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Short answer

A galaxy is a huge system of stars, planets, gas and dust held together by gravity. Most are spirals or ellipticals; the rest are called irregular. [S1]

Key points

• Size ranges from a few thousand stars to trillions. [S1]
• Most galaxies are 10 to 13.6 billion years old. [S1]
• Galaxies gather into groups, clusters and superclusters. [S1]
• The Milky Way is a barred spiral more than 100,000 light-years across; our solar system is about halfway out from the center. [S1][S2]
• Barred spirals make up about two-thirds of spirals. [S2]

Claims

• id: C1 claim: Galaxies consist of stars, planets, gas and dust bound by gravity. source: S1 evidence: "Galaxies consist of stars, planets, and vast clouds of gas and dust"
• id: C2 claim: Most galaxies are between 10 billion and 13.6 billion years old. source: S1 evidence: "Most galaxies are between 10 billion and 13.6 billion years old."
• id: C3 claim: Spiral galaxies are defined by the presence of spiral arms. source: S2 evidence: "a broad class of galaxies defined by the presence of spiral arms"
• id: C4 claim: The Milky Way and Andromeda are barred spirals. source: S2 evidence: "the Andromeda galaxies belong to a subtype known as barred spirals"

Sources

• [S1] Galaxies. NASA. https://science.nasa.gov/universe/galaxies/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Types. NASA. https://science.nasa.gov/universe/galaxies/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_qn84703r,
  author = {{iinventedlight}},
  title = {{Galaxies: what they are and the main types}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/galaxies-basics/},
  note = {Permanent ID iil:note:qn84703r. License: CC-BY-4.0.},
}

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