Astro-2 Public Affairs Status Report #20 6:00 p.m. CST ( 9/17:22 MET), March 11, 1995 Spacelab Mission Operations Control Marshall Space Flight Center Huntsville, Ala. On the tenth day of the STS-67 mission, Astro-2 scientists took their first look in the extreme ultraviolet at what may be the most massive star in the known universe. They also focused their observations on spiral galaxies, elliptical galaxies, and star clusters. HD 269810, a faint O-class star located in the Large Magellanic Cloud, is about 190 times as massive as Earth's sun and qualifies as a candidate for the most massive star ever observed. Hopkins Ultraviolet Telescope Guest Investigator Dr. Nolan R. Walborn, of the Space Telescope Science Institute, used the HUT instrument to study the star's unusually powerful stellar wind, or expanding outer layer, like it has never been seen before. Although this star probably possesses the most mass a star can have, its stellar wind is depleting this mass at an accelerated rate. These observations provide crucial information regarding the ultimate fate of the most massive stars. Dr. Walborn uses the HUT to study a sample of very hot O class stars that are currently being observed in ultraviolet wavelengths of 1200 angstroms and above by the Hubble Space Telescope. However, only HUT is able to take its unprecedented measurements of these stars in the 900-1200 angstrom wavelength range. Commenting on what he called the "beautiful" real-time data acquired from this observation, Dr. Walborn said, "Now we have new information about a current candidate for the most massive star known. This was the star I wanted to see, and now we have it." Earlier this morning, Payload Specialist Sam Durrance successfully pointed the Astro telescopes at a somewhat challenging target, the elliptical galaxy M60, to enable the science teams to obtain an overall average spectrum of stars in its core. A roughly sphere-shaped galaxy with no clearly discernible internal structure, M60 contains older, evolved stars and therefore represents a stable stage of development. Due to the faintness of this galaxy and the absence of suitable guide stars for automatic targeting, manual acquisition of this target was necessary. The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) also observed the center of the M60 giant elliptical for polarized radiation, or radiation preferentially oriented in one direction. M60 is part of rich cluster of galaxies in the Virgo constellation, containing about 2500 galaxies, and has a notable ultraviolet excess. The Ultraviolet Imaging Telescope (UIT) viewed NGC 6791, an unusual open cluster located 14,000 light years away in the Lyra constellation. Open clusters are diffuse collections of 100 to 1,000 stars and are usually thought to be young systems, less than 10 million years old. As one of the oldest known open clusters, NGC 6791 is three billion years older than our solar system. Because UIT's field of view is well matched to the sizes of these clusters, UIT scientists are able to perform ultraviolet observations of many stars at the same time. Another target for UIT, M101 in the Ursa Major constellation, was a big spiral galaxy with arms that are wide and not very tightly wound. A perfect example, or prototype, of spiral structure, it is the highest priority spiral galaxy for UIT. This galaxy contains bright regions, such as Searle 2 (named for Dr. Leonard Searle, the astronomer who first discovered that region), of glowing hydrogen ionized by hot blue stars. UIT's ultraviolet imaging offers a powerful new tool for the study of these regions, especially since it emphasizes these hot stars. Ultraviolet imaging also suppresses the cool star background of the galaxy, allowing the young hot stars to become more evident to astronomers studying the evolution of stars. Early tomorrow morning, UIT will attempt its most difficult observation when the Astro-2 telescopes will be pointing at the moon to provide the first detailed far ultraviolet photographs of our nearest neighbor in space. To follow the mission in progress, visit Astro-2's home page on the Internet World Wide Web: URL "http://astro-2.msfc.nasa.gov"