Star formation

Star formation begins when dense pockets inside interstellar molecular clouds—often called stellar nurseries—collapse under their own gravity to create stars. This field of astronomy examines the interstellar medium and giant molecular clouds as the raw materials, and studies protostars and young stellar objects as the direct results. It overlaps closely with planet formation. A complete theory of star formation must explain not only how individual stars come into being, but also the prevalence of binary systems and the range of stellar masses seen in the universe. Most stars are born in groups, forming star clusters or stellar associations rather than in isolation.

The interstellar medium of spiral galaxies like the Milky Way contains stars, stellar remnants, and a diffuse mix of gas and dust, with densities ranging from 10⁴ to 10⁶ particles per cubic centimeter. By mass, this medium is roughly 70% hydrogen, 28% helium, and 1.5% heavier elements. Those heavier elements are forged inside stars through nucleosynthesis and are released when stars end their main-sequence lives. Denser patches of the interstellar medium form clouds or diffuse nebulae, the sites where stars are born. Elliptical galaxies, unlike spirals, lose their cold interstellar medium within about a billion years, so they cannot form diffuse nebulae unless they merge with other galaxies.

In the dense nebulae where stars form, most hydrogen exists as molecules (H₂), giving these clouds the name molecular clouds. Observations by the Herschel Space Observatory show that filaments—long, dense gas structures—are common in molecular clouds and play a central role in star formation. These filaments break into gravitationally bound cores, most of which will become stars. The way filaments fragment is influenced by ongoing gas accretion, geometric bending, and magnetic fields. Supercritical filaments often display quasi-periodic chains of dense cores, spaced roughly as wide as the filament itself, and some already contain protostars with outflows.

The coldest clouds tend to produce low-mass stars, first detected by the infrared light they emit while still embedded, then in visible light once the surrounding cloud disperses. Giant molecular clouds, which are warmer, can form stars of all masses. These giants have typical densities of about 100 particles per cubic centimeter, diameters around 100 light-years, masses up to 6 million times that of the Sun, and interior temperatures averaging 10 K. Roughly half the Milky Way’s interstellar medium mass resides in molecular clouds, and the galaxy contains an estimated 6,000 such clouds, each exceeding 100,000 solar masses. The nearest site of massive star formation is the Orion Nebula, 1,300 light-years away. Lower-mass star formation occurs closer, about 400–450 light-years distant in the ρ Ophiuchi cloud complex.

Bok globules—compact, opaque clouds of dense gas and dust named after astronomer Bart Bok—are another site of star formation. They can form alongside collapsing molecular clouds or independently. Typically up to a light-year across and containing a few solar masses, they appear as dark silhouettes against bright emission nebulae or background stars. More than half of known Bok globules contain newly forming stars.

An interstellar cloud stays in hydrostatic equilibrium as long as the kinetic energy of its gas pressure balances the gravitational potential energy. This balance is described by the virial theorem, which requires that gravitational potential energy equal twice the internal thermal energy. If a cloud is massive enough that gas pressure cannot support it, it collapses. The threshold mass for collapse is called the Jeans mass, which depends on temperature and density and is usually thousands to tens of thousands of solar masses. During such a collapse, dozens to tens of thousands of stars form nearly simultaneously, producing what are known as embedded clusters. The final result is an open cluster of stars.

Triggered star formation occurs when an external event compresses a molecular cloud and initiates collapse. Molecular clouds can collide, or a nearby supernova can send shock waves into the cloud at high speed. The new stars born from such events may themselves produce supernovae, leading to self-propagating star formation. Galactic collisions can also trigger massive starbursts, as gas clouds in each galaxy are compressed and stirred by tidal forces. This mechanism may be responsible for forming globular clusters.

A supermassive black hole at a galaxy’s center can regulate star formation in the galactic nucleus. When such a black hole accretes matter, it can become active and emit a strong wind through a collimated relativistic jet, which may limit further star formation. Massive black holes ejecting radio-emitting particles at near-light speed can also suppress new star formation in aging galaxies. However, the radio emissions around these jets can sometimes trigger star formation instead. A weaker jet may also play a role.