The Milky Way galaxy is made of billions of stars, and gas and dust, all bound together by mutual gravitational attraction, as well as a lot of dark matter. The diameter of our galaxy is about 100,000 light years [e1] across (the visible material at least – the dark matter halo goes beyond that). The Milky Way is roughly shaped like a pancake with a ball fixed in the middle of the pancake. The Solar System resides in one of the Milky Way’s spiral arms, about two-thirds from the center of the Galaxy, in the more flat or “pancake” part of the Galaxy. Latest estimates show that the Milky Way is believed to be about 12,000 light years thick, from top to bottom. The “ball” of the Milky Way is known as its “bulge” and is about 10,000 light years across, containing a dense halo of stars, gas, and dust. At the center of the bulge is the area known as the Galactic Center. This area lies about 26,000 light years away (2.5x1017 km or 1.5x1017 miles) from Earth and contains a supermassive black hole, known as Sagitarrius A*, which has a mass about 4 million times that of our Sun.
How was our galaxy formed?
Dark Matter! For galaxy creation to be successful, we first need a mysterious substance known as dark matter. Astronomers don’t know what dark matter is made of exactly but they do know it’s invisible and takes up most of the matter in the Universe. Dark matter, which will be shaped as a huge sphere around your galaxy and also sprinkled throughout your galaxy, seems to trigger the growth of clouds of gas and dust. Galaxy creators would need to shape the dark matter into a super-galactic-sized sphere. This dark matter will then weakly attract hydrogen gas and trigger the birth of the first stars. Be sure to stand back, as that first generation of stars can go out with a bang, with many supernova explosions occurring.
Gas! What are the other materials that go into making a galaxy? As mentioned above, clouds of gas and dust are believed to have eventually collapsed under their own gravitational pull to form the Milky Way’s first stars. So we need enough hydrogen gas to create a few billion stars.
Spiral
The main feature of spiral galaxies is their disk, which contains most of their stars and enough to make many billions more. Galaxies such as the Milky Way are not among the largest in the Universe (compare to M87 and Hercules A at 980,000 and 1.5 million light years across respectively)
Elliptical
Category of galaxy in which the stars are distributed in an elliptical shape on the sky, ranging from highly elongated to nearly circular in appearance. In the sky, where we can only see two of their three dimensions, these galaxies look like elliptical, or oval, shaped disks. The light is smooth, with the surface brightness decreasing as you go farther out from the center.
Irregular
A strangely shaped galaxy, often rich in interstellar matter, but apparently not a member of any of the major classes of spiral or elliptical galaxies.
