For almost 400 million years the Universe was dark. There was simply no light. There were no stars to light up the heavens. Until the first stars formed; the first step in billions of years of birth and fiery death and birth again for stars great and small. The search for that first light has been going on for years, but we can be fairly certain about how stars’ form, both in the early days of the Universe and today.
Clouds of gas are common in the vast space between stars in today’s Universe, known as the interstellar medium. Commonly these are nebulae, immense gas clouds that are the remnants of supernovae, produced as huge stars reach the explosive end of their life and throw off their outer layers into space. In the early Universe these gas clouds were almost entirely Hydrogen, the simplest and lightest of the elements, but the current clouds contain more complex elements, fused in the heart of stars. It is these gas clouds that collapse to form stars.
Once an area of the cloud becomes dense enough, it will generate a net gravitational force, pulling the gas together and the cloud will begin to collapse. It may be that an external shock, like a nearby star going supernova, triggers the collapse, but the result is the same. Once the collapse has begun the cloud might fragment, becoming a few separate collapsing clouds, destined to become stellar neighbours and siblings. The collapse accelerates as the gas cloud is compressed as this increases the clouds density and thus increases the gravitational force acting to compress it.
The very centre of the cloud is heated immensely by the continued and ever faster collapse, such that the heat and pressure in the core is sufficient to halt the collapse and the cloud becomes a protostar. Now the protostar sucks in more and more gas and debris from the surroundings, some of which may not become part of the star, but instead become planets or asteroids in orbit around the star.
Eventually the gas ignites, burning deuterium, a form of Hydrogen, which further heats the star, until, at last, Hydrogen fusion begins in the core. A star is born. The properties of the star; its brightness, its temperature and how long before its inevitable fiery death, are determined by the initial state of the cloud. The birth of a star is the start of a long journey, one that lasts billions of years, and lights up our sky and that of distant worlds.