Stars also have births and deaths. They follow different evolutionary paths according to their mass and supply elements to the universe.
Basic Knowledge
Stars are born when gas and dust in space (interstellar matter) gather together under gravity (a protostar). When the temperature at the center exceeds approximately 1000万K, nuclear fusion of hydrogen (p-p reaction or the CNO cycle) begins, and the star becomes a stable main-sequence star. The Sun is currently a main-sequence star.
When the nuclear fuel (hydrogen) is used up, the star expands and becomes a red giant. Its fate depends on its mass:
① About the mass of the Sun: It ejects its outer layer and becomes a planetary nebula, while the center becomes a white dwarf.
② Eight times the mass of the Sun or more: It undergoes a supernova explosion and becomes a neutron star or a black hole.
In a supernova explosion, heavy elements (iron and heavier) are scattered into space and become material for future stars. The heavy elements in our bodies also came from the deaths of stars in the past.
Main-sequence star（A stable stage in which hydrogen fusion occurs at the center. The Sun is at this stage）
Red giant（A stage in which the outer layer expands after hydrogen is used up）
White dwarf（The final form of a low-mass star. About the size of Earth and extremely dense）
Supernova explosion（The final explosion of a massive star. Releases heavy elements into space）
Neutron star（A high-density celestial body left after a supernova. Approximately 10km in radius and about the mass of the Sun）
Black hole（A celestial body with gravity so strong that even light cannot escape）
Further Exploration (Background・Significance)
The supernova of the Crab Nebula observed in 1054 (SN1054) was also recorded in Fujiwara no Teika’s “Meigetsuki.” Chinese astronomers recorded it as a “guest star,” and it can still be observed today as a supernova remnant.
In about 50億 years, the Sun will become a red giant and expand nearly to Earth’s orbit. Eventually, it will eject its outer layer and become a white dwarf the size of Earth.
The elements in the universe were produced as follows: hydrogen and helium in the Big Bang, carbon through iron by nuclear fusion inside stars, and elements heavier than iron (such as gold and uranium) in supernova explosions. The iron and carbon in your body are “stardust” that came from stars.
In 2017, gravitational waves from a neutron-star merger were detected for the first time, confirming that the heaviest elements, such as gold and platinum, are also formed in this process.
- Star formation = gravitational contraction of interstellar matter
- Main-sequence star = hydrogen-fusion stage
- Red giant = expansion stage after hydrogen depletion
- Low-mass star (about the mass of the Sun)→white dwarf
- Massive star (eight times the mass of the Sun or more)→supernova→neutron star or black hole
- Carbon through iron = synthesis inside stars
- Elements heavier than iron = supernovae/neutron-star mergers
Points to Note (Easily Confused)
① The sequence is main-sequence star (stable)→red giant (expansion)→white dwarf or supernova. ② The fate depends on mass (eight times the mass of the Sun is the boundary). ③ A nova and a supernova are different. A nova is a partial explosion in a binary system, while a supernova is the final explosion of the entire star. ④ The boundary between a neutron star and a black hole is approximately 30 times the mass of the Sun.
Practice
- State the evolutionary stage the Sun is currently in.
- State the final form of a star with about the mass of the Sun.
- What kind of phenomenon mainly produces elements heavier than iron?