Black Hole Properties
The Galactic Center Group members have been measuring the positions of thousands of stars in the vicinity of the Galactic Center for more than 20 years. This unique data set allowed us to measure directly short-period orbits of stars.
In particular, a full phase coverage has been measured for two stars: S0-2 with an orbital period of 15.56 years, and S0-102 with 11.5 years.
At the closest approach, S0-2 is only 17 light hours away from the center of the Galaxy, about four times the distance of Neptune from the Sun. From these orbital data, we can determine the mass of the central black hole in our own Galaxy.
The Milky Way Galaxy black hole is the closest example of
the supermassive black holes, located only ~25,000 light years away from us.
Its mass is estimated to be 4 million times the mass of the sun, which implies that the Schwarzschild radius
is about 17 times that of Sun's radius. As a comparison, Mercury's orbit is located at a distance of ∼ 83 solar radii.
Because the Galactic Center is the site of the closest supermassive black hole by a factor of 100,
it is a unique laboratory for solving some of the greatest mysteries associated with the fundamental
physics of supermassive black holes and the role that they play in the fomration and evolution of galaxies.
Furthermore, it is the only galactic nucleus in which direct measurements of stellar orbits is possible,
with either the current or the next-generation instruments.
Observations of stellar orbits around the Galactic black hole also yields precision measurement of the distance to the Galactic Center, which is important as it affects almost all questions not only of Galactic structure, dynamics and mass, but those of extragalactic distance scales and the value of Hubble's constant as well.
For further reading:
• Ghez et al. 2008 Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
• Meyer et al. 2012 The Shortest-Known-Period Star Orbiting Our Galaxy’s Supermassive Black Hole