How the Universe Makes Galaxies

Galaxies Have Been Forming Throughout Cosmic Time

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Hubble Space Telescope looked at a pair of colliding galaxies (upper part of the image) that are tangling as they interact. The shock of the collision has produced blue streamers that look like clouds. They are actually giant starburst regions, where clusters of hot, massive young stars are being born. The third smaller galaxy to the right of the colliding pair may or may not be involved, nor is the galaxy at the bottom. Starburst clusters are just one of the many effects of galaxy collision. NASA/ESA/STScI

When you look at the night sky, you see stars and planets, star clusters, and nebulae. You can also see galaxies. You live inside one, called the Milky Way. It's not too difficult to spot another galaxy or two in the sky with the naked eye. In the northern hemisphere, we can see the Andromeda Galaxy in good, dark skies. The Southern Hemisphere sky sports the Large and Small Magellanic Clouds, companions to our Milky Way.

To see galaxies farther away, you need to magnify your view with telescope or binoculars.

Searching out the Origins of Galaxies

The study of galaxies has advanced quite a lot since the time when astronomers such as Charles Messier called them "fuzzy objects" and others in the 19th century called them "spiral nebulae." Today we know they are stellar cities, swarming with stars. One of the major questions astronomers are answering about these objects is, "Where do they come from?" 

Observatories as the Hubble Space Telescope excel at spotting and imaging distant “baby” galaxies that were born right after the Big Bang. In the early universe, galaxies didn’t start out the way they look now. These structures — which are the oldest in the cosmos — formed through the processes of mergers and collisions that take place over millions and billions of years.

The Earliest Seeds of Galaxies Go Back a LONG Way

The seeds of galaxies were planted just after the formation of the universe, an event called the Big Bang.

For hundreds of thousands of years, the young universe was hot and opaque. The infant universe was a soup of primordial atomic particles. Eventually, as this early universe expanded, it cooled. It began to experience tiny changes in the density of its primordial "soup". The gravitational influence of dark matter played a role in corraling the cosmic seed material into regions where the processes of star and galaxy formation could take place.

 

Eventually, about 400,000 years after the Big Bang, the first stars began to shine in coalescing infant galaxies. These massive star formation regions were the cores of the first galaxies. As time went by, the early shreds of galaxies began to collide with each other. Shocks induced by the collisions spurred the births of more stars within the resulting galaxies. This is the way the infant Milky Way Galaxy first began to form some 11 billion years ago. Today, our galaxy continues to cannibalize a few dwarf galaxies. For example, the Sagittarius Dwarf Spheroidal galaxy is orbiting our galaxy, and has very likely lost stars to the Milky Way each time it passes through. Astronomers have studied an ultracompact dwarf galaxy called M60-UCD1 and discovered a supermassive black hole at its heart, which may be a clue to the collisional history of this tiny galaxy.

Galaxy Collisions Continue

Galaxy collisions result in the formation of all the types of galaxies we see. They continue to affect galaxies in the modern universe, too. The Milky Way is a spiral galaxy and it has undergone many collisions. It's still gobbling up smaller dwarf galaxies, such as the Sagittarius Dwarf. We also know that it will likely collide with the nearby Andromeda Galaxy beginning in a few billion years.

Eventually, the two galaxies could form a giant elliptical galaxy (that is, an egg-shaped, rounded galaxy with  no spiral arms) that astronomers have pre-emptively nicknamed "Milkdromeda". The two could also combine to create a much larger, more massive disk galaxy, as well. 

Astronomers also think that the ​Large and Small Magellanic Clouds could also collide with the Milky Way in the distant future. These two neighboring galaxies are interacting with each other, leaving behind a stream of high-speed gas between them. 

The End Results of Some Collisions: Elliptical Galaxies

Elliptical galaxies are some of the oldest galaxies in the cosmos. They form directly from violent collisions of smaller galaxies and usually have supermassive black holes, perhaps as a consequence of their spectacular evolution.

 

There are at least 100 billion galaxies in the known universe, and it's very likely each one has had at least one close interaction with a neighbor. The result is a new galaxy, bursting with waves of star formation and evolution, as well as the formation of those supermassive black holes at their hearts.