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We're (basically) in the middle of an arm of our galaxy. That is to say, we're sitting in the middle of a dense disc of stars.

It would seem to me that. You should see:

the thick line of the milky way all around you, i.e. on that plane, in all four directions.

(Additionally - sure - in the particular direction of the galaxy center, you would additionally see the huge central bulge.)

However: this does not seem to be the case: when you look at the milky way from the vicinity of our solar system, you basically see it "in one direction".

What am I misunderstanding? How come the sky object "the milky way" is well-known as only a lump/strip in one direction, rather than, a lump/strip that goes right around us?

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Put it this way...

Quite simply does anyone have any photography of the anti-galactic point? (Near "Auriga" right?) Does it show any "milky way band" going through it?

If not why not? Looking outwards, we are still looking through ~30k lightyears of the dense disk we are sitting in.

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    $\begingroup$ We're 30K light years out from the center. The side looking away from the central bar is quite a bit dimmer. $\endgroup$
    – Wayfaring Stranger
    Oct 27, 2016 at 14:12
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    $\begingroup$ I would say it can be seen all around us. In the image I just linked, it does get less noticeable around the edges (i.e., away from the galactic core) but as Wayfaring Stranger points out, that's because there's significantly less stars/dust out that way, compared to the core. $\endgroup$
    – zephyr
    Oct 27, 2016 at 14:28
  • $\begingroup$ Here is another example with a more cylindrical projection. I think this shows it quite clearly (though distorted because of the tilt) $\endgroup$
    – Andy
    Oct 27, 2016 at 15:04
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    $\begingroup$ Say Andy - hmm, while that is a 360 photo, it only shows about 180? of the milkyway. The only way you could get all 360 of the milky way in one go, without the Earth blocking you, is quite far away from the Earth-Moon. You can only simulate - stitch together - a panorama of the "whole sky" (as if from a point near Earth, with no Earth there). $\endgroup$
    – Fattie
    Oct 27, 2016 at 15:59
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    $\begingroup$ Hi @Andy - actually I think the image you reference most perfectly and absolutely shows that is indeed the case. I guess it's a case of "more fool me for not realizing I can see the galaxy I am in." $\endgroup$
    – Fattie
    Oct 28, 2016 at 12:21

2 Answers 2

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I'll turn my comment into a full answer.

To put it simply, we actually do see the Milky Way all around us, even in the diametrically opposite direction from the galactic core. You can see this in the image below which is a full sky image I took from APOD.

enter image description here

If you look at the edges of the disk in that image, you're looking at what is actually the edge of our galaxy, opposite in direction from the core. In effect, this is precisely the image you requested because it does contain the part of the sky which contains the anti-core part of the galaxy. It is certainly not as bright, but there are still stars and dust out there. In fact, if you look very closely, you still see lots of dark splotches that mask distance stars and galaxies, indicating there is dust there.

I think the issue you may be having is that you expect many more stars to be within the outer regions of the disk than there actually are. The stellar density profile for our disk is roughly exponential, meaning there are literally exponentially more stars near the core than at the edges. If this means anything to you, the scale length for the exponential radial density profile is ~4 kpc.

To really get a good understanding of the stellar distribution, take a look at Jurić et al. (2008). They looked at (~48 million) stars from the SDSS and analyzed the stellar distribution across our galaxy (that is visible to us). You should find figures 10 through 18 of particular interest, however I'll present part of figure 16 here.

enter image description here

This image shows the (logarithmic) density of stars as a function radius from the galactic core. The varying shades of grey indicate varying heights above the galactic plane (numbered in parsecs). The dashed lines are various exponential decay models with differing scale heights. You can see that these star densities, even within the limited radial ranges covered by the SDSS fall off by an order of magnitude! Hopefully this helps you appreciate the significant difference between the core brightness/visibility and that of the galactic edge.

Here's a silver of Andromeda spectacularly demonstrating the falloff: even though galaxy photography tends to suggest to the casual eye an evenly-dense plate:

enter image description here

This synthetic image referenced by Andy, from the Tycho Catalog Skymap also shows the situation clearly.

enter image description here

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  • $\begingroup$ I had NO IDEA the dropoff was so strong. If you look at a random image of say Andromeda, there (a) a highly dense middle area, and then (b) the platter gives the impression of being about the same density all over. $\endgroup$
    – Fattie
    Oct 28, 2016 at 12:17
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    $\begingroup$ @JoeBlow Unfortunately, you can't really trust your eyes when it comes to things like this. Your eyes are logarithmic detectors whereas a camera is a linear detector, and your brain is really good at "tricking" you into seeing things that aren't real (e.g., this well known optical illusion). You really have to defer to the unbiased math. However, try looking at this sliver of Andromeda I extracted and observe the intensity fall off. That will reduce any optical illusion effects. $\endgroup$
    – zephyr
    Oct 28, 2016 at 12:56
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    $\begingroup$ Hey Zeph; understood (I'm not an astronomer but I do crap like image software). True, your "sliver" there is a knockout demonstration, heh, thanks for that...very wise. Thanks for that! $\endgroup$
    – Fattie
    Oct 28, 2016 at 15:29
  • $\begingroup$ i went ahead and Ticked this answer since it's so badass, even though it's early days. Cheers all! $\endgroup$
    – Fattie
    Oct 28, 2016 at 15:33
  • $\begingroup$ I don't know if this was what the OP was thinking at all, but one might imagine the visible milky way to be dominated by stars in our arm, with stars way off in the center "too far away to matter". If that were true, you really would expect it to be similarly bright all the way around. But it sounds like it's not true - all the stars towards the center accumulate to give us what we see. Don't know if this adds much, but I thought it might be worth noting. $\endgroup$
    – Mark Foskey
    Oct 7 at 18:04
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I suppose that we are so far from the other Arm that we can see only the small part of Milky Way, approximately 27,000 light-years from this center (http://solarstory.net/objects/milky-way#visual-idea) and our solar system is so small that we cannot observe all these amazing stars.

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  • $\begingroup$ its a great point that indeed we are ALSO looking all the way through to the other side. I just didn't think of that $\endgroup$
    – Fattie
    Feb 11, 2021 at 10:56

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