Barred Spiral Galaxies Are Latecomers to the Universe

07.29.08

 

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Release No.: STScI-2008-29
Credit: NASA, ESA, and Z. Levay (STScI)

A frequent sign of the maturity of a spiral galaxy is the formation of a ribbon of stars and gas that slices across the nucleus, like the slash across a "no smoking" sign.

In a landmark study of more than 2,000 spiral galaxies from the largest galaxy census conducted by NASA's Hubble Space Telescope, astronomers found that so-called barred spiral galaxies were far less plentiful 7 billion years ago than they are today, in the local universe.

The study's results confirm the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. The observations are part of the Cosmic Evolution Survey (COSMOS).

This new detailed look at the history of bar formation, made with Hubble's Advanced Camera for Surveys, provides clues to understanding when and how spiral galaxies formed and evolved over time.

A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.

Bars have been forming steadily over the last 7 billion years, more than tripling in number. "The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," Sheth explained. "They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today."

The findings, Sheth continued, have important ramifications for galaxy evolution. "We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time," he said.

COSMOS covers an area of sky nine times larger than the full Moon, surveying 10 times more spiral galaxies than previous observations. In support of the Hubble galaxy images, the team derived distances to the galaxies in the COSMOS field using data from Hubble and an assortment of ground-based telescopes.

Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. "The tiny elongations in the stars' orbits grow and they get locked into place, making a bar," explained team member Bruce Elmegreen of IBM's research Division in Yorktown Heights, N.Y. "The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy's inner region join the bar."

Added team member Lia Athanassoula of the Laboratoire d'Astrophysique de Marseille in France: "The new observations suggest that the instability is faster in more massive galaxies, perhaps because their inner disks are denser and their gravity is stronger."

Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes.

"The formation of a bar may be the final important act in the evolution of a spiral galaxy," Sheth said. "Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars."

Our Milky Way Galaxy, another massive barred spiral, has a central bar that probably formed somewhat early, like the bars in other large galaxies in the Hubble survey. "Understanding how bars formed in the most distant galaxies will eventually shed light on how it occurred here, in our own backyard," Sheth said.

Other members of the study include Debra Elmegreen (Vassar College); Nick Scoville (COSMOS principal investigator); Peter Capak, Richard Ellis, Mara Salvato, and Lori Spalsbury (California Institute of Technology); Roberto Abraham (University of Toronto); Bahram Mobasher (University of California, Riverside); Eva Schinnerer (Max Planck Institute for Astronomy, Heidelberg); Linda Strubbe and Andrew West (University of California, Berkeley); Mike Rich (University of California, Los Angeles); and Marcella Carollo (ETH Zurich).

For more images, visit:

http://hubblesite.org/newscenter/archive/releases/2008/29/image/

Most Black Holes Might Come in Only Small and Large

Until now, astronomers had suspected that globular clusters like the one pictured here were the most likely place to find medium-sized black holes -- elusive objects that have proved difficult to pin down. Globular clusters are spherical collections of stars that orbit around larger galaxies like our Milky Way. Scientists analyzed a globular cluster called RZ2109 and found it does not possess a medium-sized black hole. RZ2109 is much farther away than the globular cluster pictured here, called Omega Centauri. Credit: NASA/JPL-Caltech/NOAO/AURA/NSF
› Previously released image with caption Black holes are sometimes huge cosmic beasts, billions of times the mass of our sun, and sometimes petite with just a few times the sun's mass. But do black holes also come in size medium? A new study suggests that, for the most part, the answer is no.

Astronomers have long suspected that the most likely place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to find one conclusively.

Now, a team of astronomers has thoroughly examined a globular cluster called RZ2109 and determined that it cannot possess a medium black hole. The findings suggest that the elusive objects do not lurk in globular clusters, and perhaps are very rare.

"Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn't true," said Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, Calif. Stern is second author of a study detailing the findings in the Aug. 20 issue of Astrophysical Journal. The lead author is Stephen Zepf of Michigan State University, East Lansing.

Black holes are incredibly dense points of matter, whose gravity prevents even light from escaping. The least massive black holes known are about 10 times the mass of the sun and form when massive stars blow up in supernova explosions. The heftiest black holes are up to billions of times the mass of the sun and lie deep in the bellies of almost all galaxies.

That leaves black holes of intermediate mass, which were thought to be buried at the cores of globular clusters. Globular clusters are dense collections of millions of stars, which reside within galaxies containing hundreds of billions of stars. Theorists argue that a globular cluster should have a scaled down version of a galactic black hole. Such objects would be about 1,000 to 10,000 times the mass of the sun, or medium in size on the universal scale of black holes.

