I know we can't see the center. But do we see any of the central bulge? Are we just seeing nearby stars in the Orion Spur where we are? In the Sagittarius Arm? In the other direction, are we seeing any of the arms further out from ours? Where are the dust clouds we see, in relation to these arms?

  • 1
    $\begingroup$ Errr, it depends where you are standing at what time of year and at what time. However, a typical optical extinction length in the galactic plane is around 3000 light years. So, not to the bulge. $\endgroup$
    – ProfRob
    Sep 7 '17 at 9:06
  • $\begingroup$ But we do see a bulge, of sorts. Is that just a local bulge in the Orion Spur or on the Sagittarius Arm that just happens to be between the Sun and the galactic center? $\endgroup$
    – Launce
    Sep 7 '17 at 23:46
  • $\begingroup$ @RobJeffries -- you can see parts of the bulge (Baade's Window being a particularly low-extinction sightline). $\endgroup$ Oct 31 '17 at 13:35

But do we see any of the central bulge?

With the naked eye and a dark sky, the galactic center is brighter than other areas of the galactic plane. Some of the light from that region does make it to earth to make that direction identifiable.

If you look at the ESO night sky composite from Brunier and Tapissier (which is approximately what someone could see with the naked eye given dark skies and good viewing), you can see that while the galactic center is brighter, other directions along the plane seem fairly similar. That makes it very difficult to say what fraction of scattered glow is coming from one arm or another. It all just blends together.

ESO0932 Milky Way Galaxy https://www.eso.org/public/news/eso0932/

If all we can see are stars in our galaxy that are relatively close to us, then the Milky Way should look relatively uniform, right?

Not necessarily. The visible part of the galaxy is relatively thin. It doesn't have a fixed distance, but a commonly quoted one is 1000ly. That means that even if light were magically cut off at 3000ly or so, it could appear non-uniform.


The Universe Today has a great video on just this topic:


EDIT to clarify:
If you look in the direction of the constellation Sagittarius, you're looking towards the center of the Milky Way Galaxy. You're correct that we can't see all the way to the center because of obscuring dust. Since we're looking out from inside one of the spiral arms, we can only see about 1500 light-years, which amounts to roughly 1.5 - 2% of the Galaxy's diameter. enter image description here

  • $\begingroup$ Link-only answers are frowned upon, in particular because the linked page may disappear at any time. While providing links is good, an answer should stand on its own. See meta.stackexchange.com/questions/8231/… for more. $\endgroup$
    – chirlu
    Sep 7 '17 at 1:25
  • 3
    $\begingroup$ I don't think the waitbutwhy.com graphic is accurate-- we can see some large stars that are thousands of light years away, though admittedly not many. And, of course, there's the Andromeda Galaxy (and the Magellanic Clouds). $\endgroup$
    – user21
    Sep 7 '17 at 2:54
  • $\begingroup$ After viewing both those resources, I guess I'm still confused. If all we can see are stars in our galaxy that are relatively close to us, then the Milky Way should look relatively uniform, right? It should make no difference whether you look toward or away from the galactic center. But it isn't uniform. Is the distribution of stars within our viewing range of 3000 light years (as noted above) uniform, or are there local anomalies? EDIT - In fact, the waitbutwhy graphic suggests, if anything, that we should see a bulge when we look AWAY from the center of the galaxy. $\endgroup$
    – Launce
    Sep 7 '17 at 23:53
  • $\begingroup$ I did read that through dust-free areas like "Baade's Window," with a telescope, you can view objects 25,000 light years away, in the galactic bulge. But that still doesn't explain why, even outside that window, galactic center looks a lot brighter and wider than the other direction. Seems like we must be seeing considerably farther than 1500-3000 light years. $\endgroup$
    – Launce
    Sep 8 '17 at 1:18

So, to sum up, the answer to my question is that on a clear night, in the right position, with the naked eye you can see the central bulge of the Milky Way galaxy, even though it's up to 26,000 light years from Earth. You can't identify the particular spiral arm that you are seeing, in any direction, as such. But you can locate, at least with a telescope if not the naked eye, objects that are in our Orion Spur, the Sagittarius Arm, and the Perseus Arm in the other direction.



Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.