Why don't we see "the milky way" in both directions?

  • 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?


    --


    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.


    We're 30K light years out from the center. The side looking away from the central bar is quite a bit dimmer.

    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.

    Here is another example with a more cylindrical projection. I think this shows it quite clearly (though distorted because of the tilt)

    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).

    Actually that's precisely what @zephyr's linked image is, right zephyr? That should be an answer don't you think?

    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?

    In this winter Milky Way image, Auriga is near the center, and north is to the left.

    The Milky Way is seen as an almost continuous band all around the sky, and can be enjoyed at all times of the year from both hemispheres. In particular, it is easily visible just north of Orion. It is more prominent roughly in the direction of the GC, as you would expect.

    @JoeBlow have a look at the Magnitude "5.0" image on this page: Tycho Catalog Skymap. (Unfortunately I can't link the image directly in an answer; maybe it's too big.) It gives an idea of the overall band, though it's not in Galactic Projection again. Auriga is close to the darker bit just to the left of the "peak".

    HI @RobJeffries - I see; well if that's the case, it's settled! Looking at dark skies from all over the globe, I've never seen the "outwards" part: clearly I'm just plain wrong. Thx

    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."

  • zephyr

    zephyr Correct answer

    5 years ago

    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


    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.

    @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.

    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!

    i went ahead and Ticked this answer since it's so badass, even though it's early days. Cheers all!

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Content dated before 7/24/2021 11:53 AM