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Issue 4.12

The Astrophysics Spectator

August 8, 2007

Two new pages that describe theories of young, massive stars close to the central Galactic black hole Sagittarius A* are added to the “Milky Way Galaxy” topical path in this issue of the web site. The first of these pages describes how dynamical friction causes clusters of stars to drift inward towards the black hole. The second page describes how a star in a highly-eccentric obit around the central black hole may have its orbit dramatically altered by an encounter with a third body.

Theorists have turned to complex stellar systems to explain the numerous young, massive stars closely orbiting the central Galactic black hole. The impetus pushing theorist to these theories, and away from theories that rely on the drift of individual stars to the center of the Galaxy, is the inability of dynamical friction, the slowing of a star as it gravitationally interacts with neighboring stars, to change rapidly the orbit of an individual star. An individual star can move from its birthplace at several tens of parsecs from the black hole to an orbit only 100 AU from the black hole in 1 billion years, which is much too long to explain the stars found near the black hole, which have lifetimes as short as 10 million years.

The dynamical friction exerted by individual stars on star clusters has long been regarded the mechanism that creates the cores of galaxies. As a globular cluster flies through the center of a galaxy, it is slowed in its orbit by the gravitational scattering of cluster stars by stars in the galactic core. This friction on a star cluster shrinks the cluster's orbit and energizes the stellar motion within the cluster. The star cluster drifts inward to the central black hole in several million years. The cluster itself disintegrates over this time as it evaporates stars, leaving behind a remnant of massive stars close to the black hole.

Not all theorists ponder how a star born tens of parsecs from the central black hole can drift into an orbit that brings it to within 100 AU of the black hole. Some theorists prefer to study how the stars close to the central black hole attain small orbits. The assumption is that nature easily places a star into a highly-eccentric orbit, with the star swinging through its orbit from 1 parsec to within 100 AU of the black hole and back. The problem is then changing the star's orbit so it stays within several thousand AU of the black hole. The theories motivated by this problem involve the interaction between three bodies: the central black hole, the star, and a third body that extracts orbital energy from the star. Banging stars together near a massive black hole: what could be more fun than that?

Next Issue: The next issue of The Astrophysics Spectator is scheduled for August 22.

Jim Brainerd

Milky Way Galaxy

Sinking Star Clusters Near Sagittarius A*. The long timescale for a massive star to drift into close orbit around the central Galactic black hole can be overcome by placing the star into a massive star cluster. As a star cluster loses orbital kinetic energy through dynamical friction, it loses its least-massive stars, until there remains only a small remnant of massive stars. This mechanism has been used to explain massive stars found at parsec from the central black hole. (continue)

Three-Body Interactions Near Sagittarius A* On occasion in astrophysics, theorists become sidetracked from the principal problem in explaining an astronomical phenomenon, exerting themselves instead against an assumed secondary problem. In the study of stars orbiting the central black hole, the closeness of the star S2 to the black hole is the secondary problem attracting attention and motivating theories involving energy exchange between three bodies. (continue)

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