Transcript Lesson 9: H-R Diagram and Star Classification

2.9 x103
max ( meters ) 
T ( Kelvin)
So we can determine the
distances and characteristics
of Stars
We need a better
classification scheme
Where 4πR2 =
area of a sphere
Supergiants
Luminosity (Lsun)
106
104
102
Giants
1
102
104
40,000
White Dwarfs
20,000
10,000
Temperature (K)
5,000
2,500
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The Mass of a star is very significant in terms of
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Evolution, luminosity, lifespan, etc.
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PROBLEM: there is not a good way to
determine the mass of an isolated star.
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SOLUTION: Fortunately, about 70% of the stars
visible to Earth are part of Star Systems of 2 or
more stars that revolve about a center of mass.
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This is the Binary Star Kruger 60 (in the constellation
Cepheus)
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Period of Revolution = 44.5 years
Maximum angular separation = 2.5 arcsec.
SO WHAT????
The so what is that each star obeys Kepler’s Laws of Orbital
Motion!
Kepler's 3rd Law
for objects orbiting
each other
A3
M1  M 2  2
T
The Orbit of 70 Ophiuchi
Recall Equilibrium – the more massive object sits
nearer the COM. In Binary Star Systems we see the
same effect
Recall Equilibrium – the more massive object sits nearer the
COM. In Binary Star Systems we see the same effect. In this
picture which star is more massive?
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Using visual binaries,
we can plot a graph of
luminosity vs. mass
for Main Sequence
stars.
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Check out the trend
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Lets revisit the HR
Diagram (next slide)
L  M4
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This trend implies
that the Main
Sequence is a
progression of...
Luminosity
Temperature
Mass
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Sometimes the stars are too far away to
visually distinguish them, but...
We can use spectra
A
B
B
B
A
A
A
B
PLOT FOR HD 171978
The Binary System is
receding from Earth at
12km/sec (as indicated
by dotted line)
Spectroscopic Binary K Arietis. As with visual binaries, the masses
can be determined and plotted on the HR Diagram.
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Consider the following Binary Star system as viewed
from Earth.
We can determine some info, such as
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Ratio of surface temps
Relative sizes of both the stars and the orbits
Stellar atmospheres, etc
4
3
2
1
Sample problem: F2 M02 exam
Sample problem: F2 M02 exam
Circular or elliptical orbits drawn around the centre of mass.
Star spectra shifts towards blue when moving towards Earth
and towards red when moving away. As one star is moving
towards Earth while the other moves away, a red shift in a
binary system is always accompanied by a blue shift. No
shift occurs when stars are moving perpendicular to Earth.
20 days
Use simplified red-shift equation E6.3
We will cover this later
Mass of star / system