Issue 4.11

The Astrophysics Spectator

July 25, 2007

This issue of the web site adds another page about the bright stars in close orbit around the central black hole Sgr A^* to the “Milky Way Galaxy” path. The subject of the new page is the difficulty of rapidly transporting a star born far from the central Galactic black hole to a close orbit around the black hole.

As the page introduced last week explains, the many B main-sequence stars orbiting the central black hole present astrophysicists with a conundrum, what some in the astronomy community have called the “paradox of youth,” that has resisted explanation. The conundrum is that stars are not expected to be born near the central black hole, and the short lives of the B main-sequence stars do not seem to give them enough time to diffuse close to the central black hole. The new page discusses in more detail the basic mechanism—dynamical friction—that causes a massive star to sink to the center of the galaxy. The failure of dynamical friction to solve this problem is the starting point for all theories that suggest the stars we see close to the black hole are born far from it.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for August 7.

Milky Way Galaxy

The Sinking of Young Stars Towards Sagittarius A^* Stars are being born 1 parsec away from the central Galactic black hole, and over time, the most massive of these stars should sink central inward to the central black hole. The basic mechanism that removes kinetic energy from these massive stars is dynamical friction, which is created by the numerous weak encounters between a massive star and the smaller passing stars. Dynamical friction requires of order 1 billion years to significantly change the orbit of a star, which is too long a time to account for the bright stars in close orbit of Sgr A^*. (continue)