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Issue 4.08

The Astrophysics Spectator

May 30, 2007

This week, several pages are added to the “Milky Way Galaxy” path. The first page describes how observations of stellar motion around the black hole at the center of the Galaxy provides a means of measuring the distance from the black hole to Earth. The second page presents the basic characteristics of the Milky Way Galaxy.

The first estimate of the distance to the center of our Galaxy came from the observations of the distribution of globular cluster. Globular clusters are very massive and spherical clusters of stars, with the total mass in stars of a cluster typically between 100,000 solar masses and 1 million solar masses. These clusters are distributed in a sphere that is centered on the Galactic center. This clustering was recognized at the beginning of the 20th century, and was used to show that Earth was not at the center of the Galaxy, to determine the direction of the Galactic center, and to derive a value for the distance to the Galactic center. This first value was 15 kpc.

All through the 20th century, the measured distance to the Galactic center had decreased as the instruments and the methods of deriving a distance improved. By the time the author was a graduate student, in the first part of the 1980s, the accepted value for the distance to the Galactic center had fallen to 8.5 kpc, with contemporary x-ray observations suggesting that 8 kpc is a better value.

Within the past decade, the distance to the Galactic center has fallen to 7.6 kpc, with an error in the value of only 4%. This value comes from applying an old method to a newly-observed object: use the Keplerian orbit of one body around another to derive the physical properties of a system. This is the method to derive the mass of the Sun, Jupiter, and Earth. What makes the method unusual in the case of the Galactic center is that we are observing the orbits of stars around a massive black hole.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for June 13.

Jim Brainerd

Milky Way Galaxy

The Mass and Distance of Sagittarius A*. A massive black hole, named Sagittarius A*, sits at the center of the Milky Way Galaxy, orbited by numerous luminous main-sequence stars. The stars close to the black hole rotate around it in Keplerian orbits. By precisely measuring the positions and velocities of the stars over time, astronomers have derived an accurate mass for the black hole and an accurate distance to the black hole from Earth. (continue)

Properties of the Milky Way Galaxy. This page presents a table containing some of the principle properties of our Galaxy. Among the information contained in the table are the estimated mass of the Galaxy, the distance from the Galactic center to Earth, the velocity of the Galactic disk in its orbit of the Galactic center, and the mass of the central black hole. (continue)

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