The Milky Way and Beyond (4 page)

The Andromeda Galaxy is the closest spiral galaxy beyond the Milky Way galaxy cluster. The Andromeda Galaxy cluster is one of the most-distant objects that can be seen from Earth with the unaided eye. At one time, scientists thought the Andromeda Galaxy was a nebula in the Milky Way. However, we now know that it is about 2,480,000 light-years from Earth and is twice the size of the Milky Way galaxy. Scientists believe this galaxy has a history of “consuming” smaller galaxies. In fact, the two galaxies are moving toward each other and will some day—billions of years from now—merge to form a single galaxy.

Beyond the Andromeda Galaxy, countless other galaxies with countless stars and nebulae are scattered to the farthest corners of the cosmos. We may never travel past the distant limits of our own solar system, and those galaxies may always be just colourful specks visible only through our most-powerful telescopes. However, scientists will continue to study them and search for new stars, nebulae, and galaxies in the hope of learning more about our place in the vast cosmos. With billions upon billions of galaxies—each of which are home to billions upon billions of stars—there will no doubt be plenty for scientists to study for many years to come.

O
n a very dark clear night, if you look upward at the heavens, you will see an irregular luminous band of stars and gas clouds that stretches across the sky. This band is called the Milky Way. The Milky Way is actually a large spiral system, a galaxy, consisting of several billion stars, one of which is the Sun. Although Earth lies well within the Milky Way Galaxy (sometimes simply called the Galaxy), astronomers do not have as complete an understanding of its nature as they do of some external star systems. A thick layer of interstellar dust obscures much of the Galaxy from scrutiny by optical telescopes. Astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes, which can detect the forms of radiation that penetrate the obscuring matter.

The first reliable measurement of the size of the Galaxy was made in 1917 by American astronomer Harlow Shapley. He arrived at his size determination by establishing the spatial distribution of globular clusters. Shapley found that, instead of a relatively small system with the Sun near its centre, as had previously been thought, the Galaxy is 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. (A light-year is the distance traveled by light in one year and is roughly 9,460,000,000,000 km, or 5,880,000,000,000 miles.) His values have held up remarkably well over the years. Depending in part on the particular component being discussed, the stellar disk of the Milky Way system is just about as large as Shapley's model predicted, with neutral hydrogen somewhat more widely dispersed and dark (i.e., unobservable) matter perhaps filling an even larger volume than expected. The most distant stars and gas clouds of the system that have had their distance reliably determined 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 25,000 light-years.

The Milky Way Galaxy's structure is fairly typical of a large spiral system. This structure can be viewed as consisting of six separate parts: (1) a nucleus, (2) a central bulge, (3) thin and thick disks (4) spiral arms, (5) a spherical component, and (6) a massive halo. Some of these components blend into each other.

At the very centre of the Galaxy lies a remarkable object—in all likelihood a massive black hole surrounded by an accretion disk of high-temperature gas. Neither the central object nor any of the material immediately around it can be observed at optical wavelengths because of the thick screen of intervening dust in the Milky Way. The object, however, is readily detectable at radio wavelengths and has been dubbed Sagittarius A* by radio astronomers. Somewhat similar to the centres of active galaxies, though on a lesser scale, the galactic nucleus is the site of a wide range of activity apparently powered by the black hole.

Infrared radiation and X-rays are emitted from the area, and rapidly moving gas clouds can be observed there. Data strongly indicate that material is being pulled into the black hole from outside the nuclear region, including some gas from the
z
direction (i.e., perpendicular to the galactic plane). As the gas nears the black hole, its strong gravitational force squeezes the gas into a rapidly rotating disk, which extends outward about 5–30 light-years from the central object. Rotation measurements of the disk and the orbital motions of stars (seen at infrared wavelengths) indicate that the black hole has a mass 4,310,000 times that of the Sun.