In a previous study, Zepf and his colleagues looked for evidence of a black hole in RZ2109, located 50 million light-years away in a nearby galaxy. Using the European Space Agency's XMM-Newton telescope (which derives its name from X-ray Multi-Mirror design), they discovered the telltale X-ray signature of an active, or "feeding" black hole. But, at that point, they still didn't know its size.

Zepf and Stern then teamed up with others to obtain a chemical fingerprint, called a spectrum, of the globular cluster, using the W.M. Keck Observatory on Mauna Kea in Hawaii. The spectrum revealed that the black hole is petite, with roughly 10 times the mass of our sun.

According to theory, a cluster with a small black hole cannot have a medium one, too. Medium black holes would be quite hefty with a lot of gravity, so if one did exist in a globular cluster, scientists argue that it would quickly drag any small black holes into its grasp.

"If a medium black hole existed in a cluster, it would either swallow little black holes or kick them out of the cluster," said Stern. In other words, the small black hole in RZ2109 rules out the possibility of a medium one.

How did the scientists figure out that the globular cluster's black hole was small in the first place? Using modeling techniques, Zepf and his colleagues concluded that the spectrum taken by Keck reveals high-velocity flows of matter, or "winds," firing out of the black hole. Only a small black hole could spit out these observed high winds.

Zepf explains, "We knew from X-ray data that this black hole was actively swallowing up, or accreting, material. If an intermediate-sized black hole were accreting this material, it wouldn't be too big of a deal for it. But if a small black hole were accreting this material, it would be a lot for it to take and therefore some material would be ejected in the form of high winds. Thus, the high winds were our smoking gun showing that this black hole is small."

Is this the end of the story for medium black holes? Zepf said it is possible such objects are hiding in the outskirts of galaxies like our Milky Way, either in surrounding so-called dwarf galaxies or in the remnants of dwarf galaxies being swallowed by a bigger galaxy. If so, the black holes would be faint and difficult to find.

Other authors of this paper include: Thomas Maccarone of the University of Southampton, England; Arunav Kundu of Michigan State University; Marc Kamionkowski of the California Institute of Technology, Pasadena; Katherine Rhode and John Salzer of Indiana University, Bloomington; and Robin Ciardullo and Caryl Gronwall of Penn State University, University Park, Pa. Salzer is also with Wesleyan University, Middleton, Conn.

JPL is managed by Caltech for NASA. More information about JPL is at www.jpl.nasa.gov.

Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.b.clavin@jpl.nasa.gov

Most Black Holes Might Come in Only Small and Large

Until now, astronomers had suspected that globular clusters like the one pictured here were the most likely place to find medium-sized black holes -- elusive objects that have proved difficult to pin down. Globular clusters are spherical collections of stars that orbit around larger galaxies like our Milky Way. Scientists analyzed a globular cluster called RZ2109 and found it does not possess a medium-sized black hole. RZ2109 is much farther away than the globular cluster pictured here, called Omega Centauri. Credit: NASA/JPL-Caltech/NOAO/AURA/NSF
› Previously released image with captionBlack holes are sometimes huge cosmic beasts, billions of times the mass of our sun, and sometimes petite with just a few times the sun's mass. But do black holes also come in size medium? A new study suggests that, for the most part, the answer is no.

Astronomers have long suspected that the most likely place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to find one conclusively.

Now, a team of astronomers has thoroughly examined a globular cluster called RZ2109 and determined that it cannot possess a medium black hole. The findings suggest that the elusive objects do not lurk in globular clusters, and perhaps are very rare.

"Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn't true," said Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, Calif. Stern is second author of a study detailing the findings in the Aug. 20 issue of Astrophysical Journal. The lead author is Stephen Zepf of Michigan State University, East Lansing.

Black holes are incredibly dense points of matter, whose gravity prevents even light from escaping. The least massive black holes known are about 10 times the mass of the sun and form when massive stars blow up in supernova explosions. The heftiest black holes are up to billions of times the mass of the sun and lie deep in the bellies of almost all galaxies.

That leaves black holes of intermediate mass, which were thought to be buried at the cores of globular clusters. Globular clusters are dense collections of millions of stars, which reside within galaxies containing hundreds of billions of stars. Theorists argue that a globular cluster should have a scaled down version of a galactic black hole. Such objects would be about 1,000 to 10,000 times the mass of the sun, or medium in size on the universal scale of black holes.

In a previous study, Zepf and his colleagues looked for evidence of a black hole in RZ2109, located 50 million light-years away in a nearby galaxy. Using the European Space Agency's XMM-Newton telescope (which derives its name from X-ray Multi-Mirror design), they discovered the telltale X-ray signature of an active, or "feeding" black hole. But, at that point, they still didn't know its size.

Zepf and Stern then teamed up with others to obtain a chemical fingerprint, called a spectrum, of the globular cluster, using the W.M. Keck Observatory on Mauna Kea in Hawaii. The spectrum revealed that the black hole is petite, with roughly 10 times the mass of our sun.

