Nathan Mayne

Colour-Magnitude Diagrams(CMDs)

Nathan Mayne

Stellar Evolution (simple picture)

Three main phases:

• Pre-main-sequence (pre-MS)

• Main-sequence (MS)

• Post-main-sequence (post-MS)

Mass dominates evolution:

• Times spent in each phase

• Final state/death

For general picture, three mass categories:

• Low-Mass (LM), M*<0.08Msun

• Intermediate-Mass (IM), 0.08Msun<M*<Msun

• High-Mass (HM), M*>Msun

Nathan Mayne

Initial Collapse:

• Mcloud>MJ (Jeans mass)

• Density increase

• Size decreases

• Temperature increases

• Birth-line-Star emerges

Pre-MS:

• Star emerges from cloud

• Fully convective gas

• Gradually contracts

• Density of core increases

Timescales:

Collapes:

~Myrs

Pre-MS:

LM:~Gyrs

IM:~10-100Myrs

HM:<0.1Myrs

LM, IM and HM:

• Core becomes radiative

• Stars begin H fusion

• Stars join Zero age MS (ZAMS)

M*<LM:• Stars never fuse H • Brown dwarfs

Nathan Mayne

Red Giants (or supergiants):

• Envelope cools and expands

• Nuclear ‘Ash’ in cores, Mcore increases.

• Then envelope falls onto core, ‘bounces.’ LM, IM and HM:

• e- degeneracy

• For M*<8-12Msun

• Planetary nebulae

• White dwarf

• Cool

Very HM:

Very HM:

• p++e-=n

• Neutron degeneracy

• Supernovae (type II)

After ‘Bounce’….

Very HM:

• Neutron, Mcore<5Msun

• Black Hole Mcore>5Msun

MS & Post-MS

MS:

LM:N/A

IM:~Gyrs

HM:<10Myrs

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What’s changing?

Size and structure:

• Pre-MS, contracts and fully convective

• MS, stable size and radiative core

• Post-MS:

• Giants, expand from MS and largely convective.

• Dwarfs and Neutron stars, contract from giants and are radiative.

L=4πR2σT4eff

Convective: T4eff stable, swift cooling

Radiative: T4eff increases, slow cooling

Image from Vik Dillon, 9th May 1999

Nathan Mayne

Black Body radiation:

• Planck’s Law

• Decrease T, increase peak

• Increase Flux or L increase amplitude

PhotometryFilter:

• Integrated Flux

• Set central and Δ

Therefore:

• V or R amplitude (magnitude) Luminosity (flux)

• V-R, relative flux, shift in peak Temperature (colour)

Nathan Mayne

Stellar Evolution:

Pre-MS:

• Contract to the MS at constant T (Hayashi)

• Radiative cores, T increases size stable (Henyey)

MS:

• Roughly stable R and T

Post-MS:

• Stars expand and cool to Giants

• Contract and heat, White Dwarfs

• Very HM, Neutron stars and Black Holes

Summary

Photometry:

• Magnitude in V or R Luminosity Size (L=4πR2σT4eff)

• Colour (V-R), Temperature, depends on mechanism (Radiative or Convective)

Nathan Mayne

CMDs

Or V-R?

Image from http://www.aavso.org/images

Nathan Mayne

A real one….

Nathan Mayne

Complications

Distance:

• Decrease intensity

• 4πr2

• Affects V and R EQUALLY

• Therefore, V-R unaffected

Nathan Mayne

Nathan Mayne

Fitting Theory to observationsIsochrones:

• Theory from simulations

• Predict V and V-I (or V-R)

• Fit for age, distance and extinction

Contraction and Turn-on:

• Fit contracting Pre-MS stars

• pre-MS-MS, turn-on

• Problems with distance and age degeneracy

Nuclear and Turn-off:

• Fit expanding Giants

• MS-Post-MS, turn-off

• Few stars, sensitive

Nathan Mayne

Summary

Observe stars:

• Magnitude and colour

• Filters i.e. V and R

• Infer size and temperature

• Use to probe evolutionary state

Analyse CMD:

• Positions of MS, pre-MS and post-MS separated

• Changes in R and T ( as radiative or convective)

• Find distances and ages of stars