skip navigation
Galaxy NGC4414 from HST Astronomy 162:
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)

Key Ideas

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:

From Wien's Law, we expect:


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 stars?

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!]

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.


Harvard Classification System

In 1890, Edward Pickering and Williamina Fleming made a first attempt at spectral classification:
Problem:
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 spectra. After this, one was left with the 7 primary classes we recognize today, in order:

O B A F G K M

Later work by Cannon and others added the classes R, N, and S which are no longer in use today.

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.


Cecilia Payne-Gaposhkin

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.

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.
Why? Implications:

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 state.
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 excited state.

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 decreasing luminosity:

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:
G2v

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.



Supplement:
Stellar Spectral Type Mnemonics

The traditional mnemonics for remembering the spectral types are based on the old Harvard OBAFGKM system. Some examples:

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 Nixon Smiling
(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.


Return to [ Unit 1 Index | Astronomy 162 Main Page ]
Updated: 2006 January 8
Copyright Richard W. Pogge, All Rights Reserved.