Hubble Identifies Source Of Ultraviolet Light In An Old Galaxy

Hubble confirms that the ultraviolet light comes from a population of extremely hot helium-burning stars at a late stage in their lives. Unlike the Sun, which burns hydrogen into helium, these old stars exhausted their central hydrogen long ago, and now burn helium into heavier elements.

The observations, taken in October 1998, were made with the camera mode of the Space Telescope Imaging Spectrograph (STIS) in ultraviolet light. The STIS field of view is only a small portion of the entire galaxy, which is 20 times wider on the sky. For reference, the full moon is 70 times wider than the STIS field of view. The bright center of the galaxy was placed on the right side of the image, allowing fainter stars to be seen on the left side of the image.

These results will be published in March 1, 2000 issue of The Astrophysical Journal.

Thirty years ago, the first ultraviolet observations of elliptical galaxies showed that they were surprisingly bright when viewed in ultraviolet light. Before those pioneering UV observations, old groups of stars were assumed to be relatively cool and thus extremely faint in the ultraviolet. Over the years since the initial discovery of this unexpected ultraviolet light, indirect evidence has accumulated that it originates in a population of old, but hot, helium-burning stars. Now Hubble provides the first direct visual evidence.

Nearby elliptical galaxies are thought to be relatively simple galaxies comprised of old stars. Because they are among the brightest objects in the Universe, this simplicity makes them useful for tracing the evolution of stars and galaxies.

Credits: NASA and Thomas M. Brown, Charles W. Bowers, Randy A. Kimble, Allen V. Sweigart (NASA Goddard Space Flight Center) and Henry C. Ferguson (Space Telescope Science Institute).

NOTE TO EDITORS: For additional information, please contact Thomas Brown, NASA Goddard Space Flight Center, Greenbelt, MD 20770, phone: 301-286-5765, e-mail: tbrown@pulsar.gsfc.nasa.gov or Harry Ferguson, Space Telescope Science Institute, Baltimore, MD 21218, phone: 410-338-5098, e-mail: ferguson@stsci.edu.

Image files are available on the Internet at:http://oposite.stsci.edu/pubinfo/pr/1999/40 or via links inhttp://oposite.stsci.edu/pubinfo/latest.html a