I understand how planets and stars are limited in the range of their sizes. A planet cannot be really big or it will become a star. A star cannot become really big because then it will just burn fuel faster.
I don’t see a reason why galaxies cannot vary wildly in size. Can there be super small galaxies (a few hundred stars) (no, I don’t mean star clusters within a galaxy) or honking huge galaxies a billion times the size of our Milky Way?
I think they can vary, except maybe there has not been enough time for gravitational attraction to coalesce a structure a billion times the size of ours. On the largest scales, structures are dictated by quantum mechanical fluctuations in the early tiny universe that have gotten big with its expansion, and these fine fluctuations seeded the inherently unstable concentration of matter where denser regions become denser still.
The Magellanic Clouds are small galaxies that are hangers-on of the Milky Way. And IIRC the largest elliptical galaxies are many times the size of the MW.
Observationally, the spectrum of galactic sizes (measured in number of stars) ranges from over a trillion (e.g. the Sombrero galaxy) to a few billions (e.g. any of the dwarf galaxies).
So, the range goes from about 10[sup]6[/sup] to 10[sup]12[/sup] stars in the various galactic types.
As I understand it, although hard for us to detect, the most common type of galaxy is dwarf.
YIKES! Make that 10[sup]9[/sup] to 10[sup]12[/sup] stars (a fairly narrow range).
Actually, one of the references I consulted said “a few tens of millions” [of stars] for the smallest dwarf galaxies, so elevate that to 10[sup]7[/sup] for the bottom of the range, and you’d be accurate.
I wasn’t able to nail anything on relative sizes, but I got the impression that the smallest, dimmest dwarf galaxies were a very small fraction of the Milky Way’s 30,000-50,000 LY radius. And the giant elliptical galaxies are monsters, easily ten or a dozen times our radius and perhaps much larger.
You also need to appreciate the fact that “galaxy”, as a classification, is a very general term which bears no relation to the actual formation process. Galactic clusters, spiral galaxies, barred spiral galaxies, elliptical galaxies, anisentropic or irregular galaxies, Seyfert galaxies, ring galaxies, lenticular galaxies, and starburst galaxies all form in different ways. Since we can’t observe galactic formation, but only observe it in various (and mostly advanced) stages at different distances, our knowledge is limited to mostly inference. However, we can make a few general observations, one of which is that in order to form new stars, a galaxy must have a given density for the ill-named “planetary nebulae” to form and initate star formation, so galaxies of only a few hundred (or indeed, a few million) stars are unlikely at best.
The size of galaxies is actually very interesting, as it is the most immediately visible indicator of large scale structure of the cosmos. (The linked Wikipedia article is inexcusibly brief for such an extensive topic, but it’s the quickest thing I could find. Voyage To The Great Attractor: Exploring Intergalactic Space by Alan Dressler is a great introduction to this topic, but unfortunately is out of print. Check your local library and use interlibrary loan to get this book; it’s a worthwhile read.) We can reason as to why galaxies aren’t smaller, as mentioned above; however, the question remains as to why galaxies aren’t bigger. The expansion of space, at least on the local level, doesn’t seem to be large enough to prevent formation of supermassive galaxies, and yet, the largest galaxies are only a few orders of magnitude larger than our own modest spiral. So why do we have the structures we have (or at least the ones we see)? What’s up with the Great Attractor and the Great Wall. And why is my martini evaporating so rapidly?
Excuse me, I think I need to find another drink.
Stranger
Planetary nebulae are indeed ill-named, but I don’t think they’re what you were thinking of. A planetary nebula is the remains of a single mid-sized star, so you’re not going to get multiple stars out of one. I haven’t heard any other term for a star-forming nebula than “star-forming nebula”, or variants thereof.
So. Instead of ranging from really really really really really really big to really really really really really really really really really really really really big, even the smallest of them are actually really really really really really really really really really big?
:eek:
Yep. That about sums it up really. Just don’t ask anyone to describe a galaxy cluster to you. They’re really really really really really really really really really really really really eally really really really really really really really really big. Even the small ones. 
There’s actually a fairly broad range of sizes for stars; bigger than I expected, anyway. See here:
http://www.samtsai.com/p318
You may think it’s a long way down the road to the chemist, that’s just peanuts to space.
My bad; I wasn’t really thinking when I threw that in. Nonetheless, the ability of galaxies to perpetuate star formation is critical to their survival; a galaxy with too low a “critical mass” of material won’t survive long.
Stranger
It’s intriguing that the largest galaxies are ellipticals (though some dwarfs are elliptical too) and that elliptical galaxies have characteristics associated with age (irregulars having characteristics associated with youth and spirals being in between). Ellipticals generally have comparatively little interstellar gas and dust, and their stars are predominantly Population II, the older stars. Irregulars have a lot of gas and dust and are predominantly Population I. Spirals (including barred spirals like the Milky Way) contain both populations and quantities of gas and dust intermediate between elliptical and irregular galaxies.
Its not so surprising if you consider theories/models of structure formation in the universe; the most predominant one, and the one that reproduces observations reasonably well when used in simulations being that of ‘hierarchical structure formation’; that is to say that little things formed first, and that bigger things then grew up out of smaller things.
Thus a large elliptical galaxy will have had to have grown out of smaller galaxies such as spirals and irregulars. If we consider two galaxies a given distance away from us, an elliptical and a spiral or irregular, which we can both see. As we can see both the elliptical and the spiral/irregular now, this means that the elliptical must have formed at an earlier time than the spiral/irregular galaxy so that the elliptical galaxy could have evolved into its elliptical state by now.