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The question is a rather obvious one.

In normal English a word like "dwarf" used as an adjective is assumed to modify the following word. Thus a "dwarf galaxy" would be assumed to be a galaxy of a type which is noticably smaller than galaxies of at least one other type.

And in science there might possibly be an established dividing line between dwarf galaxies and ordinary sized galaxies, just as there might possibly be a diving line between ordinary sized galaxies and giant galaxies.

But in 2006 the International Astronomical Union created new definitions, in which a dwarf planet is not a subcatebory of planet, but a member of a totally separate class of astronomical solar system object. Dwarf planets are not included in planets, and planets do not include dwarf planets.

That is ungrammatical in English, but it follows a pattern.

A minor planet is mostly equivalent to what is usually called an asteroid.

A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet.[a] Before 2006, the International Astronomical Union (IAU) officially used the term minor planet, but during that year's meeting it reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs).1

https://en.wikipedia.org/wiki/Minor_planet[1]

So in astronomy a minor planet is not a planet, and a dwarf planet is not a planet. The category of planets does not include minor planets or dwarf planets.

And thus it seems possible to me that perhaps in astronomy a dwarf galaxy might not be a galaxy.

If dwarf galaxies are a subcategory of galaxies, it would correct to call a dwarf galaxy a "galaxy" for short. But if dwarf galaxies are a category of astronomical object separate from galaxies, it would not be correct to call a dwarf galaxy a "galaxy" for short.

So are dwarf galaxies a subcategory of galaxies, or are they considered to be a separate category from galaxies?

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  • $\begingroup$ The section on planets seems to be irrelevant, unless you are asking about IAU definitions. $\endgroup$
    – James K
    Nov 28, 2020 at 19:05
  • $\begingroup$ When we say something is a "dwarf ..." it is actually that thing; the 2006 definition on "dwarf planets" is the only dumb exception. As if the term "planetoid" wouldn't exist or they'd be unable to invent a term like "underplanet" or "subplanet" or classify them under the already extant term "protoplanet". $\endgroup$
    – Greenhorn
    Nov 29, 2020 at 12:40

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The IAU has not defined "galaxy", therefore you should not use the word (Joke)

In all seriousness, language is a tool. You are trying to dance around definitions. There are many types of galaxies: spiral and elliptical, barred and unbarred, flocculent and unflocculent, small, medium and large. They are all galaxies, of one kind or another. The IAU has chosen to define "Planet" and "Dwarf planet" in a particular way, so that children don't have to learn the names of 46 solars system bodies. They haven't chosen to give a definition of "galaxy", nor "cluster" nor "nebula". But it doesn't matter. You use language to communicate. You are not in thrall to the IAU. So you can use the word "galaxy" as they do to mean "galaxy".

Yes: dwarf galaxies are galaxies.

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  • $\begingroup$ I agree with you that dwarf galaxies are galaxies. See my post # 162 at: trekbbs.com/threads/yet-another-doomsday-machine-thread.306036/… - Saving children from having to learn the names of 46 planets doesn't seem important to me. I when was in grade school and hight school, I knew the names of the then nine planets, the first four asteroids found, and at least 24 of the then 31 known moons, pluse a number of fictioal planets.. $\endgroup$
    – M. A. Golding
    Nov 29, 2020 at 3:57
  • $\begingroup$ I agree with you that dwarf galaxies are galaxies. See my post # 162 at: trekbbs.com/threads/yet-another-doomsday-machine-thread.306036/… - Saving children from having to learn the names of 46 planets doesn't seem important to me. I when was in grade school and hight school, I knew the names of the then nine planets, the first four asteroids found, and at least 24 of the then 31 known moons, pluse a number of fictioal planets.. $\endgroup$
    – M. A. Golding
    Nov 29, 2020 at 3:58

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