Galaxies Explained

Types of Galaxies

Galaxies are classified using the Hubble sequence, a system developed by Edwin Hubble in the 1920s that categorizes galaxies by their visual appearance. Spiral galaxies like the Milky Way and Andromeda have a flat, rotating disk containing stars, gas, and dust organized into spiral arms, plus a central bulge of older stars and a surrounding halo of globular clusters. Barred spiral galaxies have an elongated bar of stars running through the center, with spiral arms extending from the ends of the bar. About two-thirds of all spiral galaxies are barred, including our own Milky Way.

Elliptical galaxies range from nearly spherical to elongated, cigar-like shapes and are classified by their apparent ellipticity from E0 (nearly round) to E7 (highly elongated). They contain mostly older, red and yellow stars with very little interstellar gas or dust, which means they have very low rates of new star formation. Elliptical galaxies are thought to form primarily through mergers of smaller galaxies, a process that disrupts the ordered rotation of spiral disks and produces the more random stellar orbits characteristic of ellipticals. The largest galaxies in the universe, found at the centers of galaxy clusters, are giant ellipticals that can contain over a trillion stars.

Irregular galaxies lack the distinct shape of spirals or ellipticals and are often the result of gravitational interactions with other galaxies. The Large and Small Magellanic Clouds, visible from the Southern Hemisphere as fuzzy patches of light, are irregular galaxies orbiting the Milky Way. Lenticular galaxies (S0) are an intermediate type with a disk and bulge like spirals but without prominent spiral arms, representing a transitional form between spirals and ellipticals.

The Milky Way

Our home galaxy, the Milky Way, is a barred spiral galaxy approximately 100,000 light-years in diameter and about 1,000 light-years thick in the disk region. It contains an estimated 100 to 400 billion stars and a comparable number of planets, with a total mass of roughly 1.5 trillion solar masses when dark matter is included. The Sun and our solar system are located in the Orion Arm, a minor spiral arm about 26,000 light-years from the galactic center, orbiting the center at a speed of about 220 kilometers per second and completing one orbit every 225 to 250 million years.

At the very center of the Milky Way lies Sagittarius A*, a supermassive black hole with a mass of about 4 million Suns. Its existence was confirmed through decades of observations tracking the orbits of nearby stars, work that earned Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in Physics. The galactic center is obscured by dense clouds of gas and dust at visible wavelengths but can be studied at infrared, radio, and X-ray wavelengths. The region immediately surrounding Sagittarius A* is a complex environment with extreme stellar densities, powerful magnetic fields, and streams of hot gas.

The Milky Way has a rich satellite system of at least 60 smaller galaxies gravitationally bound to it, the largest being the Large Magellanic Cloud at about 160,000 light-years away. These satellite galaxies are slowly being consumed by the Milky Way through tidal interactions, a process that has produced stellar streams, elongated trails of stars stripped from disrupted satellites that wrap around the galaxy like cosmic ribbons.

Galaxy Formation and Evolution

Galaxies began forming within a few hundred million years after the Big Bang, as dark matter halos gravitationally attracted ordinary matter and the first stars and protogalaxies coalesced. The hierarchical model of galaxy formation holds that small galaxies formed first and then merged over billions of years to produce the larger galaxies we see today. This model is supported by observations showing that distant (and therefore younger) galaxies tend to be smaller, more irregular, and more actively forming stars than nearby galaxies.

Galaxy mergers are a major driver of galactic evolution. When two galaxies collide, their stars rarely hit each other directly because the distances between individual stars are immense, but the gravitational interaction disrupts the structure of both galaxies. Gas clouds collide and compress, triggering bursts of intense star formation called starbursts. The merger process can take hundreds of millions of years and often results in an elliptical galaxy if the merging galaxies are of similar mass. The Milky Way is currently in the early stages of a collision with the Andromeda Galaxy, which will merge with our galaxy in approximately 4.5 billion years to form a larger elliptical galaxy sometimes called Milkomeda.

Active galactic nuclei (AGN) represent another important phase in galaxy evolution. When large amounts of gas fall toward the supermassive black hole at a galaxy's center, the material heats up and emits enormous amounts of radiation, sometimes outshining the entire rest of the galaxy. Quasars, the most luminous type of AGN, can be seen at distances of over 13 billion light-years. Most galaxies, including the Milky Way, are thought to have gone through active phases in the past and could become active again if sufficient material were to fall toward the central black hole.

Galaxy Clusters and the Cosmic Web

Galaxies are not distributed randomly through space. They are organized into a hierarchy of increasingly larger structures. Galaxy groups contain a few dozen galaxies, such as our own Local Group which includes the Milky Way, Andromeda, Triangulum, and about 80 smaller galaxies within a volume roughly 10 million light-years across. Galaxy clusters are much larger, containing hundreds to thousands of galaxies bound together by gravity within a volume roughly 10 to 30 million light-years in diameter. The Virgo Cluster, the nearest large cluster to us at about 54 million light-years away, contains over 1,300 galaxies.

On the largest scales, galaxy clusters and groups are arranged in a pattern called the cosmic web, consisting of filaments (elongated chains of galaxies), sheets or walls (flat concentrations of galaxies), and voids (enormous regions of nearly empty space). The largest known structures include the Hercules-Corona Borealis Great Wall, which spans over 10 billion light-years and is one of the largest known structures in the observable universe. This web-like pattern is a direct imprint of the tiny density fluctuations present in the early universe, amplified over 13.8 billion years by gravitational attraction.

Galaxy clusters also contain vast amounts of hot gas (the intracluster medium) heated to tens of millions of degrees that emits X-rays, and even larger amounts of invisible dark matter that can be detected through gravitational lensing. In fact, the visible galaxies themselves make up only a small fraction of a cluster's total mass. About 80 to 85 percent is dark matter, roughly 12 to 15 percent is the hot intracluster gas, and only about 3 to 5 percent is in the form of stars and galaxies.