SDSS Spectrum Graphs

The SDSS records two main types of data: photometric and spectroscopic. Preflight training in SDSS Data explains in detail how photometric (imaging) and spectroscopic data are captured, recorded, and accessed in the database. In this training session we look in detail at just one of the data products, the spectrum, and learn how astronomers might begin to interpret what they see. We begin with a graph.

An SDSS spectrum


Step 1: Know What Was Measured

The X- and Y-axis on the graph show you what properties of light were measured and reported. Wavelength is measured using very small units of length such as the nanometer (one billionth of a meter) or angstrom (one ten-billionth of a meter).
The Y-axis displays the amount or intensity of the radiation detected for each bin in wavelength. Its units are much more complicated. Astronomers refer to intensity using the word flux. A flux measurement takes into account exposure time and collecting area when describing the intensity of the measured light. Usually, the flux is measured in units of energy (ergs) in a particular period of time (seconds) within a set area (square centimeters). For now, all you need to pay attention to is the fact that the flux increases as you move up the Y-axis. Be careful to pay attention to the range; it is not all the same from one SDSS spectrum to another.

Step 2: Observe the Overall Shape of the Spectrum

After making note of the units and scale used on a spectrum, astronomers turn their attention to the overall shape of the spectrum, at first ignoring little bumps and wiggles. The question they might ask is, “What does the continuum emission of this graph look like?” The presence or absence of a peak is important. The location of the peak should be noted as well. Click here to see the continuum for the graph above.
In general, a graph where the continuum is easy to spot and has an identifiable peak is usually associated with a star. This type of curve is referred to as a blackbody curve and can be used to identify the temperature of the object that emitted it. The Launch activity, Stars as Blackbodies, explores this further. In addition to stars, the SDSS focuses on capturing spectra of galaxies and very distant objects called quasars. Each has continuum features in common as shown below.
Galaxy spectrum

Galaxy Spectrum

Galaxy Spectrum

Galaxy continuum emissions range from rather flat to slightly curved. In either case, it is difficult to identify a peak. This makes sense since the light we receive from a galaxy is a mixture of billions of stars of all different colors. However, a galaxy that appears blue would be expected to have a different shape than a red galaxy. What color galaxy do you think produced this spectrum?
Quasar spectrum

Quasar Spectrum

Quasar Spectrum

Quasar continuum emissions almost always show an obvious rise at the blue end of the spectrum (to the left on this graph). Unlike stars, the rise is sharp and does not come to a peak before reaching the end of the measurable range for SDSS spectra. Quasars are high energy objects; much of the radiation they emit is at ultraviolet wavelengths.


Step 3: Observe the Spikes and Dips

A perfect blackbody would emit a perfectly smooth continuous spectrum. In reality none of the spectra in the SDSS are perfectly smooth. They all have spikes and dips which an astronomer uses to understand the composition of the object and its environment. The spikes are referred to as emission lines and the dips, absorption lines. Which features are present? Are they large or small? Broad or narrow? Where are they? Each of the three main types of objects found in the SDSS (stars, galaxies, and quasars) has some characteristics in common. A quick inspection of a continuum emission can usually reveal which it is. Look at the spectra below. Note the features you see and then click on the image to see how you did.

Step 4: Notice Absorption and Emission Lines

You have observed that some objects have emission lines in their spectra, others have absorption lines, or still others have some combination characteristic of the type of object that emitted it. Emission lines represent places where light has been added to the continuous spectrum at specific wavelengths by hot or excited gas, which radiates light at specific wavelengths. Absorption lines indicate that light has been absorbed (removed from the spectrum) by gas or dust surrounding the source of the light.

A chart of absorption lines for different atoms

Astronomers understand that every element in the universe emits a characteristic combination of light when excited. They use this knowledge to add to their interpretation of spectra. The pattern of wavelengths they see reveals the chemical content of the object that produced it. The picture at the right shows just a few examples. For more information about how atoms absorb and emit particular wavelengths of light use this spectroscopy resource from the University of Arizona. Use the animation below to gain a general understanding of how these spectral features are produced.
For each position of the telescope, see if you can explain what is happening to the electromagnetic energy. Where is it being produced? What happens to it before reaching the telescope and the detector (spectrometer)?