Galaxies

Galaxies are large assemblage of stars, gas and dust that is held together by the mutual gravitational interaction between its constituents. Galaxies may contain anywhere between a few million and about ten trillion (10¹³) stars together with differing proportions of interstellar material (gas and dust).

Most galaxies can be categorized as normal galaxies, but a few percent (those that exhibit unusual characteristics such as having a compact, variable nucleus, radiating predominantly non-stellar radiation, and having a disturbed appearance) belong to the category of Active Galaxies. Normal galaxies may be classified in a number of ways, the most common system being the Hubble classification scheme, which divides galaxies into the following principal types: elliptical, spiral, barred spiral and irregular. Elliptical galaxies are spherical or ellipsoidal in shape, are dominated by old red stars and contain very little interstellar gas. Spiral galaxies consist of a central nucleus of relatively closely packed stars surrounded by a flattened, disk-shaped distribution of stars, gas and dust, within which luminous gas clouds and the brightest, youngest, stars are gathered into spiral-shaped ‘arms’ that appear to spread out from the nuclear region. In barred spirals, the nucleus is elongated, or bar-shaped, and the spiral arms emanate from either end of the nuclear bar. Irregular galaxies display no obvious pattern or structure. Spirals and barred spirals contain a larger proportion (typically about 10%) of gas and dust than ellipticals. The gas content of irregulars varies considerably, but is typically about 20%.

The masses of galaxies range from a few million solar masses, in the case of dwarf ellipticals, to a few trillion (10¹³ ) solar masses, in the case of the most massive ellipticals.  Their luminosities range from about 200 000 to a trillion (10¹² ) times that of the Sun and their diameters mostly range from a few thousand light-years to a few hundred thousand light-years. Although most of the ellipticals are relatively small, ‘giant’ ellipticals are found in the cores of clusters of galaxies, a subset of these, the ‘supergiant’ galaxies, having diameters of up to five million light-years.

Measurements of the rotation curves of spiral galaxies (graphs of the velocities at which stars and gas clouds revolve at different distances from the centers of galaxies) indicate that stars and gas clouds in the outer regions of galaxies are moving as fast as, or faster than, those that are located closer to their centers. These results imply that up to 90% of the mass of a galaxy consists of dark matter (matter that emits no detectable radiation but exerts a gravitational influence), much of which lies in its outer regions.

The galaxy to which the Sun belongs is known as the Galaxy (with a capital ‘G’), or the Milky Way Galaxy. It is a spiral or barred spiral system that contains more than 100 billion stars and which has an overall diameter (measured across its disk) of about 100 000 light-years.

Questions to Ponder:

• How are galaxies and solar systems similar?
• Do stars' orbits in galaxies obey Kepler's Laws?