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In various large public forums about astronomy or astrophysics, from time to time I see people using the word "galaxy" for what seems to not mean the galactic set of stars (like Milky Way or Andromeda) but our (or any) Solar system.

-> is it a "legal" use of the word galaxy in English ? is it a wrong but popular use ? Or is it just wrong thoughts by ignorant users ?

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    Welcome to ELL and thank you for your question. There are no "laws" in force, but galaxy means A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. It certainly can't describe the solar system. Our tour and Help Center pages provide advice on writing useful questions and answers. We hope you will ask more questions! – P. E. Dant Sep 14 '16 at 7:20
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    I would say "legal" is a too strong of a word in this question. – Ébe Isaac Sep 14 '16 at 7:27
  • Can you provide an example? It's hard for this native speaker to imagine any native speaker using "galaxy" to mean "solar system". – stangdon Sep 14 '16 at 11:30
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I think that it is improper to use galaxy to refer to a solar system:

Galaxy:

  • a large system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.

  • Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant their light takes millions of years to reach the Earth. Galaxies are classified in three main groups; spirals, ellipticals and irregulars.

Astronomy Dictionary

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