It’s not exactly that distance across.
Wikipedia says that it’s size is 87,400 ± 3,590 light years.
That’s an accuracy of ± one part in 25.
So in other words - they’re saying that we know that it’s at least 83,810 light years and no more than 87,400 light years.
However - we can’t send spacecraft far enough away to take a nice photo of it like this for us to measure…
…and even if we could - you can see that the galaxy has VERY fuzzy edges and an extremely irregular shape.
Furthermore - we can hardly see anything on the other side of the central bulge of the galaxy because the dense stars, gasses and interstellar dust is blocking our view.
Quite honestly - I’m shocked that any “official” measurement could possibly be as accurate as stated in the Wikipedia article…after all - if there is just one star that’s a part of the galaxy that’s further out than that - then the answer is wrong.
GIven the massive scope for error - I suspect that whatever source you saw for the 100,000 light year number was just rounding it to a single digit of accuracy - which is an entirely justifiable thing to do under the circumstances.
It’s not some very accurate (and therefore amazingly coincidental) number - it’s a very, very ill-defined number that’s been (justifiably) rounded off to a realistic amount of precision.
The number that Wikipedia uses comes from a book written in 1998 - so it’s possible that there are newer numbers - is also has note that this is measured using the “isophotal diameter”…which it then explains in painful detail:
The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness. Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2);
Yeah - so to try to unpack this…the “isophotal diameter” doesn’t mean “if you measured from the furthest star to the east to the furthest star on the west - this is the number you’d get”.
CONCLUSION:
You can’t accurately measure the diameter of a weird shaped fuzzy blob with spiral arms - especially not if you have to do it using photos taken from a single point INSIDE that fuzzy blob!
Therefore, the number “100,000 lightyears” is rounded off to one significant digit - which is a very reasonable thing to do in the face of so much uncertainty.
