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I was reading a good explanation of why we don't see the colors in the Milky Way with the naked eye, while even undoctored photos show some color. (It has to do with our colorblind rods.)

But it got me to wondering: Why, in even black-and-white photos, does the galaxy appear brighter on one side, as though there were a bright light shining on it from galactic south (or north, I don't know which is which)? Does the "shadowed" side have more dust? Is it an artifact of the photographic process? An optical illusion?

[Edit: Added the panorama. I suppose the effect is more pronounced towards galactic center. But it still seems like a non-random distribution. Of dust, if that's what it is.]

Photo From Mauna Kea, copyright as noted. Panoramic composite Credit:ESO/S. Brunier

long exposure from Mauna Kea

Panorama Composite

Launce
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    Can you be clearer about what asymmetry you are discussing. You seem aware that there is a Galactic centre and this produces an asymmetry with galactic longitude. But are you asking about an asymmetry with galactic latitude? – ProfRob Mar 08 '23 at 07:37
  • Yes, thank you. I didn't have the right terms. I am talking about galactic latitude: What produces the illusion that a vast light is shining on the galaxy from (in the photos) "below" the galactic plane? Without the flight of fancy, why is the distribution of light and dark areas, as seen from earth, asymmetric in a north-south direction? – Launce Mar 09 '23 at 13:43
  • I should've said "lit from below." But the question occurred to me when I was looking at photos that show the Milky Way stretching down the sky in a north-south direction. – Launce Mar 09 '23 at 13:47
  • Can you edit that into your post. – ProfRob Mar 09 '23 at 20:19

3 Answers3

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The density of stars in the Galactic disk is not uniform. Very roughly, it increases exponentially as you go towards the center (= decreases exponentially as you go out in radius away from the center). This means that when you look in a direction that passes through the inner disk (i.e., close to the Galactic center), there are more stars along your line of sight than if you looked in a direction that only passes through the outer disk.

This is mitigated somewhat by the fact that the density of dust also increases towards the center. So what you refer to as the "shadowed side" (i.e., the anti-center direction) actually has less dust. If there weren't any dust, the asymmetry would be even stronger.

There's an additional effect due to the presence of the bulge, which adds even more stars to your sightline if that sightline passes through the bulge (i.e., within a few degrees of the Galactic center). But the overall asymmetry you describe would still be present if the Milky Way had no bulge.

Peter Erwin
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Well, you wrote the answer in your question: light is more concentrated towards the galactic centre. Imagine the galaxy as a (huge!) pancake. In the middle of that pancake, cut a hole where you’ll put a golf-sized meatball. Obviously, the strawberry will stick out on top of the pancake—and if it were to be vertically centred with respect to it, it would also stick down through the plate, so let’s imagine it just floating around.

The galactic centre is like that meatball. And just like it versus the pancake, there are more stars in the central region than in the fluffier pancake. Because there are more stars, there is also more light.

Galactic dust (it is indeed the technical term, although its composition is not like that of the dust you sweep regularly from your floors) is more concentrated in the galactic disk, hence its more uniform spread along the band we see as the Milky Way. The Sun is about two thirds of the way from the centre to the edge of the disk, which explains how we have a “lit-from-one-side” view of our galaxy.

Pierre Paquette
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Yes, it is just to do with the distribution of dust.

As correctly pointed out by other answers, the radial distribution of stars around the Sun is quite asymmetric, since we are looking towards the Galactic centre in one direction and away from it in the other and there is a large radial gradient in stellar density moving out from the Galactic centre. That the Milky Way looks even close to uniform around the sky is because there is also more dust towards the Galactic centre than away, which mostly prevents us from seeing the vast number of stars in the central regions.

However, there are "holes" in the dust coverage that allow us to see to greater distances and hence to see far more stars. The most significant "hole" is known as Baade's window - its Galactic position is about 1 degree of longitude and -4 degrees latitude (in Galactic coordinates). The hole means that we can see many more stars, reaching even to the central bulge of the Galaxy, and the "hole" appears like a brightly lit cloud of (to the naked eye, or a panoramic photograph) unresolved stars.

It is this that gives the impression of uplighting from below (from Galatic south) near the Galactic centre.

ProfRob
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