Astronomy 100


Lectures Table of Contents Astro 100

Atoms and Star Light

Atoms and Spectral Lines


  1. Atoms and Electron Energy Levels
  2. Absorption and Emission Lines

Terms to Know

emission line
absorption line

1. Atoms and Electron Energy Levels

Atoms consist of two parts: nuclei (containing protons and neutrons), and electron clouds, with one or more electrons orbiting the nucleus.

The electrons are analogous to planets going around the Sun, except for one big difference: they are allowed in only a few special orbits . Each electron orbit corresponds to a different energy level. The orbit closest to the nucleus has the lowest energy level. If the electron gains more energy (from a collision with another atom, or by absorbing a photon), it jumps to an orbit farther away from the nucleus. If it gains a lot of energy, it jumps so far from the nucleus that it escapes completely. This is called ionization , and the leftover nucleus (which may still have other electrons) is an ion.

To jump from one energy level to a higher one by absorbing light, an electron needs just the right energy photon to come along. Later, the electron will at some point emit one or more photons, with either the same energy as the one it absorbed, or with the energy corresponding to a different orbital jump. At the same time, the electron will drop back down to a lower energy orbit.

Each element (hydrogen, helium, oxygen, etc.) has different, distinct energy levels, depending on the number of protons in the nucleus and the arrangement of the electron cloud.

2. Absorption and Emission Lines

Gas is made of atoms (often arranged into molecules). Shine a light through a cloud of gas, pass the light through a prism, and what do you see? Some colors have been removed, or absorbed from the light by the gas! This is due to electrons in the gas "stealing" energy from the passing light. The colors correspond to the allowed energy levels of the electrons for that particular element. Since each element has special energy levels, each element has its own pattern of absorption lines that makes up a unique "fingerprint" in the spectrum.

If you turn the light off, the gas may still glow -- with the same colors that were removed before! These are called emission lines.

Spectral Types of Stars; The Doppler Effect


  1. Stellar Spectra
  2. The Doppler Effect

Terms to Know

spectral type
Balmer line
Doppler shift

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:


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)
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 (Vr) small compared to the speed of light (less than, say, 1% of c), the amount of shift in wavelength is given by:

/ = Vr/c
where is the change in induced by the motion, i.e., new - old. (It gets a bit more complicated for higher velocities.)

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100