The astronomer Walter Baade (lived 1893--1960) working at Mount Wilson Observatory outside of Los Angeles was forbidden to work in the United States' military research during World War II because of his German birth. Being a good astronomer, he took advantage of the dark skies from the frequent blackouts of Los Angeles during the war. He also had plenty of time at the telescope because the other astronomers were engaged in the war effort. In 1944 he discovered that the stars in our Galaxy can be divided into two basic groups: Population I and Population II.
The Population I stars are in the disk component of the Galaxy. They have a wide range of ages, from 0 to 10 billion years old. The youngest ones are in the spiral arms. Population I star orbits are orderly: roughly circular orbits close to the mid-plane of the galactic disk. Young star clusters made of Population I stars are called open clusters because the stars are loosely bound together, in contrast to the old, concentrated globular clusters.
Population I stars have a greater abundance of elements heavier than helium than the Population II stars. Elements heavier than helium are often called metals in astronomy even though many of them are not the shiny conductors called metals in popular usage. The Population I are said to be ``metal-rich'' in comparison to the Population II stars.
There is a continuum of the metal abundance and orbit shapes. The oldest Population I stars have 0.1 times the metal abundance of the Sun and have slightly elliptical orbits that can take them up to 3000 light years from the disk plane. The middle-aged stars like the Sun have metal abundances of 0.5 to 1 times solar abundance and can be found at heights up to 1100 light years from the disk plane. The young stars have metal abundances of 1 to 2 times the Sun's abundance and stay within 650 light years of the disk plane. Stars less than 100 million years old are found in the spiral arms within 400 light years of the disk plane. They have circular orbits and metal abundances of 1 to 2.5 times solar abundance.
The Population II stars inhabit the spheroidal component (the stellar halo and bulge). They are old: ages range from 10 to 13 billion years old, so they have masses less than or equal to 0.8 solar masses. Their orbits are much more randomly oriented than the Population I orbits and are highly elliptical. The combination of all of the elliptical orbits randomly aligned produces the spherical distribution seen. As a unit the spheroidal component has little overall rotation. Old star clusters called globular clusters are in Population II. Because they swarm around the center like bees around a hive, the globular clusters were used to locate the center of the Galaxy.
The halo stars are metal-poor (0.001 to 0.03 times solar abundance), but the abundance increases inward toward the center. The bulge stars have metal abundances of 1 to 3 times that of the Sun. Early in the history of the Galaxy, there was probably a lot of star formation and death in the bulge component, so the metal content rose quickly. The star formation rate in the disk has been more steady, so it is slowly catching up to the bulge's metal abundance. Star formation now occurs in just the disk. Eventually, the new disk stars will be born with greater metal abundance than the metal-richest bulge stars.
Stellar velocities found from the doppler shifts of the spectral lines in the stars' spectra near the center indicate the presence of a large, compact mass at the center with almost 4 million solar masses. It is probably a massive black hole formed by mergers of stars and stellar remnants. As you look closer to a supermassive black hole, you should see the stars nearer the black hole moving faster than the ones farther out. Stars nearer the supermassive black hole move fast enough to balance the stronger gravity closer to the black hole.
High-resolution infrared measurements of the orbits of the stars at the center show that a very compact mass---a supermassive black hole---with about 4.5 million solar masses lies at the center (if use our distance to the center as 8.4 kpc). The picture below (courtesy of Andrea Ghez and the UCLA Galactic Center Group) shows the orbits of the stars around the black hole from the years 1995 to 2008. At a distance of 8.4 kpc for the Sun, the 0.2 arc second scale bar in the figure corresponds to about 0.027 light years or 1680 AU. She has also released a movie of the orbits through early 2008 that also shows their predicted motions into the future. The object is too compact to be a dense cluster of stars---the Chandra X-ray Observatory's observations of X-ray bursts from the object place an upper limit of the diameter of the object of the size of the Earth's orbit. An expanding ring is also seen about 9000 light years from the center. Other galaxy cores have supermassive compact objects (the Andromeda Galaxy, M32, Sombrero Galaxy, M87, and many others).
Just because there is a large black hole at the center, does not mean that the stars in the Galaxy are all orbiting around it like the planets orbit the Sun. Even if the supermassive black hole was not there, the stars would still orbit pretty much as they do now because of their mutual gravity. If you compare the enclosed mass inside the Sun's orbit (see the mass figure above) with the 4.5 million solar masses of the black hole, you will see that the central black hole's mass is insignificant compared to the entire Galaxy.
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last updated: May 9, 2013