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Colour-Magnitude Diagrams (CMDs)

Colour-Magnitude Diagrams (CMDs). 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:

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Colour-Magnitude Diagrams (CMDs)

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  1. Colour-Magnitude Diagrams(CMDs)

  2. 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

  3. Initial Collapse: • Mcloud>MJ (Jeans mass) • Density increase • Size decreases • Temperature increases • Birth-line-Star emerges Timescales: Collapes: ~Myrs • Pre-MS: • Star emerges from cloud • Fully convective gas • Gradually contracts • Density of core increases 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

  4. MS & Post-MS • Red Giants (or supergiants): • Envelope cools and expands • Nuclear ‘Ash’ in cores, Mcore increases. • Then envelope falls onto core, ‘bounces.’ MS: LM:N/A IM:~Gyrs HM:<10Myrs After ‘Bounce’…. • LM, IM and HM: • e- degeneracy • For M*<8-12Msun • Planetary nebulae • White dwarf • Cool • Very HM: • p++e-=n • Neutron degeneracy • Supernovae (type II) Very HM: • Very HM: • Neutron, Mcore<5Msun • Black Hole Mcore>5Msun

  5. 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

  6. Photometry • Filter: • Integrated Flux • Set central and Δ • Black Body radiation: • Planck’s Law • Decrease T, increase peak  • Increase Flux or L increase amplitude • Therefore: • V or R amplitude (magnitude)  Luminosity (flux) • V-R, relative flux, shift in peak  Temperature (colour)

  7. Summary • 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 • Photometry: • Magnitude in V or R  Luminosity  Size (L=4πR2σT4eff) • Colour (V-R),  Temperature, depends on mechanism (Radiative or Convective)

  8. CMDs Or V-R? Image from http://www.aavso.org/images

  9. A real one….

  10. Complications • Distance: • Decrease intensity • 4πr2 • Affects V and R EQUALLY • Therefore, V-R unaffected

  11. 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

  12. 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

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