Science-Paradoxically, white dwarf stars shrink as they gain mass

Telescope observations have confirmed a weird property of white dwarf stars: As they pack on more mass, they shrink in size.

White dwarfs, the stripped cores of dead stars, are thought to have this counter-intuitive quality. Because they contain an exotic material called degenerate electron gas. The more massive a white dwarf, the tighter its electrons must squeeze together. To create an outward pressure strong enough to prevent the star from collapsing under its own weight.

Understanding how white dwarfs contract as they gain mass. And it could give insight into the origins of type 1a supernovas, says astronomer and coauthor Hsiang-Chih Hwang. These supernovas are thought to occur when a white dwarf gets so massive and compact that it explodes. But no one knows exactly how white dwarf stars detonate (SN: 3/23/16).

For solo white dwarfs, the researchers studied an effect of general relativity on starlight called gravitational redshift (SN: 7/26/18). As light escapes a strong gravitational field, like the one around a dense white dwarf, its waves get stretched out into redder wavelengths. The bigger a white dwarf’s mass compared with its radius, the more extreme the stretching. That property allowed the researchers to estimate the masses of white dwarf stars, given their radii.

The new white dwarf measurements closely match theoretical predictions for the smaller sizes of heftier stars. White dwarfs with about half of the sun’s mass were about 1.75 times as wide as Earth, while those with slightly more mass than the sun came in closer to 0.75 times Earth’s width.