What Is #8: A White Dwarf

The 'What Is' Series

A couple of months ago I started a new series on this blog titled "What Is". Thus far the focus is on the field of astronomy where we hear all sorts of cool science taking place in space. The goal of this series is to explain the details of different objects we hear about relatively frequently.

Stellar Evolution

Stars are giant balls of gas that fuse in their cores. For most of their lives stars are fusing hydrogen into helium in their cores and releasing energy in the process. Eventually the hydrogen supply runs out and low mass stars such as the Sun evolve into Red Giants and grow very large in size. Red Giant stars were discussed in the last edition of this blog's 'What Is' series.

Eventually these stars start to fuse helium into carbon. It is possible to fuse carbon into heavier elements, but very high core temperatures are needed. This doesn't happen in low mass stars the size of the Sun. Instead, the outer layers of the star are shed outward (called a planetary nebula) and the remaining carbon core is called a White Dwarf.

A White Dwarf Star

The Sun, in about 5-6 billion years, will eventually become a White Dwarf. This is the last stage of a low mass star such as the Sun. White Dwarfs are very small for stars. The core continues to contract during the Red Giant stage and eventually becomes the size of a terrestrial planet such as the Earth. When stars finally reach the White Dwarf stage, they have shrunk, but are very hot objects. They are no longer producing energy, but instead slowly releasing their energy. Eventually they will release all of their energy, dim, and become what is called a Black Dwarf. A Black Dwarf is a White Dwarf that has released its energy. This takes a long, long, long time! In fact, it takes several billion years to reach the White Dwarf stage and several tens of billions of years more to become a Black Dwarf. Therefore, the Universe, at 13.8 billion years old, is not yet old enough for any Black Dwarfs to exist!

White Dwarf stars - NASA

White Dwarfs Can Be Weird

This isn't the end story for White Dwarfs, however. When stars are in binary systems and the conditions are just right, a White Dwarf can pull mass from the atmosphere of another star. This mass can start to fuse on the surface, drastically increasing the White Dwarf's brightness for a period of time. This is a nova. If enough mass is dumped on the surface of the White Dwarf, it can explode as a supernova! This is a different type of supernova compared to the typical supernova that is the result of high mass stars. The moral of this story is that weird things can happen to stars in binary systems.