---
source: https://iinventedlight.com/library/notes/galaxies-basics.md
title: "Galaxies: what they are and the main types"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NASA, Galaxies, US government work, public domain; NASA, Types, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: astronomy"
tags: science, astronomy, galaxies, milky-way
sources:
  - id: S1
    title: "Galaxies"
    publisher: NASA
    url: https://science.nasa.gov/universe/galaxies/
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Types"
    publisher: NASA
    url: https://science.nasa.gov/universe/galaxies/types/
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Galaxies: what they are and the main types

## 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).