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Stars form in different ways from diffuse clouds of gas and dust.As local regions of the stellar nursery collapse under gravity, the centers become denser and hotter and form protostars, which become main-sequence stars when their cores are hot enough for fusion to begin.A region of molecular clouds triggered by a nearby supernova explosion or other disturbance is called a stellar nursery, as tens of thousands of new stars start forming.Spiral galaxies such as our Milky Way contain a diffuse interstellar medium (ISM) that is mostly hydrogen and helium, with a tiny amount of heavier elements from the deaths of previous stars.

Stars form in different ways from diffuse clouds of gas and dust. Spiral galaxies such as our Milky Way contain a diffuse interstellar medium (ISM) that is mostly hydrogen and helium, with a tiny amount of heavier elements from the deaths of previous stars. In many places, gravity pulls this ISM into slightly denser regions called molecular clouds because they are mostly molecular hydrogen gas. These clouds may contain only 100 particles per cubic centimeter but can be massive (hundreds of thousands of solar masses) and huge (100 ly across). Molecular clouds do not usually contract because the force of gravity is balanced by the pressure of gas particle collisions. However, clouds can collapse if they collide or if they are compressed by a nearby supernova explosion. A region of molecular clouds triggered by a nearby supernova explosion or other disturbance is called a stellar nursery, as tens of thousands of new stars start forming. As local regions of the stellar nursery collapse under gravity, the centers become denser and hotter and form protostars, which become main-sequence stars when their cores are hot enough for fusion to begin. As a result, stars are usually not found alone in empty space but as part of clusters. Most stars form in systems of two or more stars that orbit each other.