In the center of the Milky Way lies a supermassive black hole called Sagittarius A* (Sgr A*). This black hole (inside the central white patch) has been detected by means of emissions just beyond its event horizon and through the motion of very close and very young massive stars. About 10 stars orbit the black hole within 0.04 parsecs and about 40 orbit within 0.1 parsecs, yet no model has been able to explain how these stars can be so close, so young, so massive, and in such relatively stable orbits.
Alexander and Livio propose a three-body dynamical interaction model to knock these stars into their orbits, like billiard balls. The young stars form far from Sgr A* and are scattered into eccentric orbits by the massive black hole. Over time, these stars cross paths with stellar mass black holes (SBHs) that are clustered near Sgr A*. Every so often, a star, a SBH, and Sgr A* undergo a three-body exchange, in which the SBH is ejected from the system and the star gets captured in a tight orbit around Sgr A*. Although the mass and the size of Sgr A* are being defined with increasing precision (see Bower et al., this issue, p. 704), the number and distribution of SBHs are less certain, so how broadly this ingenious model applies will depend on improved specification of the SBHs. — LR
Astrophys. J. 606, L21 (2004).