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Solar Storm Oct ‘03
29 Oct ‘03
Complex
486
Christie Ponder, Houston, Oct 29 ‘03
Class 16 : Other stars
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Beyond the solar system – other stars
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The distance to other stars.
Properties of other stars.
The Hertzsprung-Russell (HR) diagram.
I : The distance to other stars
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Recap… 1 AU = 150 million km.
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How far are the nearest stars?
Look at brightness…
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Brightness of something drops with the
square of the distance.
Brightest stars are about 1011 times fainter
that the Sun…
If same luminosity, this means that they are
about 300,000 times further away (i.e.
300,000 AU, or about 5 light years).
Stellar parallax
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Parallax:
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Stars appear to
wobble as the Earth
moves around Sun.
Can use this to
measure distance
to stars (since
Earth-Sun distance
known well).
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If star wobbles with amplitude
of 1 arc-second (1/3600th of a
degree), then it is at distance of
1 parsec (definition of parsec).
1 pc = 3.26 light years.
In general,
1
D ( pc) 
 wobble(arcsec )
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Find that nearest star is indeed
about 1.2 parsecs (250,000 AU;
4 light years) from us.
Method works out to 1000 pc.
II : Stellar properties
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Stars have various…
Luminosities
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Masses (~0.1 to 100 M)
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Feeble star = 0.0001  L.
Powerful star = 50,000  L.
Biggest stars usually the most luminous.
Colors
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Depends on surface temperature of star.
Cool star (RED) = 3000 K.
Hot star (BLUE) = 30,000 K.
“topaz & sapphire”
Beta Cygni (double star, also called Albireo)
Don’t bother copying…
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Stellar temperature/color also gives rise
to “Spectral Classes.”
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O (> 30,000 K).
B (10,000 – 30,000 K).
A (7,000 – 10,000 K).
F (6,000 – 7,000 K).
G (5,000 – 6,000 K) – the sun!
K (4,000 – 5,000 K).
M (< 4,000 K).
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Knowing luminosity and temperature, we can
calculate the radius using the “StephanBoltzmann law”:
L = 4R2T4
- or (L/L) = (R/R)2(T/T)4.
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Important points –
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 = 5.6710-8 W/m2/K4 is a number called the
Stephan-Boltzmann constant.
Luminosity increases very rapidly with temperature
(2 T gives 16 L) and radius (2 R gives 4 L).
II : The Hertzsprung-Russell
diagram
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Very important plot in astronomy.
Plot of luminosity against temperature
for stars
Find that stars fall into definite groups…
The Hertzsprung-Russell diagram