Astronomy 100

Lectures

Table of Contents

Astro 100

Lecture 10
Spectral Types of Stars; The Doppler Effect

Outline

Stellar Spectra
The Doppler Effect

Terms to Know

spectral type
Balmer line
Doppler shift
redshift
blueshift

1. Spectral types of stars

The spectra of stars provide astronomers with all sorts of crucial information, including temperatures, pressures, composition, velocities, ages, etc. Stellar spectra are mostly black body (thermal) spectra, with various absorption and/or emission lines superposed on the black body continuum.

In the 1890's and early 1900's, astronomers methodically cataloged hundreds of thousands of stellar spectra. At Harvard, astronomer Annie Jump Cannon revised an older classification scheme, arranging the spectra according to the absorption line features -- especially the Balmer lines of hydrogen -- that they showed. Eventually it was realized that the differences in absorption lines were caused primarily by different temperatures at the surfaces of stars, and the spectral sequence was reordered by temperature.

Spectral types:

O B A F G K M

hot cool

"oh be a fine gorilla: kiss me"
"oblique bats announce: frigid grapes kill mice"

NEWS FLASH: In mid 1998, a new spectral type, "L", was discovered and confirmed by a team of astronomers including several from UMass! L comes after M in the spectral sequence.)

Each 'spectral type' has a unique characteristic spectrum

Each is broken down into 10 subtypes, e.g. G1, .... G10
Each of these is broken down into 5 luminosity classes:
- I, II super-giants
- III giants
- IV sub-giants
- V ``dwarfs''

The Sun is a G2 V star

Spectral Type	Temperature (degrees Kelvin)	"Color"
O	30,000	blue/purple
B	20,000	blue
A	10,000	white
F	8,000	yellow/white
G	6,000	yellow
K	4,000	orange
M	3,000	red

2. The Doppler Effect

As an object moves towards you very fast, the light waves it emits get "squashed together," making them appear blue. Likewise, as an object moves away from you (recedes), the light waves get "stretched out," so they appear red. This is the well-known Doppler Shift (think: passing fire engines, trains, etc.).

The greater the radial velocity (speed towards or away), the greater the shift. This means you can measure an object's speed towards or away from you by measuring the wavelength of features in its spectrum! In practice, this is done using absorption or emission lines (since continuum doesn't have any features to shift). For radial velocities (V_r) small compared to the speed of light (less than, say, 1% of c), the amount of shift in wavelength is given by:

= V_r/c

where

is the change in

induced by the motion, i.e.,

_new -

_old. (It gets a bit more complicated for higher velocities.)