What is a Galaxy?

An interesting little paper by Duncan Forbes and  Pavel Kroupa appeared today on the arXiv today. It asks what you would have thought was the rather basic question “What is a Galaxy?”. Like many basic questions, however, it turns out to be much  more complicated than you imagined.

Ask most people what they think a galaxy is and they’ll think of something like Andromeda (or M31), shown on the left, with its lovely spiral arms. But galaxies exist in many different types, which have quite different morphologies, dynamical properties and stellar populations.

The paper by Forbes and Kroupa lists examples of definitions from technical articles and elsewhere. The Oxford English Dictionary, for instance, gives

Any of the numerous large groups of stars and other
matter that exist in space as independent systems.

I suppose that is OK, but isn’t very  precise. How do you define “independent”, for example? Two galaxies orbiting in a binary system aren’t independent, but you would still want to count them as two galaxies rather than one. A group or cluster of galaxies is likewise not a single large galaxy, at least not by any useful definition. At the other extreme, what about a cluster of stars or even a binary star system? Why aren’t they regarded as gaaxies too? They are (or can be) gravitationally bound..

Clearly we have a particular size in mind, but even if we restrict ourselves to “galaxy-sized” objects we still have problems. Why is a globular cluster not a small galaxy while a dwarf galaxy is?

To be perfectly honest, I don’t really care very much about nomenclature. A rose by any other name would smell as sweet, and a galaxy by any other name would be just as luminous. What really counts are the physical properties of the various astronomical systems we find because these are what have to be explained by astrophysicists.

Perhaps it would be better to adopt Judge Potter Stewart‘s approach. Asked to rule on an obscenity case, he wrote that hard-core pornography was difficult to define, but ” I know it when I see it”….

As a cosmologist I tend to think that there’s only one system that really counts – the Universe, and galaxies are just bits of the Universe where stars seemed to have formed and organised themselves into interesting shapes. Galaxies may be photogenic, nice showy things for impressing people, but they aren’t really in themselves all that important in the cosmic scheme of things. They’re just the Big Bang’s bits of bling.

I’m not saying that galaxies aren’t extremely useful for telling us about the Universe; they clearly are. They shed light (literally) on a great many things that we wouldn’t otherwise have any clue about. Without them we couldn’t even have begun to do cosmology, and they still provide some of the most important evidence in the ongoing investigation of the the nature of the Universe. However, I think what goes on in between the shiny bits is actually much more interesting from the point of view of fundamental physics than the shiny things themselves.

Anyway, I’m rambling again and I can hear the observational astronomers swearing at me through their screens, so let me move on to the fun bit of the paper I was discussing, which is that the authors list a number of possible definitions of a galaxy and invite readers to vote.

For your information, the options (discussed in more detail in the paper) for the minimum criteria to define a galaxy are:

  • The relaxation time is greater than the age of the Universe
  • The half-light radius is greater than 10 parsecs
  • The presence of complex stellar systems
  • The presence of dark matter
  • Hosts a satellite stellar system

I won’t comment on the grammatical inconsistency of these statements. Or perhaps I just did. I’m not sure these would have been my choices either, but there you are. There’s an option to add your own criteria anyway.

The poll can be found here.

Get voting!

UPDATE: In view of the reaction some of my comments have generated from galactic astronomers I’ve decided to add a poll of my own, so that readers of this blog can express their opinions in a completely fair and unbiased way:



18 Responses to “What is a Galaxy?”

  1. i remember worrying about this when the Hubble Deep Field came out in 1996 (in fact i thought some papers were written about it at the time). its of course a much harder thing to decide what is a galaxy or a part of a galaxy at high redshift – but it is important.

    …and you were right – i was shouting at the screen with your “its the empty spaces between the galaxies which are more important” tirade – remember all those “origins” motivations – our galaxy, our solar system, our planet. surely the “stuff” in the universe is more important than the non-stuff?

    • telescoper Says:

      I was trying to be a bit tongue-in-cheek, but not entirely so.

      Stars, planets and galaxies are of course interesting things to study for their own sake, but their interest is much much greater when you can use them to learn things about the fundamentals of our Universe. That is, of course, my opinion and it is biased. What I omitted to say also is that you have to study these objects in great detail before you know enough about them to be able to use them for other things. That’s why cosmology and extragalactic astronomy are so interdependent, even though the methods and ultimate goals may be different. Chess and mud-wrestling.

      I also think cars are interesting insofar as one might be able to take me from A to B, but I find them of absolutely no interest in themselves

  2. Bryn Jones Says:

    I have fretted over the issue of what exactly is meant by a galaxy for a number of years, ever since we blurred the previously sharp distinction between small (dwarf) galaxies and globular clusters.

