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Stellar Phenomenology
and Stellar Properties
• Stellar spectral classification
• Hertzsprung-Russel and color-magnitude
•
•
diagrams
Fundamental stellar properties, observables
Stellar population properties
infrared spectra
visible spectra
Spectral Classification: Temperature
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Spectral Classification: Temperature
Weak Ca+
T
Y
1,400–2,500 K
none
Molecules: H2O, hydrides
reddest star-like objects
400–1,400 K
none
Molecules: H2O, CH4
none
Molecules: H2O, CH4, NH3
<400 K
~ 0.1
10–5–10–3
>100 Gyr
<0.08
~ 0.1
10–6–10–5
N/A
<0.08
~ 0.1
<10–6
N/A
Stellar Classification: Temperature
Sun
stars
(G dwarf) M dwarf
5700 K
~3500 K
brown dwarfs
planets
L dwarf
T dwarf
Jupiter
~2000 K
~1000 K
160 K 4
Spectral Classification: Luminosity
• luminosity, radius, surface gravity, and surface pressure are
mutually related
– L = 4πR2σTeff4, 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 ~ 1.5
II: (bright) giants, log g ~ 0.5
I: 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
Color-Magnitude
Diagram (CMD)
• proxy for the (Teff-L)
Hertzsprung-Russell
diagram
• e.g., B–V vs. MV, J–K
vs. MK, etc.
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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
Messier 80
•
•
•
Pleiades
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
Stellar Phenomenology
and Properties
• Stellar spectral classification
• Hertzsprung-Russel and color-magnitude
•
•
diagrams
Fundamental stellar properties,
observables
Stellar population properties
Spectroscopic
Binary
(a)
• double-lined (SB2)
– spectra of both stars visible
(d)
(a)
(b)
(b)
(c)
(c)
(d)
• single-lined (SB1)
– only spectrum of brighter star visible
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Radial Velocity vs. Time for Doublelined SB in a Circular Orbit
Totally Eclipsing Binaries
ta – start of secondary ingress
tb – end of secondary ingress
tc – start of secondary egress
td – end of secondary egress
Luminosity-mass relation for binary stars with welldetermined orbits
Data from
Popper
(1980;
ARA&A 18,
115)
Mass vs.
Teff
Taking the Stellar Temperature
Teff
• (Fe II λ5317 / Fe I λ5328) line ratio decreases with decreasing Teff
Energy Generation: p-p Chain
Initial Mass Function
• ξL ≡ dN / (dlog M/MSun) ∝
∝ (M/MSun)–Γ
• ξ ≡ dN / (d M/MSun) ∝
(M/MSun)–α
• slope Γ ≡ α + 1
log M/MSun
Kroupa (2002)
Single Star Fraction
• lower
mass stars
are more
commonly
single
Lada (2006)