Genre: Information/Explanation

Stellar Evolution 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 tracing 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 protoplanetary 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 burn 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 . 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 ionised 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 can not 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 synthesised during the star's lifetime and in the explosion itself . These supernova remnants are the chemical distribution centres of the Universe .