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Galaxy Formation: Spiral, Elliptical & Irregular Galaxies

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  • 0:05 Galaxy Formation
  • 2:26 Spiral Galaxy
  • 3:18 Elliptical Galaxy
  • 4:31 Irregular Galaxies
  • 5:01 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

This lesson explains how galaxies form, starting with the Big Bang. You'll also learn about the solar nebula hypothesis and three galaxy types, including spiral, elliptical, and irregular galaxies.

Galaxy Formation

Look at the clear night sky. Check out our galaxy. There are so many stars up there! Too many to even count. Astronomers estimate there are more than 100 billion stars in our galaxy alone, and more than 100 billion galaxies in the whole universe! That's a lot of stars! So how did our galaxy form? How does any galaxy form? And what is a galaxy, anyway?

Let's start at the beginning. A galaxy is a group of stars, dust, and gases held together by gravity. Galaxies formed after the universe was born. The universe, which includes all of existence, including planets, stars, galaxies, the contents of intergalactic space, and all matter and energy, was born with the Big Bang. The Big Bang occurred when a small bit of mass suddenly exploded and spread outward to form the universe. When it was first born, our universe was quite homogeneous. There was little structure and no galaxies.

Photo of the Milky Way, a spiral galaxy
Spiral Galaxy

Astronomers believe that all the structures of the universe we see today began as small fluctuations or changes in the density in small areas of the universe. This is known as a solar nebular hypothesis, which states that the solar system began as a cloud of interstellar dust and gas, known as a nebula, that started to spin and condense into a solar system. These small changes in density attracted gas and dark matter - matter that makes up a majority of the universe, but is something we can't see. These attractions of matter are considered to be the first galaxies, or proto-galaxies. Soon after the proto-galaxies were formed, the hydrogen and helium gases within them condensed to make the first stars. The universe was very violent in its early years and galaxies grew quickly.

Galaxies come in three broad categories, based on the role of the bulge (the round distribution of stars at the center) and the disk (the flat distribution that includes the spiral arms). The categories are elliptical, spiral, and irregular. The one you are probably most familiar with is our own Milky Way, a pancake-flat spiral galaxy.

Spiral Galaxy

Spiral galaxies are thin, dense, highly organized disks of gas and stars that rotate very fast. The disks in spiral galaxies are very fragile, and mergers with other galaxies can quickly destroy them.

Spiral galaxies have three main parts: a bulge, disk, and halo. Think of the classic picture of an old spaceship. The bulge is a spherical structure found in the center of the galaxy. This feature mostly contains older stars. The disk is made up of dust, gas, and younger stars. The disk has spiral arms. Our sun and solar system are located in one of the arms of the Milky Way spiral galaxy. The halo of a galaxy is a loose, spherical structure located around the bulge and some of the disk.

Elliptical Galaxy

Photo of an elliptical galaxy
Elliptical Galaxy

Another type of galaxy is the massive elliptical galaxy. An elliptical galaxy is basically all bulge with no disk. They range in shape from nearly spherical to greatly elongated football shapes, and in size from smallest to biggest known galaxies. They do not have spiral arms and have little or no star formation taking place within them.

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