|Vol. 17, No. 16||Jan. 8, 1998|
Space observations of some of the tiniest stars in the cosmos- reported Jan. 7 by UD researchers and their Danish collaborators at the American Astronomical Society meeting in Washington, D.C.-have finally confirmed a Nobel Prize-winning theory on the structure of stars.
Judi Provencal and Harry Shipman, physics and astronomy, with Danish researchers Erik Hoeg and Peter Thejll, said that understanding the structure of compact stars is central to a correct picture of the nature of pulsars and black holes as well as white dwarf stars, the subject of the team's research.
Theories important enough to deserve a Nobel Prize generally have been confirmed by many observations, said Shipman. The theory of white dwarf stars, however, has been different. Until now, this theory of stellar structure-developed by the late astrophysicist Subramanyan Chandresekhar, who won a Nobel Prize in 1983-was only supported by four error-prone data points, each representing observations of one individual star, Shipman noted.
These tiny balls, only 1/100th the diameter of the Sun, are hard to see, Shipman said; their smallness and faintness explains the sparsity of information about them. "Such an important theory deserved better confirmation," he said. "We were lucky enough to be able to provide it. We now know that Chandresekhar deserved his Nobel."
By understanding these tiny stars, scientists hope to learn more about how stars live, die and shed mass into the Milky Way galaxy. "If the data had shown that Chandresekhar was wrong," said Provencal, "it would have meant that a lot of the work we've done with white dwarf stars would have been wrong, including much of our picture of the history of the galaxy."
Stars stay a certain size because they're nuclear powered, Shipman said. But when their nuclear fuel runs out, they shrink. Stars with the most mass have the most gravity, and they shrink the most, becoming black holes. Lightweight stars like our Sun shrink somewhat less, becoming white dwarf stars, in which stellar matter is compressed so much that a cupful would outweigh 24 elephants, according to Shipman. The more massive white dwarf stars shrink the most, he said, and the less massive white dwarf stars shrink a little less. "This theory of stellar structure specifies just how large a white dwarf star of a particular mass should be," he added.
The UD/Danish team used the European HIPPARCOS satellite to measure the distance to a few dozen white dwarf stars. Twenty could be used to test Chandresekhar's theory. "We used the distances to provide the radius of each white dwarf star," Provencal said. "We then compared each star's radius with its mass to see if the stars with bigger masses had smaller radii, as the theory predicts. The results fit the theory precisely."
Shipman had learned 15 years ago that the European Space Agency planned to launch a satellite to determine the distances to approximately a quarter of a million stars. He proposed 150 specific stars for study using the satellite, including nearly a few dozen white dwarf stars. Danish team member Hoeg and others on the HIPPARCOS team turned the ambitious mission into reality.
It took a decade and a half to launch the spacecraft, obtain data on a million stars and analyze those measurements to get firm distances for 118,218 stars, as well as positions for 1,058,332 stars. Shipman and Provencal finally received their data in 1996-over a decade after the quest began.
One of the first stars they studied, 40 Eridani B, is located 16.47 light years away in the constellation of Eridanus, the River. Thanks to HIPPARCOS, Shipman said, its distance is known very precisely-to within 0.07 light years, or 0.4 percent. And, its mass and radius are better understood, as a result of the satellite data. New data on 40 Eridani B fits precisely with Chandresekhar's theory, within a few percent, researchers said. The results on this star, published in the Oct. 10, 1997, issue of the Astrophysical Journal, were combined with the results from 19 other white dwarf stars to produce the results announced yesterday. (These results will be published in February, also in the Astrophysical Journal.)
Chandresekhar developed the theoretical understanding of white dwarf stars when he was still a graduate student in the 1930s. Much of it was developed on a slow boat trip, which took him from India to Cambridge, England, where he was to embark on graduate study. His theory forms the basis for scientists' current understanding of other compact stellar objects such as neutron stars and black holes, Shipman noted. (For more information, go to the web site: http://www.nobel.sdsc.edu/laureates/physics-1983.html.)
"These results mean that our understanding of white dwarfs and black holes is at last on a firm factual footing," Shipman said. "Of course, there is still more to be done. We are analyzing more stars, and we are trying to get more precision for the stars we have."
The HIPPARCOS satellite was launched, operated, and paid for by the European Space Agency. Provencal and Shipman's work on the data was supported by NASA.