Post on 17-Mar-2022
Stellar Phenomenology and Stellar Properties
• Stellar spectral classification
• Hertzsprung-Russel and color-magnitude diagrams
• Fundamental stellar properties, observables
• Stellar population properties
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Spectral Classification: Temperatureinfrared spectra visible spectra
Spectral Classification: Temperature
T
1,400–2,500 K none Molecules: H2O, hydridesreddest star-like objects ~ 0.1 10–5–10–3 >100 Gyr
400–1,400 K none Molecules: H2O, CH4 ~ 0.1 10–6–10–5 N/A<0.08
Weak Ca+
Y <400 K none Molecules: H2O, CH4, NH3 ~ 0.1 <10–6 N/A<0.08
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Stellar Classification: Temperature
Sun
M dwarf T dwarfL dwarf Jupiter
brown dwarfs planetsstars
5700 K ~3500 K ~2000 K ~1000 K 160 K
(G dwarf)
Spectral Classification: Luminosity• luminosity, radius, surface gravity, and surface pressure are
mutually related– L = 4πR2σTeff
4, g = GM/R2, P = ρgl (l is photon m.f.p.)
• define “luminosity spectral class”V: dwarfs, log g ~ 4.5 [cgs units]IV: subgiants, log g ~ 3 (approximately as on Earth)III: giants, log g ~ 1.5II: (bright) giants, log g ~ 0.5I: supergiants, log g ~ –0.5
• Sun: G2 V star (Teff = 5777K, log g = 4.43)
(figure from D. Gray)
Hertzsprung-Russell (H-R)
Diagram• log L vs. log Teff
• main sequence:– locus of most stars– bulk of stellar
lifetimes– L ∝ M3.8
– τMS ≈ 1010 yr (M/MSun)–2.8
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Color-Magnitude Diagram (CMD)
• proxy for the (Teff-L) Hertzsprung-Russell diagram
• e.g., B–V vs. MV, J–K vs. MK, etc.
Stellar Abundances• derived from spectral
lines
• compared to the Sun or to abundance of hydrogen (H) in star–or both
• a.k.a., “metallicity”
• “metal-poor” stars, a.k.a. “subdwarfs” are hotter than dwarfs of same luminosity
[Fe/H] = log [n(Fe) / n(H)]* – log [n(Fe) / n(H)]Sun
Stellar Populations
• Population I:
• low galatic scale heights, rotate with galactic disk, similar composition to Sun
• Population II:
• large scale heights, high space velocities, low mass: old stars
Star Clusters
• globular clusters (e.g., M80: Pop II stars, gravitationally bound, dense
• open clusters (e.g., Pleiades): Pop I stars, gravitationally bound, < 2 Gyr
• “O-B” associations: loose, not gravitationally bound, < 20 Myr
Messier 80 Pleiades
Stellar Phenomenology and Properties
• Stellar spectral classification
• Hertzsprung-Russel and color-magnitude diagrams
• Fundamental stellar properties, observables
• Stellar population properties
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Spectroscopic Binary
• double-lined (SB2)– spectra of both stars visible
• single-lined (SB1)– only spectrum of brighter star visible
(a)
(b)
(c)
(d)
(a)(d) (b)
(c)
Radial Velocity vs. Time for Double-lined SB in a Circular Orbit
Totally Eclipsing Binaries
ta – start of secondary ingresstb – end of secondary ingresstc – start of secondary egresstd – end of secondary egress
Luminosity-mass relation for binary stars with well-determined orbits
Data fromPopper(1980; ARA&A 18, 115)
Mass vs. Teff
Taking the Stellar Temperature
• (Fe II λ5317 / Fe I λ5328) line ratio decreases with decreasing Teff
Teff
Energy Generation: p-p Chain
• ξL ≡ dN / (dlog M/MSun) ∝ ∝ (M/MSun)–Γ
• ξ ≡ dN / (d M/MSun) ∝ (M/MSun)–α
• slope Γ ≡ α + 1
Initial Mass Function
Kroupa (2002)log M/MSun
Single Star Fraction• lower
mass stars are more commonly single
Lada (2006)