Surrounding the nucleus is an extended bulge of stars that is nearly spherical in shape and that consists primarily of old
stars, known as Population II stars, though they are comparatively rich in heavy elements. Mixed with the stars are several globular clusters of similar stars. Both the stars and clusters have nearly radial orbits around the nucleus. The bulge stars can be seen optically where they stick up above the obscuring dust of the galactic plane.

From a distance the most conspicuous part of the Galaxy would be the disk, which extends from the nucleus out to approximately 75,000 light-years. The Galaxy resembles other spiral systems, featuring as it does a bright, flat arrangement of stars and gas clouds that is spread out over its entirety and marked by a spiral structure.

The disk can be thought of as being the underlying body of stars upon which the arms are superimposed. This body has a thickness that is roughly one-fifth its diameter, but different components have different characteristic thicknesses. The thinnest component, often called the “thin disk,” includes the dust and gas and the youngest stars, while a thicker component, the “thick disk,” includes somewhat older stars.

Astronomers did not know that the Galaxy had a spiral structure until 1953, when the distances to stellar associations were first obtained reliably. Because of the obscuring interstellar dust and the interior location of the solar system, the spiral structure is very difficult to detect optically. This structure is easier to discern from radio maps of either neutral hydrogen or molecular clouds, since both can be detected through the dust. Distances to the observed neutral hydrogen atoms must be estimated on the basis of measured velocities used in conjunction with a rotation curve for the Galaxy, which can be built up from measurements made at different galactic longitudes.

From studies of other galaxies, it can be shown that spiral arms generally follow a logarithmic spiral form such that

log
r
=
a
−
b
ϕ,

where ϕ is a position angle measured from the centre to the outermost part of the arm,
r
is the distance from the centre of the galaxy, and
a
and
b
are constants. The range in pitch angles for galaxies is from about 50° to approximately 85°. The pitch angle is constant for any given galaxy if it follows a true logarithmic spiral. The pitch angle for the spiral arms of the Galaxy is difficult to determine from the limited optical data, but most measurements indicate a value of about 75°. There are five optically identified spiral arms in the part of the Milky Way Galaxy wherein the solar system is located.

Theoretical understanding of the Galaxy's spiral arms has progressed greatly since the 1950s, but there is still no complete understanding of the relative importance of the various effects thought to determine their structure. The overall pattern is almost certainly the result of a general dynamical effect known as a density-wave pattern. The American astronomers Chia-Chiao Lin and Frank H. Shu showed that a spiral shape is a natural result of any large-scale disturbance of the density distribution of stars in a galactic disk. When the interaction of the stars with one another is calculated, it is found that the resulting density distribution takes on a spiral pattern that does not rotate with the stars but rather moves around the nucleus more slowly as a fixed pattern. Individual stars in their orbits pass in and out of the spiral arms, slowing down in the arms temporarily and thereby causing the density enhancement. For the Galaxy, comparison of neutral hydrogen data with the calculations of Lin and Shu have shown that the pattern speed is 4 km/sec per 1,000 light-years.

Other effects that can influence a galaxy's spiral shape have been explored. It has been demonstrated, for example, that a general spiral pattern will result simply from the fact that the galaxy has differential rotation; i.e., the rotation speed is different at different distances from the galactic centre. Any disturbance,
such as a sequence of stellar formation events that are sometimes found drawn out in a near-linear pattern, will eventually take on a spiral shape simply because of the differential rotation. For example, the outer spiral structure in some galaxies may be the result of tidal encounters with other galaxies or galactic cannibalism. Distortions that also can be included are the results of massive explosions such as supernova events. These, however, tend to have only fairly local effects.

The space above and below the disk of the Galaxy is occupied by a thinly populated extension of the central bulge. Nearly spherical in shape, this region is populated by the outer globular clusters, but it also contains many individual field stars of extreme Population II, such as RR Lyrae variables and dwarf stars deficient in the heavy elements. Structurally, the spherical component resembles an elliptical galaxy, following the same simple mathematical law of how density varies with distance from the centre.