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Short answer

Stars are giant balls of hot gas, mostly hydrogen and helium. A star forms when a gas cloud collapses; once its core fuses hydrogen into helium, it enters the main sequence, the longest phase of its life. What comes after depends on its mass. [S1][S2]

Key points

• Lifetimes run from a few million to trillions of years. [S1]
• The Milky Way alone has more than 100 billion stars. [S1]
• The Sun, about 4.6 billion years old, is a main sequence star expected to stay one for another 5 billion years before becoming a red giant. [S2]
• Very massive stars end as supernovas and can leave black holes. [S2]

Claims

• id: C1 claim: Stars are giant balls of hot gas, mostly hydrogen. source: S1 evidence: "Stars are giant balls of hot gas – mostly hydrogen, with some helium"
• id: C2 claim: The main sequence is the longest phase of a star's life. source: S1 evidence: "This is the longest phase of a star’s life."
• id: C3 claim: The Sun should remain a main sequence star for another 5 billion years. source: S2 evidence: "will remain one for another 5 billion years before becoming a red giant"
• id: C4 claim: Star types range widely in brightness, size, color and behavior. source: S2 evidence: "The universe’s stars range in brightness, size, color, and behavior."

Sources

• [S1] Stars. NASA. https://science.nasa.gov/universe/stars/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Types. NASA. https://science.nasa.gov/universe/stars/types/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_sfaeb9eb,
  author = {{iinventedlight}},
  title = {{How stars live and change}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/star-life-cycle/},
  note = {Permanent ID iil:note:sfaeb9eb. License: CC-BY-4.0.},
}

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