What is Spectroscopy?

Astronomers study light from distant objects, and they can do this using three types of tools: images, spectra (singular is spectrum), and light curves. The image below shows an example of each as an astronomer might study them.

From left to right: a lightcurve, an astronomical image, and a spectrum. These are the basic tools that astronomers use to study objects in the Universe.

The image is of the Crab Nebula taken by Hubble. Credit: NASA, EAS, J. Hester and A. Loll (Arizona State University). Click here for more information from the Hubble web site.

In this activity, you will be working with analyses of spectra. A spectrum is a measure of the intensity of light as a function of wavelength or energy. (Recall that the wavelength of light is related to the energy by E = hc/λ. By convention, X-ray astronomers talk about light's energy rather than wavelength.) The study of spectra is known as spectroscopy.

You are probably familiar with prisms and the way they separate white light into its constituent colors – shine white light through a prism, and you'll see a rainbow (pictured below).

Illustration of a prism splitting white light into a rainbow of colors.

That rainbow is basically a spectrum – you can see the intensity of light at different wavelengths by looking at the rainbow. In the X-ray, you can't see the light, so astronomers plot out the intensity versus energy in a graph. The spectrometers on Suzaku tabulate the energy of X-rays that pass into the detector – the different X-ray energies are equivalent to the different colors of visible light.

What can we learn from spectra?

When atoms are excited, they emit and/or absorb light at certain fixed energies. These "excitations" show up in spectra as a spike or dip at a very specific energy based on the element that emitted or absorbed the light. By precisely measuring the energy of these features in a spectrum, astronomers can determine what elements were present. For example, look at the spectrum below:

X-ray spectrum of the supernova remnant in Cassiopeia A as observed by the Chandra X-ray Observatory. The "bumps" in the spectrum are signatures of different elements present in the hot gas that produced this spectrum. The elements are labeled: silicon, sulfur, argon, calcium, and iron. The y-axis is the detector "channel", which is equivalent to the energy of the light. Note that the x-axis in on a logarithmic scale.

The graph shows the "channel", which is equivalent to energy, along the x-axis and the number of X-ray photons hitting the detector (or counts) on the y-axis. Notice that there are several "bumps" on the spectrum – around channel numbers 110, 170, 205, 250, and 450. Scientists have determined that the energies where these bumps appear can be associated with transitions in silicon, sulfur, argon, calcium, and iron (respectively). This means that those elements must be present in the hot gas that produced this spectrum.

Using this information, astronomers constructed images of the object, supernova remnant Cassiopeia A, or Cas A for short. These image show them where different elements appear in the source. See the results below.

On the top, the X-ray spectrum of the supernova remnant in Cassiopeia A as observed by the Chandra X-ray Observatory. Below images of the Cas A region using the light coming from specific elements – from left to right: silicon, calcium, and iron. The approximate regions of the spectrum corresponding to each element/image is indicated on the spectrum.

Click here for more information on this image from the Chandra website.

As you can see, spectra can help you identify individual elements in an astronomical object. Different elements have emission lines (the "bumps" in the spectrum above) at very specific energies. By identifying the energies where emission lines appear, you can trace the lines back to the original element.