|
|
Line Emission
Instead of using our spectrometer on a light bulb, what if we were to use it to
look a tube of gas - for example, hydrogen? We would first need to
heat the hydrogen to very high temperatures, or give the atoms of hydrogen
energy by running an electric current through the tube. This would cause
the gas to glow - to emit radiation. 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, the unique fingerprint of hydrogen.
These bright lines are called emission lines. Remember how we heated
the hydrogen to give the atoms energy? By doing that, we excited the electrons
in the atom - when the electrons fell back to their ground state, they gave
of photons of light at hydrogen's
characteristic energies. If we altered the
amount or abundance of hydrogen gas we have, we could change the
intensity of the lines, that is, their brightness, because more photons would
be produced. But we couldn't
change their color - no matter how much or how little hydrogen gas was
present, the pattern of lines would be the same. Hydrogen's pattern
of emission lines is unique to it. The brightness of the emission
lines can give us a great deal of information about the abundance of hydrogen
present. This is particularly useful in a star, where
there are many elements mixed together.
Each element in the periodic table can appear in gaseous form and will each
produce a series of bright emission lines unique to that element. The
spectrum of hydrogen will not look like the spectrum of helium, or the
spectrum of carbon, or of any other element.
Hydrogen:
Helium:
Carbon:
We know that the continuum of the electromagnetic spectrum extends from
low-energy radio waves, to microwaves, to infrared, to optical light,
to ultraviolet, to X and gamma-rays. In the same way, hydrogen's unique
spectrum extends over a range, as do the spectra of the other elements.
The above spectra are in the optical range of light.
Line emission can actually occur at any energy of light (i.e. visible, UV, etc.
) and with any type of atom, however, not all atoms have line
emission at all wavelengths. The difference in energy between levels
in the atom is not great enough for the emission to be X-rays in
atoms of lighter elements, for example.
Different Graphical Representations of Spectra
The sample spectra above represent energy emission as lines, the amount of
photons of light represented by the brightness and width of the line.
But we can also
make a graphical representation of a spectrum. Instead of the emission of
a characteristic energy being shown as a line, it can be shown as a peak
on a graph. In this case, the height and width of the peak show its
intensity. One example of this is the very first spectrum we looked at - the
one of the supernova remnant. The peaks and bumps on the graph are simply
a graphical representation of the emission lines of different elements.
Below, you will see the spectrum of the Sun
at ultraviolet wavelengths. There are distinct lines (in the top
graph) and peaks (in the bottom one) and if you look at the X-axis,
you can see what energies they correspond to. For example, we know
that helium emits light at a wavelength of 304 Angstroms, so if we see
a peak at that wavelength, we know that there is helium present.
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.
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.
What can we learn from continuum emission?
|
