What Is #14: A Red 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.

Stars

I have recently covered Black Dwarfs, Brown Dwarfs and White Dwarfs. Brown Dwarfs are objects that were not quite large enough to fuse hydrogen, so they never reached full star status. White Dwarfs are the final stage of a low mass star such as the Sun. After these low mass stars shed away their outer layers, the stellar core leftover is called a White Dwarf. Black Dwarfs do not yet exist in the Universe, but eventually White Dwarfs will finally emit all of their energy, leaving behind a leftover core that no longer releases energy. This leftover core will be called a Black Dwarf. But there are also Red Dwarfs. How do Red Dwarfs differ from their cousins?

Red Dwarfs

Unlike Black Dwarfs, Brown Dwarfs, and White Dwarfs that are NOT fusion hydrogen to helium in their cores, Red Dwarfs are fusing hydrogen to helium. Red Dwarfs are stars on the Main Sequence (the hydrogen fusing portion of their lives). As the name implies, Red Dwarfs are red stars. Red Dwarfs are the smallest stars still able to fuse hydrogen into helium. They have low surface temperatures and give off more red light than they do blue light. Thus the redder appearance and name Red Dwarf.

Red Dwarfs have surface temperatures below 4000 K. For comparison, the Sun's surface temperature is about 5800 K. They range in mass from about 1/2 the mass of the Sun to about 8% the mass of the Sun. This is the lower mass limit for a star to be able to fuse hydrogen into helium. Every single Red Dwarf star formed in the Universe is still a Red Dwarf fusing hydrogen into helium. This includes Red Dwarfs formed as the first stars in the Universe formed. Not a single Red Dwarf has evolved into the latter stages of life. Red Dwarfs fuse their hydrogen supply so slowly that the Universe is not old enough yet for a Red Dwarf to evolve past the Main Sequence. Wow!

Isn't astronomy cool?!?!! Yes, yes it definitely is cool!