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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 24: Matter & Light

Key Ideas:

Temperature (Kelvin Scale) Kirchoff's Laws of Spectroscopy:

The Interaction of Light & Matter

Light & Matter can interact in a number of different ways:

The last two (absorption and emission) bear on the internal energy of the matter.


Temperature is a measurement of the internal energy content of an object.
Higher temperature means higher average vibrational energy per atom or molecule.
Higher temperature means more average kinetic energy (faster speeds) per atom or molecule.

Absolute Temperature

At high temperatures:
Atoms & molecules move very rapidly.
At cooler temperatures:
Atoms & molecules move more slowly.
If it gets cold enough, all motion will cease. How cold is "cold enough"?

Realistically, this represents an ultimate lower limit that is physically unobtainable (you can get close, but you never get exactly to it).

Kelvin Temperature Scale

An absolute temperature system in which the temperature is directly proportional to the internal energy.

Absolute Kelvin Scale (K):

The principal advantage of the Kelvin scale compared to the more familiar Celsius and Fahrenheit scales is that temperature measured in Kelvins is directly proportional to the amount of internal energy in an object. If you double the internal energy, you double the temperature in Kelvins. This is why the Kelvin scale is said to measure absolute temperature. Both the Celsius and Fahrenheit systems are difficult to use for relating the absolute energy content of objects because they are tied arbitrarily to the freezing and boiling points of water on the surface of the Earth.

We will primarily use the Kelvin scale in this course (and Astronomy 162), but will occasionally use Celsius (with Fahrenheit equivalents) where we are talking about planetary temperatures.

What is a Spectrum?

A spectrum is the distribution of photon energies coming from a light source:

Spectra are observed by passing light through a spectrograph:

Kirchoff's Laws of Spectroscopy

  1. A hot solid or hot, dense gas produces a continuous spectrum.
  2. A hot, low-density gas produces an emission-line spectrum.
  3. A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.
Graphic of Kirchoffs Laws

German physicist Gustav Kirchoff (1824-1887) formulated these laws empirically during the mid-19th century. While they adequately describe the different kinds of spectra that are observed, they do not explain why these spectra appear in these circumstances. A physical explanation had to wait until the 20th century for the development of quantum mechanics and modern atomic theory.

Black Body Radiation

A Blackbody is an object that absorbs all light.

As it absorbs light, it heats up.

It is also the perfect radiator:

Stefan-Boltzmann Law

Energy emitted per second per area by a blackbody with Temperature (T):
Stefan-Boltzmann Law
s is Boltzmann's constant (a number).

In Words:

"Hotter objects are Brighter at All Wavelengths"
Note the relative brightnesses at each wavelength for the three different-temperature blackbodies above.

Wien's Law

Relates peak wavelength and Temperature:
Wein's Law
In Words: Note the wavelength of peak brightness for the three different-temperature blackbodies above.


Person: Body Temperature = 310 K

Sun: surface temperature = 5770 K

Emission-Line Spectrum

A hot, low-density gas, in which the atoms are relatively isolated from each other, will emit a non-continuous emission-line spectrum

19th chemists century noticed that each element, heated into an incandescent gas in a flame, emitted unique emission lines.

Absorption-Line Spectrum

Light from a continuous spectrum through a vessel containing a cooler gas shows absoption lines.

Why does it work?

Why does each element have a characteristic line spectrum?


Discovering the reason unlocked the secret of the atom.

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