What Is a Galaxy?

If you gaze out into the night sky with a telescope, and see beyond what’s visible to the naked eye, you'll see a lot of stars that are actually imposters. Many of those points of light are actually galaxies — collections of millions to trillions of stars. Galaxies are composed of stars, dust and dark matter, all held together by gravity.

Astronomers aren't certain exactly how galaxies formed. After the Big Bang, space was made up almost entirely of hydrogen and helium. Some astronomers think that gravity pulled dust and gas together to form individual stars, and those stars drew closer together into collections that ultimately became galaxies. Others think that the mass of what would become galaxies drew together before the stars within them were created. Astronomers are also refining their techniques of measuring the mass of individual galaxies, such as this 2018 study that used the three-dimensional movements of several galaxies to better narrow down the Milky Way's mass.

The Sun is one of billions of stars in the Milky Way Galaxy.
(Image: © Caltech)

Discovering island universes

In the early 1900s, many astronomers thought that the entire universe lay within our galaxy, the Milky Way. Others, such as Harlow Shapley, a scientist and head of the Harvard College Observatory, argued that the spiral-shaped blobs thought to be dust and gas were separate and called them "island universes."

It wasn't until 1924, when Edwin Hubble identified several special pulsing stars called Cepheid variables and realized that they lay outside of the known span of the Milky Way. These celestial objects were completely unique collections of stars at distances well beyond our home galaxy.

After Hubble measured the distance to individual galaxies, he went on to measure their Doppler shift — how much light from the galaxies was stretched out due to their motion. He determined that galaxies all around the Milky Way are moving away from us at terrific speeds. The farther away the galaxies are, the faster they are fleeing. Because of this, he was able to determine that the universe itself is expanding, and years later, astronomers determined that the expansion is accelerating.

Galaxy characteristics

Most galaxies have black holes at their centers that can produce a tremendous amount of energy, which astronomers can see over great distances. In some cases, a galaxy’s central black hole is extremely large or active, even in relatively small galaxies. Material circling the black hole may be accelerated outward by its jets. Other galaxies may contain quasars — the most energetic bodies in the universe — at their core.

Galaxies are classified by their shape. Each type has different characteristics and a different history of evolution.

Some, like the Milky Way, have arms spiraling outward around their center. Known as spiral galaxies, these groups make up most of the galaxies that astronomers can see. The gas and dust in a spiral galaxy circles the center at speeds of hundreds of miles per second, creating their pinwheel shape. Some, known as "barred spirals," have a bar structure in their center, formed by dust and gas funneled into the center. The dust and gas in spiral galaxies are consistently fueling the formation of new stars.

Elliptical galaxies lack the spiral arms of their more flamboyant cousins. Their appearance ranges from circular to very stretched out. Elliptical galaxies have less dust than their spiral counterparts, and so the star-making process has all but ended. Most of their stars are older. Although they make up a smaller portion of the visible galaxies, astronomers think that over half the galaxies in the universe are elliptical.

The remaining 3 percent of the galaxies in the universe are known as irregular galaxies. They are neither round nor boast spiral arms, and their shapes lack specific definition. The gravity of other galaxies has often affected them, stretching them out or warping them. Collisions or close calls with other galaxies can also deform their shapes.

This Hubble Space Telescope image shows the full beauty of nearby spiral galaxy M83 in a mosaic of many photos stitched together. The magentas and blues indicate star-forming regions. Also known as the Southern Pinwheel, M83 is located 15 million light-years away in the constellation Hydra. Image released January 2014.
(Image: © NASA, ESA, and the Hubble Heritage Team (STScI/AURA))

When galaxies collide

Galaxies don't float through space in isolation but are bunched together in groups known as clusters. Some clusters are large, containing over a thousand galaxies while others are much smaller. The Milky Way lies within the cluster known as the Local Group, which only contains 50 galaxies.

Occasionally, galaxies slam into one another, merging their stars and dust together. This is an important step in the evolution and growth of many galaxies. In late 2018, astronomers released a study showing two supermassive black holes crashing into one another in the final stages of a galactic smashup, representing the first time such a late stage of galaxy collision was seen in action.

Individual stars generally don't collide in a galactic collision, but the influx of dust and gas bumps up the rate of star formation. The Milky Way is set to collide with the Andromeda galaxy in about 5 billion years, and collisions have occurred in its ancient past as well; its distinctive bulge may have developed after consuming a sausage-shaped galaxy. Not to be outdone, the Andromeda galaxy probably ate up one of the Milky Way's early siblings, too.

Emerging galaxy research

In recent years, astronomers have tracked galaxies and how their evolution is shaped by dark matter, a substance that cannot be sensed with traditional telescope technology. Dark matter and dark energy together are thought to make up most of the universe's mass and energy, but their existence is difficult to prove because we can only see them through their effects on more conventional objects, like galaxies.

In 2017, astronomers found two huge galaxies from the ancient universe that were formed in a sea of dark matter. The extraordinarily large size of the galaxies is making scientists wonder if they gradually grew bigger over time, or if some other process might have been responsible. Only a few months after that discovery, astronomers also found a group of galaxies that were orbiting in sync with each other in a pattern that may be explained by dark matter.

But in 2018, a group of researchers challenged the dark matter theory when they discovered that a galaxy called NGC 1052-DF2 has about 400 times less dark matter than models would predict for an object of its size, which could also change galactic evolution models. However, the results of the study are controversial and are still under debate.

Periodic galactic surveys with increasingly advanced technology have allowed scientists to identify galaxies that were previously too faint to see and learn more about galactic evolution, size and shape. For example, in 2017, the Multi Unit Spectroscopic Explorer (MUSE) instrument on the European Southern Observatory's Very Large Telescope revealed a group of 72 galaxies that were hiding in plain sight.

In 2018, an image from the Hubble Space Telescope managed to cram in some 15,000 galaxies, providing an abundant hunting-ground for future galactic studies. And in the same year, hundreds of galaxies were found behind a super-energetic black hole that was previously obscuring their presence.

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This article was updated on Jan. 4, 2019, by Space.com contributor Elizabeth Howell.

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