B. Hidden Mass
Stars move about in galaxies under the influence of gravity in different ways, depending on the type of galaxy. The stars in elliptical galaxies move in all directions. The stars in the arms of spiral galaxies move in more orderly fashion around the center of the galaxy. Stars in irregular galaxies move more or less in random fashion.
The presence of dark matter was
first discovered in 1932 by the Dutch astronomer Jan Oort. By
examining the Doppler shifts
in the spectra of stars in the Milky Way Galaxy, Oort measured their
velocities. He found that the stars moved faster than expected. Oort
expected the stars to move only as fast as what would be expected from
the gravitation force of the visible mass (stars, gas, dust) in the
Galaxy. In reality, the stars appeared to be moving faster than this
- fast enough to escape the galaxy. Since Oort knew that this
couldn't be the case, he hypothesized that there must be additional
mass in the galaxy that wasn't visible and would keep the stars bound
to the galaxy. A year later, the Swiss astronomer, Fritz Zwicky,
working in America, came to the same conclusion while measuring the velocities of galaxies in the Coma Cluster. Scientists now know that this "hidden mass", also known as dark matter, can account for nearly 90% of the total mass of a galaxy.
In spiral galaxies, stars located at greater distances from the center of the galaxy are expected to have smaller velocities than stars that are close to the center of the galaxy. What scientists observe is that velocities are constant in the arms, and rise very quickly in the bulge. Scientists can account for this if there is a very massive object at the center of the galaxy, and a large halo of invisible matter surrounding the entire galaxy. X-ray observations confirm that massive black holes lie at the center of many galaxies. In the dark halo, the amount of mass increases linearly with radius. Spiral galaxies have extended dark halos that account for 90% of the total mass of the galaxy. Scientists still know little about dark matter. It might be objects too small to become stars and hence too small to give off their own light (e.g., planets and brown dwarfs). Or it might be an entirely new type of matter made of particles which interact only gravitationally and do not give off light. (See section C below for further discussion on the possible sources of dark matter.)
Measuring rotation curves in an elliptical galaxy is difficult because of the nature of the orbits of stars in this type of galaxy and because the spectral lines are too weak to be used to measure the velocity. However, X-ray observations show that elliptical galaxies have a halo of hot gas extending well outside the optical limits of the galaxy. As an example, in one of the elliptical galaxies in the Virgo Cluster, the total mass of this hot gas can be 1010 times the mass of the sun. This is small compared to the total mass of all the stars in the galaxy - 1012 times the mass of the sun. However, in order for the gas to be bound to the galaxy, the galaxy must have a mass of 5x1012 times the mass of the sun. Because this is more than what is seen, astronomers conclude that elliptical galaxies also have halos of hidden mass. As in the case of spiral galaxies, the "dark matter" halos in elliptical galaxies may contain up to 90% of the total mass of the galaxy.
Recommended Activities: Modeling Mass in the Solar System and a Galaxy, Evidence for Hidden Mass, Getting a Feel for Rotation Curves, and Weighing a Galaxy