After a 100,000 years, the most massive O-type stars have reached the main sequence and are fusing hydrogen in their cores. Since high mass protostars contract faster that low mass protostars, after 5000 years, the high mass protostars are already moving towards the main sequence, while the low mass protostars have barely moved. As was discussed previously, low mass protostars are much more common than high mass protostars. The astronomers will publish their work in a future issue of Astrophysical Journal Letters.At time zero, all the members of the cluster are protostars in the upper right of the diagram. This is twice the age Liebert and his colleagues find for Sirius, suggesting the two have nothing to do with each other. They also put the group’s age at 500 million years. In 2003, however, Jeremy King of Clemson University and his colleagues questioned whether Sirius was truly a member of the group. In 1909, Danish astronomer Ejnar Hertzsprung said Sirius probably belonged to the so-called Ursa Major moving group, whose core is 80 light-years away. The new work casts further doubt on a long-held belief that Sirius moves through space with the five central stars of the Big Dipper. When Sirius B became a red giant, it must have lost 80 percent of its mass, because today it has the same mass as the Sun. If Sirius B shone so brightly today, it would look brighter than Venus - bright enough to cast shadows. Thus, the star shone as a main-sequence star and then a red giant for 101 to 126 million years - the expected lifetime of a star born with about 5 solar masses.Ī main-sequence star with this mass would have emitted a few times more light than Regulus in Leo, which a 2005 study found has 3.4 solar masses. From Sirius B’s mass and temperature (25,000 kelvins), the astronomers estimate the star became a white dwarf around 124 million years ago. Then it swelled into a red giant and shrank into a white dwarf. It was once a main-sequence star, like Sirius A. In contrast, the Sun is 4.6 billion years old - 20 times older. This age means Sirius has completed just one orbit around the galaxy. By modeling the evolution of a star with Sirius A’s mass, the astronomers find the star achieves its current luminosity and diameter 225 to 250 million years after birth. Recent interferometric work found Sirius A’s diameter to be 71-percent greater than the Sun’s. As a main-sequence star ages, its luminosity and diameter change. Liebert and his colleagues then determined the age of Sirius A. This orbital motion reveals that Sirius A, the brighter star, has 2.02 ± 0.03 solar masses and Sirius B, the white dwarf, has 1.00 ± 0.02 solar masses. The two stars orbit each other every 50 years. They first had to know the stars’ masses, because the more massive a star, the faster it evolves. Now, James Liebert, David Arnett, Jay Holberg, and Kurtis Williams of the University of Arizona and Patrick Young of Los Alamos National Laboratory have studied Sirius’ evolution. It is double: a bright A-type main-sequence star and a faint white dwarf - the closest white dwarf to Earth. Just 8.6 light-years away, Sirius is already the brightest nighttime star.
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