A large number of stars move with high velocities in the central arcsecond.
A large number of stars move with high velocities in the central arcsecond.
This is an illustration of (a subset of) the stellar orbits we have determined in the Galactic Center. The most prominent star is S2, the orbit of which is shown as a thick red line.
This is an illustration of (a subset of) the stellar orbits we have determined in the Galactic Center. The most prominent star is S2, the orbit of which is shown as a thick red line.
We have been observing the central arcsecond in the Galactic Center for three decades since 1992, and tracked the motions of the stars in the field. Many of the stars show curved trajectories, an unambiguous sign of the gravitational force of the central massive black hole. Combining astrometry and spectroscopy, we have measured in total roughly 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 what the projections of some of the orbits onto the plane of the sky look like.
The actual orbits are measured in three dimensions, and hence we were able to 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; however, a bit further out, there are a few stars that orbit on similar planes. The randomness of the inner orbits holds true in two senses: in the distribution of how the individual planes are oriented, and how the eccentricities are distributed:
This image displays the orientations of orbital planes in the Galactic Center by showing the direction of where the orbital angular momentum vector points over the sphere. The stars in the central arcsecond uniformly cover the sphere (i.e., they are randomly distributed), but there are 15 young stars (all at radii larger than ~1 arcsecond) in the top left quadrant of the diagram, which orbit in a common plane, the plane of the clockwise stellar disk (thick gray dot, Paumard et al. 2006, Bartko et al. 2009, von Fellenberg et al. 2022). The color of the labels indicates the stellar type (blue for early-type, young stars, red for late-type, old stars).
This image displays the orientations of orbital planes in the Galactic Center by showing the direction of where the orbital angular momentum vector points over the sphere. The stars in the central arcsecond uniformly cover the sphere (i.e., they are randomly distributed), but there are 15 young stars (all at radii larger than ~1 arcsecond) in the top left quadrant of the diagram, which orbit in a common plane, the plane of the clockwise stellar disk (thick gray dot, Paumard et al. 2006, Bartko et al. 2009, von Fellenberg et al. 2022). The color of the labels indicates the stellar type (blue for early-type, young stars, red for late-type, old stars).
This picture shows the cumulative probability density function for the eccentricities of the young stars for which we have determined orbits. The dark blue line is for the stars in the central arcsecond, the lighter blue line for the disk stars. The black dashed line is a thermal distribution, n(e) de~ede. The disk stars apparently have preferably modest eccentricities around e = 0.4, while the inner stars have an even more eccentric than thermal distribution.
This picture shows the cumulative probability density function for the eccentricities of the young stars for which we have determined orbits. The dark blue line is for the stars in the central arcsecond, the lighter blue line for the disk stars. The black dashed line is a thermal distribution, n(e) de~ede. The disk stars apparently have preferably modest eccentricities around e = 0.4, while the inner stars have an even more eccentric than thermal distribution.
The combination of random orientations and (slightly) supra-thermal eccentricity distribution means the system is close to a dynamically relaxed configuration. These stellar motions do hence not carry information about how he stars came to be so close to the black hole. The best hypothesis is that the inner S-stars have reached their orbits by means of the Hills-mechanism, a cosmic pool game, where a binary star passes close to the massive black hole, capturing one member and ejecting the other at high speed, potentially creating hypervelocity stars.
