Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds (GMC) as precursors to the star formation process and the study of young stellar objects and planet formation as its immediate products. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function.
A spiral galaxy like the Milky Way contains stars, stellar remnants and a diffuse interstellar medium (ISM) of gas and dust. The latter consists of about 0.1 to 1 particles per cm3 and is typically composed of roughly 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds. The largest such formations, called giant molecular clouds, have typical densities of 100 particles per cm3, diameters of 100 light-years (9.5×1014 km), masses of up to 6 million solar masses, and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 solar masses. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula, 1,300 ly (1.2×1016 km) away. However, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex.
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