Stellar Orbits


A large number of stars move with high velocities in the central few arcseconds


In total we have measured around 50 stellar orbits around Sgr A*. The stars on these orbits are commonly referred to as the "S-stars" (Schödel et al. 2002, Eisenhauer et al. 2005, Gillessen et al. 2009, Gillessen et al. 2017). The following figure shows how the projections of the orbits onto the plane of the sky looks like.

Illustration of the stellar orbits in the Galactic Center

Actually, we have measured the orbits in three dimensions, such that we can build a full 3D model of the motions of the S-stars:

The visual impression of this stellar system is that the inner orbits follow no apparent order, but a bit further out, there are a few stars that orbit on similar planes. The randomness of the inner orbits holds in two senses: In the distribution of how the individual planes are oriented, and how the eccentricities are distributed:

Orientations of orbital planes in the Galactic Center. The direction of where the orbital angular momentum vector points is plotted over the sphere.

The stars S66, S67, S83, S87, S91, S96, S97, and R44 are members of the clockwise stellar disk (Bartko et al. 2009), which is marked by the thick grey dot and the dashed line. The orbits of the other stars are oriented randomly. The color of the labels indicates the stellar type (blue for early-type stars, red for late-type stars).

Cumulative probability density function for the eccentricities of the stars for which we have determined orbits. The early type-stars are shown left (after exclusion of the six disk stars) and the late-type stars right (after excluding one star with a hyperbolic orbit). The distributions are compatible with a thermal distribution (black line), indicative of a relaxed stellar system.

The eccentricity distribution of both early- and late-type stars is compatible with a thermal distribution, n(e) de ~ e de. Taken together with the randomly oriented orbits, the stellar system appears to be dynamically relaxed. This means in other words that the stellar motions do not carry information about how stars came to reside so close to the black hole.

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