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Spectroscopy
Spectroscopy is a complex art - but it is 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 eneriges. 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.
Above 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 received by the ASCA
detector from the SNR. 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 constituent 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.
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.
On the next few pages, we'll go into more detail about line and continuum
emission - what mechanisms cause them, and what they can tell us about
the light-emitting object. But first, to understand the ways in which energy
is converted into light, we have to understand how the atom works.
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