According to theory, a cluster with a small black hole cannot have a medium one, too. Medium black holes would be quite hefty with a lot of gravity, so if one did exist in a globular cluster, scientists argue that it would quickly drag any small black holes into its grasp.

"If a medium black hole existed in a cluster, it would either swallow little black holes or kick them out of the cluster," said Stern. In other words, the small black hole in RZ2109 rules out the possibility of a medium one.

How did the scientists figure out that the globular cluster's black hole was small in the first place? Using modeling techniques, Zepf and his colleagues concluded that the spectrum taken by Keck reveals high-velocity flows of matter, or "winds," firing out of the black hole. Only a small black hole could spit out these observed high winds.

Zepf explains, "We knew from X-ray data that this black hole was actively swallowing up, or accreting, material. If an intermediate-sized black hole were accreting this material, it wouldn't be too big of a deal for it. But if a small black hole were accreting this material, it would be a lot for it to take and therefore some material would be ejected in the form of high winds. Thus, the high winds were our smoking gun showing that this black hole is small."

Is this the end of the story for medium black holes? Zepf said it is possible such objects are hiding in the outskirts of galaxies like our Milky Way, either in surrounding so-called dwarf galaxies or in the remnants of dwarf galaxies being swallowed by a bigger galaxy. If so, the black holes would be faint and difficult to find.

Other authors of this paper include: Thomas Maccarone of the University of Southampton, England; Arunav Kundu of Michigan State University; Marc Kamionkowski of the California Institute of Technology, Pasadena; Katherine Rhode and John Salzer of Indiana University, Bloomington; and Robin Ciardullo and Caryl Gronwall of Penn State University, University Park, Pa. Salzer is also with Wesleyan University, Middleton, Conn.

JPL is managed by Caltech for NASA. More information about JPL is at www.jpl.nasa.gov.

Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.b.clavin@jpl.nasa.gov

Sally Ride: Setting the Stage for Women in Space

S84-37256: Sally RideAstronaut Sally K. Ride. Credit: NASA
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>NASA 50th Anniversary Moment Podcast
Last week, space shuttle Discovery touched down after a historic mission to the International Space Station, a flight that not only launched the largest laboratory to date, but also the 50th female U.S. astronaut. Just eight weeks prior, astronaut Peggy Whitson returned to Earth after a six-month stay in orbit as the first female space station commander. Women have established their place in space, but it was the flight of Sally Ride 25 years ago that paved the road to the stars.

Ride was a mission specialist on STS-7, launched June 18, 1983. The mission deployed two communications satellites and collected research on a number of scientific experiments.

“The fact that I was going to be the first American woman to go into space carried huge expectations along with it,” said Ride. “And that was made pretty clear the day that I was told I was selected as a crew. I was taken up to Chris Kraft’s office. He wanted to have a chat with me and make sure I knew what I was getting into before I made sure I went on the crew. I was so dazzled to be on the crew and go into space I remembered very little of what he said.”

S83-29016: STS-7 crewThese five astronauts represent the space shuttle's first five-member crew, STS-7. Astronaut Robert L. Crippen (center, first row) is crew commander. Other crew members are astronauts Frederick H. Hauck, right, pilot; and Sally K. Ride, John M. Fabian and Norman E. Thagard, mission specialists. Credit: NASA
Ride joined NASA as part of the 1978 astronaut class, the first class to include women. Ride and five other women were selected out of 8,000 applicants, 1,500 of which were female. Twenty-nine men also were selected. The class became known as the “Thirty-Five New Guys” and reported to the Johnson Space Center the next summer to begin training. Ride would train for five years before she and three of her classmates were assigned to STS-7.

“On launch day, there was so much excitement and so much happening around us in crew quarters, even on the way to the launch pad, going up the launch pad,” Ride said. “I didn’t really think about it that much at the time… but I came to appreciate what an honor it was to be selected to be the first woman to get a chance to go into space.”

Following that historic flight, Ride flew on another shuttle mission, STS-41G in 1984. She was assigned to a third mission, but transitioned to a role on the Challenger accident investigation panel in January 1986. Once the investigation was completed, she served as a special assistant to the NASA administrator.

Since then, Ride has returned to academia and her passion for inspiring young people. She has authored numerous books and founded Sally Ride Science, a company dedicated to supporting students and their interest in math and science.

NASA Awards Contract for Constellation Spacesuit for the Moon

The Constellation Program mission requires two spacesuit system configurations to meet the requirements of Orion missions to the space station and to the moon. Configuration One will support dynamic events such as launch and landing operations; contingency intravehicular activity (IVA) during critical mission events; off-nominal events such as loss of pressurization of the Orion crew compartment; and microgravity EVAs for contingency operations. Image Credit: NASA.

