Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 9: Stellar Spectra
Readings: Readings: Chapter 19, sections 19-4 &l 19-5
Supplement: Characteristics of the Stellar Spectral Types (graphical overview)
- The color of a star depends on its Temperature
- Red Stars are Cooler
- Blue Stars are Hotter
- Spectral Classification
- Classify stars by their spectral lines
- Spectral differences mostly due to Temperature, not composition.
- Spectral Sequence (Temperature Sequence): O B A F G K M L T
Colors of Stars
Stars are hot, dense balls of gas:
- Emit a Continuous spectrum from the lowest visible layers
- Approximates a blackbody spectrum with a single temperature.
From Wien's Law,
- Hotter stars appear BLUE (T=10,000-50,000 K)
- Medium-hot stars appear YELLOWISH (T~6000K)
- Cool stars appear RED (T~3000K)
Spectra of Stars
Hot, dense lower photosphere of a star is surrounded by thinner
(but still fairly hot) atmosphere.
Can we use stellar spectra to distinguish among different types of
Spectral Classification of Stars
In 1866, Fr. Angelo Secchi, a Jesuit astronomer working in Italy, observed
prism spectra of ~4,000 stars.
[Note: Fr. Secchi was observing by eye, not using photography!]
- Divided stars into 4 broad spectral classes by common spectral
Between 1886 and 1897, the Henry Draper Memorial Survey at Harvard
carried out a systematic photographic study of stellar spectra over the
entire sky. Effort was led by Edward C. Pickering.
- Used objective prism photography from telescopes at Harvard
and Arequipa, Peru.
- Obtained spectra of 220,000 stars.
- Hired women as "computers" to analyze spectra.
Harvard Classification System
In 1890, Edward Pickering and Williamina Fleming made a
first attempt at spectral classification:
- Sorted stars by decreasing Hydrogen absorption-line strength
- Spectral Type "A" = strongest Hydrogen lines
- followed by types B, C, D, etc. (weaker)
- Other lines did not fit into this sequence.
Annie Jump Cannon
In 1901, Annie Jump Cannon noticed that stellar temperature
was the principal distinguishing feature among different
After this, one was left with the 7 primary classes we recognize today,
- Re-ordered the ABC types by temperature instead of Hydrogen
- Most classes were thrown out as redundant.
Later work by Cannon and others added the classes R, N, and S which are
no longer in use today.
O B A F G K M
Henry Draper Catalog of Stars
Cannon further refined her spectral classification system by dividing
each class into numbered ten subclasses.
For example, type A is subdivided into:
A0 A1 A2 A3 ... A9
Between 1911 and 1924, she applied this Harvard Classification scheme to
about 220,000 stars, published as the Henry Draper Catalog.
The Harvard (or Henry Draper) spectral classification system was adopted
by all astronomers.
Two New Spectral Types: L & T
These are the coolest stars, with T<2500 K.
Discovered in 1999, they are turning up in relatively large numbers in
recent digital all-sky surveys. Because the stars are extremely cool,
they emit mostly at infrared wavelengths.
Their spectra are quite different from M stars, and 2 new spectral classes
have been proposed for them:
- L Stars:
- Temperatures ~1300-2500 K
- Spectra show strong metal-hydride molecular bands (CrH & FeH),
and neutral metals, but TiO and VO bands are nearly absent.
- T dwarfs:
- Spectra show strong bands of Methane (CH4), like
the spectrum of Jupiter.
- Most likely to be failed stars (low-mass "Brown Dwarfs") with
masses too small to ignite hydrogen fusion.
Harvard graduate student in the 1920s. In 1925, her dissertation,
published as the book Stellar Atmospheres was
the breakthrough work in understanding stellar spectra.
Put our understanding of stellar spectra on a firm physical basis.
- The first comprehensive theoretical interpretation
of spectral spectra.
- It was based on the then new advances in atomic physics.
Her work showed for the first time that all stars were made of mostly
Hydrogen and Helium and small traces of all the other metals.
The Spectral Sequence is a Temperature Sequence
- The Differences among the spectral types are due to
differences in Temperature.
- Which spectral lines you see depends primarily on the state of
excitation and ionization of the gas.
- Excitation and Ionization are determined primarily by the
temperature of the gas.
- Composition differences are unimportant.
- Differences in temperature are the most important factor.
Example: Hydrogen Lines
The Hydrogen absorption lines in the part of the spectrum at visible
wavelengths all arise from H atoms with the electron in first excited
- B Stars (11,000-30,000 K):
- Most of H is ionized, so only very weak H lines (not much H around
with electrons to make any absorption lines)
- A Stars (7500-11,000 K):
- Ideal excitation conditions, strongest H lines.
- G Stars (5200-5900 K):
- Too cool, little excited H, so only weak H lines because the
electrons are mostly in the ground state instead of the first
Modern Synthesis: The M-K System
In 1943, William Morgan (Chicago) and Phillip Keenan (Ohio State) added
Luminosity as a second classification parameter.
Luminosity Classes are designated by the Roman numerals I thru V, in order of
- Ia = Bright Supergiants
- Ib = Supergiants
- II = Bright Giants
- III = Giants
- IV = Subgiants
- V = Dwarfs
We will explain these names in a subsequent
lecture once we learn more about the physics of stars.
M-K spectral classifications of familiar stars:
- The Sun:
- In Winter Sky:
- Betelgeuse: M2Ib
- Rigel: B8Ia
- Sirius: A1v
- Aldebaran: K5III
Why is this Important?
Spectral classification provides a way to estimate the physical
characteristics of stars by comparing their spectral features.
Spectral classification is a very powerful tool for understanding the physics of stars.
- Spectral differences primarily reflect differences in the temperatures
of the stellar atmospheres.
- A star's spectrum uniquely locates the star within the overall sequence of
The traditional mnemonics for remembering the spectral types are based
on the old Harvard OBAFGKM system. Some examples:
Stellar Spectral Type Mnemonics
- Harvard (1920s):
- Oh Be A Fine Girl, Kiss Me
- (this is the old (tired) classic mnemonic)
- Berkeley (late `60s):
- Oh Buy A Fine Green Kilo Man
- Caltech (late `70s):
- On Bad Afternoons Fermented Grapes Keep Mrs. Richard
- (this uses the supplementary RNS classes that are not
strictly part of the temperature sequence, and no longer used).
However, with the addition of types L and T, we need a new mnemonic, but
no good ones have emerged...
For fun, try to make up your own mnemonic for remembering the
temperature order (hottest to coolest) of the stellar spectral types.
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Updated: 2006 January 8
Copyright © Richard W. Pogge,
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