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: