Ever wonder how stars form? Astronomers have been thinking about that for quite a while, and are pretty sure they have a good idea about how it works. Stars form out of giants clouds of gas and dust in space. The tiny particles bump into each other until they stick and form a large ball of gas. This protostar, as astronomers call it, gravitationally attracts the cloud of gas surrounding it and increases in size until most of the gas has fallen onto the star. What's left is a big ball of gas surrounded by a disk of dust particles which accretes (falls onto) the star. Some of the dust in this circumstellar disk will stick together to form rocks, and a few may even grow big enough to be planets! After about 10 Million years (yes million!) the star will accrete most of the dust and become a "young adult" star, or Zero Age Main Sequence star. The star is now big enough and hot enough to burn Hydrogen in its core (left side of diagram). If the star doesn't accrete enough gas and dust to become large enough to burn hydrogen, it becomes a brown dwarf (right side of diagram). Most of the stars that you see in the night sky are average main sequence stars of various sizes and ages.
