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I'm asking how large (by which specifically I mean diameter in light-years, though a rough estimate of the number of stars would be appreciated) can the main body of the spiral galaxy get.

I'm familiar with NGC 6872, but its size is from the tip of one of the two outstretched arms to the other; I'm not interested in this size, because of the shape. I'm instead wondering how big of a diameter a spiral galaxy a la the Milky Way or the Sombrero Galaxy (as in, a circular galaxy with no arms stretching noticeably beyond the edges of the disk) could be, within whatever physical limitations of galactic formation apply to spirals.

If its at all relevant I'm intending my galaxy to be an flocculent unbarred spiral. I want it to be as big as I can get while still being a spiral galaxy.

Thanks

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    $\begingroup$ This is probably dependent on the limits of how much dark matter can be contained within a galaxy, which itself is probably constrained by the nature of dark matter that is currently a matter of debate. $\endgroup$
    – John O
    Jul 20, 2022 at 15:29
  • $\begingroup$ @Ushumgallu I don't understand your objection to NGC 6872. Why do you think that NGC doesn't have a circular disc of disc stars extending to the tips of he two spiral arms and having at least that much diameter. Spiral arms are regions of recent star formation where there are lots of young bright stars, custers, and nebulae. But those bright objects are a tiny minority among all the ordinary disc stars in a galaxy. Spiral arms are brighter than the spaces between them, but don't have a much higher density of normal disc stars than the spaces between spiral arms. $\endgroup$
    – M. A. Golding
    Jul 20, 2022 at 18:20
  • $\begingroup$ I'm just looking for the size limit on more traditionally shaped spiral galaxies $\endgroup$
    – Ushumgallu
    Jul 20, 2022 at 19:29
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    $\begingroup$ VTC Opinion-Based (I'll retract if one of our astrophysicists disagrees with this): This is a question that falls into a category I call, "How many angels can dance on the head of a pin? Answer: As many as wanting." The universe is constantly surprising us with characteristics we didn't previously believe possible. Thus, any answer you receive is more than likely to be an opinion. Out of curiosity, what's stopping you from choosing what size you want? It's not like anyone can say you're wrong. $\endgroup$
    – JBH
    Jul 21, 2022 at 3:06
  • $\begingroup$ @Ushumgallu And I am suggesting that possibly NGC 6872 has a normally shapped disc of disc stars, and that the somewhat distorted spiral arms are simply somewhat unsually distribution of the newly formed brightly shining stars and clusters within that disc. What do professional astronomers say about whether the actual distrubtion of stars is similar to or different from the disttribution of highly visible stars in NGC 6872? $\endgroup$
    – M. A. Golding
    Jul 21, 2022 at 19:09

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A spiral galaxy is limited by the accumulation of dark matter, and dark matter is still not understood to that degree. This means that there is no scientific way of answering this short of providing known examples. You can make up dark matter physics, and that will give you an answer, but nobody with a background in astrophysics would state that there was a single authoritative answer to this question.

Andromeda: 220kly

UGC 2885: 265kly (10x the mass of the Milky Way)

UGC 2885 is presumed to have gotten that way because it formed as an isolated galaxy in a large void. Maybe you could base this on the largest voids, but then you have to add in the sparseness of material in those voids, so you might not be buying anything with that.

Even when figuring out the maximum size of a stars, which is something that we do understand the physics behind, we've found cases that we've had to adjust our theories to encompass.

Thus, this is an area with much room for creative license.

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  • $\begingroup$ alright. I was worried this was going to be unanswerable; this is as good as it gets. Appreciate it $\endgroup$
    – Ushumgallu
    Jul 21, 2022 at 12:45
  • $\begingroup$ What an answer, you send me down on a good rabbit-hole about galaxies and stars, even making me re-consider to get a degree on Astronomy. $\endgroup$
    – vivus
    Jul 21, 2022 at 13:50
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A quick cite and link answer could be..

The Largest-Known Spiral Galaxy

The spectacular barred spiral galaxy NGC 6872 has ranked among the biggest stellar systems for decades. This enormous spiral is 522,000 light-years across from the tip of one outstretched arm to the tip of the other, making it about five times the size of our home galaxy, the Milky Way

https://svs.gsfc.nasa.gov/30111

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  • $\begingroup$ I specifically noted that NGC 6872 was not the shape I was looking for $\endgroup$
    – Ushumgallu
    Jul 20, 2022 at 14:42
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    $\begingroup$ @Ushumgallu it IS the largest spiral galaxy.. But if you don't want to adhere to common definitions, better put a drawing. There are a lot of these giant galaxies.. Pick and choose.. nasa.gov/feature/jpl/… $\endgroup$
    – Goodies
    Jul 20, 2022 at 14:44
  • $\begingroup$ yeah you're right. I've edited it a bit to make that clearer. $\endgroup$
    – Ushumgallu
    Jul 20, 2022 at 14:46
  • $\begingroup$ Fine.. I'll remove this answer later tonight, and we'll see what comes up. $\endgroup$
    – Goodies
    Jul 20, 2022 at 14:47

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