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Apr 19, 2013
IESO
The Sun is a Star
Part 1
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The story thus far
Masses, RadiiBinary stars (visual, spectroscopic,
eclipsing)
Temperatures, composition,
surface gravity, Luminosity class
Spectrum
Temperatures, TColour Index (B-V) (+ black body curve)
Luminosities, L (absolute
magitudes)
Flux, F, at Earth, apparent magnitude
(with d + inverse square law)
Distances, dParallax, moving clusterCalculated propertyObserved quantity
But what about ages, how stars are born, how they shine,
how they die?
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HR Diagram - introduction
Make a plot of heightvs weight for
students in class
Supermodels
New-born
babies
B-ball players
Sumo
wrestlers
Adults
Children
Mass
Height
Height increasingHeight increasing
upward - massupward - massincreasing toincreasing to
leftleft
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Hertzsprung-Russell Diagram
A Stellar Demographic Diagram
By turn century, astronomers were aware of a spectral sequence: OBAFGKM O stars are hot, luminous, most massive; M are stars cool, faint, least massive
Originally, it was proposed that the spectral sequence was also an evolutionary
sequence: start as hot O stars, use fuel, lose mass, cool to die as a dim M star
1905 - amateur astronomer, Hertzsprung, found a correlation between spectral type
and absolute magnitude - but stars G and later showed a range in MVfor same
spectral type - brighter stars called giants.
1913 - established US astronomer, Henry Norris Russell, found same result.
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Hertzsprung-Russell Diagram
Henry Norris Russellsfirst diagram: MV vs
spectral type.
None available
in sample
Old type
Spectral Type
O B A F G K M
N
-4
0
+4
+8
+12
MV
Note: T increases to leftand bright stars at the top.
TBright
Stars
Band upper left to lower
right is called the MainSequence. It contains 80-
90% of all stars.
White dwarfs at lower left.
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Hertzsprung-Russell Diagram
L = 4R2Teff4
R1/R2 = (L1/L2)1/2 x (T1/T2)-2If 2 stars have same spectral type
(Teff), brighter star is bigger.
Constant radius line has slope 4.
R must increase diagonally to upper
right in HR Diagram
Modern observers HR diagram
Modern theorists HR diagram
Supergiants few 100 - 1,000Rsun(e.g. Betelgeuse)
Main Sequence stars 0.1-10Rsun(e.g. Sun)
White Dwarfs 0.01Rsun
(e.g. Sirius B)
Giants 10 - 100 Rsun(e.g. Aldebaran)
R
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Mass-Luminosity Relation
Masses were obtained from observations of manybinaries.
Luminosities were obtained from parallax
measurements.
These are all main sequence stars
Lower mass stars on main sequence
Have cooler spectral types -
Main Sequence is a mass sequence
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Hertzsprung Russell Diagram
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Sun
Bias in HR Diagrams
= K & M dwarfs
= F & G dwarfs
= Giants
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Sun
Bias in HR Diagrams
= K & M dwarfs
= F & G dwarfs
= Giants
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Sun
Bias in HR Diagrams
= K & M dwarfs
= F & G dwarfs
= Giants
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Hertzsprung-Russell Diagram
This is a proper sample of faint
stars. Note that none of the rarer
giant stars are within this volumeof space.
Beware of the selection effects in
the HR Diagram
HR diagram of
the stars nearestto the Sun.
Luminosity Function of Stars
Near to Sun
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Hertzsprung-Russell Diagram
This is a proper sample of faint
stars. Note that none of the rarer
giant stars are within this volumeof space
The number of giant and
supergiant stars is small but they
can be seen over vast distances.
Beware of the selection effects in
the HR Diagram
HR diagram of
the stars nearestto the Sun.
HR diagram of
the brightest stars
in the night sky.
