Unformatted text preview: analogy with the denition of radians, the unit of solid angle, the steradian,
measures the area on the unit sphere that the object covers. Since a sphere
has area 4r there are 4 steradians in the whole sky.
The corresponding number of square degrees is
2 360 = 41252:96 40000
4 2 since is the \radius of a unit circle in degrees."
The Sun and Moon are both very close to 0:5 diameter, so their areas are
each (0:5) =4 = 0:20 square degrees, or 1=200000 of the celestial sphere.
(If you think hard, you will see why this tells you that full moonlight is about
10 times dimmer than full sunlight!)
2 2 5 2.2 Brightness measurements
2.2.1 Flux and UBV system
Flux is the energy arriving from a particular object (star, galaxy,...) per unit
area of detector per unit time, so it has units erg cm 2s 1 . 1 cm 2 distance D You can see that if the source emits isotropically (same in all directions),
the relation between ux F and total luminosity L (erg s ) is
F = 4D :
1 2 14 ...
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- Fall '09
- luminosity, Solid angle, square degrees