    The distinction does matter, because it raises the possibility of a continuum of properties between obvious galaxies and star clusters. That in turn redefines the observational constraints on galaxy formation/evolution theories. And the formation and evolution of galaxies is a critical problem both in astrophysics and to cosmology.

    I trust that extragalactic astrophysics will not follow the mistakes of the planetary science community in its attempts to define what is a planet, with its confusion over whether Pluto is a planet, a dwarf planet or whatever. I’m confident we shall not.

  3. Rhodri Evans Says:

    I often feel our understanding of galaxies is still at the same stage that our understanding of stars was back at the end of the 19th Century. Before the HR diagram and the understanding of thermonuclear fusion as their power source, all we had really done was classify stars into different types, based on their spectra.

    We still don’t know how galaxies evolve, what the relationship is (if any) between a spirals and ellipticals, whether globular clusters and dwarf galaxies form before giant galaxies or vice-versa. And, as Bryn says, the distinction between globular clusters and dwarf galaxies has become blurred in recent years.

    With planets, I would say we have a fairly clear definition that a planet is an object orbiting a star (or rather, orbiting their common centre of mass) which does not have, or has not had, hydrogen-burning at its core. I personally think the re-classification of Pluto to being a dwarf planet was a good thing, as the discovery of more and more Kuiper belt objects from 1992 onwards, some apparently more massive than Pluto, made Pluto’s position as a “planet” hard to justify.

  4. Too many astronomers spend too much time putting things in boxes, then fretting endlessly about the shape and size of the box. I vote for physically motivated classification.

    ps – Peter, thought you might like this headline

    Consumers kept in dark by Coles’ refusal to go online


    • telescoper Says:

      Interesting. I wonder if the existence of an Australian supermarket with the same name as me means that I have an ancestor who moved to Australia? I wonder what crime he committed?

      In Canada, Coles seems to be the name of a chain of bookshops, although they wouldn’t give me a discount there when I claimed to be the owner.

    • Rhodri Evans Says:

      When I was in Toledo (Ohio) there was a big chain of fast food restaurants there called Bob Evans. A huge number of my students thought I must be related to him – little did they know that everyone in Wales is called Evans (or Jones).

    • Rhodri Evans Says:

      Cusp – I agree that astronomers (and scientists in general) can spend too much time fussing about categorisation. But, when we don’t understand something, it is often a first step in trying to gain an understanding. When you consider the spectral classification of stars based on their Hydrogen-line strength (the Harvard system), of course it turned out that the strength of the Hydrogen line was not the most important or fundamental characteristic of a star. But it was a start in trying to put some kind of order to things.

  5. peter – when you moved to cardiff you nearly got back to your homeland:


    and apparently your surname’s mosaic type is “Just Moving In” (B08 of “Happy Families”) – for an explanation see pg26 of:


    • telescoper Says:

      Interesting, but I’m not sure what any of this has to do with what a Galaxy is!

      I believe it’s true that people from the South-West of England were more likely than elsewhere to be transported to the colonies if found guilty of a crime, so the Coles of Australian Supermarket fame may have been descended from down there…

      I do have Welsh relatives on the “Coles” side of the family, incidentally. In case you’re interested, “Coles” is actually a diminutive form of Nicholas.

    • telescoper Says:

      ps. I learned from my neighbours that the house I currently live in was once occupied by an old lady called Gwyneth Coles. She died a while ago and the house was extended and slightly altered after that. I don’t think I’m related to her, but for a while the neighbours assumed I was – which caused some confusion!

  6. if you look at the “Geographical Location” tab – you’ll see the Coles seem to be fairly uniformly spread into Australia, NZ and Canada (i guess newcastle was the equivalent in the UK) – which i assume isn’t a profile expected from deportation – its more like early-C19 emigration.

    …and if you want to get back on message – then i’d argue that your poll suffers from a selection effect due to your blog topic (selection effects being one of those things which astronomers worry about and cosmologists think [wrongly] happen to other people).

    • telescoper Says:

      I think quite a few people haven’t noticed the even more fundamental (and deliberate!) problem with my poll!

  7. […] I have a strong opinion about what should and shouldn’t be classified as a galaxy;  and like Peter Coles, you could even argue whether the discussion is even relevant. But if galaxies are your thing, join […]

  8. […] How’s this for a definition of galaxy? […]

  9. >> Cusp – I agree that astronomers (and scientists in general) can spend too much time fussing about categorisation. But, when we don’t understand something, it is often a first step in trying to gain

    I agree – but having been to some AGN talks it seems things have moved very slowly in some fields.

    • telescoper Says:

      I’d go a bit further, I think. I’d say that to some astronomers the stamp collecting is all that matters.

    • Rhodri Evans Says:

      What was Rutherford’s quote? – “There’s physics, and everything else is stamp collecting” (or something similar).

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