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More About Spectra and the Spectra of Galaxies

More About Spectra and How They are Represented

When you look at the light of the Sun through a prism, you see a wonderful display of all of the colors of the rainbow. Separating the combined colors (or energies) of the Sun like this gives you a "spectrum", which is just a measure of light emission as a function of energy (or wavelength, or frequency, which are all uniquely related). This spectrum of the Sun is known as the visible spectrum and it is a small part of the light in the electromagnetic spectrum, which spans energies from radio waves to gamma-rays.

EM Spectrum

The spectrum of the Sun is a continuous spectrum and is frequently represented as shown below. This type of spectum is called an emission spectrum because what you are seeing is the acutal radiation emitted by the radiation source. In the case of the Sun, light is emitted at almost all energies in the visible spectrum, which is why you see all of the colors in the Sun's spectrum.

Continuous Spectrum

Consider another spectrum as shown below:

Lab Spectra

Certainly this is not a continuous spectrum, yet it represents some visible radiation which has been separated by means of a prism or a diffraction grating. In this view, the horizonatal axis shows the energy (or color) of the light, the same as the spectrum above, and it's easy to see that light is emitted only at particular "discrete" energies corresponding to the bright lines, and not continuously at all energies. But what kind of light source emits this pattern of discontinuous radiation? Many lab based elements can emit this type of pattern, and in fact, each element has its own characteristic patten. The one that is shown is characteristic of the hydrogen alpha ( Ha) atom. This also is an emission spectrum and the bright colored lines which show up are called emission lines. These type of spectra can be generated for many elements by vaporizing the element in a flame.

Yet while the colors are pretty, this representation doesn't tell us much about how much light there is of the colors that we see, (they seem about equally bright, but it is hard to make a quantitative comparison) and would convey no information at all if it weren't viewed within the limits of the human eye.

Consider yet another spectrum as shown below. This is a narrow segment of the spectrum of the Sun in the ultarviolet (UV) range which has been modeled in the same manner as a continuous visible spectrum. (Remember, we can't see UV as it is above the frequencies that the human eye can see.) The spectrum covers wavelengths of energy between 300 and 350 angstroms. Again, we can see where the emission by the Sun is most active (the light regions) but again we see little about the intensity of the radiation. There is no quantitative measure of the light emitted as a function of energy.

UV Bar Spectrum
Wavelength in angstroms

But suppose we were to look at the data of the image above in a format that more closely resembles the raw data which was collected. And....instead of just indicating via light bands where there are regions of activity, we could represent the intensity of the radiation as well. Many instruments which generate spectra have the ability to measure quantitatively the amount (or intensity) of light received at each energy. Examine, for example, the spectrum below, which is of the Sun in the same range as the UV spectrum above:

UV Graph Spectrum
Wavelength in angstroms

You see...while this isn't as colorful as either of the spectra shown in a "photographic" format, this representation tells us much about the emission of the Sun in this wavelength area. Not only do we know at what wavelength emission occurs, but we also know about the intensity of the emission in those areas. Compare these two representations of the same solar spectrum. Notice that where the first spectrum shows a bright line, the second shows a peak in the graph. In the second representation, the vertical axis is in units of intensity, while the horizontal axis remains the same. This type of spectrum shows us not only where the Sun emits light, but also gives a measure of how much light is emitted as a function of energy.

Absorption Spectra

Both of the examples thus far have been examples of emission spectra. Sometimes when we view sunlight through a gas, we get a spectrum which is continuous except for the regions in which the gaseous element would have emitted its signature spectra if heated. For clarification, the diagram below of sodium is provided; the top spectrum shows the emissioni lines of heated sodium, while the bottom spectrum shows absorption by sodium of a continuous source.

Sodium Spectra

Notice that the dark bands in the emission spectrum correspond to the same exact position and therefore wavelength of the bright bands in the emission spectra.

Here is another example of an absorption spectrum presented in a simulaneous "photographic" format and "graphical" format.

Bar and Graph Spectra

At the bottom you will see a graphical representation of the radiation. In the second representation, there is a dip in the graph corresponding to energies where we see dark "absorption lines" in the first representation. This obviously corresponds to the photographic representation at the top of this diagram. This time the regions where the balck bands occur correspond to the portion of the spectrum where incident radiation is absorbed.

Generally, because scientists observe at many energies of light (and not just the visible portion of the spectrum), and because it is important to know the relative intensities and exact shape of of emission and absorption features (as well as the emission's energy), most spectra used in astronomy look like graphs, similar to the second representations above. These graphical representations have two labelled axes; energy (or equivalently frequency or wavelength), along the horizontal, and intensity (or photon counts or some such thing) along the vertical axis.

Quiz Click here for a quiz on representations of spectra!
Info Click here to learn about what spectra can tell us.
Return Click here to return to observing the spectrum of M31 to determine its velocity.
 

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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