A galaxy contains between a few million to hundreds of billions of stars, bound together by their mutual gravitation. A galaxy's stars can be part of stellar clusters, or part of a larger population of separate stars pervading the galaxy. In addition, a galaxy contains stellar remnants, dust, gas and Dark Matter. Many galaxies have a supermassive black hole at their centre.
Dark Matter is a type of matter which does not emit or interact with electromagnetic radiation, and thus is impossible to see by direct observations. Although Dark Matter cannot be seen, it has mass, and its existence is inferred from its gravitational effects on visible objects. Such effects include the motion of visible objects, or the distortion of images due to gravitational lensing. Galaxies are surrounded by a much larger halo of Dark Matter — in a sense, what we see of a galaxy is only the tip of the iceberg.
Over the first hundreds of millions of years of the Universe’s history, Dark Matter evolved into numerous large, denser regions, called halos. As hydrogen and helium gas fell onto these halos, the first galaxies and the first stars formed. Larger spiral galaxies like the Milky Way evolved as they attracted and incorporated numerous smaller galaxies. Large elliptical galaxies formed when more massive galaxies collided and merged. Depending on their gas reserves, and on heating through exploding stars or activities in the galactic centre, these galaxies formed new stars at an increased or slower rate.
Accordingly to their visual appearance, galaxies are categorised into spiral, elliptical and irregular galaxies. These types differ not only in shape but also in their contents. Spiral galaxies have flattened spiral arms formed predominantly by bright young stars and large quantities of gas and dust. In contrast, elliptical galaxies contain less gas. Their stars are mostly old and distributed in an ovoid or spherical shape. Some galaxies, including most dwarf galaxies, have neither of these two standard shapes and are called irregular.
Our Milky Way is a spiral galaxy with a bar-shaped structure at the centre. The Solar System is located at about 25,000 light-years from the centre, in a spiral arm. The visible part of our galaxy is a disk-shaped collection of stars with a diameter of about 100,000 - 120,000 light-years and a thickness of only about 2,000 light-years. In this disk, young stars and dust form spiral arms. During a dark night and from a suitably dark location, we can see a minute fraction of the more than 100 billion stars within the Galactic disk as an enormous hazy band arching across the sky. This is our view from within our home galaxy.
A widely accepted theory about the formation of spiral arms is that they are the result of a density wave moving through the disk of a galaxy, causing the stars, gas and dust to pile up in a way similar to a traffic jam on a busy highway. This gives rise to denser regions in the disk which are seen as spiral arms. These high density regions contain a lot of gas and dust, which are essential for the formation of new stars. Hence, the spiral arms contain many young bright stars, showing that these regions have a high star formation rate.
A typical galaxy contains an estimated 100 million stellar-mass black holes. These types of black holes are formed when a massive star ends its life in a supernova explosion. Supermassive black holes are found in the centres of most galaxies, and are the largest type of black hole, with masses between a few million and more than a billion solar masses. Our Milky Way has a supermassive black hole at its centre with a mass of about four million solar masses. The first direct image of the silhouette of the event horizon of a black hole, at the center of the huge elliptical galaxy M87, was achieved in 2019 by combining data from eight radio telescopes around the world.
The Milky Way’s closest neighbour is the Canis Major dwarf galaxy, at a distance of about 25,000 light-years. Distant galaxies appear very faint to us and are therefore difficult to observe. In order to obtain images of distant galaxies, it is necessary to employ large telescopes with high resolving power, and take long exposures to gather enough light from these objects.
GGalaxies are not scattered randomly throughout the Universe. Rather, the average galaxy is part of a galaxy cluster. These clusters consist of hundreds or even thousands of galaxies bound together by their mutual gravitational attraction. Clusters of galaxies themselves are also grouped together in bigger structures called superclusters. The Milky Way is part of what is called our Local Group of galaxies, which includes more than 54 galaxies. The Local Group is an outlying member of the Virgo Cluster, which is part of the Virgo Supercluster, which is in turn part of the Laniakea Supercluster.
Interactions between galaxies influence their appearance and evolution. In the past it was believed that one type of galaxy could evolve to a different type throughout its life but current scientific knowledge shows that gravitational interactions are the reason behind some types of galaxies. For example, elliptical galaxies may be created by mergers between large predecessor galaxies, and at the same time these events may trigger an intense star formation burst in the interacting galaxies.