Introduction to Spectroscopy

Spectroscopy is a complex art – but it can be very useful in helping scientists understand how an object like a black hole, neutron star, or active galaxy is producing light, how fast it is moving, and even what elements it is made of. A spectrum is simply a chart or a graph that shows the intensity of light being emitted over a range of energies. Spectra can be produced for any energy of light – from low-energy radio waves to very high-energy gamma-rays.

Spectra are complex because 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 – or using the technical term for light, electromagnetic radiation. Each of these mechanisms has a characteristic spectrum.

Let’s look at a spectrum and examine each part of it.

X-ray Spectrum of supernova remnant Cas A

To the right is an X-ray spectrum made using data from the ASCA satellite. It is of a supernova remnant (SNR) – a SNR 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 y-axis of the graph shows the intensity of the light recorded by the instrument from the SNR – – that is, the number of photons of light the SNR is giving off at each energy, multiplied by the sensitivity of the instrument at that energy. We can tell that the light, or radiation, from this SNR is very high energy – if we look at the units of the X-axis – we can see that the photons of light have energys measured in keV, or kilo-electron Volts. A kilo-electron Volt is 1000 electron Volts (eV). This puts is the X-ray range of the electromagnetic spectrum.

The graph shows a decreasing curve, with lots of bumps in it. The curve itself is called a continuum – it represents X-ray photons emitted at all energies continuously. The X-rays that are producing this continuum can be caused by several mechanism that are completely different than those producing the X-rays at the various peaks and bumps on the curve. The peaks and bumps are called line emission. Not only are these two different kind of X-ray emission (continuum and line) produced differently, but they each tell us different things about the source that is emitting them.

The Electromagnetic Spectrum

White light (what we call visible or optical light) can be split up into its colors easily and with a familiar result – the rainbow. All we have to do is use a slit to focus a narrow beam of the light at a prism. This set-up is actually a basic spectrometer.

White light spectrum

The resultant rainbow is really a continous spectrum that shows us the different energies light (from red to blue) present in visible light. But the electromagnetic spectrum encompasses more than just optical light – it covers all energies of light extending from low-energy radio waves, to microwaves, to infrared, to optical light, to ultraviolet, to very high-energy X- and gamma-rays.

Line Emission


Spectrum of Hydrogen




Spectrum of Hydrogen


Spectrum of helium


Spectrum of carbon


Different Graphical Representations of Spectra



Ultraviolet spectrum of the Sun

Spectra and Astronomy

In a star, there are actually many elements present. The way we can tell which ones are there is by looking at the spectra of the star. In fact, the element helium was first discovered in the Sun, before it was ever discovered on Earth. The element is named after the Greek name for the Sun, Helios. The science of spectroscopy is quite sophisticated. From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star. Emission lines can also tell us about the magnetic field of the star. The width of the line can tell us how fast the material is moving, giving us information about stellar wind. If the lines shift back and forth, it means that the star may be orbiting another star – the spectrum will give the information necessary to estimating the mass and size of the star system and the companion star. If the lines grow and fade in strength we can learn about the physical changes in the star.

Spectral information, particularly from energies of light other than optical, can tell us about material around stars. This material may have been pulled from a companion star by a black hole or a neutron star, where it will form an orbiting disk. Around a compact object (black hole, neutron star), the material in this accretion disk is heated to the point that it gives off X-rays, and the material eventually falls onto the black hole or neutron star. It is by looking at the spectrum of X-rays being emitted by that object and its surrounding disk, that we can learn about the nature of these objects.

Continuum Emission

White light spectrum

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.

Like line emission, continuum X-ray emission involves charged particles. Continuum emission is a result of the acceleration of a population of charged particles. All X-ray sources contain such particles. These particles must be at least partially ionized – their electrons need to be unbound from their nuclei to be free to zip around when they are heated to extreme temperatures. For an electron to radiate X-rays, the gas containing the electron must have extreme conditions, such as temperatures of millions of degrees, super-strong magnetic fields, or the electrons themselves must be moving at nearly the speed of light. Extreme conditions can be found in disks of matter orbiting black holes or in supernova remnants. Strong magnetic fields, like those created in the wake of a supernova explosion, can also accelerate fast moving ions in spirals around the field lines to the point of X-ray emission. Electrons can be accelerated to nearly the speed of light in the shockwave created by a supernova explosion.

There are three mechanisms that will produce a continuum X-ray emission. They are Synchrotron Radiation, Bremsstrahlung, and Compton Scattering. The radiation produced is continuous, and not at the discreet energies of line emission because the populations of electrons have a continuous range of energies, and they can be accelerated through a range of energies.

Sychrotron radiation
Image courtesy of University of Hertfordshire

Sychrotron radiation is emitted when a fast electron interacts with a magnetic field. A magnetic field in an area an electron is traveling in will cause the electron to change direction by exerting a force on it perpendicular to the direction the electron is moving. As a result, the electron will be accelerated, causing it to radiate electromagnetic energy. This is called magnetic bremsstrahlung or synchrotron radiation (after radiation observed from particle accelerators by that name). If the electrons and the magnetic field are energetic enough, the emitted radiation can be in the form of X-rays.

Bremsstrahlung emission
Image courtesy of University of Hertfordshire

Bremsstrahlung occurs when an electron passes close to a positive ion, and the strong electric forces cause its trajectory to change. The acceleration of the electron in this way causes it to radiate electromagnetic energy – this radiation is called bremsstrahlung, (literally, from the German meaning ‘braking radiation’). Thermal bremsstrahlung occurs in a hot gas, where many electrons are stripped from their nuclei, leaving a population of electrons and positive ions. If the gas is hot enough (millions of degrees Kelvin), this kind of radiation will primarily take the form of X-rays.

Compton Emission
Image courtesy of University of Hertfordshire

Comptonization is when a photon collides with an electron – the photon will either give up energy to or gain energy from the electron, changing the electron’s velocity as a result.

What Are Some Examples of Continuum Emission?

Gas that is hotter than 10 million degrees, such as the gas heated by a supernova explosion, produces most of its emission in X-rays from thermal Bremsstrahlung. Gas can be heated to these temperatures by the outward moving shock of a supernova explosion, or in an accretion disk around a black hole or neutron star. Synchrotron radiation can produce X-rays around supernova remnants (SNR), where the magnetic fields are strong and ions have been accelerated by the shock wave to high energies. X-rays produced by SNR require electrons with energies of about 104 GeV (Giga electron-Volts) each (you would have to heat an electron to a temperature of about ten trillion degrees for it to have this much energy)! Synchrotron radiation and Compton scattered radiation are major components of the diffuse X-ray background and emission from active galaxies.

For the Student

Using the text, define the following terms: spectroscopy, keV, continuum, continuum emission, line emission, electromagnetic spectrum, synchrotron radiation, bremmstrahlung, comptonization.

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