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I recently saw the below image circulating around twitter/Facebook/reddit. It is titled "Andromeda's actual size if it was brighter":

enter image description here

One of the comments provides a link to another comparison provided by APOD, and the following comment mentions that the first one I linked seems a bit off.

Is the first image accurate in term of relative sizes? What is the relative angular size of Andromeda's galaxy and the Moon?

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  • $\begingroup$ The biggest difference in images like this comes from different exposure times. Since the brightness of galaxies falls off with the distance from the center, longer exposure times capture an increasingly larger part of the galaxies' light. That is, the longer you expose, the larger the galaxies look. $\endgroup$
    – pela
    Nov 2, 2017 at 13:58
  • $\begingroup$ When it's dark enough to see M31 with a naked eyeball, it looks lie a fuzzy patch about as wide as the moon in its longest dimension. These pics with sensitive digital cameras or telescopes make it appear larger, by bring out the fainter stars around the galaxy's edges. That pic looks like a high res image of M31, imposed on a near twilight lunar sky. If real, you'd see a lot more stars from within the Milky way. $\endgroup$ Nov 2, 2017 at 14:53
  • $\begingroup$ I wish I could figure out where this got linked recently $\endgroup$
    – Federico
    Oct 21, 2022 at 6:53
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    $\begingroup$ @Federico in case you're still wondering: news.ycombinator.com/item?id=33281682 $\endgroup$ Oct 21, 2022 at 9:00

2 Answers 2

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The Andromeda Galaxy's angular size is about 178x63 arc-minutes and the Moon is about 31 arc-minutes wide. Relatively speaking, the Andromeda Galaxy's apparent size is nearly 6x2 Moons.

So from the photo, their relative sizes in the sky appears to be very roughly accurate. The galaxy appears to have been superimposed from another image just to show what the galaxy might look like in the sky if it were brighter.

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  • $\begingroup$ Ok, I see. (I don't fully agree on the accurateness though. Roughly looking at the pixels, the image depicts Andromeda with ~319 pixels on the major axis, and the moon with a diameter of ~40 pixels, that's roughly 8 times, vs the actual 6 you mention. $\endgroup$
    – Federico
    Nov 2, 2017 at 11:27
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    $\begingroup$ @Federico I find, like you, that in that image, Andromeda is about 8 times wider than the moon. However, you have to take into account that the angular size of Andromeda is not well defined because there is no clear cutoff for where Andromeda ends. If you look at wikipedia, you'll find a quoted size of 3.167 x 1 degree which is slightly wider than Kozaky's source. I'll admit that Andromeda seems to be slightly too big, but it's still mostly accurate. Note the moon also changes size up to 14% as it orbits. $\endgroup$
    – zephyr
    Nov 2, 2017 at 15:30
  • $\begingroup$ Now that is interesting. I always knew the Andromeda galaxy was big but dim, but I didn't appreciate just how big. $\endgroup$ Nov 4, 2017 at 15:33
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From the duplicate I have closed:

Unfortunately, galaxies don't have a neat edge (and the Moon is not at a fixed distance) so your question is difficult to answer in absolute terms.

However, a commonly quoted diameter for the disk of the Andromeda galaxy is something like 70 kpc (about 220,000 light years), which at a distance of 750 kpc (about 2.4 million light years, e.g. see Andromeda galaxy) gives an angular diameter of about 5 degrees, which is about 10 times the diameter of the full Moon.

Thus I think the picture is reasonably justified.

On the other hand, is it realistic to expect to be able to see Andromeda this big? I'm not so sure and have asked a linked question about this. For example, this (well-known) picture from the digitised sky survey shows an Andromeda disk that is less than 3 degrees across!

M31 picture

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