What Is #9: A Brown 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

In my discussion of other objects in this "What Is" series, I've made mention that an object in space becomes a star when it starts fusing hydrogen into helium in the core. It takes high temperatures of nearly 10 million K to start fusion, but once started, the object is an official star. Later on, the star can stop fusing hydrogen as it enters its latter evolutionary stages, but it is always referred to as a star. What happens though if the object trying to become a star is not hot enough to fuse hydrogen into helium in the core? If it's very close to another star, it will orbit that star and be a planet. This is similar to Jupiter in our Solar System, although Jupiter needed much more mass to become a star. It isn't very close. If the object is by itself and not gravitationally bound to another star, we call it a Brown Dwarf.

Brown Dwarfs

Sometimes would-be stars do not acquire enough mass to have core temperatures high enough to fuse hydrogen into helium. If not part of another solar system, these objects are Brown Dwarfs, or often referred to as "failed stars". It's much easier to make small stars than big stars, so there are far more small stars than big stars in our galaxy. Thus there are likely many, many more Brown Dwarfs than there are regular stars in the galaxy. Brown Dwarfs are very difficult to observe as they are very dim and give off very little light. They have energy from their formation, but don't emit much at a time. As telescopes improve, more Brown Dwarfs will be discovered, but the numbers at this time are small. The first Brown Dwarf was discovered in 1995 and there are only about 3000 confirmed observations of Brown Dwarfs to date. There are billions of stars and likely billions of Brown Dwarfs in the galaxy, so this shows how difficult they are to find. Below is a comparison of the Sun to Brown Dwarfs and a few other astronomical objects.

MPIA/V. Joergens - First published in "Joergens, Viki, 50 Years of Brown Dwarfs - From Prediction to Discovery to Forefront of Research, Astrophysics and Space Science Library 401, Springer, ISBN 978-3-319-01162-2.

As you can see, the Sun is much bigger than a large Brown Dwarf, although Jupiter is still smaller than a very cool Brown Dwarf.

Conclusion

Sometimes there just isn't enough material to make a star, but an object, a Brown Dwarf, still forms. Could there be planets around Brown Dwarfs? Yes! Could there be life on one of these planets? Possibly!