<p>From the duplicate I have closed:</p>
<p>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.</p>
<p>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 <a href="https://en.wikipedia.org/wiki/Andromeda_Galaxy" rel="nofollow noreferrer">Andromeda galaxy</a>) gives an angular diameter of about  5 degrees, which is about 10 times the diameter of the full Moon.</p>
<p>Thus I think the picture is reasonably justified.</p>
<p>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 <a href="https://astronomy.stackexchange.com/questions/49370/how-could-i-see-andromeda-as-large-as-in-this-picture?noredirect=1&amp;lq=1">linked question</a> about this.
For example, this (well-known) picture from the digitised sky survey shows an Andromeda disk that is less than 3 degrees across!</p>
<p><a href="https://i.sstatic.net/4P1mp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4P1mp.png" alt="M31 picture" /></a></p>

CC BY-SA 4.0
2023-05-02T11:35:24.787Z
2023-05-02T11:35:24.787Z
ProfRob
<p>I recently saw the below image circulating around twitter/Facebook/reddit. It is titled &quot;Andromeda's actual size if it was brighter&quot;:</p>
<p><a href="https://i.sstatic.net/NIsYM.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/NIsYM.jpg" alt="enter image description here" /></a></p>
<p>One of the <a href="https://www.reddit.com/r/spaceporn/comments/1u0dxs/andromedas_actual_size_if_it_was_brighter/cedhp35/" rel="nofollow noreferrer">comments</a> provides a link to <a href="https://apod.nasa.gov/apod/ap130801.html" rel="nofollow noreferrer">another comparison</a> provided by APOD, and the following comment mentions that the first one I linked seems a bit off.</p>
<p>Is the first image accurate in term of relative sizes? What is the relative angular size of Andromeda's galaxy and the Moon?</p>

CC BY-SA 3.0
2017-11-02T13:58:08.370Z
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](https://en.wikipedia.org/wiki/Sersic_profile), longer exposure times capture an increasingly larger part of the galaxies' light. That is, the longer you expose, the larger the galaxies look.
pela
CC BY-SA 3.0
2017-11-02T14:53:41.413Z
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.
Wayfaring Stranger
CC BY-SA 4.0
2022-10-21T06:53:39.267Z
I wish I could figure out where this got linked recently
Federico
CC BY-SA 4.0
2022-10-21T09:00:48.697Z
@Federico in case you're still wondering: https://news.ycombinator.com/item?id=33281682
Eduardo Ivanec
<the-moon><galaxy>
Is this photocomposition of Andromeda's galaxy and the Moon accurate?
Federico
# Is this photocomposition of Andromeda's galaxy and the Moon accurate?

(Asked by: username_0 on 2017-11-02)

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](https://i.sstatic.net/NIsYM.jpg)](https://i.sstatic.net/NIsYM.jpg)

One of the [comments](https://www.reddit.com/r/spaceporn/comments/1u0dxs/andromedas_actual_size_if_it_was_brighter/cedhp35/) provides a link to [another comparison](https://apod.nasa.gov/apod/ap130801.html) 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?

**Tags:** the-moon, galaxy
> **Comments:**
> **username_1**: 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](https://en.wikipedia.org/wiki/Sersic_profile), longer exposure times capture an increasingly larger part of the galaxies' light. That is, the longer you expose, the larger the galaxies look.  
> **username_2**: 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.  
> **username_0**: I wish I could figure out where this got linked recently  
> **username_3**: @Federico in case you're still wondering: https://news.ycombinator.com/item?id=33281682  

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## 1 answers

### Answer by username_4 on 2023-05-02. Score: 2

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](https://en.wikipedia.org/wiki/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](https://astronomy.stackexchange.com/questions/49370/how-could-i-see-andromeda-as-large-as-in-this-picture?noredirect=1&lq=1) 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](https://i.sstatic.net/4P1mp.png)](https://i.sstatic.net/4P1mp.png)


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