Spiral galaxies are galaxies with a central, dense area and spiraling arms (which are often sites of star formation). These common galaxies have two major parts:
- a central, flat disk containing a dense cloud of interstellar matter and young star clusters (mostly on the arms)
- a central bulge (or nucleus) containing older stars
There are two types of spiral galaxies, “S” (normal spiral) and the less common “SB” (barred spiral, with an elongated center).
The Milky Way and the Andromeda Galaxy (M31) are two of a multitude of known spiral galaxies.
A globular star cluster is a spherical group of up to a million stars held together by gravity. These remote objects lie mostly around the central bulge of spiral galaxies.
Some spiral galaxies
Milky Way Galaxy
The Milky Way Galaxy is a spiral galaxy; our sun and solar system are a small part of it. Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-years in diameter and 1,000 light-years thick. There are about 2 x 10 11 stars in the Milky Way. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-years from the center of the Milky Way Galaxy.
The Earth, Sun and the rest of our solar system are a tiny part of the Milky Way Galaxy, a spiral galaxy. The center of the Milky Way galaxy is moving towards the constellation Sagittarius.
The Local Arm is the arm of the Milky Way Galaxy where our solar system is located. It is also called the Orion Arm.
The Andromeda Galaxy (also known as M31 and NGC 224) is the closest major galaxy. It is a spiral galaxy (like our galaxy) and is in the Local Group. It is flanked by two dwarf elliptical companion galaxies (M32 and M110). It is part of the Local Group, a cluster of galaxies to which we (in the Milky Way) belong.
The Andromeda Galaxy can just be seen with the naked eye in the constellation Andromeda. Andromeda is the farthest object that can be seen with the naked eye.
It is about 2,500,000 light-years from Earth. It is 141,000 light-years wide. Recently, the Hubble Space Telescope found that Andromeda has a double nucleus. This second nucleus is probably from an ancient collision with a smaller galaxy.