The masses of galaxies are found from the orbital motion of their stars. Stars in a more massive galaxy will orbit faster than those in a lower mass galaxy because the greater gravity force of the massive galaxy will cause larger accelerations of its stars. By measuring the star speeds, you find out how much gravity there is in the galaxy. Since gravity depends on mass and distance, knowing the size of the star orbits enables you to derive the galaxy's mass.
For spiral galaxies the rotation curve is used to measure their masses like is done to find the mass of the Milky Way. The rotation curve shows how orbital speeds in a galaxy depend on their distance from the galaxy's center. The mass inside a given distance from the center = (orbital speed)2 × (distance from the center)/G. The orbital speed is found from the doppler shifts of the 21-cm line radiation from the atomic hydrogen gas. The angular distance of the piece of the disk from the center is measured, but to use the enclosed mass formula, the piece of the disk's actual linear distance from the center must be found.
Remember way back in the planetary science chapter that the linear distance can be found from the angular distance if you know the distance to the object? The linear distance from the galaxy center = [(2p × (distance to the galaxy) × (angular distance in degrees)] / 360°. This is why you must first know the distance to a galaxy if you want to measure its mass.
For elliptical galaxies the width of the absorption lines from all of the stars blended together is used to measure the mass of elliptical galaxies. The width of the absorption lines depends on the spread of the distribution of the velocities---the velocity disperion. The elliptical galaxy's mass = k × (velocity dispersion)2 × (the distance the stars are from the galaxy center)/G, where k is a factor that depends on the shape of the galaxy and the angle the galaxy is from Earth.
The stars and gas in most galaxies move much quicker than expected from the luminosity of the galaxies. In spiral galaxies, the rotation curve remains at about the same value at great distances from the center (it is said to be ``flat''). This means that the enclosed mass continues to increase even though the amount of visible, luminous matter falls off at large distances from the center. In elliptical galaxies, the gravity of the visible matter is not strong enough to accelerate the stars as much as they are. Something else must be adding to the gravity of the galaxies without shining.
That something else is called dark matter. It is material that does not produce detectable amounts of light but it does have a noticeable gravitational effect. Astronomers are not sure what the dark matter is made of. Possibilities range from large things like planets, brown dwarfs, white dwarfs, black holes to huge numbers of small things like neutrinos or other particles that have not been seen in our laboratories yet. The nature of dark matter is one of the central problems in astronomy today.
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last updated: 26 May 2001