A hydrogen gas cloud, so cold that at atmospheric pressures air would be ice, begins to condense under its own gravity. As the cloud contracts, gravitational potential energy is released - heating the cloud and radiating thermal energy. The center becomes hotter than the outer regions. As the star heats up, gas and radiation pressure build up which slows the contraction. The proto-star heated by gravitational collapse is a cool red but it is very large so it has a high luminosity and appears at the upper right in the Heretzsprung-Russell diagram.
Contraction slows down, but the star continues both to radiate energy and to be heated further by the release of gravitational potential energy. When the core reaches a high enough temperature nuclear fusion reactions begin. During this phase the star lies above the main sequence. Pre-main sequence stars are observed as T-Tauri Stars, [inset over Orion nebula image where stars are forming]. Material is still falling inward onto the star, but the star is also spewing material outward in strong winds.
It takes another several million years for the star to settle down on the main sequence. Because the fusion of Hydrogen to Helium is the most exothermic of the nuclear reactions, the main sequence phase is the longest phase of a star's life [about 12 billion years for a star with 1 solar mass]. The sun is half way through its time on the main sequence. It has 6 000 000 000 years to go before it reaches the next stage in its life - the red giant phase.
A main sequence star is stable because it is in dynamic equilibrium. If the nuclear reactions in the core produce less heat the internal radiation pressure drops and the star contracts - heating the core and increasing the reaction rate. If the core reaction rate rises the pressure increases - the star expands and the core will cool - slowing the rate. Dramatic oscillations of this type are observed in the outer layers of old yellow giant stars known as Cepheid variables that have moved off the main sequence.
Main sequence stars at the left hand upper limit have masses as high as 100 times that of the Sun and are rare because they 'burn' fast and have short lives. Stars at the extreme right hand lower limit are red dwarfs, slow burning and long lived they are the most common stars in the galaxy. Only the closest are visible in even the most powerful telescopes. Proxima Centauri (the closest star) is a red dwarf.
The star runs out of hydrogen in the core but nuclear reactions continue in a shell surrounding the core. The remaining helium core contracts and the star interior rises in temperature causing an increase in the reaction rate in the remaining hydrogen burning shell. The extra energy output increases the temperature further causing the outer atmosphere to grow by as much as a factor of 200. The star is now a red giant with a huge cooler surface. When the sun reaches this stage its outer atmosphere will be somewhere between the orbits of Earth and Mars!
The core temperature rises and helium fusion takes place to form carbon via the triple-alpha process. This fusion reaction releases only about 20% of the energy of hydrogen fusion per kilogram so the life-time on the helium burning phase is only about one fifth of the main sequence life time.
During the helium burning phase some carbon and helium will fuse to make oxygen, resulting in the formation of a carbon-oxygen core. When the helium is newly exhausted in the core, helium burning is still taking place in a shell surrounding the core. If a large star becomes unstable during the final helium shell burning phase a planetary nebula is formed from matter ejected in a series of events which vary in number and intensity from star to star.
Very large red giants (super red giants) continue burning carbon in the core to heavier elements up to iron, the element with the highest binding energy per nucleon. These extreme stars with upwards of ten solar masses do not form carbon 12 white dwarfs via the planetary nebula stage but explode as a Nova. The inner iron core collapses to a neutron star and in extreme case of very large mass directly to a black hole.
Note: some authorities (including IB examiners) refer to nova explosions of this type as supernovas of Type 1b and 1c. A true supernova (the thermonuclear detonation of a whiter dwarf) is referred to as a supernova of Type 1a.
The collapse to a white dwarf takes place over tens of thousands of years, while the star blows off its outer envelope to form a planetary nebula. A white dwarf is small, about the size of the Earth, with a density of the order of one million grams per cubic centimeter, about a ton per teaspoonful! The companion star to Sirius (Sirius B) is a white dwarf with a surface temperature of 25 000 °C and the mass of the Sun.
Supernovas of Type 1a
If it has a very close companion star, a white dwarf may accrete matter from a companion star until it reaches the Chandrasekhar limit, (1.4 solar masses) at which point gravitational collapse takes over again. The white dwarf does not collapse to the next stage (neutron star) but instead undergoes runaway carbon fusion, blowing completely apart in a Type Ia supernova that may outshine an entire galaxy for a week. The entire star becomes a gigantic carbon bomb, producing about one fifth of the energy per kilogram of hydrogen bomb.
 Subramanyan Chandrasekhar was a famous Indian mathematician - astronomer - cosmologist who worked for most of his life at Cambridge in England. He published his celebrated work on collapsing stars in 1935 and continued working actively in physics until his mid eighties. |
A white dwarf star will take billions of years to radiate away its store of thermal energy because of its small surface area. The dwarf star will slowly move down and to the right on the H-R diagram as it cools until it fades from view as a black dwarf. We cannot see black dwarfs, but we can sometimes detect their gravitational effects.
The Sun is a medium sized star. It is expected to pass through the red giant stage and gradually contract to end is 'life' as a white dwarf.
More on the H-R diagram
Bellatrix (on one shoulder of Orion) is one of the hottest naked eye stars with a surface temperature of about 30 000 K. It is a main sequence star with 8-9 times the mass of the Sun and is of spectral type B.
The central stars of planetary nebulae (up to 250 000 K) are the hottest known conventional stars. Peak emission is in the UV, so the remaining white dwarf is often not obvious in visual images of the nebula and the light from the surrounding nebula is partly provided by fluorescence. Neutron stars that are accreting matter may have surface temperatures as the result of the release of gravitational energy of ~2 million K and emit in the x-ray region. The upper atmosphere of the Sun has a similar temperature and emits thermal radiation in the x-ray region. The mechanism for transferring energy into the upper atmosphere is improperly understood.
Largest - most luminous - and fastest rotation
Small stars
Red dwarfs on the main sequence have recently been found. In the local star field, out to ten light years, they outnumber the brighter well known stars. They were not previously not seen because they have such low luminosities. Smaller Brown dwarfs that fuse only deuterium are even less luminous. Recently a few close ones have been found. As detection improves even fainter examples will become known. It is interesting to speculate on the possibility of tiny failed stars the size of Jupiter orbiting in the galaxy without large companions. Their detection would be almost impossible since they do not shine with light of their own.