Stars come from large clouds of gas and dust in space called molecular clouds or nebulae. These clouds are mostly hydrogen and helium with some other elements and can be very massive, spanning many light-years. Gravity causes regions of higher density within these clouds to collapse, forming clumps that heat up and eventually create a protostar—a baby star. Over millions of years, as the core temperature rises, nuclear fusion of hydrogen into helium begins, releasing energy that keeps the star stable. This process marks the birth of a star, which then enters its main phase of life. These star-forming regions are sometimes called stellar nurseries, and many stars form in groups called star clusters. The leftover gas and dust around a new star can form planets and other bodies in a surrounding disk.
