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(as you read this, please note that the term "burn" is referring to the process of fusion. No real burning, as we know it on Earth, occurs in stars)
Stellar evolution is a description of the way that stars change with time. On human timescales, most stars do not appear to change at all, but if we were to look for billions of years, we would see how stars are born, how they age, and finally how they die.
The primary factor determining how a star evolves is its mass as it reaches the main sequence. The following is a brief outline of the evolution of a low-mass and a high-mass star.
The life of a star
Stars are born out of the gravitational collapse of cool, dense molecular clouds. As the cloud collapses, it fragments into smaller regions, which themselves contract to form stellar cores. These protostars rotate faster and increase in temperature as they condense, and are surrounded by a proto-planetary disk out of which planets may later form.
The central temperature of the contracting protostar increases to the point where nuclear reactions begin. At this point, hydrogen is converted into helium in the core and the star is born onto the main sequence. For about 90% of its life, the star will continue to fuse hydrogen into helium and will remain a main sequence star.
Once the hydrogen in the core has all been burned to helium, energy generation stops and the core begins to contract. This raises the internal temperature of the star and ignites a shell of hydrogen burning around the inert core. Meanwhile, the helium core continues to contract and increase in temperature, which leads to an increased energy generation rate in the hydrogen shell. This causes the star to expand enormously and increase in luminosity – the star becomes a red giant.
Eventually, the core reaches temperatures high enough to burn helium into carbon. If the mass of the star is less than about 2.2 solar masses, the entire core ignites suddenly in a helium core flash. If the star is more massive than this, the ignition of the core is more gentle. At the same time, the star continues to burn hydrogen in a shell around the core.
The star burns helium into carbon in its core for a much shorter time than it burned hydrogen. Once the helium has all been converted, the inert carbon core begins to contract and increase in temperature. This ignites a helium burning shell just above the core, which in turn is surrounded by a hydrogen burning shell.
What happens next depends on the mass of the star
Stars less than 8 solar masses
The inert carbon core continues to contract but never reaches temperatures sufficient to initiate carbon burning. However, the existence of two burning shells leads to a thermally unstable situation in which hydrogen and helium burning occur out of phase with each other. This thermal pulsing is characteristic of asymptotic giant branch stars.
The carbon core continues to contract until it is supported by electron degeneracy pressure. No further contraction is possible (the core is now supported by the pressure of electrons, not gas pressure), and the core has formed a white dwarf. Meanwhile, each thermal pulse causes the outer layers of the star to expand, resulting in a period of mass loss. Eventually, the outer layers of the star are ejected completely and ionized by the white dwarf to form a planetary nebula.
Stars greater than 8 solar masses
The contracting core will reach the temperature for carbon ignition, and begin to burn to neon. This process of core burning followed by core contraction and shell burning, is repeated in a series of nuclear reactions producing successively heavier elements until iron is formed in the core.
Iron cannot be burned to heavier elements as this reaction does not generate energy – it requires an input of energy to proceed. The star has therefore finally run out of fuel and collapses under its own gravity. The mass of the core of the star dictates what happens next. If the core has a mass less than about 3 times that of our Sun, the collapse of the core may be halted by the pressure of neutrons (this is an even more extreme state than the electron pressure that supports white dwarfs!). In this case, the core becomes a neutron star. The sudden halt in the contraction of the core produces a shock wave which propagates back out through the outer layers of the star, blowing it apart in a core-collapse supernova explosion. If the core has a mass greater than about 3 solar masses, even neutron pressure is not sufficient to withstand gravity, and it will collapse further into a stellar black hole.
The ejected gas expands into the interstellar medium, enriching it with all the elements synthesized during the star’s lifetime and in the explosion itself. These supernova remnants are the chemical distribution centers of the Universe.
An important tool in the study of stellar evolution is the Hertzsprung-Russell diagram (HR diagram), which plots the absolute magnitudes of stars against their spectral type (or alternatively, stellar luminosity versus effective temperature). As a star evolves, it moves to specific regions in the HR diagram, following a characteristic path that depends on the star’s mass and chemical composition.
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