Larry Klaes ( )
Wed, 26 May 1999 10:34:34 -0400

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>EMBARGOED UNTIL: 11:00 AM (EDT) May 25, 1999
>CONTACT: Donald Savage
> NASA Headquarters, Washington, DC
> (Phone: 202/358-1547)
> Nancy Neal
> Goddard Space Flight Center, Greenbelt, MD
> (Phone: 301/286-0039)
> Ray Villard
> Space Telescope Science Institute, Baltimore, MD
> (Phone: 410/338-4514)
>The Hubble Space Telescope Key Project team today announced that it has
>completed efforts to measure precise distances to far-flung galaxies, an
>essential ingredient needed to determine the age, size and fate of the
>"Before Hubble, astronomers could not decide if the universe was 10
>billion or 20 billion years old. The size scale of the universe had a
>range so vast that it didn't allow astronomers to confront with any
>certainty many of the most basic questions about the origin and
>eventual fate of the cosmos," said team leader Wendy Freedman, of the
>Observatories of the Carnegie Institution of Washington. "After all
>these years, we are finally entering an era of precision cosmology.
>Now we can more reliably address the broader picture of the universe's
>origin, evolution and destiny."
>The team's precise measurements are the key to learning about the
>expansion rate of the universe, called the Hubble constant. Measuring
>the Hubble constant was one of the three major goals for NASA's Hubble
>Space Telescope before it was launched in 1990.
>For the past 70 years astronomers have sought a precise measurement of
>the Hubble constant, ever since astronomer Edwin Hubble realized that
>galaxies were rushing away from each other at a rate proportional to
>their distance, i.e. the farther away, the faster the recession. For
>many years, right up until the launch of the Hubble telescope - the
>range of measured values for the expansion rate was from 50 to 100
>kilometers per second per megaparsec (a megaparsec, or mpc, is 3.26
>million light-years).
>The team measured the Hubble constant at 70 km/sec/mpc, with an
>uncertainty of 10 percent. This means that a galaxy appears to be moving
>160 thousand miles per hour faster for every 3.3 light-years away from
>"The truth is out there, and we will find it," said Dr. Robert Kirshner,
>of Harvard University. "We used to disagree by a factor of 2; now we are
>just as passionate about 10 percent. A factor of two is like being
>unsure if you have one foot or two. Ten percent is like arguing about
>one toe. It's a big step forward."
>Added Dr. Robert Kennicutt of the University of Arizona, a co-leader of
>the team: "Things are beginning to add up. The factor of two controversy
>is over."
>The team used the Hubble telescope to observe 18 galaxies out to 65
>million light-years. They discovered almost 800 Cepheid variable stars,
>a special class of pulsating star used for accurate distance
>measurement. Although Cepheids are rare, they provide a very reliable
>"standard candle" for estimating intergalactic distances. The team used
>the stars to calibrate many different methods for measuring distances.
>"Our results are a legacy from Hubble telescope that will be used in a
>variety of future research," said Dr. Jeremy Mould, of the Australian
>National University, also a co-leader of the team. "It's exciting to see
>the different methods of measuring galaxy distances converge, calibrated
>by the Hubble Space Telescope."
>Combining the Hubble constant measurement with estimates for the density
>of the universe, the team determined that the universe is approximately
>12 billion years old - similar to the oldest stars. This discovery
>clears up a nagging paradox that arose from previous age estimates. The
>researchers emphasize that the age estimate holds true if the universe
>is below the so-called "critical density" where it is delicately
>balanced between expanding forever or collapsing. Or, the universe is
>pervaded by a mysterious force pushing the galaxies farther apart, in
>which case the Hubble measurements point to an even older universe.
>The universe's age is calculated using the expansion rate from precise
>distance measurements, and the calculated age is refined based on
>whether the universe appears to be accelerating or decelerating, given
>the amount of matter observed in space. A rapid expansion rate indicates
>the universe did not require as much time to reach its present size, and
>so it is younger than if it were expanding more slowly.
>The Hubble Space Telescope Key Project team is an international group of
>27 astronomers from 13 different U.S. and international institutions.
>The Space Telescope Science Institute is operated by the Association of
>Universities for Research in Astronomy, Inc. for NASA, under contract
>with NASA's Goddard Space Flight Center in Greenbelt, MD.
