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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 '17 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. $\endgroup$ – Wayfaring Stranger Nov 2 '17 at 14:53
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    $\begingroup$ @WayfaringStranger I thought it was clear I know that's not a real image? I'm asking if it is accurate. $\endgroup$ – Federico Nov 2 '17 at 15:25
  • $\begingroup$ @Federico Camera-wise, yes. Eyeball-wise, no. $\endgroup$ – Wayfaring Stranger Nov 2 '17 at 21:28
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    $\begingroup$ Accurate in terms of relative size - close enough I guess. Accurate in terms of relative brightness - not even close. It's a composite. $\endgroup$ – Florin Andrei Nov 3 '17 at 18:04
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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 '17 at 11:27
  • $\begingroup$ I never took a ruler to my screen to check, but if you look on Google images for 'size of Andromeda and moon', there are several examples of more accurate comparisons. $\endgroup$ – user10106 Nov 2 '17 at 11:42
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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 '17 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. $\endgroup$ – John Duffield Nov 4 '17 at 15:33

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