NASA has awarded a contract to Oceaneering International Inc. of Houston, for the design, development and production of a new spacesuit system. The spacesuit will protect astronauts during Constellation Program voyages to the International Space Station and, by 2020, the surface of the moon.

The subcontractors to Oceaneering are Air-Lock Inc. of Milford, Conn., David Clark Co. of Worcester, Mass., Cimarron Software Services Inc. of Houston, Harris Corporation of Palm Bay, Fla., Honeywell International Inc. of Glendale, Ariz., Paragon Space Development Corp. of Tucson, Ariz., and United Space Alliance of Houston.

"The award of the spacesuit contract completes the spaceflight hardware requirements for the Constellation Program's first human flight in 2015," said Jeff Hanley, Constellation program manager at NASA's Johnson Space Center in Houston. Contracts for the Orion crew capsule and the Ares I rocket were awarded during the past two years.

The cost-plus-award-fee spacesuit contract includes a basic performance period from June 2008 to September 2014 that has a value of $183.8 million. During the performance period, Oceaneering and its subcontractors will conduct design, development, test, and evaluation work culminating in the manufacture, assembly, and first flight of the suit components needed for astronauts aboard the Orion crew exploration vehicle. The basic contract also includes initial work on the suit design needed for the lunar surface.

"I am excited about the new partnership between NASA and Oceaneering," said Glenn Lutz, project manager for the spacesuit system at Johnson. "Now it is time for our spacesuit team to begin the journey together that ultimately will put new sets of boot prints on the moon."

Configuration Two will build upon Configuration One and will support lunar surface operations. While preparing to walk on the moon, the astronauts will construct Configuration Two by replacing elements of Configuration One with elements specialized for surface operations. Image Credit: NASA.

Suits and support systems will be needed for as many as four astronauts on moon voyages and as many as six space station travelers. For short trips to the moon, the suit design will support a week's worth of moon walks. The system also must be designed to support a significant number of moon walks during potential six-month lunar outpost expeditions. In addition, the spacesuit and support systems will provide contingency spacewalk capability and protection against the launch and landing environment, such as spacecraft cabin leaks.

Two contract options may be awarded in the future as part of this contract. Option 1 covers completion of design, development, test and evaluation for the moon surface suit components. Option 1 would begin in October 2010 and run through September 2018, under a cost-plus-award fee structure with a total value of $302.1 million.

Option 2 provides for the Orion suit production, processing and sustaining engineering under a cost-plus-award fee or a firm-fixed-price, indefinite-delivery, indefinite-quantity contract structure with a maximum value of $260 million depending on hardware requirements. Option 2 would begin at the end of the basic performance period in October 2014, and would continue through September 2018.

For The Love of Hubble

The Hubble Space Telescope, the most productive scientific instrument of all time, is slated for its fifth and final repair mission later this year. The space shuttle astronauts will launch from Kennedy Space Center in Florida, match orbits with the telescope, capture it, service it, upgrade it, and replace its broken parts—on the spot.

Roughly the size of a Greyhound bus, Hubble was launched aboard the space shuttle Discovery in 1990 and already has outlived its 15-year life expectancy. Students in high school today have never known a time without Hubble as their conduit to the cosmos. This new servicing mission will extend Hubble’s life several more years. It also will replace burned-out circuit boards to the Advanced Camera for Surveys. That’s the instrument responsible for Hubble’s most memorable images since it was installed in 2002.

Servicing Hubble is a task that requires exquisite dexterity. I recently had the opportunity to visit NASA’s Goddard Space Flight Center in Maryland. There, I donned puffy, pressurized astronaut gloves, wielded a space-age portable screwdriver, stuck my head in a space helmet, and attempted to extract a faulty circuit board in a mock-up of the Advanced Camera for Surveys, which was embedded within a full-scale model of the Hubble telescope. This was a darn-near impossible feat. And I wasn’t weightless. I was not wearing the full-body spacesuit. Nor were Earth and space drifting by.

We normally think of astronauts as brave and noble. But, in this case, having the “right stuff” includes being a hardware surgeon extraordinaire.

Perhaps you didn’t know, but Hubble is not alone up there. About two dozen space telescopes of assorted sizes and shapes orbit the Earth and the Sun. Each of them provides a clear view of the cosmos that is unobstructed, unblemished, and undiminished by Earth’s turbulent and murky atmosphere. But most of these telescopes were launched with no means of servicing them. Parts wear out. Gyroscopes fail. Batteries die. These hardware realities limit a telescope’s life expectancy to anywhere from three to seven years.

These telescopes all advance science, but most perform their duties without the public’s awareness or adulation. They are designed to detect bands of light invisible to the human eye, some of which never penetrate Earth’s atmosphere. Entire classes of objects and phenomena in the cosmos reveal themselves only through one or more of these invisible cosmic windows. Black holes, for example, were discovered by their X-ray calling card—radiation that was generated by the surrounding, swirling gas just before it descended into the abyss. Telescopes also have captured microwave radiation—the primary physical evidence for the Big Bang.

