What are Galaxies

Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebula.

We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses; our Sun is one of several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant (and closer in case of some dwarfs which are currently merged with the Milky Way), while the nearest giant neighbor, the Andromeda Galaxy(M31), also a spiral, is about 2-3 million light years distant. 

 

Spiral (and Barred-Spiral) galaxies are shaped like pinwheels, with arms that spiral outward. The barred spiral has an elongated bar shape across the middle. Examples of the spiral are our own Milky Way and Andromeda. A barred spiral Galaxy is NGC 1300.

Andromeda-Galaxy
NGC 1300-Galaxy

Elliptical galaxies look like flattened spheres rather than the thinner spiral form. They are observed to have comparatively little interstellar matter. An example is the Maffei 1 galaxy.

Maffei 1-Galaxy

Lenticular galaxies are flattened galaxies without an obvious spiral structure. An example is the Spindle Galaxy in Draco.

NGC 6643-Galaxy

Irregular galaxies do not have any of the common shapes, and may have been disrupted by various forces. These include Hoag's Galaxy (a ring), the Magellanic Clouds, and NGC 1427A (which is speeding toward the Fornax cluster).

Hoag's-Galaxy
Magellanic Cloud's-Galaxy
NGC 1427A-Galaxy