Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram

Section 8.2 Section 8.2

The story thus farThe story thus farThe story thus farThe story thus far

Masses, RadiiMasses, RadiiBinary stars (visual, Binary stars (visual, spectroscopic, eclipsing)spectroscopic, eclipsing)

Temperatures, Temperatures, composition, surface composition, surface gravity, Luminosity classgravity, Luminosity class

SpectrumSpectrum

Temperatures, TTemperatures, TColour Index (B-V) (+ Colour Index (B-V) (+ black body curve)black body curve)

Luminosities, L Luminosities, L (absolute magitudes)(absolute magitudes)

Flux, F, at Earth, apparent Flux, F, at Earth, apparent magnitude (with d + inverse square magnitude (with d + inverse square law) law)

Distances, dDistances, dParallax, moving clusterParallax, moving clusterCalculated propertyCalculated propertyObserved quantityObserved quantity

But what about…… ages, how stars are born, how they shine, how they die?

HR Diagram - introductionHR Diagram - introductionHR Diagram - introductionHR Diagram - introduction

Make a plot of Make a plot of height vs height vs weight for weight for students in students in classclass

Supermodels

New-born babies

B-ball players

Sumo wrestlers

Adults

Children

Mass

Height

Height Height increasing increasing upward - upward - mass mass increasing increasing to leftto left

Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram A “Stellar Demographic Diagram”A “Stellar Demographic Diagram”

By turn century, astronomers were aware of a By turn century, astronomers were aware of a spectral sequence: OBAFGKM spectral sequence: OBAFGKM O stars are hot, luminous, most massive; O stars are hot, luminous, most massive; M are stars cool, faint, least massiveM are stars cool, faint, least massive

Originally, it was proposed that the spectral Originally, it was proposed that the spectral sequence was also an evolutionary sequence: sequence was also an evolutionary sequence: start as hot O stars, use fuel, lose mass, start as hot O stars, use fuel, lose mass, cool to die as a dim M starcool to die as a dim M star

1905 - amateur astronomer, Hertzsprung, found a 1905 - amateur astronomer, Hertzsprung, found a correlation between spectral type and absolute correlation between spectral type and absolute magnitude - but stars G and later showed a range in magnitude - but stars G and later showed a range in MMVV for same spectral type - brighter stars called for same spectral type - brighter stars called “giants”. “giants”.

1913 - established US astronomer, Henry Norris 1913 - established US astronomer, Henry Norris Russell, found same result.Russell, found same result.

Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram

Henry Norris Russell’s first 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 left and bright stars at the top.

TBright Stars

Band upper left to lower right is called the Main Sequence. It contains 80-90% of all stars.White dwarfs at lower left.

Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram

L = 4R2Teff

4

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

Modern observer’s HR diagram

Modern theorist’s 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

Mass-Luminosity RelationMass-Luminosity RelationMass-Luminosity RelationMass-Luminosity Relation

Masses were obtained from Masses were obtained from observations of many binaries.observations of many binaries.

Luminosities were obtained from Luminosities were obtained from parallax measurements. parallax measurements.

These are all main sequence These are all main sequence starsstars

Lower mass stars on main Lower mass stars on main sequencesequence

Have cooler spectral types -Have cooler spectral types -Main Sequence is a mass sequenceMain Sequence is a mass sequence

Hertzsprung Russell DiagramHertzsprung Russell DiagramHertzsprung Russell DiagramHertzsprung Russell Diagram

Sun

Bias in HR DiagramsBias in HR DiagramsBias in HR DiagramsBias in HR Diagrams

= K & M dwarfs

= F & G dwarfs

= Giants

Sun

Bias in HR DiagramsBias in HR DiagramsBias in HR DiagramsBias in HR Diagrams

= K & M dwarfs

= F & G dwarfs

= Giants

Sun

Bias in HR DiagramsBias in HR DiagramsBias in HR DiagramsBias in HR Diagrams

= K & M dwarfs

= F & G dwarfs

= Giants

Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram

This is a proper sample of faint stars. Note that none of the rarer giant stars are within this volume of space.

Beware of the selection effects in the HR Diagram

HR diagram of the stars nearest to the Sun.

Luminosity Function of Stars Near to Sun

Hertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell DiagramHertzsprung-Russell Diagram

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.

Beware of the selection effects in the HR Diagram

HR diagram of the stars nearest to the Sun.

HR diagram of the brightest stars in the night sky.