Hubble, on the other hand, is the first and only space telescope to observe the universe using primarily visible light. Its stunningly crisp, colorful, and detailed images of the cosmos make Hubble a kind of supreme version of human eyes in space. Yet Hubble’s appeal to us comes from much more than parades of pretty portraits. Hubble came of age in the 1990s, during an exponential growth of access to the Internet. That’s when its digital images were first cast into the public domain. As we all know, anything that’s fun, free, and forwardable spreads rapidly online. Hubble images, one more splendorous than the next, became screen savers and desktop “wallpaper” for computers owned by people who never before would have had the occasion to celebrate, however quietly, our place in the universe.

Indeed, Hubble brought the universe into our backyards. Or, rather, it expanded our backyards to enclose the universe itself. It did that with images so intellectually, visually, and even spiritually fulfilling that most don’t even need captions. No matter what Hubble reveals—planets, dense star fields, colorful interstellar nebulae, deadly black holes, graceful colliding galaxies, the large-scale structure of the universe—each image establishes your own private vista on the cosmos.

Hubble’s scientific legacy is unimpeachable. More research papers have been published using its data than have ever been published for any other scientific instrument in any discipline. Among Hubble’s highlights is settling the decades-old debate about the age of the universe. Previously, the data were so bad that astrophysicists could not agree. Some thought 10 billion years. Others, 20 billion. Yes, it was embarrassing. But Hubble enabled us to measure accurately how the brightness varies in a particular type of star that resides in a distant cluster of galaxies. That information, when plugged into a simple formula, tells us their distance from Earth. And because the entire universe is expanding at a known rate, we can then turn back the clock to determine how long ago everything was in the same place. The answer? The universe was born 14 billion years ago.

Another result, long suspected to be true but confirmed by Hubble, was the discovery that every large galaxy, such as our own Milky Way, has a supermassive black hole in its center that dines on stars, gas clouds, and other unsuspecting matter that wanders too close. The centers of galaxies are so densely packed with stars that Earth-based telescopes see only a mottled cloud of light—the merged image of hundreds or thousands of stars. From space, Hubble’s sharp imagery allows us to see each star individually and to track its motion around the galactic center. Behold, these stars move much, much faster than they have any right to. A small, unseen yet powerful source of gravity must be tugging on them. Crank the equations, and we are forced to conclude that a black hole lurks in their midst.

In 2004, a year after the Columbia tragedy, NASA announced that Hubble would not receive its last servicing mission. Curiously, the loudest voices of dissent were not from the scientists but from the general public. Akin to a modern version of a torch-wielding mob, angry editorials, snippy letters to the editor, and no end of radio and television talk shows all urged NASA to restore the funding and keep Hubble alive. Congress ultimately listened and reversed the decision. Democracy had a shining moment: Hubble would indeed be serviced, one last time.

For the first time in the history of civilization, the public took ownership of a scientific instrument—they took ownership of the Hubble Space Telescope.

Of course, nothing lasts forever—except, perhaps, the universe itself. So Hubble eventually will die. But in the meantime, NASA is building the James Webb Space Telescope, specially designed to see deeper into the universe than Hubble ever could. When launched early next decade, it will allow us to plumb the depths of gas clouds in our own Milky Way galaxy in search of stellar nurseries, as well as probe the earliest epochs of the universe in search of the formation of galaxies themselves.

Meanwhile, NASA plans to retire the aging space shuttle by 2010. This step will enable its aerospace engineers, assembly lines, and funding streams to focus on a new suite of launch vehicles that will do what the shuttles are not designed to do—return us to the Moon and take us on to Mars and beyond.

The march of discovery continues, driven by our timeless and collective urge to explore.

What We’ve Learned From Hubble
The Hubble has yielded an unprecedented scientific legacy. Among its top achievements:
* It allowed us to accurately measure the age of the universe.
* It confirmed that every large galaxy has at its center a massive black hole.
* It was key to the discovery of the role of “dark energy” in the expanding universe.

Where to Find Hubble Images:
The scientists who work at Hubble’s base-camp, the Space Telescope Science Institute in Baltimore, Maryland, are deeply aware of Hubble’s inspirational power. They identify the best of all images, and sometimes create images that they know in advance will have strong appeal. Visit the Hubble Heritage Project.

Will meteors from Halley's comet surge?

A possible flurry of "shooting stars" makes this year's Eta Aquarid meteor shower worth a look.

Francis Reddy

Will meteors from Halley's comet surge?

A possible flurry of "shooting stars" makes this year's Eta Aquarid meteor shower worth a look.

