APOD: Moon over Andromeda (2020 Sep 25)

[APOD Robot]

Moon over Andromeda

Explanation: The Great Spiral Galaxy in Andromeda (also known as M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (below and right).

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[Ann]

It's fascinating that the Andromeda galaxy is so large in the sky, and the Moon is so relatively small.

I remember seeing a wide-angle picture of Sagittarius. We could see all those familiar sights, the Lagoon Nebula, the Trifid Nebula, star clusters M6 (image by Rolando Ligustri, APOD October 17, 2014) and M7, and maybe we could see parts of Scorpius too, with the conspicuous "False Comet".

Anyway, there was a small deep orange speck among all the sights of Sagittarius, and I couldn't figure out what it was. It couldn't be Mars, because Mars isn't that red. Guess what it was? It was the eclipsed Moon. It blew me away how small it looked, when its light was so sharply dimmed.

Imagine that it looked something like the picture I posted here. The photo is by Alan Dyer, and I added the orange dot!

Ann

[Buzzer2]

It was a bit confusing trying to find M32. I think it is to the left of centre of the Andromeda galaxy

[Ann]

It was a bit confusing trying to find M32. I think it is to the left of centre of the Andromeda galaxy

Exactly. It is the fuzzy white "ball" directly below the Moon in this composite image.

Wikipedia wrote about M32:

The galaxy is a prototype of the relatively rare, compact elliptical (cE) galaxy class. Half the stars concentrate within an effective radius of only 100 parsecs. Densities in the central stellar cusp increase steeply, exceeding 3×10⁷ M⊙ pc⁻³ at the smallest radii resolved by HST, and the half-light radius of this central star cluster is around 6 parsec.
...
The structure and stellar content of M32 are difficult to explain by traditional galaxy formation models. Theoretical arguments and some simulations suggest a scenario in which the strong tidal field of M31 can transform a spiral galaxy or a lenticular galaxy into a compact elliptical. As a small disk galaxy falls into the central parts of M31, much of its outer layers will be stripped away.

So it seems likely that M32 is really the just bulge of what was once a considerably larger disk galaxy. The stars and gas in the original disk of M32 have been absorbed into the big bully of M31, the Andromeda galaxy.

Ann

[BDanielMayfield]

It's fascinating that the Andromeda galaxy is so large in the sky, and the Moon is so relatively small.

It's all about distance and timing.

[Ann]

It's fascinating that the Andromeda galaxy is so large in the sky, and the Moon is so relatively small.

It's all about distance and timing.

Well, it's a composite image, so timing is not that important.

Ann

[ToddT]

I know the caption explains that it is a composite image, but it might be good to explicitly point out that the juxtaposition that is shown never actually occurs. The Moon is never in the same part of the sky as Andromeda. M 31 has an ecliptic latitude around 30 degrees while the Moon's orbit is only tilted relative to the ecliptic plane by about 5 degrees so it never gets that far from the ecliptic.

[orin stepanek]

I think it quite amazing how dominant Andromeda is in our sky; and
enhanced brightening if it shows what a wonderful, beautiful galaxy
it is!

[starsurfer]

Adam Block is one of my favourite astrophotographers! I wish he had an OIII filter and imaged more planetary nebulae.

[De58te]

Re, it is a bit confusing finding M32. I think I know where the confusion stems. In this APOD mirror image from 2006. https://apod.nasa.gov/apod/ap061228.html (Or is today's the mirror image?) M32 is to the right of Andromeda, and M110 below. Perhaps in the reprint they decided to flip the picture but forgot to flip the description.

[neufer]

Re, it is a bit confusing finding M32. I think I know where the confusion stems. In this APOD mirror image from 2006. https://apod.nasa.gov/apod/ap061228.html (Or is today's the mirror image?) M32 is to the right of Andromeda, and M110 below. Perhaps in the reprint they decided to flip the picture but forgot to flip the description.

Parity is preserved...it is rotated (not a flipped/mirror image).

[scr33d]

This kinds of artificial photoshopped images would never be needed if every single astro photo comes with an angular scale. The reason they don't is to accommodate people's revulsion to number, and people's laziness from making their own comparison.

