THE GALAXIES

Since time immemorial, the hazy swath of millions of stars visible in the night sky has been called the Milky Way. During the course of the twentieth century, astronomers realized that these stars form but a small part of our own vast Milky Way Galaxy.

A galaxy is a large collection of stars in space having a distinct identity and resembling a luminous island. Galaxies are often classified according to their different shapes. The most conspicuous of these are the spiral galaxies, which tend to present a beautiful rotational symmetry. They are also classified in terms of their size. The largest known galaxies are the huge elliptical ones that lie at the center of dense clusters of galaxies. Some of these have a diameter on the order of six million light-years, while the smallest galaxies may have a diameter of only a few thousand light-years. The third way that galaxies are classified is in terms of their mass, which is directly related to the number of stars they contain. In this classification scheme, the mass of our own sun is used as a standard; galactic masses are typically expressed in terms of solar masses. The least massive galaxies may contain only a few million solar masses—and stars, while the most massive contain several trillion.

Fig. 8.1. Atlas Image mosaic courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.

The Milky Way Galaxy is a typical spiral galaxy containing some two hundred billion stars and has a diameter of approximately one hundred thousand light-years. Our solar system lies about two-thirds of the way from the galactic center toward the edge of the luminous disk, and is part of one of the spiral arms. Shown here is an infrared image of the Milky Way Galaxy taken by NASA’s Background Explorer (launched in 1989).

Although it is generally believed that galaxies were first discovered during the twentieth century, there is textual evidence to support that the ancients knew about them thousands of years ago.