HST Key Project Observation of M81 Cepheids

A team of astronomers working with NASA's Hubble Space Telescope announced results today of a major step in the Key Project to measure the Hubble Constant and the age of the universe. The team has discovered variable stars in its first target, the spiral galaxy M81, and measured the distance of the galaxy to be 11 million light-years (3.4 megaparsecs). They quote a 10% uncertainty in this result (plus or minus approximately one million light-years). Previous estimates of the galaxy's distance have ranged from 4.5 to 18 million light-years (1.4 to 5.6 megaparsecs). Team member Dr. Wendy Freedman of Carnegie Institution of Washington said, "In our two observed fields in M81 we have found a total of 32 Cepheids (variable stars). Decades of previous work from the largest ground-based telescopes have only succeeded in measuring periods for two Cepheids. HST's superior resolution and its ability to schedule observations when and where they are required give it a special advantage in this work." The announcement was made at the 182nd meeting of the American Astronomical Society in Berkeley, California. Their results are detailed in several presentations by team members at that meeting and are being submitted for publication in the Astrophysical Journal. The team includes Laura Ferrarese and Dr. Holland Ford of Space Telescope Science Institute, Baltimore, Maryland. The astronomers used the Hubble' 5 Wide Field & Planetary Camera to study two fields in M81. In each field they took 22 twenty-minute exposures spread over 14 months to find the variable stars and measure their periods and brightness. Cepheids are pulsating stars that become alternately brighter and fainter with periods ranging from 10 to 50 days. Astronomers have known for over 50 years that the periods of these stars precisely predict their total luminous power, which allows their distance to be measured. Messier 81 is a large spiral galaxy in the constellation Ursa Major. It is a rotating system of gas and stars similar to our own Galaxy, the Milky Way, but approximately twice as massive. This galaxy achieved prominence three months ago when the brightest northern supernova this century was discovered. In the expanding universe, the Hubble Constant (H0) is the ratio of the recession velocities of galaxies to their distances. The age of the universe can be estimated from the Hubble Constant. The age is currently thought to lie between 10 and 20 billion years, but a more precise measurement of the Hubble Constant is required to narrow this range. Key projects are designated top priority scientific goals for the Hubble Space Telescope. This extragalactic distance scale Key Project aims to discover Cepheids and measure the distances to galaxies in order to determine an accurate value of the Hubble Constant. Dr. Jeremy Mould, Principal Investigator for the team, said, "This is the first step in a major program of measuring distances of galaxies with the Hubble Space Telescope. When the telescope is serviced later this year, and the new Wide Field & Planetary Camera is installed with its corrective optics, we plan to use the same technique on galaxies up to 50 million light-years away, which will allow us to measure the Hubble Constant, the rate of expansion of the universe. We have three years of work ahead of us, and until the project is substantially complete, I won't speculate on what value of HO this work will yield." Although this HST Key Project has the explicit goal of getting H0, other astronomers have used Hubble to search for Cepheids. Previous HST observations carried out by a different group also demonstrated HST's unique capability by resolving 27 Cepheids in another galaxy. The team, led by Jeremy Mould (Caltech), consisted of Sandra Faber and Garth Illingworth (Univ. of California, Santa Cruz), Wendy Freedman, John Graham, and Robert Hill (Carnegie Institution of Washington), John Hoessel ~niv. of Wisconsin, Madison), John Huchra (Center for Astrophysics, Cambridge, MA), Shaun Hughes (Caltech), Robert Kennicutt (Univ. of Arizona), Myung Gyoon Lee (Carnegie), Barry Madore (Caltech), Peter Stetson (Dominion Astrophysical Observatory, Victoria, British Columbia), and Anne Turner (U. Arizona), Laura Ferrarese and Holland Ford (Space Telescope Science Institute).