MadSci Network: Astronomy

Re: Why can't stars be seen during the day?

Date: Thu Nov 26 07:10:43 1998
Posted By: Michael Martin-Smith, Other (pls. specify below), Family Physician, Fellow,BIS, amateur astronomer( BAA), British Interplanetary Society
Area of science: Astronomy
ID: 907114094.As

Stars subtend a very small angle in the sky, so that, although they are intrinsically brighter than the Moon, the effective radiating surface area in our sky is much less - the Moon being billions of times nearer! The Moon occupies a half degree of area, and thus has a total magnitude of -12.7. This is much greater than any star!

Red Giant stars are cooler by far than white dwarfs, and, area for area, radiate far less energy - but being grossly distended (hundreds of millions of miles diameter as compared with a few thousand for a white dwarf, appear brighter by far. Thus, although their brightness per unit area as seen from Earth is lower than for White Dwarfs, their total luminosity is much greater because their total surface area is so much greater, emitting more photons, in absolute numbers, towards the Earth. In astronomical parlance, we say that a Red Giant has a much greater apparent brightness than a White Dwarf, but a much lower intrinsic surface brightness. Or, put another way, energy emission /unit area is higher in White Dwarfs >> Red Giants, but total energy recieved is greater due to greater surface area of Red giants.

The same applies, a fortiori, to the Moon relative to any star- except the Sun!!

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