Galaxy

Galaxy The spiral galaxy containing the Sun and all the stars visible to the naked eye at night; it is written with a capital ‘G’ to distinguish it from other galaxies. Its disk is visible to the naked eye as a faint band of light around the sky, the Milky Way; hence the Galaxy itself is also often referred to as the Milky Way.

 Our Galaxy has three principal components. One is a rotating disk of about 6 × 10¹⁰ solar masses consisting of relatively young stars (Population I), open clusters, gas, and dust, with some concentration of the young stars and interstellar material into spiral arms. The disk is quite thin, about 1000 l.y., compared with its diameter of over 100 000 l.y. Active star formation continues in the disk, particularly in giant molecular clouds. The second main component is a faint, roughly spherical halo with perhaps 15–30 % the mass of the disk. The halo is composed of old stars (Population II), a few per cent of them in globular clusters, plus small amounts of hot gas, and it merges into a more conspicuous central bulge of stars, also of Population II. The third main component is an unseen halo of dark matter with a total mass of at least 4 × 10¹¹ solar masses. In all, there are probably about 2 × 10¹¹ stars in the Galaxy, most with masses less than the Sun's. The age of the Galaxy remains somewhat uncertain, but the disk is at least 10 billion years old, while the globular clusters and most of the halo stars are believed to be 12–14 billion years old.

 The Sun lies about 26 000 l.y. from the centre, in the Orion Arm, which lies between the Sagittarius and Perseus Arms. The galactic centre itself lies in the constellation Sagittarius and is thought to harbour a supermassive black hole of 3–4 million solar masses. The Milky Way is a spiral, probably of Hubble type Sbc (i.e. intermediate between types Sb and Sc), but observations of its structure and attempts to measure the dimensions of individual spiral arms are hampered by obscuring dust in the disk and by the difficulty of estimating distances. It is possible that the Galaxy is a barred spiral, since there is some evidence for bar-like structure in the central regions and bulge.