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Trail of Dying Green Stars Leads to Millions of Stars Hiding in Galaxy Clusters

6 January 1999

Early results from the largest new survey of dying stars in the Virgo Cluster reveal the existence of large numbers of stars in areas of space that previously appeared to be empty. The discovery, which uses dying stars known as planetary nebulae as tools for probing the universe, indicates that at least 22 percent of Virgo's light is coming from previously unknown stars that populate the space between the cluster's galaxies.

The discovery was made by John J. Feldmeier, a Penn State graduate student, Robin Ciardullo, associate professor of astronomy and astrophysics at Penn State, and George H. Jacoby, staff astronomer at Kitt Peak National Observatories, using observations from the National Science Foundation's (NSF) Kitt Peak National Observatory in Tucson, Arizona. The astronomers will present their discovery at 9:20 a.m., Central time, on Thursday, January 7, 1999, during the 193rd National Meeting of the American Astronomical Society in Austin, Texas.

"Planetary nebulae are dying stars having what I call a stellar heart attack — a very short phase that most stars will go through at the end of their lifetime," Feldmeier explains. When a star is in the planetary nebula phase, its outside layers of gas come off, exposing its very hot inner core, which ionizes and lights up the gas around it in only a few specific wavelengths. "A huge amount of green light comes out at the 5007-angstrom wavelength, but not, for example, at 5006 angstroms or 5008 angstroms," Feldmeier says.

The astronomers used a technique, which Ciardullo and Jacoby developed, that takes advantage of this strange green starlight. "We take a picture through a filter that accepts only 5007 angstroms, and another picture through a filter that accepts only a different wavelength, and when we subtract the two all the planetary nebulae just pop right out as distinct points of light," Feldmeier explains.

The astronomers have used their technique to discover 130 previously unknown planetary nebulae in six small patches during the initial phase of their large-scale survey of the space between galaxies in the Virgo Cluster. "We are able to see the planetary nebulae even though they are literally 48 million light years away," Feldmeier explains. "We expect to find tens of thousands of planetary nebulae as we piece together a more complete picture of the Virgo Cluster, and to use them to know Virgo better than any other galaxy cluster in the universe," Ciardullo adds.

Feldmeier, Ciardullo, and Jacoby are using the green glow of the planetary nebulae as beacons to shine light on a number of astronomical mysteries, including how much mass might be hiding between galaxies throughout the universe, how galaxies form and change in a cluster, how long ago the Virgo Cluster formed, the three-dimensional shape of the Virgo Cluster, and where the orphan stars were born.

"For every planetary nebula we see during its short stellar-heart-attack phase, there are literally millions of other normal stars in other phases of their life that we can't see out there," Ciardullo says," so we think the stellar mass of this cluster has been underestimated. The researchers say their findings could affect estimates of the amount of mass the universe contains, which is a critical element in attempts to model the evolution and eventual fate of the universe. "Our research indicates that at least some of the "missing mass" needed for some models is not some form of exotic matter but just normal stars that we could not detect before, which could affect existing models of how the universe works," Ciardullo comments.

The astronomers say their evidence also tentatively supports the idea that one way galaxies change in clusters is by disrupting each other during grazing near-miss encounters.  The idea is that when galaxies in a crowded cluster careen past each other, some of their stars can be tugged so strongly by the gravity of the passing mass that they get ripped out of the parent galaxy, leaving an arching trail of stars hanging in space.  "Basically, it's like demolition derby," Feldmeier explains.  "The longer the galaxies have had to run into each other the more beaten up they get and the more they make what I call orphan stars," Feldmeier says.

"We think we will be able to tell something about the age and energy of a galaxy cluster by using planetary nebulae to gauge the distribution and number of the intergalactic orphan stars it contains," Ciardullo adds. Well-formed trails of green-glowing stars would indicate a cluster of moderate age — old enough to have had interactions between its galaxies but not so old as to have destroyed the evidence. "We are just starting to piece together a larger patchwork picture of Virgo, and one of our first results is that these planetary nebulae are not randomly distributed but appear to be clustered in some way that could possibly be these arching trails," Ciardullo says. "Eventually, when we have a more complete picture of the cluster, we hope to be able to see these trails of green stars and, perhaps, follow the trail from one galaxy to the other finding intracluster stars along the whole way," Feldmeier adds.

Another clue the astronomers discovered is that the planetary nebulae contain oxygen, which can be created only deep in the cores of a previous generation of stars that have exploded into space at the end of their lifetime and then been compressed again by gravity into their present form.  The researchers reason that, because the intracluster stars formed from the recycled stardust of a previous generation of stars, they are more likely to have formed where there are lots of other stars--in a galaxy--than alone in an isolated area of space. "It is rather remarkable, I think, that we can now say it looks like these stars have been ripped out of galaxies in the not-so-distant past," Ciardullo remarks. "As we gather more data during this survey, we hope to be able to tighten up that argument."

The researchers also learned something about the three-dimensional shape of the Virgo Cluster by using another unusual characteristic of planetary nebulae, which is that there is a maximum limit to how bright they can get. "We were able to map the area around the galaxy named M87, which people generally accept as Virgo's core, and we found that Virgo is shaped more like a football than a sphere — it's longer pointing toward us than side to side," Ciardullo says. Because the astronomers make the assumption that all the planetary nebulae in the Virgo cluster attain the same maximum possible brightness, they can use the stars' apparent brightness when observed from Earth to judge their relative distances — those that appear to be dimmer are assumed to be farther away from Earth. "If it actually is fainter than you assume, it could be even closer but not farther away," Ciardullo explains.

This spring, the team plans to use a much larger camera at the Kitt Peak observatory, which will allow them to survey five times as much area in the same amount of time. "Instead of finding 130, we could find 600," Feldmeier speculates. "We are probing this cluster in a way that no one has done before — by using planetary nebulae to look between the galaxies at the stars that have been orphaned — and are discovering things that we could not have learned another way," Ciardullo says.

This research was supported, in part, by the National Aeronautical and Space Administration and the National Science Foundation. The Kitt Peak National Observatory is one of four divisions of the National Optical Astronomy Observatories (NOAO). NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation.

Contact at Penn State: Barbara Kennedy, 814-863-4682,
Contact at NOAO: Bruce Bohannan, 520-318-8157,
Contact at AAS meeting: Lynn Cominsky, press room 512-404-4650 (fax

High-resolution images will be available on the World Wide Web beginning on
January 7, 1999, at

This press release is issued jointly by Penn State and the National Optical Astronomy Observatories.