The number of young stars of different masses shows that the formation of less massive stars is more common than the formation of massive ones. Binary or multiple stars form more often than single ones. Many stars form together in a star formation region. The flat shape and regular rotation of the solar system show that the material from which it formed also was flat and rotating.
Stars form in the cores of clumps of material within giant molecular clouds. Massive stars form within warm, massive cores whereas less massive stars form within cold cores. Cores collapse under their own weight. Magnetic fields within the cores may slow the collapse.
A protostar develops at the center of a collapsing core. The protostar is originally transparent, but eventually becomes opaque. At this point, the protostar stops collapsing and begins to grow in mass by accumulating infalling material. It begins a period of slow contraction that ends when the star becomes hot enough for hydrogen fusion to occur. As a protostar collapses, it rotates ever more rapidly. Rotation eventually causes infalling material to accumulate in a rotating nebular disk.
Young stars first become visible when they shed their surrounding gas and dust. For most stars, this takes place before they reach the main sequence. These stars, called T Tauri stars and Ae and Be stars, have vigorous surface activity and are orbited by disks of material. Massive stars, on the other hand, become visible only after they have already reached the main sequence.
The vigorous surface activity of a young star produces a wind that eventually blows away infalling material. At the same time, friction within the star's gas and dust disk causes its center to spin more slowly and carries angular momentum outward. At first the disk prevents the wind from blowing outward. This produces a collimated wind along the star's polar axis. Eventually, the disk is blown away, but sometimes this doesn't happen until after a companion star or a planetary system forms.
The abundances of atomic isotopes in meteorites suggest that the solar nebula existed 4.6 billion years ago and that the formation of the planets took 100 million years or less to complete. The solar nebula contained an amount of material equal to between a few hundredths and a few tenths of the mass of the Sun.
As the solar nebula cooled, solid particles and liquid droplets condensed from the gas. The condensation continued until the gas in the solar nebula was blown away by a strong wind from the Sun. By the time this happened, only metals, oxides, and silicates had condensed in the hot, inner part of the solar nebula. In the outer nebula, temperatures had dropped enough that ices had condensed as well.
After dust particles formed, they accumulated into planetesimals, which ranged between a few millimeters and hundreds of kilometers in diameter. These then collided with each other to produce the planets. In the outer solar system, the cores of the planets Jupiter, Saturn, Uranus, and Neptune captured gas from the solar nebula to produce their massive gaseous envelopes. Alternatively, the giant planets may have formed when parts of the solar nebula collapsed under their own weight.
The regular satellites of the outer solar system probably formed in a process similar to that which formed the planets. Some satellites, however, may be planetesimals captured by the planets while they still had gaseous disks. The disks were able to slow down approaching planetesimals enough for the planetesimals to be captured into orbits about the planets.
