Credit: Jose-Luis Olivares, MIT
Raining onto a White Dwarf
A white dwarf is the a burnt-out core of a relatively low-mass star left behind after the star runs out of fuel. Most white dwarfs are very dense balls of carbon atoms, so dense that a spoonful of white dwarf matter at the surface of the earth would weigh as much as a pickup truck. White dwarfs are supported by a "electron degeneracy pressure" produced by a quantum-mechanical effect that occurs when matter is highly compacted and compressed. They are also ticking time bombs, since there's a limit to how much matter this degeneracy pressure can support; if the mass of a white dwarf reaches this limit (called the Chandrasekhar limit, after the scientist who discovered it) the white dwarf will collapse and explode. So understanding how the mass of a white dwarf can increase is an important topic. If the white dwarf is isolated, its mass will only slowly increase due to accretion of small bits of interstellar matter that may fall onto the white dwarf. But if the white dwarf has a companion star in orbit around it, the white dwarf can steal matter from the companion. Usually the stream of stolen matter will form a thin disk around the white dwarf as it spirals onto the white dwarf's surface. But if the white dwarf has a powerful magnetic field, the magnetic field can be strong enough to lift the accreting material into a narrow column of material flowing onto the white dwarf at the magnetic poles, as shown in the colorful artist illustration above. This illustration shows the stream of material from the companion star on the right forming an accretion disk around the white dwarf, while the white dwarf's bipolar magnetic field pulls the material at the inner edge of the accretion disk into two columns before it falls onto the poles of the white dwarf. For the first time, astronomers have been able to study this process in fine detail using the Imaging X-ray Polarimeter Explorer (IXPE). IXPE is unique among X-ray satellite observatories in that it can measure not only the brightness and energy of cosmic X-rays, but also the orientation, or polarization, of the electromagnetic waves of X-rays as well. IXPE's observation of accretion by a magnetized white in a binary system called Ex Hydrae showed that the X-ray emission had a surprisingly high degree of polarization. The IXPE data suggest that the columns are huge, about 2000 miles high, roughly half the radius of the white dwarf itself, much larger than theory predicted.
Published: November 24, 2025
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 01-Dec-2025 08:08:49 EST