Francis Reddy

Eta Aquarid Meteors will pepper the sky before dawn May 5. With the Moon at new phase, observing conditions should be ideal. Astronomy: Roen Kelly

April 29, 2008 Be on the lookout for a rush of meteors before dawn Monday morning. That's when the annual Eta Aquarid meteor shower reaches maximum activity. Seeing the shower with no interference from the Moon is nice, but there's a possible bonus. Astronomers think the Eta Aquarids could produce more than twice the usual number of meteors. Meteors are fleeting fiery trails — "shooting stars" — that occur as small solid particles burn up in Earth's atmosphere. Comets shed dust as ice boils off their surfaces and litter their orbits with debris. Meteor showers result when Earth grazes a comet's dusty path and sweeps up some of these particles. Dust shed by Comet 1P/Halley creates the Eta Aquarid shower, so named because the meteors seem to emanate from a common point, or radiant, near the star Eta in the constellation Aquarius. Meteor-watching is a minimalist activity. No equipment is required — skygazers just need to know when and where to look. Dress warmly, relax in a comfortable chair, and keep an eye on the southeastern sky. It's kind of like fishing.

In Halley's dust Astronomers give a shower's meteor rate using numbers that express the number of meteors seen each hour by an observer viewing under a clear, dark sky when the radiant is overhead. In most years, by this measure, the Eta Aquarid shower rates 30 meteors per hour. But the radiant never gets overhead before dawn, so observers typically will see far fewer meteors. This year, though, the rate could more than double. Studies suggest the shower's rates rise and fall in a 12-year cycle. This period hints that Jupiter, the solar system's largest planet, is affecting the debris that creates the shower. Jupiter orbits the Sun in just under 12 years. Every time it passes closest to the Eta Aquarid track, the orbiting particles feel an extra-strong tug. This results in a wavy track that sometimes places extra dust in Earth's way.

Catch a falling star The Eta Aquarid shower is best for Southern Hemisphere observers, and the view gets worse the farther north you go. In the United States, the radiant stands only about 15° high in the southeast at 4 A.M. local daylight time. This low altitude will cut the number of visible meteors significantly. Even so, observers can expect a nice show. The shower produces pleasingly fast and often bright meteors. About 30 percent of the meteors leave behind dimly glowing trails called persistent trains. Some can be seen for as long as a minute. Although the radiant's low altitude reduces the number of observable meteors for northern observers, there is compensation. Eta Aquarid meteors tend to follow long paths across the sky.

Eta Aquarid meteor shower fast facts

The Eta Aquarids are the first of two annual showers produced by Halley's Comet. The other is the Orionid shower in late October. Astronomers discovered the shower in 1870 and linked it to Comet Halley just six years later. Its meteors are among the fastest, entering the atmosphere at 151,000 mph (243,000 km/h). The meteors average magnitude 3. The brighter ones display a yellowish color.

Astronomer Carter W. Roberts passes

Carter W. Roberts (1946-2008)

Posted 04-25-2008 by Jeremy McGovern

We received sad news last night that Carter Roberts passed away about a long battle with colon cancer.

Roberts was one of the true heavyweights of West Coast astronomy. He served on the board of Oakland’s Chabot Space and Science Center since 1994 and as president of the Eastbay Astronomical Society since 1988. He was also instrumental in coordinating events like the Riverside Telescope Makers Conference and Astronomy Day in Northern California.

In 2007, the Western Astronomical Association named Roberts winner of the G. Bruce Blair Award — the highest honor the WAA bestows on an individual — for his dedication to the hobby. Notably, he was instrumental in Chabot’s relocation and the restoration of the observatory’s original instruments.

Although we are saddened that Roberts is no longer with us, we can find comfort in his dedication to bringing the curious into our hobby. Those people, who in turn share their love of the night sky with future generations, are Roberts’ lasting legacy.

Galaxies gone wild

To mark the 18th anniversary of Hubble's launch, scientists have released 59 images of galaxies merging.

