa and her team in the Sloan Digital Sky Survey archive caught their attention because of its superstrong iron lines: the strongest (relative to oxygen emission) that were ever observed in a galaxy. In them the authors see an evidence for a molecular torus that plays an important part in so-called unified models of active galaxies.

The unified model postulates that all active galaxies are made of identical components and that the perceived differences are just due to the different directions from which we view the galaxies. An important element of this model is the molecular torus, which surrounds the black hole and its accretion disk and covers them when viewed from certain directions. Also, the breadth of the spectral lines that the scientists measure is influenced by the viewing direction and that means by the molecular torus.

Should the expectations of Komossa and her colleagues be confirmed, this will be the first time that scientists have seen such a strong time-variable signal from a molecular torus. From the light echo, the torus can be mapped and its geometry inferred, something that has not been possible up to now.

Along the same lines is the detection of variable emission in the infrared: It can be interpreted as the "last cry for help" of the heated dusty torus matter before the dust is destroyed by the flash.

In addition to the remarkably strong iron lines, the scientists also noticed a very peculiar shape of the hydrogen emission lines that had never been seen before. This line hints at activities of the disk of matter around the black hole, which consists mainly of hydrogen.

"Probably we are seeing the debris of the disrupted star here which is just being accreted by the black hole," explains Hongyan Zhou from the MPE, co-author of the research paper.

The recently discovered light echo still continues and is being traced with powerful telescopes. The burst itself has faded away. The first observations with the X-ray satellite Chandra show measurable but already faint X-ray light from the galactic nucleus. "Reverberation-mapping of light echoes opens up new possibilities to study galaxies," concludes Komossa. The team now wants to use this method to explore the physical conditions in the circumnuclear material in active and non-active galaxies

Stellar birth in the galactic wilderness

Astronomy on 04/24/2008 at 12:47pm (UTC)

A new image from NASA's Galaxy Evolution Explorer shows baby stars sprouting in the backwoods of a galaxy — a relatively desolate region of space more than 100,000 light-years from the galaxy's bustling center.

The striking image, a composite of ultraviolet data from the Galaxy Evolution Explorer and radio data from the National Science Foundation's Very Large Array in New Mexico, shows the Southern Pinwheel galaxy, also known simply as M83.

In the new view, the main spiral, or stellar, disk of M83 looks like a pink and blue pinwheel, while its outer arms appear to flap away from the galaxy like giant red streamers. It is within these so-called extended galaxy arms that, to the surprise of astronomers, new stars are forming.

"It is absolutely stunning that we find such an enormous number of young stars up to 140,000 light-years away from the center of M83," says Frank Bigiel of the Max Planck Institute for Astronomy in Germany, lead investigator of the new Galaxy Evolution Explorer observations. For comparison, the diameter of M83 is only 40,000 light-years across.

Some of the "outback" stars in M83's extended arms were first spotted by the Galaxy Evolution Explorer in 2005. Remote stars were also discovered around other galaxies by the ultraviolet telescope over subsequent years. This came as a surprise to astronomers because the outlying regions of a galaxy are assumed to be relatively barren and lack high concentrations of the ingredients needed for stars to form.

The newest Galaxy Evolution Explorer observations of M83 (colored blue and green) were taken over a longer period of time and reveal many more young clusters of stars at the farthest reaches of the galaxy. To better understand how stars could form in such unexpected territory, Bigiel and his colleagues turned to radio observations from the Very Large Array (red). Light emitted in the radio portion of the electromagnetic spectrum can be used to locate gaseous hydrogen atoms, or raw ingredients of stars. When the astronomers combined the radio and Galaxy Evolution Explorer data, they were delighted to see they matched up.

"The degree to which the ultraviolet emission and therefore the distribution of young stars follows the distribution of the atomic hydrogen gas out to the largest distances is absolutely remarkable," says Fabian Walter, also of the Max Planck Institute for Astronomy, who led the radio observations of hydrogen in the galaxy.

The astronomers speculate that the young stars seen far out in M83 could have formed under conditions resembling those of the early universe, a time when space was not yet enriched with dust and heavier elements.

"Even with today's most powerful telescopes, it is extremely difficult to study the first generation of star formation. These new observations provide a unique opportunity to study how early generation stars might have formed," says co-investigator Mark Seibert of the Observatories of the Carnegie Institution of Washington in Pasadena.

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