Spectral Classification of Stars

Stellar 

Classification 

Spectral Lines 

H 

Fe 

Na 

H 

Ca 

H 

Thursday, September 29, 2011

Spectral Classification of Stars 

Timeline: 

1890s 

Edward C. Pickering (1846-1919) and Williamina P. Fleming 

(1857-1911) label spectra alphabetically according to strength 

of Hydrogen (Balmer) lines, beginning with “A” (strongest). 

1890s 

Antonia Maury (1866-1952) developed a classification scheme based on 

the “width” of spectral lines. Would place “B” stars before “A” stars. 

1901 

Annie Cannon (1863-1941), brilliantly combined the above. 

Rearranged sequence, O before B before A, added decimal 

divisions (A0...A9) and consolidated classes. Led to 

classification scheme still used by astronomers today! 

OBAFGKM (Oh Be A Fine Guy/Girl, Kiss Me) 

“Early Type” 

Stars 

“Late Type” 

Stars 


Timeline: 


1901 

Annie Cannon (1863-1941), brilliantly combined the above. 

Rearranged sequence, O before B before A, added decimal 

divisions (A0...A9) and consolidated classes. Led to 

classification scheme still used by astronomers today! 

OBAFGKM (Oh Be A Fine Guy/Girl, Kiss Me) 

“Early Type” Stars : Stars near the beginning 

of Sequence 

“Late Type” Stars : Stars near the end of 

the Sequence. 

One can mix the definitions: K0 star is an “early-type” K star. B9 is a “late-type” B star. 

1911-1914 

During 1990s 

Annie Cannon classified 200,000 spectra, listed in the Henry Draper Catalog. 

Catalog ID’s are “HD 39801” (ID for Betelgeuse in the constellation Orion). 

Two new letters added to Sequence for very cool, Brown-Dwarf stars. 

“L” spectral types (T=1300-2500 K) and “T” types (T < 1300 K). 

OBAFGKMLT (Oh Be A Fine Guy/Girl, Kiss Me - Less Talk !) 



Hotter 

Cooler 

Spectral Type 

O 

B 

A 

F 

G 

K 

M 

L 

T 

Characteristics 

Hottest blue-white stars, few lines. Strong He II (He + ) absorption 

lines. He I (neutral helium) stronger). 

Hot blue-white. He I (neutral Helium), strongest at B2. 

H I (neutral Hydrogen) stronger. 

White stars. Balmer absorption lines strongest at A0 (Vega), 

weaker in later-type A stars. Strong Ca II (Ca + ) lines. 

Yellow-white stars. Ca II lines strengthen to later types. F-stars. 

Balmer lines strengthen to earlier type F-stars. 

Yellow stars (Sun is a G5 star). Ca II lines become stronger. Fe I 

(neutral iron) lines become strong. 

Cool orange stars. Ca II (H and K) lines strongest at K0, becoming 

weaker in later stars. Spectra dominated by metal absorption lines. 

Cool red stars. Spectra dominated by molecular absorption bands, 

e.g., TiO (titanium oxide). Neutral metal lines strong. 

Very cool, dark red (brown dwarfs). Brighter in Infrared than 

visible. Strong molecular absorption bands, e.g., CrH, FeH, water, 

CO. TiO weakening. 

Coolest stars. Strong methane (CH4), weakening CO bands. 


Thursday, September 29, 2011 

Spectral Classification of Stars










Physical Description 

Hydrogen when T < 9900 K 

Majority of electrons in 

ground state, n=1 

Hydrogen when T = 9900 K 

Majority of electrons in first excited, n=2 state, 

and capable of producing Balmer lines 

Hydrogen when T > 9900 K 

Majority of electrons unbound, 

ionized hydrogen. 




Stars are not composed of pure hydrogen, but 

nearly all atoms (mostly H, He, and metals = 

anything not H or He). 

Typically 1 He atom for every 10 H atoms 

(and even fewer metals). 

Helium (and metals) provide more electrons, 

which can recombine with ionized H. 

So, it takes higher temperatures to achieve 

same degree of H ionization when He and 

metals are present. 

Abundance is = log10(Nelement/NH) + 12. 

I.e., Abudance of Oxygen = 8.83, which means: 

8.83 = log10(NO/NH) + 12 

NO/NH = 10 8.83 - 12 = 0.000676 ≈1/1480 

There is one Oxygen atom 

for every 1480 H atoms ! 

Most Abundant Elements in the Solar Photosphere. 

Element Atomic # Log Relative Abundance 

H 1 12.00 

He 2 10.93 ± 0.004 

O 8 8.83 ± 0.06 

C 6 8.52 ± 0.06 

Ne 10 8.08 ± 0.06 

N 7 7.92 ± 0.06 

Mg 12 7.58 ± 0.05 

Si 14 7.55 ± 0.05 

Fe 26 7.50 ± 0.05 

S 16 7.33 ± 0.11 

Al 13 6.47 ± 0.07 

Ar 18 6.40 ± 0.06 

Ca 20 6.36 ± 0.02 

Ng 11 6.33 ± 0.03 

Ni 28 6.25 ± 0.04 




In 1925, Cecilia Payne (1900-1979) calculate the relative abundances of 18 elements in 

stellar atmospheres (one of the most brilliant PhD theses ever in astronomy). 




