The Long Answer: What is a galaxy? The short answer: A galaxy is an immense collection of dust, gas, and millions of stars and their individual solar systems, held together mostly by gravity. A spiral galaxy can be either closed (elliptical) or open (circular). A spiral galaxy, the Milky Way, has a giant black hole at the center. Stars in the evening sky also belong to the Milky Way galaxy.
There are several different kinds of spiral and elliptical galaxies. Close-to-spherical spiral galaxies like the Mauna Planum are extremely compact, with stars approximately the same size as the Earth’s sun. The other major kind of spiral galaxy are the bulge and Open clusters, with their individual gas giants, stars, and white dwarf stars much bigger than our own sun.
How long does a galaxy last? The age of a galaxy can be determined by looking at how many stars it contains. A very old galaxy (a barred galaxy) will have a very few stars close to the central region where the black hole is, while a younger galaxy (which has not developed black holes or bulges) will have very many stars within a large area where the galaxy bulge or disk is nearby. The largest galaxy in our own galaxy, the Milky Way has about 100 billion stars, making it the most massive known. It is estimated that this galaxy contains about twice the mass of our own sun. Thus, a million years is the average age of a galaxy, although exact ages can not be established since new ways of analysis are constantly being developed.