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Comment by ianbicking | Hacker News Reader

I'm having a hard time understanding this picture along with what I understand of the scale of space. The Andromeda Galaxy is big, and it has a lot of stars. Which is to say, it's REALLY big, and it merely has a bunch of stars, so the size of the stars should be very small compared to the empty space between the stars, even if you are looking through the galaxy. I would assume that if you get a better and better image, you'll be seeing the holes better just like you'll see the stars better. Kind of a fractal effect: when you look closer, you see the same kind of density but at different scales.

But if I zoom into this image it just feels like I'm seeing a mush of speckled light. What's up with that? How much of the image is an artifact of the telescope?

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A couple things to keep in mind:

Andromeda is so big that to see it, you're looking through a large portion of the Milky Way. Some of the really bright looking stars in this picture are actually in the Milky Way.

There is a ton of space between stars, but they're also not completely flat. It's less like a sheet of paper and more like a thick pancake.

So-called "empty" space isn't empty. Some of it is mostly just a vacuum, but lots of empty space is filled with gas which is glowing due to latent light/heat/radiation. In that regard, parts of this picture are glowing similarly to shining a flashlight in fog.

Even if you managed to zoom in close enough to an empty spot between stars in our galaxy, which also was an empty spot between the stars and gas in Andromeda, you would still start seeing the distant galaxies far beyond Andromeda. The Hubble Deep Field [1] is a good demonstration of this effect. They pointed the Hubble at a previously thought "empty" region of space and held it there for about 10 days. After gathering all the received photons collected over that time into a single image they realized that even "empty" space is full of a lot of stuff if you look far enough out.

[1] https://en.wikipedia.org/wiki/Hubble_Deep_Field

Not an astronomer, but I'll give it a shot.

If you do the zoom thing, at the highest level? Yes, it's a mush of speckled light -- looks a lot like a noisy or grainy RGB image.

But what I think you're seeing are the actual stars -- thousands or maybe ten thousands of them in a full screen at the highest zoom. They're all clumped together because you're looking cross-wise at the galaxy, through the disc. So there so many stars at the highest level of zoom that the image looks grainy.

As a somewhat astronomer, this explanation seems reasonable.

Usually you can't discern individual stars in another galaxy, like in the center of the Andromeda galaxy (But doing statistics is still possible). However, the Andromeda galaxy seems close enough that at the outer edge of it, you can isolate them to do astronomy on individual stars (like spectroscopy).

Edit: Also, you can find stars in almost all colors. The average of the colors depends on the age of the stars in the galaxy. If there are only a few new stars born, the galaxy will appear redder. If there are many new stars born, it appears bluer (see Hertzsprung-Russel diagram). The way the stars appear in different colors means that they have different ages and masses.

Noise in a telescope (except for high energy particles hitting your detector) is thermal noise. It will only appear if there's almost no light detected. So if it were noise, it should be the other way round: Noisy where there are little stars and no noise where there are a lot of stars. If you'd ask me, I'd say there's exactly zero visible noise in there.

As a former astronomer, this is correct. Even in extremely high resolution images each pixel contains many, many stars. This fact has actually been used to determine the distances to galaxies. The basic idea is that if the galaxy is nearby, then there will be fewer stars per pixel (say, only a few dozen), but if the galaxy is very far away, there will be many thousands of stars per pixel. As a result, if the galaxy is nearby, there will be a lot of variation in brightness from pixel to pixel because the Poisson fluctuations are proportionally much larger. If the galaxy is more distant, the brightness at each pixel will be more homogenous. This is called the surface brightness fluctuation technique.

There's a nice page about it here: https://ned.ipac.caltech.edu/level5/Jacoby/Jacoby9_1.html

those specks are not noise, they are stars! if you zoom in to the darkest top right corner, you'll see that there's in fact no noise in the image.

On that note, the stars all have a 'stellation' along what appear to be the same axes. What is this effect? On the surface of the Earth my assumption was that the stellation/twinkling is an atmospheric thing. In this case is it an effect of gases or some telescopic thing?

Actually if it were telescopic it could probably be analyzed out.

In telescope photos it's often caused by diffraction off the 'spider' that holds optical elements in the centre of the telescope's barrel.

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