The vast majority of the stars discovered are part of huge systems called galaxies. Millions or billions of stars - tiny red dwarfs, sunlike stars, supergiants, neutron stars, nebulae, clusters and many other kinds of stellar objects - are bond together in a massive gravitational system that spans across eons and eons of space, sometimes more than 100 000 light years.
In 1924 Edwin Hubble measured the distance to the Andromeda galaxy. He found that it was about hundred thousand times more distant than the nearest stars, hence he concluded that it was a separate galaxy, like our own galaxy, the Milky Way.
Edwin Hubble changed our view of the universe. Later on he made a system that classified the galaxies depending on their shapes. This system is used even today by astronomers. There are many kinds of galaxies: some are elliptic, others are formed like a spiral, and yet other have these irregular shapes. Irregular galaxies usually originally fit within Hubble's galaxy classification,
but they get their shapes distorted when a nearby galaxy passes by, or even when colliding with another galaxy.
Our own galaxy is a giant spiral galaxy, with 100 - 200 billion stars. It has many smaller galaxies orbiting it, like moons orbiting a planet, but on an immensely larger scale. Around spiral galaxies there are also globular clusters that hover above the spiral galaxy disc, which they orbit in a similar way as the satellite galaxies.
Among the closest neighbouring spiral galaxies is the famous Andromeda galaxy, located in the constellation of Andromeda.
Just like stars sometimes come in clusters our galaxy is located in a cluster of galaxies called The Local Group, which harbors about 30 galaxies, including the Andromeda galaxy and a number of dwarf galaxies. The Local Group spance a distance of about 10 million light years and is a part of yet another, much larger cluster of galaxies called the Virgo Supercluster.
Hubble's Galaxy Classification
Formation of Galaxies
Active Galactic Nuclei
The Milky Way