Expert: Tom Whiting - 3/10/2004

Question
Hi Tom,
 In pictures we see of galaxies throught telescopes or Hubble, they are of course these awe-inspiring sights.  I saw a pic of the Sombrero galaxy on the Hubble sight and well, it was really majestic.  My q= on a dark night in the desert, I've seen our own Milky Way "band" and it's impressive but certainly nowhere as blindingly bright as the pics we see of other spirals.  Obviously theres the aspect of time exposure in photos.  Or is it a question of some "dark matter/dust" in between us and the edge on view of our own galaxy that prevents us from seeing the full illumination?  Another view, if we were magically transported to a view right above (not in the plane) a typical spiral looking downward, would it be some incredible sight like the pics we see?

Tim

Answer
Tim,
Interesting questions!  Actually, had you stayed out at
night in the desert long enough to become fully dark

adapted, (a full 30 minutes in total darkness with zero
moon out), and then tried to use your night vision to find a
faint fuzzy, say the double open cluster in Perseus, you would find our Milky Way very annoyingly bright!  Enough to affect your very deep night vision which, you can tell,
under those conditions, is being affected.

Actually, its all of the above, and one more you didn't
think of, which is the biggie!
Yes, there is that obvious dust lane (the Cygnus rift)
up thru the middle of our view of the Milky Way which blocks some of the starlight....very minor.
Yes, the longer the time exposure, naturally the more light
one gathers on the film. Remember though, the real naked
eye view of M-104 (Sombrero Galaxy) is well below naked-eye
at 8th magnitude.  But check out an equivalent time exposure
very wide angle photo of the Milky Way; its quite bright!
But the real WINNER to your question is:  SB  "Surface Brightness"!

The Milky Way is so spread out on our sky, that its
enormous amount of light covers many degrees of sky, thus
any small region that you observe, has a very small surface
brightness.
Whereas, say with the Sombrero Galaxy, we see
the entire light of a whole galaxy (200 billion stars) compressed into an area less than 1 minute of arc in diameter....or 1/60th of one degree!  Very high surface
brightness, and thus a very easy object in a scope.
 
In fact, with all DSO's, (Deep Sky Objects) it's the surface brightness that either makes them easy naked-eye thru a
telescope, or difficult.  Take M-33, the Pinwheel galaxy,
a face-on spiral galaxy; its total brightness makes it just barely a naked-eye fuzzy spot in very dark skies, but thru a scope, its a very difficult dim object, compared to the Sombrero, because its light is spread over almost 1/2 degree of sky.
Same is true for the Helix Nebula in Aquarius (NGC 7293);
again, its light covers nearly 1/4 of a degree, and yet its
listed in most star atlases as 6th magnitude...barely naked
eye planetary nebula...the closest at only 450 ly distant.
But it is very difficult and dim in a telescope because of its very low surface brightness.

Question 2; incredible?  Probably, area wise...but
brightness?  No.  Remember, as you get closer, again the
object will spread out in area, and its light remains the same or even drops, due to decreasing surface brightness.

Even viewing galaxies face on from the Earth as described
above, they appear much dimmer (naked-eye thru a scope, not a time exposure which the eye cannot do) than ones that are edge-on, or tilted by up to about 45 degrees to our line of sight. Even though the two comparison galaxies are the same size and same distance.  So no, most objects would appear
no different than how we see them from the Earth because
as you approach them, the increased brightness is almost
exactly offset by the spreading out of all that light, as the area increases, decreasing the surface brightness....and thus, on very colorful objects, say the Great Nebula in Orion, you would actually *lose* some color, because it takes lots of light to see color....spread the light out, and you actually lose on color.  Sorry to 'burst your bubble', but I have to be honest with you.  But now, at least you know why.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA