A Beast at the Heart of the Galaxy
Animation: Observations of the Galactic Center with and without adaptive optics (AO). The current AO system increases the image resolution of the Keck Telescopes by a factor of 25. Animation created by Dr. Andrea Ghez and her research team at UCLA from data sets obtained with the W. M. Keck Telescopes. Send your comments on this issue of Cosmic Matters to .(JavaScript must be enabled to view this email address).
Image: Supermassive black holes, like the one hidden by a girdle of dust in this artist’s impression, have the mass of up to a billion suns. They may lie at the center of most, if not all, large galaxies. Image and caption courtesy of Monster of the Milky Way.
A black hole forms when gravity overcomes all other known forces. It then collapses to an infinitesimally small size. Although there is some physical size associated with a black hole, it is difficult to say what that size actually is. So, formally, a black hole has no size.
Animation: Images taken from the years 1995 through 2006 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy. These orbits, and a simple application of Kepler’s Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun. Animation created by Dr. Andrea Ghez and her research team at UCLA from data sets obtained with the W. M. Keck Telescopes.
In fact, a black hole has only three physical principles: mass, spin, and electric charge. Only one of these, mass, can be measured confidently. The orbit of a star can be used to determine how much mass is inside its orbit. Scientists prove the existence of a black hole by showing that there is a lot of mass inside a small volume, as determined by the radii of the orbits of neighboring stars.
To measure the mass of the black hole at the center of the Milky Way Galaxy, scientists needed to be able to see the stars as close as possible to the center of the Galaxy. By tracking specific stars orbiting the proposed black hole from 1995 through 2006, Dr. Andrea Ghez provided evidence which established the existence of Sgr A*, a supermassive black hole at the Galactic Center. Dr. Ghez and her team have now established that Sgr A* has a mass 4 million times the mass of the Sun.