Stars reach completely different final stages depending on their mass. The Sun becomes a white dwarf, while massive stars become neutron stars or black holes.
Basic Knowledge
A star forms when interstellar gas contracts under gravity to become a protostar, and then becomes a main-sequence star when nuclear fusion begins.
Evolution of stars about the mass of the Sun (0.5〜8M☉):
Hydrogen depletion in the core → core contraction and expansion of the outer layers → red giant (helium fusion begins) → outer layers are expelled as a planetary nebula → the core remains as a white dwarf (less than 1.4M☉, the Chandrasekhar limit 8).
Evolution of massive stars (8M☉ or more):
Fusion proceeds from hydrogen → He → C → O → Ne → Mg → Si → Fe (iron) (fusion beyond Fe cannot release energy) → the iron core undergoes gravitational collapse → a Type II supernova explosion → when the core density reaches nuclear density, a neutron star (equivalent to M < 3M☉) or a black hole (equivalent to M > 3M☉) remains.
Red giant (an evolved star whose outer layers have expanded because of hydrogen-shell fusion; the Sun will become one in 5 billion years)
Planetary nebula (a glowing, ring-shaped cloud of gas formed when a red giant ejects its outer layers)
White dwarf (the burnt-out core of a star about the mass of the Sun; less than 1.4M☉, Earth-sized, and high-density)
Chandrasekhar limit (the maximum mass of a white dwarf, about 1.4M☉; exceeding it leads to a Type Ia supernova)
Supernova (a huge explosion caused by the collapse of the iron core of a massive star; heavy elements are dispersed into space)
Neutron star (a high-density object left after a supernova; radius about 10km, density 10¹⁴g/cm³)
Further Study (Background and Significance)
A Type Ia supernova occurs when a white dwarf reaches the Chandrasekhar limit and explodes. Because its luminosity is nearly constant (a standard candle), it is used to measure distances in the universe. In 1998, observations of Type Ia supernovae led to the discovery of accelerating cosmic expansion, which contributed to the Nobel Prize (2011).
A pulsar, a type of neutron star, regularly emits beams of electromagnetic radiation because of its strong magnetic field and rapid rotation. It was discovered by Jocelyn Bell in 1967 and initially attracted attention because it was considered possible artificial radio emission (the LGM hypothesis).
Heavy elements synthesized inside stars are dispersed into space by supernova explosions and form the next generation of stars, planets, and life. The statement “We are made of star stuff” (Carl Sagan) is a scientific fact. The iron, calcium, oxygen, and other elements in the human body are all products of supernova explosions.
- Stars about the Sun’s mass (〜8M☉): main sequence → red giant → planetary nebula → white dwarf
- Massive stars (8M☉〜): main sequence → red supergiant → supernova → neutron star/BH
- Fusion beyond iron does not release energy
- Chandrasekhar limit = 1.4M☉
- Type Ia supernova = standard candle → contributed to the discovery of accelerating cosmic expansion
- Neutron star: radius about 10km, density 10¹⁴g/cm³
- Supernova explosions disperse heavy elements into space
Points of Caution (Easily Confused)
① A planetary nebula is unrelated to planets (it was named because its appearance resembled a planet). ② A Type II supernova (gravitational collapse of a massive star) and a Type Ia supernova (thermonuclear explosion of a white dwarf) are different phenomena. ③ The order of the limiting masses is white dwarf (less than 1.4M☉), neutron star (equivalent to 1.4〜3M☉), and BH (3M☉ or more). ④ Red giants and red supergiants are similar types, but the latter have greater mass and size.
Practice
- What is the name of the object left at the end of the evolution of a star about the mass of the Sun?
- What is the explosion caused by the gravitational collapse of an iron core called?
- What is the Chandrasekhar limit in M☉?