What are Galaxies?

Galaxies are defined as large groupings of stars, dust, and gas held together by gravity. They vary greatly in size and shape. Most of the objects we know of in space are contained within galaxies. They contain stars, planets, moons, comets, asteroids, nebulae, dust, neutron stars, and black holes. Many probably even contain large amounts of unseen dark matter. Since most of the space between galaxies is thought to be empty, a galaxy is essentially an oasis in space. Our own solar system is located within a galaxy. Our Sun is only one of over 100 billion stars in a galaxy called the Milky Way. The Milky Way is a gigantic spiral disk with a bright, central bulge. Our solar system is located about 3/4 of the way out from the center in one of the galaxy's spiral arms. All of the stars we see in the night sky are part of the Milky Way. And just like our solar system, our galaxy is in motion. The stars within the Milky Way revolve around the central core. The Milky Way itself is moving as well. In fact, all of the galaxies in the universe seem to be moving away from each other at tremendous speeds. After studying galaxies for many years, astronomer Edwin Hubble decided to classify galaxies according to their shape. This classification system is known as the Hubble Sequence. It divides galaxies into three main classes with a few variations. Today, galaxies are divided into four main groups: spiral, barred spiral, elliptical, and irregular.

There are innumerable galaxies in the universe

Types of Galaxies

Astronomers classify galaxies into three major categories: elliptical, spiral, and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.

Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars, and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.

Spiral galaxies appear as flat, blue-white disks of stars, gas, and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.

Irregular galaxies, which have very little dust, are neither disk-like nor elliptical. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe before spirals and ellipticals developed.

Aside from these three classic categories, astronomers have also identified many unusually shaped galaxies that seem to be in a transitory phase of galactic development. These include those in the process of colliding or interacting, and those with active nuclei ejecting jets of gas.

Different types of galaxies

How do galaxies form?

The appearance and make-up of galaxies are shaped over billions of years by interactions with groups of stars and other galaxies. While we don't know for certain how galaxies formed and took the many shapes that we presently see, we have some ideas about their origins and evolution. Using supercomputers, scientists can look back in time and simulate how a galaxy may have formed in the early universe and grown into what we see today.

Astronomer Edwin Hubble’s observations led to the idea that the universe is expanding. Scientists estimate the age of the universe at 13.8 billion years based on the rate of expansion. Because the deeper you look into space, the further you see back in time, we can conclude that galaxies several billions of light-years away formed fairly soon after the big bang. While most galaxies formed early, data indicates that some galaxies have formed within the past few billion years — relatively recently in cosmic terms.

The early universe was filled mainly with hydrogen and helium, with some areas slightly denser than others. These dense areas slightly slowed the universe’s expansion, allowing the hydrogen and helium to accumulate into small clouds swirling through space.  Gravity caused the gas in these clouds to collapse and form the first generation of stars. These first stars rapidly burned out. 

Gravity continued to collapse the clouds. As other clouds came close to each other, gravity sent them careening into one another and knitted the clouds into larger, spinning packs.  As the clouds further collapsed, they became rotating disks, which amassed more gas and dust. New stars formed, creating extensive spiral arms filled with colonies of stars. Sprinkled along the periphery were globular clusters, along with a halo of gas, dust, and dark matter.

A galore of galaxies as seen using a space telescope

What is Dark Matter?

In the late 1970s, astronomer Vera Rubin made the surprising discovery of dark matter. She was studying how galaxies spin when she realized the vast spiral Andromeda Galaxy seemed to be rotating strangely. In an apparent violation of Newton and Kepler’s Laws, the material at the galaxy’s edges was moving just as fast as the material near the center, even though most of the mass she could see was concentrated at the center. Some extra non-visible mass, dubbed dark matter, appeared to be holding the galaxy together. She soon discovered that a huge halo of dark matter was present in the galaxy after galaxy that she examined. Nearly half a century later, scientists still don’t know what dark matter is. They do know, however, that dark matter comprises some 84 percent of the universe’s material. Its invisible and ubiquitous presence affects how stars move within galaxies, how galaxies tug on each other, and how matter clumped together in the early universe.

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