A fellow postdoc, Michael Cushing, is giving this week's colloquium on his research, brown dwarfs. Brown dwarfs are "failed stars," stars that do not have enough mass to keep nuclear fusion going in their cores.
One interesting fact about low-mass stars (red dwarfs) and brown dwarfs is that they are all about the same size as the planet Jupiter. This is due to a property of matter at very high pressures, like you find at the centers of these planets/stars called "degeneracy."
This neat fact inspired the following two artistic renditions comparing our sun, a red dwarf, two brown dwarfs, and the planet Jupiter. This art is from Dr. Robert Hunt at the Infrared Processing and Analysis Center in Pasadena, CA.
The first picture is what each of these objects look like in optical light. From left to right, we see the Sun, a red dwarf, a hot brown dwarf, a cool brown dwarf, and Jupiter. Jupiter is only visible due to light reflected from the sun, while the brown dwarfs glow feebly due to heat left over from their formation. The red dwarf and the sun produce their own light by nuclear fusion in their cores.
The second picture is what these objects look like in infrared light. In the infrared, even Jupiter glows due to heat left over from its formation five billion years ago. The sun is blue because it is "hot" for an infrared object. The red dwarf and hot brown dwarf are colored yellowish because they emit most of their light in the infrared, like our yellow sun emits most of its light in the optical. The cool brown dwarf is blue but hard to see, because most of its infrared light is absorbed by methane gas in its atmosphere. Finally, Jupiter is a rainbow of colors, as it has lots of different chemicals and clouds that have their own unique infrared characteristics.