The Hertzsprung Russell Diagram

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The Hertzsprung Russell Diagram

• Dr Bryce

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Class Notices

• Homework due today
• Remember that there is homework due next Friday
  as well!

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Measuring stellar masses
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The orbit of a binary star system depends on
strength of gravity
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Types of Binary Star Systems

• Visual Binary
• Eclipsing Binary
• Spectroscopic Binary
• About half of all stars are in binary systems

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Visual Binary
We can directly observe the orbital motions of
these stars
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Eclipsing Binary
We can measure periodic eclipses
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Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
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We measure mass using gravity Direct mass
measurements are possible only for stars in
binary star systems p period a
average separation
4p2 G (M1 M2)
p2 a3
Isaac Newton
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Need 2 out of 3 observables to measure mass

• Orbital Period (p)
• Orbital Separation (a or r radius)
• Orbital Velocity (v)
• For circular orbits, v 2pr / p

v
M
r
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Most massive stars 100 MSun Least
massive stars 0.08 MSun (MSun is the
mass of the Sun)
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An H-R diagram plots the luminosity and
temperature of stars
Luminosity
Temperature
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Most stars fall somewhere on the main sequence of
the H-R diagram
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Large radius
Stars with lower T and higher L than
main-sequence stars must have larger radii
giants and supergiants
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Stars with higher T and lower L than
main-sequence stars must have smaller radii
white dwarfs
Small radius
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A stars full classification includes spectral
type (line identities) and luminosity class (line
shapes, related to the size of the star) I
- supergiant II - bright giant III -
giant IV - subgiant V - main
sequence Examples Sun - G2 V Sirius - A1
V Proxima Centauri - M5.5 V Betelgeuse - M2 I
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H-R diagram depicts Temperature Colour
Spectral Type Luminosity Radius
Luminosity
Temperature
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C
B
Which star is the hottest?
D
Luminosity
A
Temperature
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C
B
Which star is the most luminous?
D
Luminosity
A
Temperature
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C
B
Which star has the largest radius?
D
Luminosity
A
Temperature
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Main-sequence stars are fusing hydrogen into
helium in their cores like the Sun Luminous
main-sequence stars are hot (blue) Less luminous
ones are cooler (yellow or red)

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Mass measurements of main-sequence stars show
that the hot, blue stars are much more massive
than the cool, red ones

High-mass stars
Low-mass stars
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The mass of a normal, hydrogen-burning star
determines its luminosity and spectral type!

High-mass stars
Low-mass stars
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Core pressure and temperature of a higher-mass
star need to be larger in order to balance
gravity Higher core temperature boosts fusion
rate, leading to larger luminosity
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Main-Sequence Star Summary
High Mass High Luminosity Short-Lived
Large Radius Blue Low Mass Low
Luminosity Long-Lived Small Radius Red