>- end -
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>EMBARGOED UNTIL: 11:00 A.M. (EDT) May 25, 1999
>A NASA Hubble Space Telescope (HST) view of the magnificent spiral
>galaxy NGC 4603, the most distant galaxy in which a special class of
>pulsating stars called Cepheid variables have been found. It is
>associated with the Centaurus cluster, one of the most massive
>assemblages of galaxies in the nearby universe. The Local Group of
>galaxies, of which the Milky Way is a member, is moving in the direction
>of Centaurus at a speed of more than a million miles an hour under the
>influence of the gravitational pull of the matter in that direction.
>Clusters of young bright blue stars highlight the galaxy's spiral arms.
>In contrast, red giant stars in the process of dying are also found.
>Only the very brightest stars in NGC 4603 can be seen individually, even
>with the unmatched ability of the Hubble Space Telescope to obtain
>detailed images of distant objects. Much of the diffuse glow comes from
>fainter stars that cannot be individually distinguished by Hubble. The
>reddish filaments are regions where clouds of dust obscure blue light
>from the stars behind them.
>This galaxy was observed by a team affiliated with the HST Key Project
>on the Extragalactic Distance Scale. Because NGC 4603 is much further
>away than the other galaxies studied with Hubble by the Key Project
>team, 108 million light-years, its stars appear very faint from the
>Earth, and so accurately measuring their brightness, as is required for
>distinguishing the characteristic variations of Cepheids, is extremely
>difficult. At this distance some non-variable stars may by chance appear
>to grow brighter and fainter in the same fashion as Cepheids due to the
>physical impossibility of perfect measurements of such dim objects.
>Determining the distance to the galaxy required an unprecedented
>statistical analysis based on extensive computer simulations.
>Researchers found 36-50 Cepheids and used their observed properties to
>securely determine the distance to NGC 4603. These measurements indicate
>that when the expansion of the Universe and the motion of the Local
>Group are accounted for, the Centaurus cluster is very nearly at rest
>compared to the surrounding regions. It is part of the cause of the
>rapid motions in the nearby universe, rather than being strongly pulled
>by other concentrations of matter. Observations of distant Cepheids
>such as those in NGC 4603 also help astronomers to precisely measure the
>expansion rate of the Universe.
>Credit: Jeffrey Newman (Univ. of California at Berkeley) and NASA
>Albert Einstein rejected it, Edwin Hubble embraced it, and many
>astronomers since then have debated it. Now a team of astronomers using
>the Earth-orbiting Hubble Space Telescope, named after Edwin, has
>refined the value for the expansion rate of the cosmos, called the
>Hubble constant. This number holds the key to other fundamental
>astronomical questions. By nailing down the Hubble constant, astronomers
>can figure out the size of the universe and work backward to determine
>how long it has been around. Here is a thumbnail history of the Hubble
>1908: Harvard Observatory astronomer Henrietta S. Leavitt makes the
>first crucial step in establishing the distances to nearby "spiral
>nebulae." Studying variable stars in the Magellanic Clouds, she
>discovers the presence of rhythmically pulsating stars, known as Cepheid
>variables, which brighten and dim over a period of days. By observing
>the relationship between a Cepheid's brightness and its pulsation rate,
>astronomers can calculate how much light it emits and then use that
>number to estimate its distance.
>1912: Vesto M. Slipher of Lowell Observatory studies the motion of about
>50 "spiral nebulae." He notices that most of them appear to be fleeing
>away from Earth at a very fast rate.
>1916-1927: Einstein applies his newly published general theory of
>relativity to the structure of the universe. In Einstein's universe,
>space remains static, neither expanding nor contracting. Willem de
>Sitter counters in 1917 with an expanding universe model, also
>consistent with Einstein's theory. De Sitter's universe, however, is
>devoid of matter. Aleksandr Friedmann and Georges Lemaitre also join the
>theoretical jousting match, providing their own models for an expanding
>1923: Edwin Hubble, working at the Carnegie Institution's Mount Wilson
>Observatory in California, pinpoints 12 Cepheid variable stars in the
>"spiral nebulae" M3 and M22 in the Triangulum nebula. By deducing the
>distances, he establishes that they are individual galaxies, far outside
>the Milky Way galaxy.
