By the 20th century it was found that several of the spiral nebula had variable stars in them, stars that change in brightness acoording to their total luminosity. Their apparent luminosity was such that spiral nebula must be very far away and were comprable in size to our own Galaxy, the Milky Way. A galaxy is a collect of stars, gas and dust bound together by their common gravitational pull. Galaxies have certain features in common. Gravity holds the billions of stars together, and the densest region is in the center, called a core or bulge.
Some galaxies have spiral or pinwheel arms.
All galaxies have a faint outer region or envelope and a mysterious dark matter halo. Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by American astronomer Edwin Hubble in 1926. In the 1930's, Edwin Hubble discoveried that all galaxies have a positive redshift . In other words, all galaxies were receding from the Milky Way. Where the constant is called Hubble's constant and relates distance to velocity in units of light-years .