White Dwarfs

A dead star in Aquila, the eagle, is a double record holder. It’s the heaviest star of its type yet discovered — and also the smallest. It’s only about 130 light-years away. But it’s so tiny and faint that you need a big telescope to see it.

The star is a white dwarf — the dead core of a once-normal star — or in this case, two stars. It probably formed when two white dwarfs merged. The combined star is 1.35 times the mass of the Sun. That’s almost as heavy as a white dwarf can get. Yet it’s only about as big as the Moon.

The core of a star up to about eight times the mass of the Sun is kept “puffed up” by the radiation from its nuclear reactions. When they end, gravity wins out, and the core collapses to form a white dwarf.

But it only collapses so far. That’s because the charged particles known as electrons exert a pressure that keeps them apart.

The heavier the white dwarf is, though, the tighter it’s squeezed by gravity. Its electrons have to move faster and faster to limit the star’s collapse.

Eventually, though, the electrons just can’t move any faster. They combine with protons to form neutrons. The neutrons are perfectly happy to jam together, so at about 1.44 times the mass of the Sun, a core collapses to form a neutron star — a body about the size of a small city.

The star in Aquila isn’t massive enough to become a neutron star. Instead, it’ll remain a small but heavy white dwarf — the remnant of two dead stars.
 

Script by Damond Benningfield