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Axel Bonacic Marinovic's model help Astronomers better understand the evolution of Binary Star

It's the brightest star in the sky, and it's less than 9 light-years away. Yet Sirius, the blue-white gem of the winter sky, harbors many secrets. Now, a Ph.D. student at Utrecht University in the Netherlands has solved one of those nagging problems. In a paper accepted for publication by Astronomy & Astrophysics, Axel Bonacic Marinovic explains why Sirius and its small companion revolve around each other in a highly elongated orbit, instead of the circular one theorists would expect. His model will help astronomers better understand the evolution of binary stars.
Sirius is a hot, bluish star, twice as massive and almost twice as large as our own sun. In 1844, slow wobbles in Sirius's position led German astronomer Friedrich Bessel to predict the existence of a companion. And sure enough, 18 years later, American telescope builder Alvan Clark discovered Sirius B, which turned out to be a faint but compact white dwarf star--the mass of the sun packed into a sphere the size of Earth. Every 50 years, the two stars complete an orbit around their common center of gravity.

The trip, however, is extremely elongated: The distance between Sirius A and Sirius B varies from 1.2 billion to 4.7 billion kilometers. That's strange, says astrophysicist James Liebert of the University of Arizona, Tucson, who reconstructed the evolution of the binary with colleagues 2 years ago. They concluded that Sirius B must have been a hefty, bloated, red giant some 100 million years ago, before it shrunk into a white dwarf. Because of that enormous size, the orbit was much smaller back then; Sirius A was practically skimming the outer layers of Sirius B, Liebert says. Calculations reveal that the resulting tidal effects on Sirius B would have made any eccentric orbit circular.

So how did it stay elongated? Bonacic Marinovic says that the tidal effect was in a "delicate contest" with another mechanism. During Sirius A's closest approaches to the red giant's tenuous envelope, gravitational perturbations would have led Sirius B to lose mass even faster. According to Bonacic Marinovic's calculations, this variable mass loss would tend to elongate the binary orbit. The same mechanism might explain the unexpected eccentricity of dozens of other tight binary stars that contain a white dwarf, he says.

Although Liebert remains puzzled by the fact that Sirius A apparently wasn't completely engulfed by its bloated companion, the paper has won him over. "There had to be some explanation of this type," he says, "and this one makes sense."

About Sirius
Sirius is the brightest star in the night-time sky with a visual apparent magnitude of −1.47. It is also known as the "Dog Star"; its constellation, Canis Major (English: Big Dog), is named after it.

What to the eye appears as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, named Sirius A, and a faint white dwarf companion of spectral type DA2, named Sirius B. The reason for its brightness is not its intrinsic luminosity but its closeness to earth; at a distance of 2.6 pc or 8.6 light years, Sirius is one of our near neighbours. Sirius A is only about twice as massive as the Sun and, with an absolute magnitude of 1.42, is far less bright than Canopus or Rigel.

About Binary star
A binary star is a stellar system consisting of two stars orbiting around their center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries.

Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but may not be bound noticeably by gravity. Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. If binaries happen to orbit in a plane containing our line of sight, they will eclipse each other; these are called eclipsing binaries.

Systems consisting of more than two components, known as multiple stars, are also not uncommon and are generally classified under the same name. The components of binary star systems can exchange mass, bringing their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole).

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