UW ASTRONOMERS IDENTIFY COLDEST KNOWN STAR April 23, 1998 -- The coldest known star has been discovered by an international team of astronomers led by David Ciardi and Steve Howell of the University of Wyoming Department of Physics and Astronomy. Ciardi, a postdoctoral researcher working with Howell, an assistant professor, led the research effort along with collaborators Peter Hauschildt of the University of Georgia, Vik Dhillon of the Royal Greenwich Observatory and the University of Sheffield, and France Allard of Wichita State University. Their results will be reported in the September 1998 edition of the Astrophysical Journal. Work by the international team was aimed at understanding the life cycles of each of the two stars contained within very old binary systems. They estimate the temperature of a secondary star in an observed binary system to be no hotter than 1,700 degrees Kelvin or 3,600 Fahrenheit. "For comparison, the temperature of the sun is about 6,000 degrees K or 11,000 F and until this discovery, the coldest star known was about 2,600 degrees K or 5,400 F," Ciardi says. The binary system, known as WZ Sagittae, contains a more massive white dwarf star and a less massive "normal" star. The extremely dense white dwarf, about the mass of the sun but in a sphere about the size of the Earth, has drained material from the smaller secondary star over the span of billions of years. During that time, the secondary star has slowly become smaller and smaller. As a result its temperature has been greatly reduced. Ciardi says many stars form originally as binary systems, where mutual gravitational attraction holds two stars in orbit for eternity. Stars in a binary system are near enough that the more massive star drains the other star of its material. Recent work by an international team led by Howell predicted, then confirmed that after billions of years the less massive star becomes progressively smaller and colder, ending its life as a unique type of stellar end-product. "These cooler stars are as different from other stellar end-products, like black holes, pulsars and white dwarfs, as they are from normal stars," Ciardi says. "We now believe we have found a binary system with the smallest and coldest star yet identified." Because binary systems take billions of years to evolve, Ciardi, Howell and their collaborators have used the binary evolution theory to estimate that our galaxy is at least 10 billion years old. The discovery of a binary system with an extremely cold secondary star is a major observational confirmation of their theories. The astronomers used observations from some of the world's largest telescopes, including the United Kingdom Infrared Telescope on Mauna Kea, Hawaii, the William Herschel telescope in the Canary Islands, the Wyoming Infrared Observatory on Jelm Mountain, Wyo., and the Hubble Space Telescope. They also ran theoretical models on supercomputers to model the emissions from these old binary systems. For more information, contact Ciardi or Howell at the UW Department of Physics and Astronomy, (307) 766-6150; or by e-mail at: ciardi@hedorah.uwyo.edu. -PCD-