Provided by Hubble Europe

Interacting galaxies are found throughout the universe, sometimes as dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that result in new galaxies. This collection shows 12 of the most dynamic images of the overall 59. NASA, ESA, Hubble Heritage Team (STScI/AURA)-ESA and A. Evans/NRAO, K. Noll, and J. Westphal [View Larger Image] April 24, 2008 Fifty-nine new images of colliding galaxies make up the largest collection of Hubble images ever released together. As this astonishing Hubble atlas of interacting galaxies illustrates, galaxy collisions produce a remarkable variety of intricate structures. Interacting galaxies are found throughout the universe, sometimes as dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that result in new galaxies. A series of 59 new images of colliding galaxies has been released from the several terabytes of archived raw images from the NASA/ESA Hubble Space Telescope to mark the 18th anniversary of the telescope's launch. This is the largest collection of Hubble images ever released to the public simultaneously. Galaxy mergers, which were more common in the early universe than they are today, are thought to be one of the main driving forces for cosmic evolution, turning on quasars, sparking frenetic star births and explosive stellar deaths. Even apparently isolated galaxies will show signs in their internal structure that they have experienced one or more mergers in their past. Each of the various merging galaxies in this series of images is a snapshot of a different instant in the long interaction process. Our own Milky Way contains the debris of the many smaller galaxies it has encountered and devoured in the past, and it is currently absorbing the Sagittarius dwarf elliptical galaxy. In turn, it looks as if our Milky Way will be subsumed into its giant neighbour, the Andromeda galaxy, resulting in an elliptical galaxy, dubbed "Milkomeda", the new home for the Earth, the Sun and the rest of the Solar System in about two billion years time. The two galaxies are currently rushing towards each other at approximately 500,000 kilometres per hour. Cutting-edge observations and sophisticated computer models, such as those pioneered by the two Estonian brothers Alar Toomre and Juri Toomre in the 1970s, demonstrate that galaxy collisions are far more common than previously thought. Interactions are slow stately affairs, despite the typically high relative speeds of the interacting galaxies, taking hundreds of millions of years to complete. The interactions usually follow the same progression, and are driven by the tidal pull of gravity. Actual collisions between stars are rare as so much of a galaxy is simply empty space, but as the gravitational webs linking the stars in each galaxy begin to mesh, strong tidal effects disrupt and distort the old patterns leading to new structures, and finally to a new stable configuration. The pull of the Moon that produces the twice-daily rise and fall of the Earth's oceans illustrates the nature of tidal interactions. Tides between galaxies are much more disruptive than oceanic tides for two main reasons. Firstly, stars in galaxies, unlike the matter that makes up the Earth, are bound together only by the force of gravity. Secondly, galaxies can pass much closer to each other, relative to their size, than do the Earth and the Moon. The billions of stars in each interacting galaxy move individually, following the pull of gravity from all the other stars, so the interwoven tidal forces can produce the most intricate and varied effects as galaxies pass close to each other. Typically the first tentative sign of an interaction will be a bridge of matter as the first gentle tugs of gravity tease out dust and gas from the approaching galaxies (IC 2810). As the outer reaches of the galaxies begin to intermingle, long streamers of gas and dust, known as tidal tails, stretch out and sweep back to wrap around the cores (NGC 6786, UCG 335, NGC 6050). These long, often spectacular, tidal tails are the signature of an interaction and can persist long after the main action is over. As the galaxy cores approach each other their gas and dust clouds are buffeted and accelerated dramatically by the conflicting pull of matter from all directions (NGC 6621, NGC 5256). These forces can result in shockwaves rippling through the interstellar clouds (ARP 148). Gas and dust are siphoned into the active central regions, fuelling bursts of star formation that appear as characteristic blue knots of young stars (NGC 454). As the clouds of dust build they are heated so that they radiate strongly, becoming some of the brightest (luminous and ultraluminous) infrared objects (APG 220) in the sky. These objects emit up to several thousand billion times the luminosity of our Sun. They are the most rapidly star-forming galaxies in today's universe and are linked to the occurrence of quasars. Unlike standard spiral galaxies like the Milky Way, which radiate from stars and hot gas distributed over their entire span of perhaps 100 000 light-years, the energy in luminous and ultraluminous infrared galaxies is primarily generated within their central portion, over an extent of 1000 to 10,000 light-years. This energy emanates both from vigorous star formation processes, which can generate up to a few hundred solar masses of new stars per year (in comparison, the Milky Way generates a few solar masses of new stars per year), and from massive accreting black holes, a million to a billion times the mass of the Sun, in the central region. Intense star formation regions and high levels of infrared and far- infrared radiation are typical of the most active central period of the interaction and are seen in many of the objects in this release. Other visible signs of an interaction are disruptions to the galaxy nuclei (NGC 3256, NGC 17). This disruption may persist long after the interaction is over, both for the case where a larger galaxy has swallowed a much smaller companion and where two more closely matched galaxies have finally separated. Most of the 59 new Hubble images are part of a large investigation of luminous and ultraluminous infrared galaxies called the GOALS project (Great Observatories All-sky LIRG Survey). This survey combines observations from Hubble, the NASA Spitzer Space Observatory, the NASA Chandra X-Ray Observatory and NASA Galaxy Explorer. The Hubble observations are led by Professor Aaron S. Evans from the University of Virginia and the National Radio Astronomy Observatory (USA). A number of the interacting galaxies seen here are included in the The Atlas of Peculiar Galaxies, a remarkable catalog produced by the astronomer Halton Arp in the mid-1960s that built on work by B.A. Vorontsov-Velyaminov from 1959. Arp compiled the catalogue in a pioneering attempt to solve the mystery of the bizarre shapes of galaxies observed by ground-based telescopes. Today, the peculiar structures seen by Arp and others are well understood as the result of complex gravitational interactions.

