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SEE BJV EDIT BELOW.

I came across this image which purports to show what the size of the Andromeda galaxy would be in the night sky if we could see its dim stars with the naked eye. Is this image reasonably correct with respect to size? enter image description here

EDIT BJV 08-23-2022: A friend asked me 'So... where is Andromeda anyway and what would it look like if I could see it all?" I was at a loss to explain in terms he would totally grasp.

AND SO MY ASK... could somebody with the right combination of competencies create an image showing Andromeda (in its entirety) in the night sky in its correct location between Cassiopeia and Pegasus with trees/hills/buildings/whatever in the image to give a real sense of normal viewing scale??

Extra credit... provide same image with 'unaided human eye under IDEAL conditions'.

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    $\begingroup$ Seems Andromeda is too big. The Andromeda Galaxy is about 3deg wide, the moon is about 1/2deg wide. So it would be 6 times the width of the moon. Here it looks like it's more like 8. $\endgroup$
    – Greg Miller
    Commented May 18, 2022 at 14:40
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    $\begingroup$ @Greg miller so... in your estimation image is NOT off by a factor of 100, or 10, or even a factor of 2 $\endgroup$
    – BradV
    Commented May 18, 2022 at 16:05
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    $\begingroup$ No, it's certainly in the ballpark. But like ProfRob said, it would depend exactly on where you define the edge of the galaxy. I think a more precise method would be to compare the image above to other images of it of known scale. But if you're just looking for ballpark estimates, it's certainly in it. $\endgroup$
    – Greg Miller
    Commented May 18, 2022 at 19:04

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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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    $\begingroup$ thank you so very much for calculation. If I had my wits about me this morning I could have done the simple math myself. (embarrassed emoji here). I am constantly being amazed by the cosmos around us that is unseen by the unaided eye!! $\endgroup$
    – BradV
    Commented May 18, 2022 at 16:13
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    $\begingroup$ For reference, the Moon is less than a degree wide in angular diameter, making the scaling in the original picture perfectly plausible $\endgroup$
    – adrian
    Commented May 22, 2022 at 1:18
  • $\begingroup$ @pela and ProfRob ... care to take up the task?? $\endgroup$
    – BradV
    Commented Aug 24, 2022 at 1:55
  • $\begingroup$ FWIW, in my own shot here Andromeda fills the vast majority of an (astrometically determined) 3.8º wide shot flickr.com/photos/raybellis/52647556258/in/… $\endgroup$
    – Alnitak
    Commented Jun 28, 2023 at 21:38

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