NOTES: Star Characteristics:

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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/4pd2
Power/unit area of sphere. From d, the distance,
we get L, the luminosity (watts of source).
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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).
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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.)
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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 4pR2. f L/(4pR2) ? L
f(4pR2).
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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.
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We can see about 1,000 stars in Northern
Hemisphere with the naked eye.
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Again Flux (new 'apparent brightness')
f(watts/m2) L/4pR2 Power/unit area of
sphere. From R, the distance, we get L, the
luminosity (watts of source). L 4pR2f
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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 Wiens Law.
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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.
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We can use this flux, b, to find the radius of a
star R from f(surface) L/4pR2.
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b is flux f brightness
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At this point you may be really CONFUSED!?!
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Arent you glad I dont require you to learn all
that math?