For most of recorded history, the universe was assumed to be the Milky Way and nothing else. The faint elliptical smudges catalogued by early observers were classified as nebulae — clouds of gas within our own system.
The question of whether they were something else was settled in the 1920s, when Edwin Hubble identified Cepheid variable stars in the Andromeda nebula and used them to measure its distance. The answer came back far beyond any plausible boundary of the Milky Way. Andromeda was not a cloud in our galaxy. It was another galaxy, and the universe was suddenly enormous.
What a galaxy is
A galaxy is a gravitationally bound system of stars, gas, dust, and dark matter. The range is vast: dwarf galaxies contain a few million stars, while the largest ellipticals hold many trillions.
The Milky Way is a fairly typical large spiral, containing on the order of a few hundred billion stars, spanning roughly a hundred thousand light years, with the Sun sitting about halfway out in one of the spiral arms.
The visible matter is a minority component. Rotation measurements and gravitational lensing both indicate that galaxies sit inside halos of dark matter extending well beyond the visible disk and accounting for the bulk of their mass.
The main types
The classification scheme still in use descends from Hubble’s original diagram, though the evolutionary sequence he suggested has not held up.
Spirals
A flattened rotating disk with arms winding outward from a central bulge, containing substantial gas and dust and forming new stars. Many, including the Milky Way, have a bar-shaped structure of stars through the centre.
The arms are not fixed structures of the same stars. They are density waves — regions of compression that move through the disk at a different rate than the stars do, triggering star formation as gas is squeezed. The arms look bright because they contain hot, short-lived, luminous young stars, not because they hold more material overall.
Ellipticals
Smooth, featureless, roughly ellipsoidal collections of stars with little gas and minimal ongoing star formation. Their stars orbit in randomly oriented paths rather than a shared plane.
Ellipticals include the largest galaxies known, and they are concentrated in the centres of galaxy clusters. The prevailing explanation is that they are merger products: when two large spirals collide, the ordered rotation is destroyed, gas is consumed or expelled in a burst of star formation, and what remains is a red, quiescent elliptical.
Irregulars and dwarfs
Galaxies without clear symmetry, often gas-rich and actively star-forming, and frequently distorted by gravitational interaction with a larger neighbour. Dwarf galaxies are by far the most numerous type, though they are faint and easily missed.
Nearly every large galaxy hosts a supermassive black hole at its centre, and the black hole’s mass correlates tightly with properties of the surrounding bulge.
That correlation is one of the more suggestive results in extragalactic astronomy. It implies that the growth of the central black hole and the growth of the galaxy around it are coupled, most likely through energy the black hole injects into its surroundings when it accretes.
Galaxies are not isolated
Galaxies cluster. The Milky Way and Andromeda dominate a small collection called the Local Group, alongside dozens of dwarf companions. The Local Group sits on the outskirts of the larger Virgo Cluster, which is itself part of a supercluster.
On the largest scales, galaxies trace a structure often described as the cosmic web: filaments and sheets of galaxies surrounding vast, nearly empty voids. This pattern was not designed into the models — it emerges naturally from simulations of gravity acting on the small density fluctuations visible in the cosmic microwave background.
The agreement between those simulations and the observed distribution of galaxies is one of the strongest arguments for the standard cosmological model, and one of the strongest arguments for dark matter, since simulations using only ordinary matter fail to produce the observed structure in the available time.
Collisions and what they do
Galaxy interactions are common, and the outcomes are dramatic on paper and gentle in practice.
Stars almost never collide. The spacing between stars is so vast relative to their size that two galaxies can pass through each other with essentially no stellar impacts. What does collide is the gas, and gas clouds meeting at high speed compress and ignite intense bursts of star formation.
The gravitational effects reshape both galaxies, drawing out long tidal tails and eventually, in a full merger, destroying the disk structure entirely.
The Milky Way and Andromeda are approaching each other and are expected to merge in roughly four billion years. The likely result is a single large elliptical. The Sun will still exist at that point, though Earth’s habitability will already have been compromised by the Sun’s own evolution.
Why they matter
Galaxies are the unit in which the universe organises its ordinary matter. Understanding how they form, how they acquire and lose gas, how star formation switches on and off, and how their central black holes regulate them is effectively the project of understanding how the universe went from a nearly uniform plasma to a structured place containing planets and observers.
That project is not finished.
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