Size and mass

The first reliable measurement of the size of the Milky Way Galaxy, sometimes simply called the Galaxy, was made in 1917 by Harlow Shapley. He arrived at his size determination by establishing the spatial distribution of globular clusters. Instead of a relatively small system with the Sun near its centre, as had previously been thought, Shapley found that the Galaxy was immense, with the Sun nearer the edge than the centre. Assuming that the globular clusters outlined the Galaxy, he determined that it has a diameter of about 100,000 light-years and that the Sun lies about 30,000 light-years from the centre. His values have held up remarkably well over the years. Although dependent in part on the particular component being discussed, with neutral hydrogen somewhat more widely dispersed and dark (i.e., nonobservable) matter perhaps filling an even larger volume than expected, the stellar disk of the Milky Way system is just about as large as Shapley's model predicted. The most distant stars and gas clouds of the system that have had their distance determined reliably lie roughly 72,000 light-years from the galactic centre, while the distance of the Sun from the centre has been found to be approximately 27,000 light-years.

The total mass of the Galaxy, which had seemed reasonably well established during the 1960s, has become a matter of considerable uncertainty. Measuring the mass out to the distance of the farthest large hydrogen clouds is a relatively straightforward procedure. The measurements required are the velocities and positions of neutral hydrogen gas, combined with the approximation that the gas is rotating in nearly circular orbits around the centre of the Galaxy. A rotation curve, which relates the circular velocity of the gas to its distance from the galactic centre, is constructed. The shape of this curve and its values are determined by the amount of gravitational pull that the Galaxy exerts on the gas. Velocities are low in the central parts of the system because not much mass is interior to the orbit of the gas; most of the Galaxy is exterior to it and does not exert an inward gravitational pull. Velocities are high at intermediate distances because most of the mass in that case is inside the orbit of the gas clouds and the gravitational pull inward is at a maximum. At the farthest distances, the velocities decrease because nearly all the mass is interior to the clouds. This portion of the Galaxy is said to have Keplerian orbits, since the material should move in the same manner that the German astronomer Johannes Kepler discovered the planets to move within the solar system, where virtually all the mass is concentrated inside the orbiting bodies. The total mass of the Galaxy is then found by constructing mathematical models of the system with different amounts of material distributed in various ways and by comparing the resulting velocity curves with the observed one. As applied in the 1960s, this procedure indicated that the total mass of the Galaxy was approximately 200,000,000,000 times the mass of the Sun.

During the 1980s, however, refinements in the determination of the velocity curve began to cast doubts on the earlier results. The downward trend to lower velocities in the outer parts of the Galaxy was found to have been in error. Instead, the curve remained almost constant, indicating that there continue to be substantial amounts of matter exterior to the measured hydrogen gas. This in turn indicates that there must be some undetected material out there that is completely unexpected. It must extend considerably beyond the previously accepted positions of the edge of the Galaxy, and it must be dark at virtually all wavelengths, as it remains undetected even when searched for with radio, X-ray, ultraviolet, infrared, and optical telescopes. Until the dark matter is identified and its distribution determined, it will be impossible to measure the total mass of the Galaxy from the rotation curve, and so all that can be said is that the mass is perhaps five or 10 times larger than thought earlier. That is to say, the mass, including the dark matter, must be about 1,000,000,000,000 times the mass of the Sun, with considerable uncertainty.

The nature of the dark matter in the Galaxy remains one of the major questions of galactic astronomy. Other galaxies also appear to have such matter in their outer parts. The only possible kinds of material that are consistent with the nondetections are all rather unlikely, at least according to present understanding in physics and astronomy. Planets and rocks would be impossible to detect, but it is extremely difficult to understand how they could materialize in sufficient numbers in the outer parts of galaxies where there are no stars or even interstellar gas and dust from which they could be formed. Massive neutrinos and other exotic, hypothetical subatomic particles also might be difficult to detect, but there is no good evidence that they even exist, and therefore they can only be considered a highly conjectural solution to the puzzle. It will take considerable effort to identify the dark matter with any degree of certainty. In the meantime it must be said that astronomy does not know what makes up much of the universe.

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