Star formation

New stars are born in space.

They grow in big clouds of gas and dust. These clouds are like nurseries.

Gravity pulls the gas together. This makes the cloud break into small pieces. The pieces get smaller and smaller. They turn into spinning balls of gas. These balls grow into new stars. Most stars are born in big groups. These groups are called star clusters. It is amazing how stars begin!

Stars are born in space. They start in giant molecular clouds. These are huge clouds of gas and dust.

Much of the gas is hydrogen.

Gravity pulls the gas together. This makes the cloud break into small pieces. These pieces turn into spinning balls of gas.

As they spin, they grow. The center gets very hot and dense. This is called a protostar. A protostar is a young star that is still growing.

Sometimes, a big explosion can help. A supernova is a massive star explosion. It can send shocks into a cloud. These shocks push the gas together to start star birth.

Most stars do not live alone. They form in big groups. We call these star clusters.

Some stars are very young. Others are older. The Sun is a young star. It is a Population I star. This means it has many heavy elements in it.

Stars are not born in empty space. They begin inside giant molecular clouds in interstellar space. These clouds are huge collections of gas and dust.

Most of the gas is hydrogen, which makes up about 70% of the cloud. Helium makes up about 28% of the mass. The rest is made of heavier elements. These clouds are often called stellar nurseries. They are the places where new stars are made.

Star formation happens through a step-by-step collapse. First, gravity pulls dense regions of a cloud together. This can happen if clouds collide or if a nearby supernova explosion sends shocks through the gas.

As the cloud collapses, it breaks into smaller pieces. These pieces become spinning spheres of gas. The center of the sphere gets very hot and dense. This growing object is called a protostar.

Eventually, the core gets hot enough to start nuclear fusion. This creates the outward pressure needed to stop the collapse.

Astronomers study different types of stars based on when they formed. The very first stars are called Population III stars. They formed from hydrogen after the Big Bang. They are not well understood yet. Next are Population II stars. These formed from the debris of the first stars. Finally, there are Population I stars. These are young stars like our Sun. They are rich in metals, which are elements heavier than hydrogen and helium.

Space is full of different star-forming sites. The Orion Nebula is a famous place where massive stars form. It is located about 1,344 light-years away.

Another site is the ρ Ophiuchi cloud complex. This area forms lower mass stars about 400 to 450 light-years away. Astronomer Bart Bok gave his name to Bok globules. These are small, dark clouds of gas and dust. They can be up to one light-year across. Over half of these globules contain new stars.

You can think of a molecular cloud like a giant factory. Instead of making cars, it makes stars. Most stars do not live alone like we might think. Instead, they form in groups called star clusters.

Some clusters are very large and bright. Others are smaller and more compact. This process links the tiny particles of gas to the huge galaxies we see. The same gravity that builds a star also helps build entire galaxies. The life of a star is tied to the gas around it.

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.