Ask Astro: What will white dwarfs, neutron stars and brown dwarfs look like at the end of their lives?

What will white dwarfs, neutron stars and brown dwarfs look like at the very end of their lives, when they no longer emit radiation?

Bill Zuna

Tallahassee, Florida

Let’s first look at a white dwarf. These stellar remnants are the remnants of Sun-like stars and consist mostly of carbon and oxygen. The outer shell of a white dwarf is so hot that it will emit visible light for about 10^ten — it’s 1 followed by 10 zeros — years. By then, the atoms will have cooled enough to crystallize into a giant diamond. However, this diamond does not last forever. As time passes another 10³⁸ years, the protons and neutrons inside all atoms will decay and produce faint traces of light. First, all atoms break down into hydrogen. Then these hydrogen atoms will slowly disappear through a hypothetical process known as proton decay.

To understand how long that would take, imagine if you were to count every person on Earth (about 8 billion) at the rate of one per year. By the time you’re done, the white dwarf would no longer glow. Then if you were to count every atom in every person on Earth – there are about 1028 atoms in a human body – at the rate of one atom per year, the star would have practically disintegrated by the time you’re done.

A neutron star is the remnant of a massive star that has run out of fuel, exploded, and collapsed into a super dense star. Like a white dwarf, a neutron star cools in about 10^ten years until it no longer emits visible light. However, unlike white dwarfs, neutron stars have a thin crust surrounding densely packed neutrons. Over the next 10³⁸ years, scientists think that the crust will disintegrate thanks to the decay of protons. Eventually the gravitational pull drops and the star expands into something reminiscent of a white dwarf (which is only 10³⁸ years to live).

Finally, a brown dwarf is barely visible at first. It does not fuse hydrogen in its core, which means it is not a real star. It could fuse deuterium or heavy hydrogen, but this fusion will stop after only 10⁸ years. Then it will slowly succumb to the decay of protons in 10³⁸ years.

It’s a very bleak future, but there’s plenty of time until then!

CHS Cabot

Graduate Student, Department of Astronomy, Yale University, New Haven, Connecticut