What is a massive cloud, believed to be the birthplace of the new stars?
Our galaxy, along with many others, contains many large clouds of gas and dust, mostly made up of hydrogen. These clouds are called "nebulae." If the cloud becomes large enough, then its own gravity begins to overcome the gas pressure, and the cloud can begin to collapse. As the cloud collapses, gravity, temperature, and pressure increase, until the cloud has collapsed enough to raise the temperature to that required to fuse (burn) the hydrogen. Once that fusion begins, the energy released halts the contraction, and the outer layers of gas are blown away. What's left is an incandescent ball of mostly hydrogen, set aglow by the fusion reactions in its core: a star.
This page updated on June 27, 2015