To better understand the way white dwarfs emit light, experimental astronomers have recreated the photosphere of these stars using the world's most powerful X-ray machine.
White dwarfs are the glowing lumps of carbon left over after stars have used up all their fuel. They are hot, dense and small, typically with the mass of the Sun packed into the volume of the Earth.
The structure of these objects is complex. Astronomers cannot see the glowing carbon embers because white dwarfs are always surrounded by a thin, dense layer of gas, drawn in by the star's intense gravity.
It is this gas that glows with an intense white light at temperatures usually between 8000K and 16,000K--by comparison the Sun's atmosphere is about 6000K. The gas is largely hydrogen but can also contain helium, various metals and carbon. These elements are all easy to identify by looking at the characteristic frequencies at which the elements emit and absorb light, a technique known as spectroscopy. However, the intense heat and pressure at the surface of these stars distorts the spectra, causing the lines to spread, for example. That's handy for astronomers because they can use these distortions to work out the pressure at the surface, which depends on the surface gravity. When combined with other data, such as temperature measurements, this allows them to work out the radius and mass of the star.