Star formation

Star formation is the complex process where dense regions within molecular clouds collapse to create stars. These regions are often called stellar nurseries or star-forming regions. This field of astronomy studies the interstellar medium (ISM), which is the matter between stars. It also examines giant molecular clouds (GMC) that act as precursors to stars. Astronomers also study protostars and young stellar objects, which are the immediate products of this process. Star formation is closely linked to the study of how planets form. Most stars do not form in isolation. Instead, they emerge as part of groups called star clusters or stellar associations.

The process begins within the interstellar medium of spiral galaxies like the Milky Way. This medium contains stars, stellar remnants, and a diffuse mix of gas and dust. The ISM typically consists of 10% to 10^6 particles per cm3. It is composed of roughly 70% hydrogen and 28% helium by mass. The remaining 1.5% consists of heavier elements. These trace elements are produced via stellar nucleosynthesis inside stars. When stars reach the end of their lives, they eject these elements back into space. Higher density regions of this medium form clouds known as diffuse nebulae. In these dense nebulae, much of the hydrogen exists in molecular (H2) form.

Star formation occurs through a hierarchical collapse of these molecular clouds. A cloud remains in hydrostatic equilibrium if gas pressure balances internal gravity. This balance is described by the virial theorem. The theorem states that gravitational potential energy must equal twice the internal thermal energy. If a cloud is massive enough, its gas pressure cannot support it. This leads to a gravitational collapse once the mass exceeds the Jeans mass. The Jeans mass depends on the cloud's temperature and density. During this collapse, the cloud breaks into smaller pieces. These fragments eventually reach stellar mass and form embedded clusters.

As a fragment collapses, it must manage its internal energy. The collapsing gas radiates away energy gained from gravitational potential energy. As density increases, the fragments become opaque. This makes it harder to radiate energy away, which raises the temperature. This heat can actually inhibit further fragmentation. The fragments then condense into rotating spheres of gas. Within these clouds, filaments or elongated gas structures are very common. These filaments fragment into gravitationally bound cores. Most of these cores will eventually evolve into stars.

Inside the collapsing core, a protostar begins to form. As the density increases, the center becomes optically opaque. A region called the first hydrostatic core forms when the collapse is temporarily halted. The temperature rises as gas falls toward this opaque region. This creates shock waves that further heat the core. When temperatures reach about 2,000 K, H2 molecules dissociate. This is followed by the ionization of hydrogen and helium atoms. These processes absorb contraction energy, allowing the collapse to continue. Eventually, the density reaches 10^-8 g/cm3. At this point, the protostar becomes transparent enough for radiation to escape.

As the protostar continues to grow, it enters a stage of accretion. Material from the surrounding cloud continues to fall onto the object. This can include material from a newly formed circumstellar disc. During this phase, the star may produce bipolar jets called Herbig–Haro objects. These jets help expel excess angular momentum from the system. When the surrounding gas and dust envelope finally disperses, the object becomes a pre-main-sequence (PMS) star. These stars are powered by the Kelvin–Helmholtz mechanism, which is gravitational contraction. They follow specific paths on the Hertzsprung–Russell (H–R) diagram, such as the Hayashi track.

Astronomers categorize stars into three distinct populations based on their history. Population III stars were the very first stars. They formed from primordial hydrogen after the Big Bang. They are thought to contain only hydrogen and helium. Population II stars formed from the debris of those first stars. These stars helped create elements with higher atomic numbers. Finally, Population I stars are young and metal-rich. These stars contain elements other than hydrogen and helium. Our Sun is a Population I star.

There are many famous sites of star formation across the universe. The Orion Nebula is a nearby region where massive stars form. It is located about 1,344 light-years away. Lower mass stars form in the ρ Ophiuchi cloud complex, about 400–450 light-years distant. Astronomer Bart Bok identified compact, opaque clouds called Bok globules. These are up to one light-year across and contain a few solar masses. In the Milky Way, about half of the ISM mass is in molecular clouds. There are an estimated 6,000 molecular clouds in our galaxy. Each of these clouds contains more than 10^4 solar masses.