Stellar Phenomenology and Stellar Properties

• Stellar spectral classification

• Hertzsprung-Russel and color-magnitude diagrams

• Fundamental stellar properties, observables

• Stellar population properties

2

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)