A galaxy is a group of many stars, along with gas, dust, and dark matter. The name ‘galaxy’ is taken from the Greek word galaxia meaning milky, a reference to our own galaxy, the Milky Way.
Gravity holds galaxies together.
There are galaxies of different sizes. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with a hundred trillion (1014) stars, all orbiting the galaxy’s center of mass.
The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Star clusters are not galaxies, they are inside galaxies.
Within galaxy clusters, galaxies move relative to other galaxies. They can and do collide. When this happens, the stars generally move past each other, but gas clouds and dust interact, and can form a burst of new stars. Gravity pulls both galaxies into somewhat new shapes, forming bars, rings or tail-like structures.
Types of Galaxies
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters. Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.
Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter that extends beyond the visible component, as demonstrated by the universal rotation curve concept.
It appears the reason that some spiral galaxies are fat and bulging while some are flat discs is because of how fast they rotate.