Howell, an assistant professor in UW's Department of Physics and Astronomy, collaborated with Saul Rappaport of the Massachusetts Institute of Technology, Mike Politano of Arizona State University, Peter Hauschildt of the University of Georgia, and Vik Dhillon of the Royal Greenwich Observatory in a project to understand the life cycles of two stars contained within very old binary systems.
Many stars form as binary systems, where two small stars orbit around each other for an eternity, held together by their mutual gravitational attraction. Howell says some binary stars are so close together that their orbit could fit within the size of our sun. This nearness affects each star's life cycle.
Howell and his collaborators used observations from some of the world's largest telescopes, including the William Herschel Telescope in the Canary Islands, and theoretical models run on supercomputers, to develop new theories about the evolution of these old binaries. They now believe a new type of star exists within some of these binary systems.
"The discovery of a new type of star doesn't happen everyday," Howell says. "Early this century, astronomers used both theory and observation to discover white dwarfs, neutron stars, and black holes. Until now, no other stellar types were found."
Stars shine due to energy production in their cores, mainly by conversion of hydrogen to helium via nuclear reactions. When the hydrogen supply is depleted, a star must readjust its overall structure to ward off collapse due to the ever present force of gravity. After a relatively brief period of millions of years, stars generally end their lives as one of three types of stellar end-products: white dwarfs, neutron stars, or black holes -- the latter being the end state of most massive stars.
"In many binary systems, the initially more massive star ends its life and becomes a white dwarf, while the initially less massive star tries to evolve normally, but all the while loses mass to the white dwarf," Howell says. "Eventually, all that remains of the less massive star is an exposed stellar core about the size of the planet Jupiter with a mass of only 5/100th or so of its original value. Having used up or lost essentially all of its hydrogen, this very small star has no remaining energy generation. It can never become one of the usual stellar end-products. Therefore, it has a structure unlike any other kind of known star."
While the work that led to the discovery of this new type of star is over, Howell says further observational and theoretical work will continue to understand its stellar state.
"It is pleasing to know that the universe still holds mysteries," he says, "and with diligence and patience, we get to peek inside and discover some small new part for ourselves."
Howell says the theory work was published in June 1997 in the monthly notices of the Royal Astronomical Society. Also, a refereed paper has been sent to the Astrophysical Journal and he expects it to be published within the next month or two.
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