University of Wisconsin

Space Astronomy Laboratory

ASTRO - 2

WUPPE PROGRAM CLASS 2.6

MAGNETIC WHITE DWARF

Objects: #2609 G70D8247 (GRW)

UW Astronomer: Art Code


Magnetic white dwarfs are stationary dwarfs that have extremely large magnetic fields. In fact, magnetic white dwarfs have magnetic fields stronger than any that can be formed in laboratories on the earth. To give you an idea how strong these fields are, we can compare them to objects we know. The magnetic field of the Earth is about 1 gauss. The magnetic field of the Sun varies from about 1 gauss in most areas to about 1000 gauss in sunspots. The strongest magnetic fields that can be made in laboratories on Earth, the superconducting electromagnets, are 1,000,000 gauss. The most strongly magnetic white dwarfs have field strengths up to 1,000,000,000 gauss! The reason they are so strong is that these stars are old and have shrunk down to a fraction of their original size. A white dwarf with the mass of the Sun is the size of the Earth! That's one million times smaller than the volume of the sun. As a magnetic star collapses to a fraction of its original volume, the magnetic field lines squeeze together, so in effect the field becomes stronger. This explains why the field is so extremely strong.

Magnetic white dwarfs are interesting to us because they provide a "laboratory" for studying the effects of strong magnetic fields on radiation. One such effect is that light coming from these objects is highly polarized. Magnetic white dwarfs are in the class of rare objects in the universe that show large amount of circular polarization. Since electrons like to spiral around field lines, they tend to make the electric field of a light wave do the same thing, and that is what circular polarization is. In very strong fields, the light becomes completely linearly polarized. That means the electric field of the light wave only moves back and forth in the direction of the magnetic field. These effects are so out of the ordinary that it's a fascinating subject to explore.