By Sarah Readdean
When we close our eyes and imagine a galaxy, we typically imagine the galaxy in which we live. Our Milky Way galaxy, as well as the Andromeda galaxy — our neighbor with whom we will eventually collide — are both spiral galaxies. But not all galaxies have this characteristic shape. There are also elliptical and irregular galaxies.
- Spiral Galaxies
Spiral galaxies consist of three components: the disk, halo and bulge. The disk is the part of the galaxy that is seen edge-on as a streak of dust, and from face-on as the spiral arms. In the disk are very young stars and very old stars, as well as gas clouds that haven’t yet formed into stars.
The stars that lie in this plane orbit around the center of the galaxy — our Sun orbits the black hole at the center of the Milky Way once every 230 million years.
The spherical bulge at the center of the galaxy and the halo surrounding the disk and bulge are composed primarily of old stars. In a barred spiral galaxy, the arms extend outward from a line of stars that stretch across the diameter of the disk.
- Elliptical Galaxies
Elliptical galaxies have a spherical halo and bulge but lack a disk. They contain older stars, so they appear more red or yellow in color, as opposed to young stars that are blue and hot.
Lenticular galaxies contain all three components of a spiral galaxy, except they do not have spiral arms. This makes lenticular galaxies an intermediate between elliptical and spiral galaxies.
- Irregular Galaxies
Irregular galaxies come in all shapes and sizes, each unique and following no recurrent pattern.
They often consist of scattered areas of young stars and a lot of dust and gas. The dust clouds indicate star formation, so they are seen as bluer because young stars are hot. Since we see young stars in irregular galaxies, scientists can conclude that because they are so far away, these were prevalent at the start of the universe.