APOD: Moon Over Andromeda (2013 Aug 01)

[Beyond]

Maybe I should be careful what I wish for.

As Mr. Spock once said... "The wanting, is not like the having".

[rstevenson]

I'd often wondered what it would be like to live on a planet where at least a couple big, bright galaxies were not only naked-eye visible in the sky at night, but easily and spectacularly so. And now I learn we have one, but it has "low surface brightness". How can an immense galaxy have low surface brightness ? What is a galactic "surface", the dust clouds surrounding it ? A dark matter/energy cocoon messing with the light? Maybe I need to get out in Death Valley on a cool Fall night and take another gander, but my wish has always been to be clobbered over the head by a nearby galaxy sparkling hugely and brightly in our sky, though I suppose that means we would already be dangerously interacting, gravitationally .

Well, even if our eyes could collect light a lot more efficiently than they do, Andromeda wouldn't be all that spectacular since it's only about 4 times the width of the Moon.

But in a very real sense we are far, far closer to a very spectacular galaxy -- our own Milky Way. How well can we see much of that in the night sky? That's what is meant by low surface brightness. Each star is a single point of light, and the vast majority of them are too faint to see well, whether or not they are near, as in the Milky Way, or far, as in Andromeda. So even though Andromeda has roughly a half a trillion stars glaring at us, they are each very faint so that even under the best conditions it is not much more than a fuzzy patch in the sky. Binoculars give a better view, though the limiting factor is still our eyes. The best way to see galaxies, ours or any other, is by taking a lot of exposures through a telescope -- or much easier, by visiting the APOD and the Asterisk regularly.

Rob

[Ann]

If a nearby galaxy was sufficiently bright to look truly conspicuous in our skies, it would have to be packed with (moderately bright) stars much closer together than the average distance between stars in our own galaxy. Not only that, but even in the bulge of that galaxy, where stars in most galaxies are usually rather close together, the average distance would have to be shrunk considerably. Personally I believe that this kind of proximity might cause the production of extra dust, which would reduce the optical brightness of the galaxy.

Bear in mind that it wouldn't be enough to just pack the stars of a galaxy closer together, because that way you would just get a high surface brightness but very small galaxy. (And just possibly, you'd get a galaxy-sized black hole!) You have to increase the number of stars in this galaxy very considerably and then to pack them close together. But this stellar concentration would still have to be extended, so that you got an "extended but packed" "body" of stars. Obviously you'd need loads of dark matter to achieve this kind of galactic profile, if it is possible at all.

I believe that the gravitational effects of such a galaxy would be weird and very strong, and I wouldn't want to be so near it that it would look obvious in my skies when I looked up!

Ann