David Aguilar
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Christine Pulliam
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CfA Press Release

AUSTIN, TX-- Serendipitous before-and-after observations of a distant galaxy containing an unusual exploding star known as Supernova 1998bu may help determine exactly how fast the universe is expanding. The observations were presented today at the American AstronomicalSociety meeting in Austin, TX, by a team including Saurabh Jha, Peter M. Garnavich, Peter M. Challis, and Robert P. Kirshner, all of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. Their result may also lead to better estimates of the age of the universe and the distances to remote galaxies.

Supernova 1998bu was discovered by the Italian amateur astronomer Mirko Villi on May 9, 1998. The supernova occurred in the galaxy M96, in the constellation Leo, and is located about 39 million light-years from Earth. By good fortune, the distance to that galaxy had been already measured with HST observations in 1995 by Prof. Nial R. Tanvir at the University of Cambridge, England.

In 1929, astronomer Edwin Hubble discovered that other galaxies were moving away from our own galaxy, the Milky Way. In addition, he found that the more distant galaxies were from Earth, the faster they appeared to recede. This observation, known as Hubble's Law, was explained in Albert Einstein's Theory of General Relativity as resulting from the expansion of the universe. Since Hubble's time, astronomers have been trying to measure the rate of this expansion, known as Hubble's Constant.The results presented today use exploding stars to measure Hubble's Constant. These supernovae, of a specific class called ``Type Ia,'' are thought to occur when a dying star called a white dwarf becomes too massive to support itself, and undergoes a colossal thermonuclear explosion, briefly and spectacularly brightening some ten billion times more than normal before gradually fading away over a period of a few months. At its peak, such a supernova can almost become as bright as its entire surrounding galaxy.

All supernovae of this class are thought to be very similar, reaching very nearly the same intrinsic peak brightness. Thus, they are used as cosmic ``standard light-bulbs,'' i.e., by measuring how bright a supernova appears, astronomers can determine how far away it is. In simplest terms, the dimmer it looks, the farther away it is.

However, while astronomers think that all these supernovae have the same intrinsic brightness, they don't know exactly how bright that is. The situation is analogous to knowing that in a collection of light bulbs each has the same brightness, but not being able to read the exact wattage labels. To determine the supernovae's intrinsic brightness, astronomers need to observe examples in nearby galaxies whose distances are known with great accuracy, preferably from observations with the Hubble Space Telescope (HST).

Only three other Type Ia supernovae have been observed with modern equipment and have had their distances measured using the HST. Normally, a supernova is discovered and studied first and then the distance is measured with HST. However, in the case of SN 1998bu, the galaxy was measured three years before the supernova was discovered. ``We were very lucky to catch a supernova in a galaxy already well studied with the Hubble Space Telescope,'' says Saurabh Jha, a graduate student at Harvard University and lead author of the study.

Using SN 1998bu and other Type Ia supernovae, the group estimates the universe is expanding at a rate of 64 kilometers per second per megaparsec (44,000 miles per hour per million light-years). This means that a galaxy which is 100 megaparsecs away (326 million light-years) is moving away from us at a speed of 6,400 kilometers per second (14 million miles per hour). When combined with results from even more distant supernovae, the team can estimate how long the expansion has been going on since the Big Bang at the beginning of the universe. They conclude that the universe is likely between 12.5 and 15.6 billion years old.

The research group also includes Alexei V. Fillipenko, Adam G. Riess, Weidong Li, Maryam Modjaz and Richard R. Treffers of the University of California at Berkeley; Eva K. Grebel of theUniversity of California at Santa Cruz;. Patrick Seitzer at the University of Michigan; George H. Jacoby of the Kitt Peak National Observatory; Priscilla J. Benson of Wellesley College ; and Akbar Rizvi and Laurence A. Marschall of Gettysburg College.

Another team, led by Nicholas Suntzeff of the Cerro-Tololo Inter-American Observatory in Chile, studied SN 1998bu using telescopes in the southern hemisphere. Suntzeff's group came to asimilar conclusion for the rate of expansion of the Universe.``It's great that both groups agree," says team member Dr. Peter Garnavich." I think we are finally closing in on the true expansionrate of the Universe."

The astronomers began observations of SN 1998bu soon after it was discovered, realizing that this object was very important since the distance to the galaxy M96 was already measured. Using these supernovae to measure the expansion rate of the universe requires careful observations of the supernova as it brightens and fades. The team obtained data from a large number of telescopes: the 1.2-m (48 inch) and 1.5-m (60 inch) diameter telescopes at the Fred L. Whipple Observatory on Mt. Hopkins, AZ; the Michigan-Dartmouth-MIT 2.4-m (94 inch) telescope, the Wisconsin-Indiana-Yale-National Optical Astronomy Observatories 3.5-m (138 inch) telescope, and the National Science Foundation's Kitt Peak National Observatory 0.9-m (36 inch) telescope,all on Kitt Peak, AZ;, the Whitin Observatory 0.6-m (24inch) telescope at Wellesley College in Wellesley, MA; the Gettysburg College Observatory 0.4-m (16 inch) telescope in Gettysburg, PA; and the Katzman Automatic Imaging Telescope on Mt. Hamilton, CA.

Images and captions accompanying this release are available at:


For more information, contact:

Peter Garnavich, Harvard-Smithsonian Center for Astrophysics,617-495-9245, pgarnavich@cfa.harvard.edu
Robert P. Kirshner, Harvard-Smithsonian Center for Astrophysics,617-495-7519, rkirshner@cfa.harvard.edu

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