What is Spectroscopy

Spectroscopy is a very important tool in astronomy. It is detailed study of the light from an object. Light is energy that moves through space and can be thought of as either waves or particles. The distances between the peaks of the waves of light are called the light's wavelength. Light is made up of many different wavelengths. For example, visible light has wavelengths of about 1/10th of a micrometer - ten thousand wavelengths would be the width of a dime.

Spectrometers are instruments which spread light out into its wavelengths creating a spectra. Within this spectra, astronomers can study emission and absorption lines which are the fingerprints of atoms and molecules. An emission line occurs when an electron drops down to a lower orbit around the nucleus of an atom and looses energy. An absorption line occurs when electrons move to a higher orbit by absorbing energy. Each atom has a unique spacing of orbits and can emit or absorb only certain energies or wavelengths. This is why the location and spacing of spectral lines is unique for each atom.

Astronomers can learn a great deal about an object in space by studying its spectrum, such as it's composition (what its made of), temperature, density, and it's motion (both it's rotation as well as how fast it is moving towards or away from us).

There are three types of spectra which an object can emit: continuous, emission and absorption spectra. The examples of these types of spectra shown below are for visible light as it is spread out from purple to red, but the concept is the same for any region of the electromagnetic spectrum.

Continuous spectra

Continuous spectra (also called a thermal or blackbody spectra) are emitted by any object that radiates heat (has a temperature). The light is spread out into a continuous band with every wavelength having some amount of radiation. For example, when sunlight is passed through a prism, it's light is spread out into it's colors.

A continuous visible light spectra

Absorption spectra

If you look more closely at the Sun's spectrum, you will notice the presence of dark lines. These lines are caused by the Sun's atmosphere absorbing light at certain wavelengths, causing the intensity of the light at this wavelength to drop and appear dark. The atoms and molecules in a gas will absorb only certain wavelengths of light. The pattern of these lines is unique to each element and tells us what elements make up the atmosphere of the Sun. We usually see absorption spectra from regions in space where a cooler gas lies between us and a hotter source. We usually see absorption spectra from stars, planets with atmospheres, and galaxies.

Detailed image of our Sun's visible light spectra

The absorption spectra of hydrogen - can you see this pattern in the solar spectrum above this image? Hint: hydrogen is the most abundant element in the sun - look at the darkest lines.

Emission spectra

An emission spectra occurs when the atoms and molecules in a hot gas emit extra light at certain wavelengths, causing bright lines to appear in a spectra. As with absorption spectra, the pattern of these lines are unique for each element. We can see emission spectra from comets, nebula and certain types of stars.

The emission spectra of hydrogen

To learn more about spectroscopy, emission, absorption and continuous spectra and how atoms and molecules produce spectral lines see the following sites:

  • Production of Light - Nick Strobel's Astronomy Notes    
  • Spectroscopy - Astronomy Camp.    
  • University of Tennessee - Astro 162    

  • In practice, astronomers rarely look at spectra the way they are displayed in the above images. Instead they study plots of intensity, signal or flux versus wavelength. These plots show how much light is present or absent at each wavelength. A peak in the plot shows the position of an emission line and dip shows where an absorption line is. The spacing and location of these lines are unique to each atom and molecule.

    The shape of the continuous spectra (often refered to as the continuum) on a plot is dependent on temperature and motion of the emitting gas. In this simple plot it is shown as a flat line - in reality it is usually a curved line. Also, many of the real data plots you will see have the wavelength or frequency on a logarithmic scale.

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