From birth to death, stars follow a set path determined by their mass. A star's mass is the most important parameter determining the speed of its evolution and its final form.
Basic Knowledge
Stars are born when an interstellar cloud (gas and dust) contracts under gravity.
Evolution of a star with about the Sun's mass:
Protostar → main-sequence star (hydrogen fusion, about 10 billion years) → red giant (hydrogen depletion, expansion of outer layers) → planetary nebula (outer layers ejected) → white dwarf (the core remains and continues to cool)
Evolution of a star with at least eight times the Sun's mass:
Main-sequence star → red supergiant → supernova explosion (Type II supernova) → neutron star or black hole (depending on mass)
A neutron star is an extremely dense celestial object about 20 km in diameter, with a density similar to that of an atomic nucleus (1cm³ is about 1 billion tons). It may be observed as a pulsar (a rotating neutron star that emits regular radio pulses).
A black hole is a region from which even light cannot escape because its escape velocity exceeds the speed of light.
Main-sequence star (A stable stage during which hydrogen fusion is occurring. It occupies most of a star's life)
Red giant/red supergiant (The expansion stage after hydrogen depletion. Solar-type/massive stars)
Planetary nebula (A gaseous celestial object formed when a solar-type star ejects its outer layers)
White dwarf (The final form of a star about equal to or less massive than the Sun. Earth-sized, hot, and dense)
Supernova explosion (The final enormous explosion of a massive star. It spreads heavy elements through space)
Neutron star/black hole (Remnants after a supernova. Determined by mass)
Further Exploration (Background and Significance)
Stellar evolution is not simply "death"; it is part of the matter cycle of the universe. Nuclear fusion inside stars synthesizes elements through H→He→C, N, and O→heavier elements, and during supernova explosions, elements heavier than iron (such as gold, platinum, and uranium) are also synthesized and dispersed into space. These elements become the materials for the next generation of stars and planets, and for life. The fact that "the heavy elements in our bodies were made by stars" is an important point showing our connection with the universe.
In 2017, the electromagnetic counterpart of a neutron-star merger (the gravitational-wave event GW170817) was observed, capturing for the first time the moment when r-process elements such as gold and platinum were synthesized.
In about 5 billion years, the Sun will become a red giant and expand to about 200 times its current radius. Earth's orbit will be near its outer edge, causing the water on Earth's surface to evaporate and making life unable to survive. On the scale of the universe, however, this is merely an ordinary episode of stellar evolution.
- Sun-like star → red giant → planetary nebula → white dwarf
- Massive star of at least 8 times the Sun's mass → red supergiant → supernova → neutron star or black hole
- Supernova explosions disperse heavy elements through space
- Neutron star = about 20 km in diameter and extremely dense
- Black hole = even light cannot escape
- The Sun has about 5 billion years remaining
- The heavy elements in our bodies came from stars
Points of Caution (Easy to Confuse)
① A planetary nebula has nothing to do with planets. It is a celestial object in which outer-layer gas becomes ionized and emits light. ② Distinguish a white dwarf (the final form of a solar-type star) from a neutron star/black hole (the final forms of massive stars) by mass. ③ Supernovae include Type Ia (explosion of a white dwarf) and Type II (massive star), but introductory high-school studies focus on Type II. ④ A red giant (solar-type star) and a red supergiant (massive star) are different stages.
Practice
- What is the final form of a star with about the Sun's mass?
- What two types of celestial objects can a star with at least eight times the Sun's mass become after a supernova explosion?
- Why are supernova explosions important to the universe?