Therefore, if we can measure the various speeds with which the stars are moving in their orbits around the center of the galaxy, we can calculate how much mass the galaxy must contain in order to hold the stars within it. Since elliptical galaxies contain stars that are billions of years old, we can assume that the galaxies themselves are not flying apart. Their stars are still orbiting the galactic center, but not in the organized way that characterizes spirals. The mass of the Milky Way Galaxy is estimated to be 8.5 × 10 11 M Sun, and so our Milky Way is turning out to be somewhat smaller than Andromeda.Įlliptical galaxies do not rotate in a systematic way, so we cannot determine a rotational velocity therefore, we must use a slightly different technique to measure their mass. Indeed, using Kepler’s third law and the velocities of its satellite galaxies, the Andromeda galaxy is estimated to have a mass closer to 1.4 × 10 12 M Sun. Recent studies show that the amount of dark matter beyond the visible edge of Andromeda may be as large as the mass of the bright portion of the galaxy. Fortunately, there is a handful of objects-such as isolated stars, star clusters, and satellite galaxies-beyond the visible edge that allows astronomers to estimate how much additional matter is hidden out there. The total mass of the Andromeda galaxy is greater than this, however, because we have not included the mass of the material that lies beyond its visible edge. Kepler’s law, together with such observations of the part of the Andromeda galaxy that is bright in visible light, for example, show it to have a galactic mass of about 4 × 10 11 M Sun (enough material to make 400 billion stars like the Sun). Remember that the faster something is moving toward or away from us, the greater the shift of the lines in its spectrum. Masses of GalaxiesĪstronomers can measure the rotation speed in spiral galaxies by obtaining spectra of either stars or gas, and looking for wavelength shifts produced by the Doppler effect. We measure how fast objects in the outer regions of the galaxy are orbiting the center, and then we use this information along with Kepler’s third law to calculate how much mass is inside that orbit. The technique for deriving the masses of galaxies is basically the same as that used to estimate the mass of the Sun, the stars, and our own Galaxy. Characterize each type of galaxy by its mass-to-light ratio.Describe the methods through which astronomers can estimate the mass of a galaxy.By the end of this section, you will be able to:
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