Amount of light reaching earth from MW and others?
Posted 03 January 2013 - 10:58 PM
Stars within Milky Way: 80%, avg. distance 15,000 ly.
Light from other galaxies: 20%, avg. distance ?
Some explanation of how the figures are derived, or further breakdown (e.g. in different directions) would be interesting.
Posted 03 January 2013 - 11:41 PM
All stars brighter than 6.5m taken together shine at -5m.
The glow of the milky way is probably about -7m (the bulk of the light coming from the Sagittarius region.) and so the milky way's light is some 6X brighter than that from the stars resolved by the unaided eye.
The light from external galaxies is rather less than either of the above, probably around -1m or so, with the bulk coming from just the brightest handful--the Magellanic clouds and M31/33. The Milky Way, in spite of the extensive mid-plane obscuration, bathes us with some 6 magnitudes, or 250 times more light than that from the external galaxies.
Incidentally, from even the darkest site, night sky airglow is fully -7m.
Posted 04 January 2013 - 02:42 PM
In my experience the biggest factors at what would be considered a class 1 site are the galactic core, its stellar plane followed by the zodiacal light and the zodiacal band.
Airglow can add to the party considerably, I've seen it at over 30 degrees above the horizon in the Kalahari on a regular winter night
Posted 04 January 2013 - 04:08 PM
But both of these are generated by our sun, and so beside the original question about the relative amounts of light coming from *outside* our solar system.
Glenn, do you know if your breakdown of Milky Way glow v. resolved stars separates the resolved stars in the plane of the MW out of the background glow? It seems to me that a lot, if not the preponderance, of light from resolved stars would also be in that plane.
If most of the light coming from outside the MW is from the closest few galaxies, we don't have a lot of "old" light reaching Earth, do we? That is, most light is from MW sources, and then most of the rest comes from less than 3 million ly away (that being quite young light in my thinking).
Posted 05 January 2013 - 02:05 AM
Yes, the milky way glow is separate from the light of the naked-eye stars superimposed upon it. If my surmise that the milky glow is about 2 magnitudes brighter than all stars brighter than 6.5m, then said starlight is only 1/6 that from the milky way. The approx. 1/4 of those stars which are superimposed upon the milky way would then amount to only 1/24, or about 4% the milky way's light.
Posted 05 January 2013 - 04:15 AM
Integrated absolute visual magnitude: -20.9
Integrated apparent visual magnitude: -6.6
Average surface brightness: ~22.7 magnitudes per square arc second
Highest surface brightness: > 20.5 magnitudes per square arc second
Posted 05 January 2013 - 07:13 AM
I note that Brian Skiff says in the article above that the "main contribution" to night sky brightness is the zodiacal light, followed by airglow, but the wikipedia article on sky brightness swaps these two.
I have wondered about that. The classic study on this subject is a 1973 monograph entitled The Light of the Night Sky, by Gordon and Roach. It ranks the zodiacal light as #1, followed by airglow. Modern studies reverse those.
In any case, these sources are certainly brighter than the Milky Way, which is responsible for almost all the rest of the light. And airglow and zodiacal light have very different characteristics.
Unlike all other sources of natural "light pollution," airglow increases toward the horizon. It varies on every time scale for reasons that are poorly understood. But it varies relatively little across the dome of the sky.
The zodiacal light is concentrated in the zodiacal band, especially near the Sun -- where it does little harm to stargazer. There is a secondary concentration (the gegenschein) opposite the Sun. But Gordon and Roach point out that it's surprisingly strong everywhere, even at the ecliptic poles.
The zodiacal light is a broadband source, whereas airglow is mostly emission lines. So which one is stronger is bound to depend on the wavelength you're measuring.
Posted 05 January 2013 - 05:57 PM
As I understand it, the integrated absolute magnitude of our Galaxy (-20.9) is not really applicable in this instance because it includes all visible light as seen under conditions of minimal obscuration, i.e., face on. From our vantage point within the dust laden disk, a considerable fraction of that light is attenuated.
Posted 08 January 2013 - 09:32 PM
I didn't mean to imply that it was applicable. I just thought I'd throw those factoids in as general information.
Posted 11 January 2013 - 12:17 AM
Posted 11 January 2013 - 07:28 PM
Posted 12 January 2013 - 08:31 PM
Thanks Glenn. Since this is not apparent to me eyeballing it in a planetarium program, I'll have to break down and do some work using measured distances.
Among the naked eye stars, there is a general concentration of their light toward the galactic plane, which continues to rather fainter magnitudes, too.