Milky Way

The Milky Way is the galaxy in which the Sun and Earth reside. Like other galaxies, it is a gravitationally bound system of stars, gas, and dust. Its name also describes the faint, milky-looking band that stretches across the sky on a dark night: from our location inside the galaxy's disc, we see the disc's many distant stars along the plane of the sky. The Milky Way is one member of the Local Group, a collection of more than 50 galaxies that includes Andromeda, the nearest large galactic neighbour.

The Milky Way is a barred spiral galaxy. Most of its stars lie in a flattened disc roughly 100,000 light-years across, with spiral arms traced by stars, gas, and dust. At the centre is a dense bulge whose elongated shape forms a bar. A much more extended stellar halo surrounds the disc and bulge; it contains old stars and globular clusters. Astronomers also infer a larger halo of dark matter from its gravitational effects, although dark matter does not emit light that telescopes can observe directly.

The Solar System lies in the galaxy's stellar disc, in the Orion Spur, about 26,000 light-years from the Galactic Centre—roughly halfway from the centre to the outskirts of the disc. The Sun and its planets orbit the centre together. One complete circuit takes about 240 million years, a span sometimes called a galactic year. This motion is part of the larger rotation of the Milky Way's stars and gas around its centre.

The inner Milky Way is far denser with stars than the region around the Sun. At its centre is Sagittarius A-star, a supermassive black hole with a mass about four million times that of the Sun. It is relatively quiet compared with actively feeding black holes in some other galaxies. Dust and crowded stellar fields make the Galactic Centre difficult to study in visible light, so astronomers use other wavelengths, including infrared, to investigate it.

Because observers are inside the Milky Way, they cannot view its overall shape from outside. Dust also blocks much visible light across parts of the disc, making radio and infrared observations especially useful for studying obscured regions. Space-based astrometry adds another perspective: the European Space Agency's Gaia survey measured the positions and motions of almost two billion stars, about one per cent of the galaxy's estimated stellar population. Combining such observations helps astronomers refine maps of the disc, its arms, and the stars around us.