Observations of the white dwarf star, Sirius B, made with NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite give astronomers firm new evidence that mathematical models widely used to predict white dwarf star mass and radius are correct.
Jay B. Holberg of the University of Arizona Lunar and Planetary Laboratory is presenting the result today at the American Astronomical Society in San Diego.
The FUSE result is important because Sirius B is one of the few stars that astronomers have to test their ideas on the relationship between mass and radius for white dwarf stars. White dwarf stars are small but astonishingly dense stars. Sirius B is the size of the Earth and as massive as the sun.
Theory that describes how white dwarf stars can exist emerged in the early 1930s, when Subramanyan Chandrasekhar – or Chandra, as he was known – calculated the limit to a white dwarf's mass by applying Einstein's theory of special relativity. It was one of the first applications of quantum mechanics to large physical systems in the sky.
No white dwarf star could be more than 1.4 times as massive as the sun or it will collapse, Chandra predicted.
"Chandra was the first person to lay out the essential details of how white dwarfs sustain themselves, and it is very, very different from the sun or any other stars," Holberg said.
Unlike most white dwarfs, Sirius B is part of a binary system, and astronomers can determine the mass of stars in a binary system.
"Having a binary system – when two stars orbit one another - is virtually the only way you can fundamentally measure the mass of a star," Holberg said. "You observe their orbits, get the period, know how far away they are, and you can find the sum of the two star masses. If you can time the orbits and know how far apart the stars are, you can determine the individual star masses. That's the most accurate way, the acceptable way to determine star masses.
"But this star has always been devilishly difficult to observe," Holberg said. The primary star in the system, Sirius A, is 8 light years from Earth and has twice the mass of the sun. It is the brightest star in the night sky, visible below Orion. Sirius B is 10,000 times dimmer than Sirius A. Astronomers can’t even see the white dwarf companion when it comes closest to the primary star during its 50-year, very elongated orbit around Sirius A.
For the post several years, Holberg and colleagues have observed Sirius B with the Voyager and Extreme Ultraviolet Explorer spacecrafts. They have refined the star's temperature and gravity - gravity being the gravitational field at the surface of the star - to refine estimates of its mass and radius.
"The methods we're using are spectroscopic. They infer the mass from synthetic models that we produce from measurements of temperature and gravity, the only two parameters of matter for a white dwarf."
Holberg and his colleagues published the best determination of Sirius B's mass-radius relationship in 1998, but that was "still far from definitive," Holberg said. "That is, the uncertainties are so large, that while these studies define the basic relationship, they don’t tell you lots of details we need to know about these stars."
The FUSE observations gave Holberg and his colleagues better spectral data on Sirius B's gravitational field and temperature needed to calculate mass. "And this is a very clean spectrum. We rolled the FUSE spacecraft to keep Sirius A from contaminating the spectrum, and we succeeded very well.
"The mathematical model very well predicts our results on the gravitational field, temperature and brightness of this white dwarf star," Holberg said. "That helps us determine the radius of the star. What we really want to do is determine mass and radius to within one percent. By verifying the Chandrasekhar limit, you put a great deal of astrophysics on much firmer footing," he added.
"Astronomy has reached the level where you can make very definitive comparisons between the models and the observations. And it looks like we are going to come out to what we expected," Holberg said.
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