Brown Dwarf

The true definition of a star is any object that has enough mass to support fusion in its core. That’s what’s happening in the Sun, and even in the smaller red dwarf stars. But if a star is too small, it lacks the mass to support fusion. Instead of blazing with the light of a fusion at its core, it smolders with the heat of gravitational collapse of its mass. These objects are called brown dwarf stars.

You can look through these books and instructional materials from Amazon.com for more information about stars.

The smallest possible mass of a star is about 75-80 times the mass of Jupiter. In other words, if you could collect together another 80 planets the mass of Jupiter and mashed them together, the object would ignite with fusion at its core. This is why Jupiter will never be a star. And so, brown dwarfs sit in between large gas giants and red dwarf stars on the mass scale. In fact, the line between a really massive planet and a brown dwarf is pretty blurry.

Brown dwarfs, like regular stars, form at the heart of collapsing clouds of gas and dust. Most of the mass collects at the center, with a disk of material surrounding it. Mutual gravity causes the material to collapse inward, and the friction of this gravitational collapse causes it to heat up. After about 100 million years, stars will actually ignite fusion at their cores and begin the path towards becoming a main sequence star. Brown dwarfs, on the other hand, can never ignite fusion.

For the first billion years or so, brown dwarfs and regular stars are hard to distinguish. The heat from the gravitational collapse of a star can still make it shine with the same temperature as a regular star. Over time, though, the brown dwarf continues to cool and becomes a dull red color, while a star continues to shine brightly.

In order to distinguish regular stars from brown dwarfs, astronomers look for the presence of lithium in the star. All stars form with some lithium, which is an remnant left over from the Big Bang. Main sequence stars will rapidly consume their lithium, turning it into helium. But brown dwarfs lack the mass to do this reaction. So the presence of lithium is one way to determine a brown dwarf.

We have written many articles about stars on Universe Today. Here’s an article about the color of brown dwarfs, and here’s an article about the coolest brown dwarfs ever discovered.

Want more information on stars? Here’s NASA’s World Book on Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?