[triastro@oregontrail.net]

I am happy to see this juxtaposition of the Moon and Andromeda galaxy.

I used to use this example in my astronomy class to demonstrate how SMALL the universe was and how easy it was to comprehend the size of the universe.

Everyone is familiar with how big the sun or moon looked on the horizon at sunset or sunrise. I then asked them to imagine the Andromeda galaxy which was 8X larger -- how big it would look rising or setting, compared to the moon or sun. Then, I noted that the Andromeda galaxy was 2.5 million light years away, so far away, yet when setting or rising, it looks 8X bigger than the moon or the sun and thus looks like it is only a short distance away in our celestial back yard. Finally, I note that the edge of the observable universe was 13.8 billion light years away, only 5,550X the distance to the Andromeda galaxy -- very comprehensible!

For some in class, this worked to make the universe size comprehensible. Of course, for others, they preferred to keep the universe size incomprehensible.

Thanks for the image in APOD!

[BDanielMayfield]

It's fascinating that the Andromeda galaxy is so large in the sky, and the Moon is so relatively small.

It's all about distance and timing.

Well, it's a composite image, so timing is not that important.

Ann

Well, I knew that Andromeda isn't on the ecliptic and so the Moon never enters that constellation, so even before reading the explanation I knew this APOD had to be a composite. This is an astronomical forum after all, so I was using timing in the astronomical sense. On astronomical time scales, the Andromeda galaxy is approaching while the Moon is receding. But, in the deep past, our Moon would have looked big enough as viewed from Earth to have been "larger" than a slightly "smaller" Andromeda way back when.

[Ann]

It's all about distance and timing.

Well, it's a composite image, so timing is not that important.

Ann

Well, I knew that Andromeda isn't on the ecliptic and so the Moon never enters that constellation, so even before reading the explanation I knew this APOD had to be a composite. This is an astronomical forum after all, so I was using timing in the astronomical sense. On astronomical time scales, the Andromeda galaxy is approaching while the Moon is receding. But, in the deep past, our Moon would have big enough as viewed from Earth to have been "larger" than a slightly "smaller" Andromeda way back when.

Good point, Bruce!

But there were probably no humans around when the Moon looked bigger in the sky than Andromeda. Or actually - there were no humans around when the Moon looked bigger in the sky than Andromeda.

(And as for timing... well, good thing I didn't have to try to survive during the latest ice age. Or during the second half of the 14th century, when the first, terrible wave of plague hit Europe. And a good thing I wasn't alive during the 17th century here in Sweden - Sweden was almost constantly at war, we were suffering through "the little ice age", people had lousy houses and little food to protect themselves against the cold, and the penal law was worse than anything you find in the most barbaric countries today...)

Ann

[BDanielMayfield]

... actually - there were no humans around when the Moon looked bigger in the sky than Andromeda.

Certainly true.

[Cousin Ricky]

This kinds of artificial photoshopped images would never be needed if every single astro photo comes with an angular scale. The reason they don't is to accommodate people's revulsion to number, and people's laziness from making their own comparison.

It’s more than that. Our brains are just more wired to deal with pictures than with numbers, and there are simply too many matters in everyday life to deal with than to check every number and every fact on everything they encounter every day. This is why our brains take shortcuts. If people weren’t “lazy,” then they literally would not be able to function for the information overload.

[scr33d]

This kinds of artificial photoshopped images would never be needed if every single astro photo comes with an angular scale. The reason they don't is to accommodate people's revulsion to number, and people's laziness from making their own comparison.

It’s more than that. Our brains are just more wired to deal with pictures than with numbers, and there are simply too many matters in everyday life to deal with than to check every number and every fact on everything they encounter every day. This is why our brains take shortcuts. If people weren’t “lazy,” then they literally would not be able to function for the information overload.

"Our brains are just more wired to deal with pictures than with numbers"
You are not helping yourself by invoking hardwiring of your brain to explain "information overload".
A scale is simply a picture with number. Is it too much to ask that you assimilate two things into a single concept and use it to interpret an image?