Accretion (astrophysics)

Space things grow big by pulling bits together.

This is how stars and planets are made.

Stars start in giant clouds of gas. The gas pulls inward and gets hot. It forms a flat disk around a center.

This disk helps the star grow.

Planets grow in a different way. Tiny bits of dust and rock hit each other. They stick and get bigger. They become large rocks called planetesimals.

These rocks hit each other too. They grow into huge worlds. This can take millions of years.

Everything in space is built this way. It is how the stars and planets form.

In space, things grow by pulling matter toward them. We call this accretion. This way of growing helps make stars, planets, and even whole galaxies.

Stars often start inside giant clouds of gas. These clouds are very cold. Over millions of years, parts of the cloud collapse. This means the gas falls inward. As it falls, it heats up. The gas forms a flat, spinning shape. We call this an accretion disk.

Small bits of dust also help make planets. These bits hit each other and stick. They grow into large rocks called planetesimals.

These rocks then crash into one another. They grow into Moon-sized objects called protoplanets. Finally, these protoplanets collide to form big planets. This can take 10 to 100 million years.

Some planets are made of rock and metal. These are terrestrial planets. Other giant planets start as icy rocks. They then pull in gas from the space around them. This is how our solar system was built.

Accretion is a way that space objects grow larger. It happens when gravity pulls matter toward a central object. This matter is usually gas or dust. This process helps build almost everything we see in space. It is how stars, planets, and even huge galaxies are made.

Stars usually begin inside giant clouds of cold hydrogen gas. These clouds are very big and can be millions of miles wide. Over millions of years, parts of these clouds collapse inward. As the gas falls, it loses energy and heats up. The spinning motion causes the gas to flatten into a disk. Scientists call this an accretion disk.

People have studied how these objects form for a long time. In 1944, Otto Schmidt suggested that Earth formed from meteoric material. Later, William McCrea proposed the protoplanet theory in 1960. Michael Woolfson also shared a capture theory. In 1969, Viktor Safronov used math to show the stages of how rocky planets form.

Building a planet is a very long process. First, tiny dust grains stick together to form small particles. These particles grow into mountain-sized rocks called planetesimals. These rocks then crash into each other to make Moon-sized objects called protoplanets. This stage takes about 0.1 to 1 million years. Finally, these embryos collide to become full planets over 10 to 100 million years.

There are different ways to make different kinds of planets. Terrestrial planets are made of rock and metal. They form in the warmer parts of a solar system. Giant planets, like Jupiter, start as large icy rocks. These icy objects capture gas like hydrogen and helium. This happens because they are past the frost line where it is cold enough for ice to stay solid.

In astrophysics, accretion is the process where a massive object grows by gravitationally attracting surrounding matter. This matter is typically in the form of gas or dust. As gravity pulls these particles inward, they often form a rotating structure called an accretion disk. This fundamental process is responsible for the formation of nearly all major astronomical objects, including galaxies, stars, and planets.

Scientists have spent decades developing models to explain how this accumulation happens. In 1944, Otto Schmidt proposed that Earth formed from meteoric material. Later, William McCrea introduced the protoplanet theory in 1960, and Michael Woolfson proposed a capture theory. In 1969, Viktor Safronov provided a quantitative look at how terrestrial planets form through specific stages. Modern science now uses intensive numerical simulations to study how these small pieces, called planetesimals, eventually build up into large worlds.

Star formation begins within giant molecular clouds made of cold hydrogen. These clouds can be massive, spanning roughly 10 to 100 light-years in diameter. Over millions of years, parts of these clouds fragment and collapse under their own gravity. These fragments create dense cores known as protostellar nebulae. As the cloud collapses, it loses potential energy and heats up, gaining kinetic energy. Because of the conservation of angular momentum, the collapsing cloud flattens into an accretion disk.

The birth of a star follows a specific sequence of stages. Initially, the central part of the nebula forms a hot, non-contracting core called a hydrostatic core. This core acts as the seed for the future star. As the surrounding envelope of gas falls inward, it accelerates, further shaping the disk. Eventually, the envelope becomes thin enough for a young stellar object (YSO) to be seen in infrared or visible light. At this point, the protostar begins to fuse deuterium.

Depending on the mass of the protostar, different outcomes occur. If the object has a mass above 0.08 solar masses, it will begin hydrogen fusion. If the mass is lower, it becomes a brown dwarf. During the Class I protostar stage, the central object has already gathered most of its mass. The remaining disk and envelope usually account for only 10% to 20% of the total mass. Later, the object becomes a classical T Tauri star, which features a prominent accretion disk and strong emission lines in its spectrum.

Planet formation is a complex journey from dust to worlds. It starts when microscopic dust grains collide and stick together through electromagnetic or van der Waals forces. These grains grow into larger aggregates, but they face the "meter size barrier." This is a problem where growing particles move too fast and collide destructively rather than sticking. To overcome this, particles may concentrate into pebbles that gravitationally collapse, or they may use a feedback mechanism called streaming instability to form massive filaments.

Once particles reach the size of mountains, they are called planetesimals. Over 0.1 to 1 million years, these planetesimals collide to form Moon-sized planetary embryos. Finally, these embryos collide over 10 to 100 million years to create full planets. There is also a process called pebble accretion, where gas drag slows down small pebbles, causing them to spiral into a growing planet. This can be 1,000 times faster than traditional planetesimal accretion.

Different environments create different types of planets. Terrestrial planets are composed of rock and metal that condensed in the warmer inner regions of a solar system. In contrast, Jovian or giant planets form further out, past the frost line. These planets begin as large, icy planetesimals that capture massive amounts of hydrogen and helium gas from the surrounding nebula. This distinction in composition is a direct result of where the accretion took place relative to the temperature of the disk.