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Spectral Analysis

Spectral Analysis



As discussed in the introduction to spectra, a spectrum is simply a chart or graph of the intensity of light of a source as a function of the energy of that light. Each spectrum holds a wide variety of information. For instance, there are many different mechanisms by which an object, like a star, can produce light. Each of these mechanisms has a characteristic spectrum.

Look at the sample spectrum below.

spectrum of Cas A

X-ray spectrum of supernova remnant Cas A from ASCA data. (Credit: Holt et al., PASJ 1994)

This spectrum was taken by the ASCA satellite. It is an X-ray spectrum of a supernova remnant (SNR), which is a huge cloud of gaseous matter swept up from the explosion of a massive star. The x-axis shows the range of energy of light that is being emitted (the units are keV, or kilo-electron Volts, which are the common units astronomers use to talk about X-ray light). The y-axis of the graph shows the intensity of the light being emitted by the SNR — that is, how many photons of light the SNR is giving off at each energy.

There are two main types of spectra in this graph – a continuum and emission lines. Notice that the overall curve decreases with higher energies. This decreasing curve is called a continuum, and is produced by a process that emits photons at all energies. That curve, though, has bumps on it, which are marked by the symbols for various elements like neon (Ne), calcium (Ca), and iron (Fe). These bumps are called line emission and occur at very specific energies. These two different types of X-ray emission (continuum and line) are produced by different processes and each tells us different things about the source emitting them.

Below we'll go into more detail about line and continuum emission, such as what mechanisms cause them, and what they can tell us about the light-emitting object. However, you might first want to understand a little more about atoms and about how spectra are represented graphically. Use the links below to read more about those topics.

* Tell Me More About Understanding the Atom!
* Tell Me More About Graphical Representations of Spectra!

Line Emission

Remember that if we use a spectrometer on white light (like an ordinary light bulb), we'll see a rainbow. However, what if instead we used our spectrometer to look at a tube of pure gas, like hydrogen? First, we need to heat the hydrogen to a very high temperature, or give the atoms of hydrogen energy by running an electric current through the tube. This would cause the gas to glow, or in other words, to emit radiation.

Hydrogen emission lamp

A hydrogen emission lamp. (Credit: Photo by Wilco Oelen via Wikimedia Commons

If we looked at the spectrum of light given off by the hydrogen gas with our spectroscope, instead of seeing a continuum of colors, we would just see a few bright lines. Below we see the spectrum, which shows the unique fingerprint of hydrogen.

Hydrogen spectrum

The spectrum of hydrogen. The bright lights are emission lines, and the pattern is unique to hydrogen. Other elements show different patterns of emission lines, making this spectrum a "fingerprint" of the element hydrogen.



Hydrogen spectrum

Helium spectrum

Carbon spectrum


Continuum Emission

We'll focus this discussion of continuum emission on those processes that produce X-rays. Just like visible light, with its range of energies from red to blue, X-rays have a continuum, or a range of energies associated with it. X-rays usually range in energy from around 0.5 keV up to around 1000 keV.



diagram of Synchrotron radiation

Illustration of the process of synchrotron radiation. (Credit: NASA's Imagine the Universe)



diagram of Bremsstralung

Illustration of the process of Bremsstralung radiation. (Credit: NASA's Imagine the Universe)


Supernova remnants (SNRs) have strong magnetic fields where ions can be accelerated by the shock wave of the supernova to high energies, producing X-ray synchrotron radiation. X-rays produced by SNR require electrons with energies of about 104 GeV each. We would have to heat an electron to a temperature of about ten trillion degrees for it to have this much energy!

Video illustration of Bremsstralung radiation


Compton scattered radiation and synchrotron radiation are major components of the diffuse X-ray background and emission from active galaxies.

diagram of Comptonization

Illustration of the process of Compton radiation. (Credit: NASA's Imagine the Universe)

Video illustration of Compton radiation

* Use Hera to analyze spectra.

Updated: August 2013



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