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 * <0.08M sun Intermediate-Mass (IM), 0.08M sun <M * <M sun High-Mass (HM), M * >M sun

Nathan Mayne Initial Collapse: M cloud >M J (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, M core increases. Then envelope falls onto core, ‘bounces.’ LM, IM and HM: e - degeneracy For M * <8- 12M sun Planetary nebulae White dwarf Cool Very HM: p + +e - =n Neutron degeneracy Supernovae (type II) After ‘Bounce’…. Very HM: Neutron, M core <5M sun Black Hole M core >5M sun MS & Post-MS MS: LM:N/A IM:~Gyrs HM:<10Myrs

Nathan Mayne 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=4πR 2 σT 4 eff Convective: T 4 eff stable, swift cooling Radiative: T 4 eff increases, slow cooling Image from Vik Dillon, 9 th May 1999

Nathan Mayne Black Body radiation: Planck’s Law Decrease T, increase peak Increase Flux or L increase amplitude Photometry Filter: 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=4πR 2 σT 4 eff ) Colour (V-R),  Temperature, depends on mechanism (Radiative or Convective)

Nathan Mayne CMDs Or V-R? Image from

Nathan Mayne A real one….

Nathan Mayne Complications Distance: Decrease intensity  4πr 2 Affects V and R EQUALLY Therefore, V-R unaffected

Nathan Mayne

Fitting Theory to observations Isochrones: 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