>1929: Hubble delivers the observational evidence that space is
>expanding. Studying 18 spiral galaxies, Hubble discovers a connection
>between the motions of galaxies and their distances from our galaxy. He
>proposes that the farther a galaxy is from us, the faster it is speeding
>into space. For example, a galaxy 10 times farther away than another
>would be moving 10 times faster. He calls this relationship the
>"velocity-distance relation." Today astronomers call it Hubble's law,
>and the value that relates the velocity to the distance is called the
>Hubble constant.
>1931: Hubble and Milton L. Humason determine the brightness of Cepheid
>variable stars in the Local Group of galaxies and other stars in M81,
>M101, and NGC 2403. They calculate a Hubble constant of 558 kilometers
>per second per megaparsec. In other words, galaxies appear to be
>receding from us at a rate of 383,000 mph for every one million
>light-years farther out we look.
>1954: The Hubble constant tumbles from 558 to 280 when Walter Baade
>shows that Hubble had unknowingly used two distinct populations of stars
>- with different relationships between pulsation rate and light output -
>to calibrate distance. He had therefore underestimated the distances to
>nearby galaxies and hence the size of the universe.
>1956: After Hubble's death, Allan Sandage of the Carnegie Observatories
>in Pasadena, Calif., takes up the quest for the Hubble constant. He
>discovers that many of the objects Hubble had regarded as the brightest
>stars in nearby galaxies are in fact groups of stars or clouds of
>illuminated gas. Sandage slashes the Hubble constant to 75, further
>increasing the distance scale. A lower Hubble constant implies that the
>universe is expanding slowly and that it has taken a longer time to
>reach its current size.
>1956-1994: Over the next four decades, several astronomers pursue the
>Hubble constant, most notably Sandage, Gustav Tammann of the University
>of Basel in Switzerland, Gerard de Vaucouleurs of the University of
>Texas, and Sidney van den Bergh of Dominion Astrophysical Observatories
>in Canada. Sandage and Tammann chip away at the number, arriving at a
>value of around 50. De Vaucouleurs and van den Bergh, Sandage's primary
>sparring partners, reach values around 100. A number of younger
>astronomers, among them Brent Tully, Richard Fisher, Marc Aaronson,
>Jeremy Mould, John Huchra, Rob Kennicutt, Barry Madore, and Wendy
>Freedman, enter the field and begin to derive values between 50 and 100.
>Astronomers realize that they have hit a cosmological brick wall.
>Ground-based telescopes can only resolve Cepheid variables, the
>cosmological "milepost markers," in nearby galaxies. To obtain an
>accurate value for the Hubble constant, astronomers recognize that they
>must peer farther across space. One of the mandates for the
>Earth-orbiting Hubble Space Telescope - launched in 1990 - is to catch
>the pulsating rhythms of Cepheids at greater distances. The telescope is
>expected to collect Cepheids in galaxies 10 times farther away than
>ground-based telescopes can spot them.
>1994: Wendy Freedman of the Carnegie Observatories in Pasadena, Calif.,
>Jeremy Mould of the Australian National University, Robert Kennicutt of
>the University of Arizona in Tucson, and an international team of
>astronomers announce that the Hubble telescope had "pushed the
>envelope," detecting Cepheid variable stars farther out in space than
>ever before. The telescope had spied these "milepost markers" in the
>remote spiral galaxy M100, a member of the Virgo cluster. This
>preliminary observation establishes the distance to the cluster as about
>56 million light-years and a Hubble constant of 80.
>1994-1999: While Cepheid variables are useful "cosmic yardsticks," even
>the Hubble telescope can't pick them out of far-flung galaxies. So, the
>Freedman, Mould, and Kennicutt team refines a technique dubbed the
>"cosmological distance ladder" to gauge distances to galaxies far across
>the cosmos. The team uses Cepheids from nearby galaxies and "secondary
>distance markers" - such as a special class of exploding star called
>Type Ia supernovae - to determine distances to faraway galaxies. In 1996
>a separate team led by Sandage reports a value of 57, and subsequently a
>value of 59.
>1999: The Freedman, Mould, and Kennicutt team announces its final
>measurement for the universe's expansion rate. The astronomers determine
>a value of 70, with an uncertainty of 10 percent. Using the Hubble
>telescope to observe 18 galaxies - the farthest of which is 65 million
>light-years away - they discover about 800 Cepheids. These predictable
>stars are then used to measure even farther distances with the
>"secondary distance markers."