The spectrum looks like this:

What is the function which describes the shape of this
spectrum? It is called the **Planck function**
and it looks like this:

where h is Planck's constant, c is the speed of light, and k is Boltzmann's constant.

The units of the Planck function are **erg/s/cm ^{2}/Angstrom/steradian**. That
is, it is the amount of energy (erg) put out each second (s) in a wavelength
range (Angstrom) which is radiated by a surface area (cm

- The spectrum peaks at different wavelengths for different
temperature blackbodies. This is known as
**Wein's law**:

- High temperatures lead to higher intensities. The integrated
luminosity is given by the
**Stefan-Boltzmann law**:

Here's where that came from. If we integrate the Planck
function over the whole surface area of the star, and over all "solid angle"
of space, we will get the **monochromatic luminosity**:

which has units of **erg/s/Angstrom**.
If we now integrate over all wavelengths we get the **total luminosity** of the star:

with units of **erg/s**.
And that's the Stefan-Boltzmann law...

Let's see Planck's law in action, courtesy of an applet by Mike Guidry at UTKnoxville