ASTR 1230 (O'Connell) Lecture Notes
7.2 ASTRONOMICAL SPECTROSCOPY
Spectrum of the Sun (wrapped) from near-ultraviolet
(lower left) to near-infrared wavelengths (upper right). From NOAO.
Spectroscopy is the study of the distribution with wavelength
(or the "spectrum") of the EM energy released by a cosmic source.
Typical astronomical spectrographs use prisms or diffraction
gratings to disperse light according to its wavelength. The drawing
below illustrates a prism spectrograph:
B. THE POWER OF SPECTROSCOPY
How is this useful?
- We already saw in
Lecture 5 that the shape and peak wavelength of the EM spectrum of
a star are related to its temperature.
- This is true of any self-luminous, dense object: its EM
spectrum is smooth or continuous and changes only slowly with
wavelength. You can think of the atoms and electrons in such an
object as interacting so strongly that the individual signatures of
each type of atom are blended away.
- On the other hand, in a dilute or thin gas---the
atmosphere of the Sun, for example---the atoms do not interact
strongly, and if there are enough atoms of a given type, they will
impress their individual signatures on the emergent spectrum. The
internal electronic structure of each type of atom consists of
discrete energy states, and these states are unique
to that type.
- In the spectrum of a dilute gas, the states appear as
sharp features called "spectral lines." They are seen either in
emission (if the gas is observed in isolation) or
absorption (if it is observed against a continuous source).
The 3 general kinds of spectra are summarized in the illustration
- Because each type of atom produces a unique spectrum, the
chemical composition of cosmic objects can be deduced from
their spectra even though they may be millions of light years away.
The figure below shows optical-band spectra of 13 kinds of stars. The
dark lines are produced by atoms of hydrogen, calcium, magnesium,
iron, sodium, titanium, and other elements. Quantitative analysis of
such spectra allows us to deduce the detailed chemical makeup of the
Sun and other stars.
For example, the element helium (the
second lightest after hydrogen) was discovered in the spectrum of the
Sun before it was studied in an Earth-bound laboratory; several
absorption features of helium (labeled "He") are identified in the
The figure shows how the prominent absorption lines change with the
temperature of the star. The labeling at the left hand side gives the
type of each spectrum. From top to bottom the type listing runs
from high (30,000o) to low (3,500o)
temperatures. Click for an enlargement.
- In addition, spectra also contain information on the velocity,
pressure, density, temperature, ionization state, turbulence, magnetic field
strength, and other physical characteristics of cosmic objects.
Spectroscopy is therefore an exceedingly powerful tool. It is the
source of much of the astrophysical information we have about the
- Perhaps the most famous application of astronomical spectroscopy
is the use of the "Doppler
shift" in the spectra of distant galaxies to demonstrate that the
universe is expanding.
- In modern professional observatories, spectrographs are frequently the
largest, most sophisticated, and costly instruments used with telescopes.
For a representative sampling of recent state-of-the-art spectrographs,
see this lecture from my ASTR 5110 course.
September 2015 by rwo
Spectral class/absorption feature diagram © M.
Briley. Text copyright © 2001-2015 Robert W. O'Connell. All
rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 1230 at the
University of Virginia.