1. The solar nebular disk model ↑
The first explanations for how the Solar System formed appeared in the 1700s. In 1734, Swedish natural philosopher Emanuel Swedenborg suggested that the Sun and the planets could have once originated from the same mass and, in 1755, German philosopher Immanuel Kant suggested that the Solar System had once been a large cloud of gas, a nebula. French mathematician Pierre-Simon Laplace popularised this theory in 1796.
Russian astronomer Viktor Safronov eventually adapted the Laplacian model to form the currently accepted model - the solar nebular disk model (SNDM). Safronov's work was publicised after it was translated into English in 1972.
2. Stars and elements ↑
The problem of how stars are fuelled was solved in the early 20th century. In 1925, British-American astronomer Cecilia Payne-Gaposchkin used spectroscopy to show that the majority of the Sun's mass is made of hydrogen and helium.
British physicist Robert d'Escourt Atkinson and Dutch-Austrian-German physicist Fritz Houtermans first suggested that a large amount of energy could be released by fusing hydrogen nuclei together in 1929, and a decade later, German-American physicist Hans Bethe and Indian-American physicist Subrahmanyan Chandrasekhar showed that stars are fuelled by nuclear fusion.
Before stars existed, there were only four elements in the universe, hydrogen and helium, and some trace amounts of lithium and beryllium. Heavier elements were first created in the first generation of stars, which formed about 200 million years after the big bang, before the formation of galaxies.
Planets began to form after the most massive first-generation stars had exploded in supernovae, spreading heavy elements across the universe. This would have occurred after a few million years. The Sun is at least a second-generation star, having formed about 4.6 billion years ago.
3. Protostars ↑
The SNDM suggests that stars form in regions known as stellar nurseries. These are massive, dense clouds of gas that are mostly made of molecular hydrogen - H2. Shockwaves can cause the clouds to become unstable, with matter falling together to make dense clumps, which become protostars.
Shock waves are produced by the arms of spiral galaxies or by supernova explosions. The first stars may have formed because of slight asymmetries in the distribution of matter after the big bang.
A protostar in a stellar nursery. Image credit: NASA/JPL-Caltech/W. Reach (SSC/Caltech)/Public domain.
When a protostar forms, it's orbited by a cloud of hydrogen gas, which can contain smaller clumps. The rotational energy causes the cloud to flatten into a disc, known as a protoplanetary disc. The largest clumps in the protoplanetary disc become planets, and the smaller clumps become asteroids or comets.
Trace amounts of other elements, including oxygen, can be mixed with the hydrogen in the protoplanetary disc, and so water can form. On objects close to the protostar, all of the water boils away, and so they are rocky. On objects further away, all of the water freezes, and so they are icy. It is not yet known exactly how gaseous planets form.
4. Main sequence stars ↑
As a protostar gets denser, gravitational potential energy is converted to kinetic energy. This causes the hydrogen nuclei to increase in velocity, and, if the protostar is massive enough, they eventually crash into each other with enough force for nuclear fusion to occur. This produces energy, in the form of photons - particles of light.
The force produced by nuclear fusion would blow the star apart if it weren't held together by the force of gravity. When the two forces balance, the star becomes stable, and is said to be in hydrostatic equilibrium. It is then known as a main sequence star. The main sequence period of a star's life lasts as long as it is fusing hydrogen to helium in its core.
In stars, the force caused by nuclear fusion, and the force of gravity balance. This is known as hydrostatic equilibrium. Image credit: Helen Klus/CC-NC-SA.
Protostars with masses less than 8% the mass of the Sun (which is about 80 times the mass of Jupiter), never become stars, and are known as brown dwarfs.
In stars about 1.3 times the mass of the Sun or less, hydrogen fusion occurs via the proton-proton chain reaction.
In stars that are more massive, and therefore hotter, the carbon-nitrogen-oxygen (CNO) cycle is more dominant. The CNO cycle produces more energy, but would not be possible in the first generation of stars, since there was no carbon, nitrogen, or oxygen in the early universe.
4.1 The proton-proton chain ↑
In the proton-proton chain, two hydrogen nuclei (protons) collide, producing a deuterium nucleus (made of one proton and one neutron), a neutrino, and a positron. The positron can collide with an electron to produce a gamma ray.
When the deuterium nucleus collides with another hydrogen nucleus, it produces a helium nucleus (made of two protons and one neutron), and a gamma ray. When two helium nuclei collide, they produce two protons, and a complete helium nucleus (made of two protons and two neutrons).
This process is extremely slow because the first and last stages occur very rarely.
4.2 The CNO cycle ↑
In the CNO cycle, a carbon nucleus (made of six protons and six neutrons - the equivalent of three helium nuclei) collides with a hydrogen nucleus (a proton), producing a gamma ray, and a nitrogen nucleus (made of seven protons and six neutrons). This decays into a carbon nucleus (made of six protons and seven neutrons), a positron, and a neutrino.
The carbon nucleus collides with a hydrogen nucleus, producing a gamma ray and a nitrogen nucleus (made of seven protons and seven neutrons). This collides with another hydrogen nucleus, producing a gamma ray and an oxygen nucleus (made of eight protons and seven neutrons).
The oxygen nucleus decays into a positron, a neutrino, and a nitrogen nucleus (made of seven protons and eight neutrons). This collides with another hydrogen nucleus, to produce a helium nucleus (made of two protons and two neutrons), and a carbon nucleus. The cycle then begins again.
Despite its complexity, this process is much faster than the proton-proton chain reaction.
4.3 The H-R diagram ↑
The term 'main sequence' refers to a star's position on the H-R diagram. The H-R diagram is a scatter graph that plots the luminosity of stars, which is related to their mass, against their temperature, which is related to their colour.
Main sequence stars are divided into seven categories known as O, B, A, F, G, K, and M-type stars. This is known as the Harvard Classification Scheme, which was devised by American astronomer Annie Jump Cannon in the 1920s.
O-type stars are the hottest, bluest, and most massive main sequence stars. They also have the shortest main sequence lifetimes, lasting just a few million years or so. M-type stars are the coolest, reddest, and least massive. They remain on the main sequence for hundreds of billions of years. The Sun, a G-type star, is somewhere in the middle.
The sizes of the Sun and other stars to scale. Image credit: NASA/Public domain.
The Sun is just over 100 times as wide as the Earth and over 300,000 times as massive, accounting for over 99% of the total mass of the Solar System and fuelling almost all life on Earth. It is about half way through its 10 billion year lifetime.
The sizes of the Sun and planets to scale. Image credit: NASA/Public domain.
The final stages in a star's life occur when it runs out of hydrogen to fuse in its core. Stars up to about 10 times the mass of the Sun will become red giants and then white dwarfs. Stars that are more massive than this will become supergiants, and then undergo a supernova, becoming either a neutron star or a black hole.