Radiation from Different
Types of Sources (Kirchhoff's Laws)
- How do we analyze it? We can
separate wavelengths with a spectroscope.
- Low-density hot gas: Produces
an emission line spectrum, atoms emit individually at specific
- Low-density cooler gas: Produces
an absorption spectrum by removing light from continuum.
- Hot solid or dense fluid: Produces
a continuous spectrum, including all wavelengths.
- Individual particles: If they
are moving in a magnetic field, they also produce a continuous
spectrum, but it does not look like the ones from dense hot matter;
it is not "thermal".
Continuous Spectra: Temperature
- What we see: A hot solid or
dense fluid produces a continuous spectrum, including all wavelengths;
also called "thermal spectrum".
- Heat and temperature: Heat corresponds
to the motion energy of atoms or molecules. The lowest possible
temperature is absolute zero, 0 K = -273°C = -460°F.
- Temperature effects: When T
increases, the radiation emitted increases at all wavelengths
[Stefan's law], and the frequency at which most of the radiation
is emitted also increases [Wien's law].
- Conclusion: Get T from
an object's peak emission frequency or wavelength! Once you know
T, find its size from its brightness.
How Can We Explain Them?
- What we see: A hot low-density
gas produces an emission line spectrum; atoms emit individually,
and light comes out only at specific wavelengths.
- Atomic structure: Nucleus (protons
and neutrons), electrons; energy levels, different for different
- What is going on? Individual
atoms and ions absorb/emit only at specific, characteristic wavelengths
because fixed energy packets (photons) are absorbed/emitted
when an electron jumps between energy levels, and an atom is
- Conclusion: Light comes out
of atoms in packets. We also have evidence that it acts as if
made of individual particles or quanta when hitting matter [photoelectric
effect]. So, photons = both particles and waves;
their particle and wave properties are related by Energy = h (frequency).
Line Spectra: What Additional
Factors Affect Them?
- Rotation effect: Lines are spread
out as a result of different velocities on opposite sides.
- Temperature effect 3: Width
of spectral lines increased by random motion.
- Conclusion: If you know T,
the line width tells you the rotation rate!
Other Types of Radiation;
- Radio waves from H atoms: When
the direction of the electron's spin changes; wavelength l = 21 cm; detected from all interstellar
gas which is not too cold [it takes an atom roughly 11 Myr to
emit one such photon, but there are lots of H atoms!].
- Radio waves from molecules:
Atoms in them can be in different rotation and vibration states;
changes between states usually involves less energy (infrared,
- Conclusion: Using spectroscopy,
we can get information on kind of source, radial velocity, composition,
(including ions and molecules) and temperature, from positions
and intensities of lines [and rotation from shape of line]; Examples
from the Sun, and other stars.
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012