Milky Way's black hole wakes up

Our galaxy's giant black hole erupted 300 years ago.
Provided by NASA's Goddard Space Flight Center
milky way's center
This Chandra image shows our galaxy’s center. The location of the black hole, known as Sagittarius A*, or Sgr A* for short, is arrowed. NASA/CXC/MIT/Frederick K. Baganoff et al. [View Larger Image]
April 15, 2008
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.
Sagittarius B2
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.

Smallest black hole found

NASA scientists identify the lowest-mass black hole known.

Provided by NASA's Goddard Space Flight Center

The lowest-mass known black hole belongs to a binary system named XTE J1650-500. The black hole has about 3.8 times the mass of our Sun, and is orbited by a companion star, as depicted in this illustration. NASA/CXC/A. Hobar [View Larger Image] April 1, 2008 Using a new technique, two NASA scientists have identified the lightest known black hole. With a mass only about 3.8 times greater than our Sun and a diameter of only about 15 miles, the black hole lies very close to the minimum size predicted for black holes that originate from dying stars. "This black hole is really pushing the limits. For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question," says lead author Nikolai Shaposhnikov of NASA's Goddard Space Flight Center. The tiny black hole resides in a Milky Way Galaxy binary system known as XTE J1650-500, named for its sky coordinates in the southern constellation Ara. NASA's Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001. Astronomers realized soon after J1650's discovery that it harbors a normal star and a relatively lightweight black hole. But the black hole's mass had never been measured to high precision. Shaposhnikov and his Goddard colleague Lev Titarchuk presented their results on Monday, March 31, at the American Astronomical Society High-Energy Astrophysics Division meeting in Los Angeles.

In this top-down illustration of a black hole and its surrounding disk, gas spiraling toward the black hole piles up just outside it, creating a traffic jam. The traffic jam is closer in for smaller black holes, so X-rays are emitted on a shorter timescale. NASA [View Larger Image] The method used by Shaposhnikov and Titarchuk has been described in several papers in the Astrophysical Journal. It uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before making the fatal plunge. When the feeding frenzy reaches a moderate rate, hot gas piles up near the black hole and radiates a torrent of X-rays. The X-ray intensity varies in a pattern that repeats itself over a nearly regular interval. This signal is called a quasi-periodic oscillation, or QPO. Astronomers have long suspected that a QPO's frequency depends on the black hole's mass. In 1998, Titarchuk realized that the congestion zone lies close in for small black holes, so the QPO clock ticks quickly. As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower. To measure the black hole masses, Shaposhnikov and Titarchuk use archival data from RXTE, which has made exquisitely precise measurements of QPO frequencies in at least 15 black holes. Last year, Shaposhnikov and Titarchuk applied their QPO method to three black holes whose masses had been measured by other techniques. In their new paper, they extend their result to seven other black holes, three of which have well-determined masses. "In every case, our measurement agrees with the other methods," says Titarchuk. "We know our technique works because it has passed every test with flying colors."

The measurement of the black hole's mass is due to high-precision timing observations made by NASA’s Rossi X-ray Timing Explorer satellite, shown here prior to launch. NASA [View Larger Image]

When Shaposhnikov and Titarchuk applied their method to XTE J1650-500, they calculated a mass of 3.8 Suns, with a margin of uncertainty of only half a Sun. This value is well below the previous black hole record holder with a reliable mass measurement, GRO 1655-40, which tips the scales at about 6.3 Suns.

Below some unknown critical threshold, a dying star should produce a neutron star instead of a black hole. Astronomers think the boundary between black holes and neutron stars lies somewhere between 1.7 and 2.7 solar masses. Knowing this dividing line is important for fundamental physics, because it will tell scientists about the behavior of matter when it is scrunched into conditions of extraordinarily high density.

Despite the diminutive size of this new record holder, future space travelers had better beware. Smaller black holes like the one in J1650 exert stronger tidal forces than the much larger black holes found in the centers of galaxies, which make the little guys more dangerous to approach. "If you ventured too close to J1650's black hole, its gravity would tidally stretch your body into a strand of spaghetti," says Shaposhnikov.

Shaposhnikov adds that RXTE is the only instrument that can make the high-precision timing observations necessary for this line of research. "RXTE is absolutely crucial for these black hole mass measurements," he says.

A new nova in Cygnus

V2491 Cygni is buffered by the two yellow lines. The star SAO 68730 star is to the top right. David Haworth April 14, 2008 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.

Use this chart to find V2491 Cygni through binoculars. Astronomy: Roen Kelly [View Larger Image]

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.