Hipparcos Data Could Change Universe's Size
(From the May/June 1997 issue of StarDate magazine)
On one day in February, Polaris, the North Star, moved about 80 light-years farther away while one of the Milky Way's satellite galaxies, the Large Magellanic Cloud, receded several thousand light-years. The universe's rate of expansion didn't changed, but the way astronomers measure its size and age may have.
The early results are in from the most ambitious and accurate effort ever undertaken to study the positions, distances and motions of stars. It seems that astronomy's tried and true distance standards may be in need of some revision. The measurements upon which the new distances are based come from the Hippocarcos satellite, a European Space Agency project that operated from 1989 to 1993.
Hipparcos used the seasonal shift of star positions as Earth orbits the Sun to fix the positions of more than one million stars. The results will be compiled into two new star catalogs: the Hipparcos Catalog of 120,000 objects, accurate to within one one-thousandth of an arcsecond, and the Tycho Catalog of one million stars, accurate to about 20 to 30 arcseconds. The data improves on existing star surveys by a factor of 100.
The revised distances to some of the night sky's most familiar objects in turn demands a revision of the estimated size of the universe and the age of its oldest stars. Based on the preliminary data, Hipparcos investigators put those figures at 12 billion and 11 billion years, respectively, closely matching the most recent age estimate made with Hubble Space Telescope observations.
The astronomers who directed the Hipparcos survey base their findings on measurements of two classes of stars: Cepheid variables and Mira variables. The rate of a Cepheid's pulsing brightness correlates directly to its distance, a relationship astronomers have long used to calibrate astronomical distance scales. The Hipparcos measurements indicate that Cepheids are brighter and more distant than previously thought, which, if true, lengthens the cosmic yardstick based on Cepheids and adds about 10 percent to the size of the universe.
Michael Feast of the University of Cape Town and Robin Catchpole of the Royal Greenwich Observatory used Hipparcos to measure 26 Cepheids within about 2,000 light-years of Earth -- including Polaris. They then applied their findings to Cepheids in the Large Magellanic Cloud to propose a revised distance of 179,000 light-years from the commonly accepted value of 163,000 light-years.
Faster-pulsing Miras, on the other hand, had not been used to make cosmic measurements before Hipparcos, but the results support the Cepheid-based findings. Using Hipparcos measurements of 16 Miras within 1,000 light-years, Patricia Whitelock of the South African Astronomical Observatory calculated two revised distances to the Large Magellanic Cloud; one method added 3,000 light-years, the other 8,000 light-years.
Observations by Floor van Leeuwen of Greenwich of the more typical stars in the Pleiades Cluster, whose location also is used as a gauge of other distances, showed the cluster's members to be dimmer than expected. Van Leeuwen used the Hipparcos results to propose a revised Pleiades distance of 360 light-years from 400 light-years, posing a challenge to the increases based on variable stars.
Astronomers won't accept any revised estimates of the size and age of the universe without a great deal of review and debate. Their first chance comes when final Hipparcos data are released during a May ESA conference in Venice. -- Doug Addison
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