Observations of a remote galaxy in the Virgo cluster by the Jet Propulsion Laboratory's Wide Field and Planetary Camer on board NASA's Hubble Space Telescope -- have yielded the most accurate estimate yet of the distance to that galaxy and the rate at which the universe is expanding.

Images of Messier 100, a large spiral galaxy in the constellation Virgo, revealed bright Cepheid stars which were used to pinpoint the galaxy's distance at approximately 51 million light-years from Earth. The observations mark the completion of a major milestone for the recently refurbished Hubble telescope, designed to measure distant galaxies and determine the true value of the Hubble constant, first demonstrated in 1929 by the American astronomer Edwin Hubble.

The findings were presented in this week's issue of Nature in an article entitled, "Distance to the Virgo Cluster Galaxy M100 from Hubble Space Telescope Observations of Cepheid Variables." An international team of astronomers, led by Dr. Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif. and Dr. Barry Madore of the California Institute of Technology/JPL Infrared Processing and Analysis Center (IPAC), coauthored the paper.

"Although this is only the first step in a major systematic program to securely measure the dimensions and age of the universe, a firm distance to the Virgo cluster is a critical milestone for the extragalactic distance scale," Freedman said. "This accomplishment has major implications for our determination and understanding of the Hubble constant."

Freedman and her colleagues made the determination during a 60-day observing window which began in April 1994 and ended in June 1994. Other members of the science team included Drs. Robert Kennicutt of Steward Observatory, University of Arizona and Jeremy Mould of Mount Stromlo and Siding Springs Observatories at the Australian National University.

The team used JPL's Wide Field and Planetary Camera-2 to image a field of stars set off from the nucleus of M100 but, nevertheless, including regions rich in recent star formation and potential Cepheid candidates.

Cepheids are pulsating, very luminous, supergiant stars that regularly dim and brighten in a characteristic "saw tooth" pattern of light variation. Periods range from a few days to a few hundred days.

Astronomers have known for more than half a century that the period of variation in a Cepheid precisely predicts its total luminosity. By comparing the predicted brightness with the observed brightness, they are able to calculate distances in the universe.

"Only the orbiting Hubble Space Telescope can make these types of observations routinely," Freedman said. "Typically, Cepheids are too faint and the resolution is too poor as seen from ground-based telescopes to clearly detect them in a crowded region of a distant galaxy."

Twelve one-hour exposures, strategically timed within the two-month observing window, did, in fact, result in the discovery of 20 new Cepheids and the simultaneous determination of periods and measurements of average brightness.

More than 40,000 stars were measured in the search for these rare but bright variables, said Dr. Barry Madore of NASA's Infrared Processing and Analysis Center at Caltech. Once identified, intermittent observations made at near infrared wavelengths were able to account for the dimming effects caused by dust and gas found along Hubble's line of sight out of the Milky Way galaxy and into the Virgo galaxy.

The target galaxy, M100, is a large, face-on rotating system of gas and stars similar to the Milky Way galaxy in which the solar system resides. This galaxy has been host to several massive star explosions, called supernova, which are considered significant distance indicators among astronomers and cosmologists. Agreement between the Cepheid star distances and M100's supernova distances adds significant weight to the astronomers' new calculations of the Hubble constant.

"Now that we've got an accurate measurement of a galaxy in the Virgo cluster, we will begin to observe other, more distant galaxies and get a handle on other Cepheid distances," Madore said. "This is an objective that was nothing more than a wish and a dream to astronomers five years ago."

In the expanding universe, the Hubble constant is the ratio of the velocity of recession of galaxies to their distances. The dynamic, ever-changing age of the universe can be estimated from the Hubble constant. From their observations, Freedman's team has concluded that the Hubble constant has a value of 80 kilometers per second per mega parsec (1 parsec equals 3.26 light years), with an estimated uncertainty of plus or minus 17 kilometers per second. That makes the universe younger than the generally held estimate of about 20 billion years old.

The new estimate of age brings into question other age estimates, the astronomers contend, including stellar evolution dating of globular star clusters and the timescales thought to be necessary for the formation of large-scale structures in galaxies throughout the universe.

The science observations were made as part of one of several key project priorities using the Hubble Space Telescope. The project, called the Extragalactic Distance Scale Key Project, is discovering Cepheids in a variety of important galaxies in order to determine their individual distances and to arrive at an accurate value of the Hubble constant in general.

The relative ease with which the discovery of Cepheids in M100 was accomplished now suggests to the scientific community that the Hubble telescope may be able to determine distances to remote galaxies that were before inaccessible because they had been blurred and distorted by the Earth's atmosphere.

"The Cepheid variables in M100 popped out at us like sparkling gems when we were reviewing the images, telling us that we will probably be able to use the telescope to observe galaxies as far as 150 million light-years away," Madore said.

"Hubble is also allowing us to make observations in a few short months that used to take a decade or more to complete," he added. "At this rate, we're going to find the answer to the Hubble constant in our lifetimes."

The Wide Field and Planetary Camer the primary imaging camera on board the Hubble Space Telescope -- was built by JPL for NASA's Office of Space Science, Washington, D.C.

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