Stellar evolution

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The lifespan of a star depends greatly on its mass. The larger the mass, the shorter the star’s lifespan, and the larger it will be.

Nebulae

  • All stars begin in stellar nurseries called nebulae. These nebulae can be of many colors, depending on the gases contained within the nebula. A well-known nebula where stars were formed is the Pillars of Creation (which could have been destroyed by a supernova). Other examples include the Great Orion and Rosette Nebulae.
  • After many years, the nebulae break apart into segments, each about a tenth of a light year in diameter. These are smaller but giant clouds of hot gas and are called protostars. Examples of protostars include V1647 Orionis and IRS2. Protostars are not packed together tightly to form a definite shape, and are much larger.
  • Originally, it was thought that stars were powered by the Kelvin-Helmholtz mechanism. Following the protostar stage, all stars would start as infrared stars, an example of which was Epsilon Aurigae B (actually an ectoplasm star), before contracting into red giants, then further to become blue main-sequence stars and finally contract down the main sequence to become red dwarfs. This theory became obsolete when it was discovered that stars were powered by nuclear fusion, and that red giants are in fact dying stars. The theory also gave astronomy the terms “early-type stars” and “late-type stars”; early-type stars were younger and corresponded to classes O, B, and A and late-type stars were older (already having been “contracted”) and corresponding to classes F, G, K, and M.
  • Now it is known that a protostar may contract to become an infrared star or ectoplasm star. An ectoplasm star remains an ectoplasm star. But an infrared star will continue to contract before it becomes one of the following phases:

Herbig Ae/Be stars (pre-main sequence early-type stars)

  • Herbig Ae/Be stars are the pre-main sequence form of B and A class stars. They are already at an expected level of solar masses, but are not yet burning hydrogen as fuel in their cores.

T Tauri stars (pre-main sequence late-type stars)

  • T Tauri stars are the pre-main sequence form of classes F, G, K, and M. They are much more luminous than their main-sequence counterparts because they are significantly larger (for example, an M-type T Tauri star will have a radius equal to that of our Sun.) Their cores are still too cool to undergo nuclear fission. Many of them are known to have exoplanets.

These stars then contract to the point that their cores become hot enough to undergo nuclear fusion. A main-sequence star is born!

Brown Dwarfs

  • If the protostar’s mass is less than 0.8 solar masses, then it will not become hot enough to start nuclear fusion. It ends up becoming a planet-like object called a brown dwarf. Most protostars will end up becoming a brown dwarf.

Lifespan of an M-type main-sequence star

  • Most stars will end up a red dwarf. These are very faint and cool stars. These have the longest lifespan, much longer than that of the universe.
  • Once they begin to reach the end of their lifespan, they radiate more luminosity and become blue dwarfs.
  • Once their hydrogen fuel is completely exhausted, they become white dwarfs which cool to black dwarfs.

Lifespan of a K-type main-sequence star

  • Orange dwarfs have enough mass (0.25 solar masses) to become a red giant. They also have a very long lifespan. The resulting red giants will not be as large as 10 solar diameters.
  • The red giant then expels its outer layers, forming a planetary nebula and subdwarf O or B star.
  • The resulting star becomes a white dwarf which cools to a black dwarf.

Lifespan of a G-type main sequence-star

  • The lifespan of a yellow dwarf such as the Sun is not simple. They live for many years before their hydrogen supply runs out.
  • Once that happens, they swell through the subgiant stage to become red giants. But they do not stop there. They become smaller helium-fusing yellow giants; a process called the Blue Loop.
  • After the helium is exhausted, the yellow giant becomes a much larger and intense red giant called a post-AGB star.
  • The post-AGB star then expels its outer layers as a planetary nebula. If for some reason, this process happens before the fusion of hydrogen, the stellar remain becomes a subdwarf O or B star. If the process occurs after the post-AGB stage, the remnant becomes a PG 1159 star.
  • The remnant finally becomes a white dwarf which cools to a black dwarf.

Lifespan of an F-type main sequence star

  • The lifespan of a yellow-white dwarf is almost identical to that of a yellow dwarf, though the blue loop stage occurs before the hydrogen fuel is exhausted.

Lifespan of an A-type main sequence star

  • A-type main sequence stars are much more massive (~5 times) that of the Sun. When the star leaves the main-sequence, it “hooks” shortly on the HR diagram by becoming hotter for a while.
  • It then becomes a subgiant and slowly swells to become a red giant, then undergoing a blue loop and becoming a yellow supergiant post-AGB star.
  • The star sheds its outer layers to form a planetary nebula, leaving a white dwarf which cools to a black dwarf.

Lifespan of a B-type main sequence star

  • B-type main sequence stars are very massive and short lived. About a thousand B-type star lives can fit in that of a red dwarf.
  • When a B-type main sequence star exhausts its hydrogen supply, it will swell through the subgiant phase to become a red supergiant.
  • This red supergiant can either heat up to become a blue supergiant; while doing so it becomes a yellow hypergiant.
  • The red supergiant or once-red blue supergiant can explode as a supernova.

Lifespan of an O-type or Wolf-Rayet main sequence star

  • These are the rarest and most massive main-sequence stars. They can reach 100 solar masses and their lifespans are much shorter than even B-type main sequence stars.
  • Like a B-type main sequence star, an O or W-type main sequence star will swell very quickly to become a blue or red hypergiant, or swing between the two phases via the yellow hypergiant stage. They can either die violently at these phases, or contract back into a Wolf-Rayet star.
  • At this point the star is very unstable. It will then shed off its outer layers in a hypernova explosion. An even more dangerous byproduct of a hypernova is a gamma-ray burst.

Supernovae and Hypernovae

  • Supernovae and Hypernovae occur when the cores of stars collapse due to them not being hot enough to fuse iron. Gravity dominates the star’s outward pressure, causing the core to collapse on itself. A flood of neutrinos causes the star to violently throw it its remains in a supernova or hypernova explosion. Hypernovae are 10,000 times more powerful than ordinary supernovae.

Neutron Stars

  • If the star overall is between 10 to 29 solar masses and its core is between 1.4 to 2.9 solar masses, then the star’s core is compressed until the protons and electrons in their atoms combine creating neutrons, filling up all the empty space. A neutron star has been formed.
  • Neutron stars no longer actively generate heat and cool over time, though this process can be delayed by accretion or collisions. As they cool, they move to the right of the Hertzsprung-Russell diagram.

Quark Stars

  • If the overall star is between 30 to 50 solar masses and the core is between 2.9 to 3 solar masses, then the core’s collapse breaks down the neutrons into their composite quarks (or even strange quarks) forming a quark star or strange star.
  • Just like neutron stars, these stars no longer generate heat and cool over time, though they cool much more quickly.

Black Holes

  • If the star’s overall mass is over 50 solar masses and its core’s mass exceeds the Tolman–Oppenheimer–Volkoff limit (~3 solar masses), then the quarks get crushed into a space infinitely smaller than an atom, called a singularity. This has a gravity field and escape velocity so strong that light cannot escape it. A stellar black hole (very rarely a collapsar) is formed.
  • The black hole eventually evaporates over billions of years, due to Hawking Radiation.

Supernova Remnants

  • The supernova remnant eventually forms a new star-forming nebula, completing the cycle of stellar evolution. Notable examples of supernovae remnants are SN 1987A (pictured), which is believed to have formed a quark star, the Crab Nebula, and the Veil Nebula.