What's happening with those galaxies?
Find a clear night sky and get out a good pair of binoculars or a telescope. You can see this feature (although not quite as well). The Whirlpool galaxy has an enhanced spiral structure due to its interactions with its companion galaxy NGC 5195.
Galaxies are the biggest groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye, the closest major galaxy — the Andromeda Galaxy, shown in Figure below — looks like only a dim, fuzzy spot. But that fuzzy spot contains one trillion — 1,000,000,000,000 — stars!
The Andromeda Galaxy is a large spiral galaxy similar to the Milky Way.
Galaxies are divided into three types according to shape: spiral galaxies, elliptical galaxies, and irregular galaxies.
Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy shown in Figure below. Several arms spiral outward in the Pinwheel Galaxy (seen in Figure below) and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars.
(a) The Sombrero Galaxy is a spiral galaxy that we see from the side so the disk and central bulge are visible. (b) The Pinwheel Galaxy is a spiral galaxy that we see face-on so we can see the spiral arms. Because they contain lots of young stars, spiral arms tend to be blue.
Figure below shows a typical egg-shaped elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish in color because they contain mostly old stars.
The large, reddish-yellow object in the middle of this figure is a typical elliptical galaxy. What other types of galaxies can you find in the figure?
Most elliptical galaxies contain very little gas and dust because the gas and dust have already formed into stars. However, some elliptical galaxies, such as the one shown in Figure below, contain lots of dust. Why might some elliptical galaxies contain dust?
Astronomers believe that these dusty elliptical galaxies form when two galaxies of similar size collide.
Is the galaxy in Figure below a spiral galaxy or an elliptical galaxy? It is neither one! Galaxies that are not clearly elliptical galaxies or spiral galaxies are irregular galaxies. How might an irregular galaxy form? Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a larger galaxy or by a collision with another galaxy.
This galaxy, called NGC 1427A, has neither a spiral nor an elliptical shape.
Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we don’t see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies.
Look back at the picture of the elliptical galaxy. In the figure, you can see two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the right of center. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors.
- A galaxy is composed of millions to billions of stars.
- Galaxies can be spiral, elliptical or irregular. Dwarf galaxies are smaller, but are more common than other galaxies.
- Galaxies that have lots of dust and gas are locations where stars are forming.
- What makes a galaxy different from other galaxies or types of astronomical objects?
- What makes irregular galaxies take an irregular shape?
- How do dwarf galaxies interact with other galaxies?