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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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.