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From: jay@npois.UUCP (Anton Winteroak)
Newsgroups: net.astro
Subject: re how many photons
Message-ID: <469@npois.UUCP>
Date: Tue, 3-Dec-85 15:42:02 EST
Article-I.D.: npois.469
Posted: Tue Dec  3 15:42:02 1985
Date-Received: Thu, 5-Dec-85 05:35:31 EST
Organization: ATTIS, Neptune, NJ
Lines: 37


	A few weeks ago, someone asked how many photons per square meter
per second did we receive for objects of various stellar magnitudes. I have
computed that from the following facts:
	1. The sun is at apparent magnitude -26 to us.
	2. The sun is 1.5 e11 meters away from us.
	3. The sun gives off 4.0 e33 ergs per second.
	4. The average energy of a photon from the sun is 2.1 eV.
	  4a. Therefore there are 2.9 e11 photons per erg.
	5. I counted photons of all wavelengths (including ir, uv, etc.)
	  5a. For visable light, divide by about two.

Apparent Mag.  Photons/meter square per sec    comments.

	-26	4e21				this is what we get from the
						sun.
	-25	2e21	
	-20	2e19
	-15	2e17				as bright as a half moon.
	-10	2e15				Comet Kohoutek was supposed
						to get this bright.
	-5	2e13				Venus gets this bright now and 
						then. Halley's Comet was this
						bright in 867.
	0	2e11				Several Stars look this bright.
	5	2,000,000,000                   dimmest naked eye stars.
	10	20,000,000			dim stars in big binoculars.
	15	200,000				the planet Pluto, on a good      						night.
	20	2,000				visable on photographs and
						image intensifiers only.
	25	20				near the limit of what can
						be photographed from through   
						our atmosphere.

remember to divide by two to get visable photons, and by two again for
atmospheric absorbtion.