Hertzsprung-Russell Diagram 

How would you measure a star’s mass ? 





Answer: Kepler’s Laws. Works well for binary stars. 

m1 

r1 

+ 

= 

r2 

m2 

R = 

m1r1 + m2r2 

m1 + m2 

= 0 

m1 

m2 

r2 

= = 

r1 

a2 

a1 


Animations of binary Stars: 

http://astro.ph.unimelb.edu.au/software/binary/binary.htm 

http://abyss.uoregon.edu/~js/applets/eclipse/eclipse.htm 


Binary Stars 

Term binary was first used by 

Sir Williams Herschel in 1802. 

"If, on the contrary, two stars should really be situated very near each 

other, and at the same time so far insulated as not to be materially 

affected by the attractions of neighbouring stars, they will then compose 

a separate system, and remain united by the bond of their own mutual 

gravitation towards each other. This should be called a real double star; 

and any two stars that are thus mutually connected, form the binary 

sidereal system which we are now to consider." 



Two Stars in Albireo 

system. 



Sirius A 

brightest star in the sky 

m = -1.46. 

In 1844, Friedrich Bessel deduced 

it was a binary. 

In 1862 Alvan Graham Clark 

discovered the companion. 

Sirius B 

m = 8.30 





angle of inclination 

For circular orbits, v1 = 2πa1/P 

m1 

= 

v2 

Plane of Sky 

Orbital Plane 

m2 

m2 

Determine radial component of velocities, 

vr, using doppler-shifted spectral lines. 

v1r=v1 sin(i) & v2r=v2 sin(i) 

v1 

To Earth 

m1 

m2 

= 

v2r / sin(i) 

v1r / sin(i) 

= 

v2r 

v1r 

Therefore, we can determine the mass ratio 

without knowing the angle of inclination ! 

m1 





Need one other equation to relate m1 and m2. Comes from Kepler’s 3rd law, replace a = a1 + a2 

a = a1 + a2 = (P/2π) v1 + (P/2π)v2 = (P/2π) (v1+v2) 

Kepler’s 3rd law, (P 2 = 4π 2 a 3 / GM ), becomes 

m1 + m2 = (P/2πG) (v1+v2) 3 

Substituting v1 = v1r /sin(i), the angle of inclination. 

m1 + m2 = (P/2πG) (v 1r+v2r) 3 

sin 3 (i) 





Combine our two formula: 

m1 

m2 

= 

v2r / sin(i) 

v1r / sin(i) 

= 

v2r 

v1r 

m1 + m2 = (P/2πG) (v 1r+v2r) 3 

sin 3 (i) 

To get (rearranging terms): 

m2 3 

(m1 + m2) 2 sin 3 (i) 

P 

= 

2πG 

v1r 3 

This is the mass function, depends only on the period, P, and radial 

velocity, v1r. In practice, only a spectrum for one star in binary pair is 

available. Mass function sets a lower limit on m2. In rare cases of eclipsing 

spectroscopic binaries, i ≈90 o and both masses can be measured directly. 


Eclipsing Binaries: 

http://www.astro.cornell.edu/academics/courses/astro101/herter/ 

java/eclipse/eclipse.htm 




Mass-Luminosity relation



This is the Hertzprung- 

Russell (HR) diagram, which 

is a stellar classification 

system developed by Ejnar 

Hertzprung and Henry 

Norris Russel in Denmark 

around 1910. 

Ejnar Hertzsprung 

Henry Norris Russell 

The HR diagram relates the 

magnitudes and colors of 

stars as a function of their 

temperature and luminosity. 


Henry Norris Russell’s 

first diagram 



Enormous Range in Stellar Radii ! 

If stars cool over time as they contract, there should be a 

relation between their temperatures and luminosities. 

R = 

1 

T 2 

√ 

L 

4πσ 

Hertzsprung (1873-1967) found that stars of Late type 

(G and later) have a large range in luminosity. If two 

stars of the same spectral type (same Temperature) 

then more luminous star is larger. 

Giants: Stars with big radii & Dwarfs: Stars with small radii. 

Our Sun is a G5 dwarf. 

Similar Conclusions reached by Henry Russell (1877-1957) 




Hertzsprung-Russell Diagram



Hertzsprung-Russell Diagram

Brighter 

30,000 K 10,000 K 7500 K 6000 K 5000 K 4000 K 3000 K 

Luminosity 

Text 

HR diagram where data points 

show measurements from 

22,000 real stars from the 

Hipparcos satellite. 

(Lines are Theoretical, 

expected luminosities and 

temperatures of stars) 

Temperature: Hotter 

Color Index: B-V 


Brighter 

HR diagram 

Absolute Magnitude (M) 

(Luminosity) 

Spectral 

Type 

Hotter 

Temperature (Color, B-V)

Spectral Classification of Stars

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