With media you are modeling a sphere
that is filled with a gas that varies in
color...
The final color depends on the background,
for example a red glow will appear purple
against a blue sky.
You can keep tweaking the size of the
sphere and the color map until you get
somewhat what you'd like.

In the real world, bright spots come from
a lens distortion effect at your eye.

Chris Colefax's include file models this by
placing a 2 dimensional disc with an
appropriate semi-transparent texture
near the camera. Some values are passed
by #declare to set the apparent location
of the glow, etc. In general for the type
of situation you are describing it would
be the better approach.
http://www.geocities.com/ccolefax/lenseffects.html

There's nothing keeping you from modeling
both the gas and the lens effect.

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