1. Hertzsprung-Russell Diagram
Section 8.2

2. The story thus far Observed quantity Calculated property
Parallax, moving cluster
Distances, d
Flux, F, at Earth, apparent magnitude (with d + inverse square law)
Luminosities, L (absolute magitudes)
Colour Index (B-V) (+ black body curve)
Temperatures, T
Spectrum
Temperatures, composition, surface gravity, Luminosity class
Binary stars (visual, spectroscopic, eclipsing)
Masses, Radii
But what about…… ages, how stars are born, how they shine,
how they die?

3. HR Diagram - introduction
Make a plot of height vs weight for students in class
Supermodels
New-born babies
B-ball players
Sumo wrestlers
Adults
Children
Mass
Height
Height increasing upward - mass increasing to left

4. 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
amateur astronomer, Hertzsprung, found a correlation between spectral type and absolute magnitude - but stars G and later showed a range in MV for same spectral type - brighter stars called “giants”.
established US astronomer, Henry Norris Russell, found same result.

5. Hertzsprung-Russell Diagram
None available in sample
Henry Norris Russell’s first diagram: MV vs spectral type.
Old type
Spectral Type
O B A F G K M N
Note: T increases to left and bright stars at the top. T
Bright Stars -4 +4 +8
+12
Band upper left to lower right is called the Main Sequence. It contains 80-90% of all stars. MV
White dwarfs at lower left.

6. Hertzsprung-Russell Diagram
Modern observer’s HR diagram
L = 4R2Teff4
 R1/R2 = (L1/L2)1/2 x (T1/T2)-2
If 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 R
Supergiants few ,000Rsun
(e.g. Betelgeuse)
Giants Rsun
(e.g. Aldebaran)
Main Sequence stars Rsun
(e.g. Sun)
White Dwarfs 0.01Rsun
(e.g. Sirius B)
Modern theorist’s HR diagram

7. Mass-Luminosity Relation
Masses were obtained from observations of many binaries.
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

8. Hertzsprung Russell Diagram

9. Bias in HR Diagrams
Sun
= K & M dwarfs
= F & G dwarfs
= Giants

10. Bias in HR Diagrams
Sun
= K & M dwarfs
= F & G dwarfs
= Giants

11. Bias in HR Diagrams
Sun
= K & M dwarfs
= F & G dwarfs
= Giants

12. Hertzsprung-Russell Diagram
Beware of the selection effects in the HR Diagram
Luminosity Function of Stars Near to Sun
HR diagram of the stars nearest to the Sun.
This is a proper sample of faint stars. Note that none of the rarer giant stars are within this volume of space.

13. Hertzsprung-Russell Diagram
Beware of the selection effects in the HR Diagram
HR diagram of the brightest stars in the night sky.
HR diagram of the stars nearest to the Sun.
This is a proper sample of faint stars. Note that none of the rarer giant stars are within this volume of space.
The number of giant and supergiant stars is small but they can be seen over vast distances.