Transcript Lecture 24 - Empyrean Quest Publishers

NOTES: Star Characteristics:
How far (d in parsecs)?
Distance to nearby star determined from stellar parallax, p,
which is ½ the maximum angular difference in position:
d (in parsecs) = 1/p (p in arc seconds)
1 parsec is the distance at which the parallax of a star is 1 arcsec.
Parallax method works for stars closer than about 100 parsecs.
(1 parsec = 3.26 LY.)
How bright (L in watts)? Luminosity at the source is determined
from apparent brightness and distance (d).
Apparent magnitude (old way). We can see about 1,000 stars in
Northern Hemisphere with naked eye. Hipparchus rated them from
1 to 6. A '1' is 2.52 x brighter than a '2', etc. Range in brightness
from the sun at '-26' magnitude to the faintest objects seen at about
'26' magnitude.
Flux (new 'apparent brightness'):
b (watts/m2) = L/4πd2 = Power/unit area of sphere.
From d, the distance, we get L, the luminosity (watts of source).
How far (d in parsecs)?
Distance to nearby star determined from stellar parallax, p,
which is ½ the maximum angular difference in position (seen
6 months later).
distance (in parsecs) = 1/p (p in arc seconds)
1 parsec is the distance at which the parallax of a star
is 1 arc second. Parallax method works for stars closer
than about 100 parsecs = 326 LY. (1 parsec = 3.26 LY.)
How bright (L in watts)? Luminosity at the source is
determined from apparent brightness (flux, f) and distance (R).
For the math oriented:
f = L/A, A = area
of surface of sphere
or A = 4πR2.
f = L/(4πR2) 
L = f(4πR2).
Apparent magnitude (old apparent brightness).
Hipparchus rated stars he could see from 1 to 6.
A '1' is 2.52 x brighter than a '2', etc.
We can see about 1,000 stars
in Northern Hemisphere with
the ‘naked eye’.
Again: Flux (new 'apparent brightness'):
f(watts/m2) = L/4πR2 = Power/unit area of sphere.
From R, the distance, we get L, the luminosity (watts of source).
L = 4πR2f
How Big (radius of star in meters)?
We get the temperature, T, of the photosphere of a star
From the peak wavelength of the black body spectrum.
This we called ‘Wien’s Law’.
Ludwig Boltzmann
We then can plug T into:
The Stefan-Boltzmann Law which gives the surface flux
from surface temperature, T.
f(surface) = constant x T4 for a black body.
We can use this flux, b, to find
the radius of a star R from:
f(surface) = L/4πR2.
b is flux = f
= brightness
At this point you may be really CONFUSED!$%*?!
Aren’t you glad I don’t require
you to learn all that math?