Stellar Evolution

Changes in a stars luminosity and temperature over its lifetime; conventionally, plotted on an Hertzsprung-Russell (HR) diagram. All stars, irrespective of their mass spend most of their lifetime on the main sequence. The more massive a star, the more luminous and hotter it is. As all stars age, they enter a giant phase (their brightness remains constant, but the effective surface temperature decreases, 7 → 9). This reflects a change in the fusion processes at work within the star: outer layers expand and are no longer of sites nuclear burning. As these layers cool, the star drifts towards the right side of the HR diagram. Subsequent evolution depends on the mass of the star.


Image source: http://www.astro.ljmu.ac.uk/courses/phys134/hrdiag.html.

For a ~1 Msun star, when H burning ends, the core temperature is insufficient for He burning to occur. With no source of energy production in the core there is no longer any outward radiative pressure to resist gravitational collapse, and the outer regions of the star start to collapse (9 → 10). Collapse raises the temperature in the H shell and H fusion occurs.
Luminosity increases as the core continues to collapse and the temperature in the H shell keeps increasing. The burning shell also provides pressure on the outer layers of the star and causes them to expand. As the layers expand they cool and the star appears to become redder.


Image source: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redgia.html#c1.

After just a few million years the H shell runs out of fuel. Once again the star contracts under its own weight. The compact core may flash into life for a short period and He be fused into C (10). The energy released in the He flash reaches the outer layers and the star becomes a red supergiant (11). Up to half its mass is thrown out into space and seen as a planetary nebula (12) leaving a white dwarf behind.


Image source: http://www.nicolascretton.ch/Astronomy/images/HR_post_MS_sun_track.jpg.

Stars with a mass of between 8 and 20 Msun have a more complex evolution. Initially, they evolve in the same way as low mass stars, turning into red giants and undergoing a core He burning phase. However, He burning is no longer the end phase of stellar evolution. When He in the core is exhausted, the additional mass allows stellar collapse to take place and the outer layers to reignite. A cross section through the star at this point would show an outer shell of H burning, an inner shell of He burning, and the core where C burning is taking place. Once the C supply is exhausted, O begins to fuse into Ne; the He shell becomes a C burning shell, the H shell a He burning shell and a new outer layer of H burning forms. Subsequently, Ne can fuse into Mg, into Si, and so on to Cr and Fe.


Image source: https://lasers.llnl.gov/programs/science_at_the_extremes/laboratory_astrophysics/.

Each of these stages produces less energy than the previous one and lasts for a shorter. During these final stages the star expands to thousands of times the diameter of the Sun, becoming a red supergiant like Betelgeuse. Iron is the end of the exothermic fusion road: to fuse iron into heavier elements is an endothermic reaction. Fission of Fe into lighter elements also requires an input of energy. The core cools, drawing heat from its surroundings to power the fusion; the outward radiative pressure, which had supported the star for many millions of years, ceases and the star undergoes free fall gravitational core collapse until it reaches nuclear densities (~1014 g/cm3). The core, which represents a large percentage of the stellar mass, exceeds the 1.44 Chandrasekhar limit for a white dwarf. Protons and electrons in the core are compressed into neutrons, yielding a
sphere the size of a large city and the density of an atomic nucleus, held up by neutron degeneracy pressure: a neutron star. Core collapse produces a shock wave that blasts out through the star releasing an enormous amount of energy in a few seconds, equivalent to ~1028 Mton of TNT. The outer layers of the star become superheated plasmas with temperatures high enough to fuse Fe and heavier elements. These outer layers brighten rapidly and are ejected into the interstellar medium at speeds approaching the speed of light. Such an event is a Type II supernova.


Image source: http://www.onafarawayday.com/Radiogenic/Ch1/Ch1-2.htm.

Stars with over 20 Msun evolve in the same way as their slightly less massive companions dying in a supernova explosion, but the core becomes a black hole. A neutron star can mass up to around 3 Msun. After this point neutron degeneracy pressure is no longer sufficient to prevent core collapse. With nothing left to resist collapse the core condenses into an infinitely small, infinitely dense point called a black hole (singularity).