When we see the Milky Way on a dark night are we seeing the bulk of the galaxy, or just our local arm? How much of the milky way is visible to the naked eye on a dark night?
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$\begingroup$ Similar to this question: astronomy.stackexchange.com/questions/6005/… - maybe considered close enough to be a duplicate? $\endgroup$– joseph.hainlineMar 25, 2015 at 2:21
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3 Answers
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At any one time, an average observer can see about 2,500 stars in a clear dark sky. Note that eyesight varies and sharp-eyed individuals may be able to see a half-magnitude dimmer stars than the average eye (apparent magnitude is a scale in which each integer is $2.51$ ($100^{0.2}$) times brighter or dimmer than the next consecutive integer.) A very dark sky may enable magnitude $+7.5$ or even $+8$ stars to be seen, but in a typical "dark" non-urban sky the limit is often $+5.5$ to $+6.5$.
Supernovae can potentially be seen as far as 13 billion light-years (ly) away, essentially from the 'edge' of our observable universe. So, it's not saying much to say that a supernova might be seen across a distance of 100,000 ly.
Some types of supernovae can be over $-22$ in absolute magnitude — where absolute magnitude is defined as their apparent magnitude if they were observed from a 32.6 ly distance (to be honest, I'm not sure if supernovae absolute brightness is defined in exactly this same way). By way of comparison, the Sun's abs. mag. is $+4.8$ (lower numbers indicate brighter stars).
Because of the dust and interstellar medium, the possible distance we can see is just a small fraction of the size of the Milky Way. In reality, few stars are bright enough to be seen over 400 ly away. Deneb which has an estimated distance of 2,600 ly (but may be as close as 1,550 ly, the large uncertainty is due to its variation in brightness). Only 6 visible stars are thought to be farther from us than 1,000 ly.
So, to sum up, looking inwards into the Milky Way, our visibility is very restricted to the nearest 1,000–2,000 ly (while the MW disk's radius is 50,000 to 90,000 ly and we're about 27,000 ly on this side of center, but only the brightest stars are visible from more than about 400 ly away.
While looking away, we can see the Andromeda Galaxy (but not its individual stars) which is 2,500,000 ly from us. In other words, most people blame dust for the poor visibility, but that's mostly only relevant for telescopes. For us, our limited eye-sight is the real barrier (not to mention the dearth of really dark skies).
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1$\begingroup$ Some of the facts are incorrect. The median naked eye stars is at a distance of 400 light years. About 10% of naked eye stars are further than 1000 light years. See physics.stackexchange.com/questions/164720/… Supernovae cannot be "seen" at distances of 13 billion light years. Most supernovae in our Galaxy could not be "seen". $\endgroup$– ProfRobAug 21, 2016 at 20:50
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An average eye can see about 4000 objects in the night sky. The brightest are planets from the Solar system, and almost all others are stars from our galaxy, the Milky Way. Some other galaxies may be spotted, but very few.
Even accounting for just half the sky, that is a very very tiny proportion of our galaxy, estimated to contain between 100 and 400 billion stars, around .000003%! The galaxy itself being an average one among more than 100 billion galaxies in the observable universe...
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$\begingroup$ When we see that small percentage, are those all stars local to our area? We don't see a general smear of stars from the core of the galaxy, for example? $\endgroup$ Mar 25, 2015 at 2:00
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$\begingroup$ Most stars are probably close by, but we see the giants and super-giants across the galaxy, and of course the super-novae. From time to time, a super-nova from another galaxy can be seen with the naked-eye, as happened in 1885 (Andromeda), and more recently in 1940 (NGC 253), 1972 (NGC 5253), 1993 (M81) and 2004 (NGC 2403). $\endgroup$– chqrlieMar 25, 2015 at 2:10
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$\begingroup$ @joseph.hainline We can't see the core of the galaxy because there is a dust cloud blocking the view. $\endgroup$– LDC3Mar 25, 2015 at 2:42
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1$\begingroup$ Sorry, missed a critical number out - the median distance of naked eye resolved stars is about 400 light years. $\endgroup$– ProfRobMar 25, 2015 at 12:02
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1$\begingroup$ I suppose what I mean is your eye probes a much bigger distance actually. Dust basically cuts the Milky Way off at about 2kpc (well beyond where you see individual stars), so you can "see" about 6% of the MW interior to the Sun's orbit. $\endgroup$– ProfRobMar 25, 2015 at 23:01
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I'm not sure exactly how to answer this, but certainly most of what you see is local. In the southern hemisphere we see the "milky Way" as a kind of lighter smudge across the sky: http://s.ngm.com/2010/12/milky-way/img/milky-way-galaxy-615.jpg, but that's probably still only a small fraction of the Milky-way, probably mostly the closest arm. The farthest star we can see easily from the eye (some people might be able to see further ones), but that's Deneb, about 1,550 light years away, about 2% the distance of the other side of the Milky Way.
http://www.armaghplanet.com/blog/how-far-away-is-the-farthest-star.html
If you know where to look, you can see Andromeda, but that's cause it's a mostly unobstructed view. http://www.public.asu.edu/~rjansen/localgroup/NVWS99B_03.gif
We could likely see a milky way super-nova much more distant than Deneb and the Milky Way should have about 1 supernova every 50-100 years or so. The last milky-way supernova that was seen was the Kepler supernova in 1604, about 21,000 light years away and at the time it was brighter than all the other stars in the sky briefly. http://phys.org/news/2012-09-kepler-supernovae.html
but however you calculate it, we only see a small percentage of the Milky Way.
