What counts as a galaxy
A galaxy is a huge collection of stars, gas, dust and dark matter bound together by gravity. Galaxies range from small dwarf systems with perhaps a few million stars to giants with a trillion or more. Our own is one of many, and astronomers estimate that the observable universe contains at least hundreds of billions of galaxies, though such counts are uncertain.
For a long time, astronomers did not know that galaxies were separate from our own. Fuzzy patches in the sky were called nebulae, and many were thought to lie inside the Milky Way. In the 1920s, Edwin Hubble used variable stars in the Andromeda nebula to estimate its distance and showed that it lay far outside the Milky Way, which settled the question.
Hubble’s sorting scheme
Hubble also grouped galaxies by their appearance. His scheme, often drawn as a “tuning fork,” remains a useful starting point, even though it describes shape rather than explaining origin.
Spiral galaxies
Spiral galaxies have a flat, rotating disk with curving arms, a central bulge, and a halo around them. The disk contains plenty of gas and dust, so new stars keep forming, especially in the arms, and these give the arms a bluish tint. The bulge and halo tend to hold older, redder stars. About two thirds of spirals show a bar-shaped structure across the center, and these are called barred spirals. The Milky Way is generally thought to be one.
The spiral pattern is not a fixed set of stars. Stars move in and out of the arms, which are better understood as regions where gas and stars are temporarily more crowded, a bit like a traffic jam on a road, while cars keep moving through it.
Elliptical galaxies
Elliptical galaxies look like smooth, rounded or egg-shaped balls of stars, with no obvious arms or disk. Their stars orbit in many directions, and they generally contain little cool gas, so they form few new stars. Their light is dominated by older, redder stars. Ellipticals range from small to the largest known galaxies, and these giants are often found near the centers of galaxy clusters.
Lenticular galaxies
Between the two lies a class of lenticular (lens-shaped) galaxies. They have a disk and a bulge but no clear spiral arms and little star formation.
Irregular galaxies
Irregular galaxies have no regular shape. Many are small, gas-rich and actively forming stars. Some are irregular because they have been disturbed by a close encounter with a neighbor, while others are small systems that never organized into a disk. The Magellanic Clouds, near the Milky Way, are often cited as examples.
Why the shapes differ
Astronomers do not think shape is a permanent property. It reflects a galaxy’s history, which can be divided into a few factors.
Spin and gas supply. In the commonly used picture, a collapsing cloud of gas with some rotation flattens into a disk, like a spinning ball of dough. Disks form when gas settles smoothly and keeps a steady supply for making stars.
Mergers. When two galaxies of similar size collide, the gravitational forces scramble their orbits. The galaxies’ stars rarely hit one another, because there is so much empty space between them, but their paths get thrown into chaotic orbits. Simulations show that a merger of two disk galaxies can produce an elliptical-like result. A merger also pushes gas toward the center, which can trigger a burst of star formation and feed a central black hole. Not every elliptical forms this way, and the relative importance of different paths is still debated.
Running out of fuel. A galaxy stops forming stars when it runs out of cold gas or the gas gets too hot. Reasons include stars exploding and heating the gas, energy released by a central black hole, and gas being stripped away as a galaxy moves through a crowded cluster. Over time, its remaining stars age and redden.
Environment. Spiral galaxies are more common in sparse regions, and ellipticals in dense clusters, where encounters are frequent. This pattern supports the idea that surroundings influence the outcome.
Growth over cosmic time
By looking at very distant galaxies, whose light left them long ago, astronomers can see what galaxies looked like earlier. Distant galaxies tend to be smaller, clumpier and more irregular, with high rates of star formation. This fits the view that galaxies grow through accretion of gas, and by merging with smaller companions. Dark matter is thought to provide the gravitational scaffolding on which this growth happens.
Many galaxies also host a supermassive black hole at the center, and their masses are loosely correlated with the properties of the surrounding bulge. This suggests that black holes and galaxies influence each other’s growth, though the details are still being worked out.
Galaxy futures
Galaxies continue to change. The Milky Way is expected to merge with Andromeda in several billion years, perhaps forming a larger, rounder galaxy, though the timing is uncertain. This is a reminder that the shapes we see are snapshots in a long process, and that place in the universe is shaped by gravitational forces acting over billions of years.