Issue 4.10

The Astrophysics Spectator

July 11, 2007

This issue continues the discussion on the Galactic center with two pages on the large stars orbiting the central black hole. The first page describes the difficult problem the existence of these stars presents to astrophysicists. The second page is a table of the basic orbital parameters for these stars. In future issues I intend to add pages that discuss the more reasonable theories for how these stars came into close orbit of Sgr A^*.

The very center of the Galaxy contains many very large, young stars orbiting Sagittarius A^*, the massive black hole at the center of the Milky Way. Specifically, at least 10 stars within 0.7 arc-seconds (0.025 parsecs, or 5,000 AU) of the black hole are B main-sequence stars, stars that are between 3 and 15 times the mass of the Sun. Familiar examples of B main-sequence stars in the night sky are Spica (α Vir) in the Virgo constellation and Regulus (α Leo) in the Leo constellation. Finding such stars in close orbit of the central black hole was a surprise, because theorists had believed that the tidal force of the central black hole prevents the formation of stars close to the black hole, and that the time for a star to drift inward from regions where star formation do occur is much longer than the lifetime of a B main-sequence star. The existence of these stars in contradiction to the theoretical expectations has been called by some astronomers “the paradox of youth,” giving the problem an epic echo reminiscent of “the riddle of steel” (“Steel isn't strong, boy, flesh is stronger!”).

Why large, young stars should sit at the center of the galaxy remains unexplained. The explanation will come most likely from studies of how kinetic energy is exchanged among stars at the galactic center—hardly a sexy problem. But the theoretical literature on the young stars at the Galactic center appears to me to favor exotic and speculative mechanisms—the formation of stars in transient accretion disks, the shrouding of black holes in envelopes of gas, the destruction of stellar clusters as they fall to the galactic center, etc.—over this mundane bit of physics. As so often happens in astrophysics, when a difficult problem is encountered, many theorists run with the exotic ideas.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for July 25.

Milky Way Galaxy

The Massive Stars Orbiting Sagittarius A^*. The bright stars found orbiting the massive central black hole are large main-sequence stars with lifetimes as short as 10 million years. The mystery of how these stars came to orbit a massive black hole comes down to a single problem: what causes the orbit of a star at the galactic center to change so dramatically in less than 10 million years? (continue)

Tables

Orbital Parameters of Stars Orbiting Sgr A^*. Several B main-sequence stars at the Galactic center have had their orbital parameters derived. Orbital values are give in the table on this page for the stars with the best-determined orbits. Among these parameters are those for S2, the star with the best-determined orbit, an orbit that is completed once every 15 years. (continue)