Brown Dwarf Star: Definition & Overview

Instructor: Suzanne Rebert

Suzanne has taught college economics, geography, and statistics, and has master's degrees in agricultural economics and marine affairs (marine resource management).

If you failed to become a star, could you still be cool? Brown dwarf stars are. Read about how these unusual sub-stellar objects were discovered and what we know about them today. You can also take a quiz to test your understanding.

Definition

A brown dwarf star is a sub-stellar object whose mass ranges between that of gas giant planets and small stars. Its mass is too low for it to fuse hydrogen, as main-sequence stars like our Sun do. Brown dwarfs can be thought of as a kind of 'missing link' between planets and stars.

Because they are a relatively recent discovery, scientists are still debating about exactly where to draw the lines between these objects, other stars, and large planets, as well as whether an object must have been able to burn hydrogen at some time in its life to qualify as a brown dwarf.

As a general rule, scientists believe the smallest brown dwarfs could be roughly the same mass as Jupiter, while the boundary between the most massive brown dwarfs and true stars is thought to be around 90 Jupiter masses. In practice, however, confirmed brown dwarfs range from around 13 to 80 Jupiter masses.

Three brown dwarfs -- Teide 1, Gliese 229B, and WISE 1828 -- with our Sun, a red dwarf star, and Jupiter for comparison
brown dwarf comparison

Theoretical Background

Scientists first theorized in the 1960s that relatively small, dark sub-stellar objects might exist throughout our galaxy, but simply be too difficult to detect through methods available at the time. Some might be 'free-floating,' while others might be companions of larger stars. These bodies could have formed, as true stars do, through the collapse of gas clouds, but be too small to sustain the conditions that allow stars to fuse hydrogen atoms into helium, emitting powerful radiation in the form of light and heat.

At first, these hypothetical objects were called black dwarfs, but that term was already used for old white dwarf stars that had cooled too much to shine brightly. Other names were proposed, but astronomer Jill Tarter suggested 'brown dwarf' in 1975, and this was generally adopted as a good approximation of the variety of magenta, red, and purple shades these objects would actually appear.

Compared to Stars

Brown dwarfs are much smaller and cooler than main-sequence stars. They are expected to range from several thousand degrees Celsius down to even lower than the freezing point of water. Lithium, methane, or even water vapor can be present in the relatively cool atmosphere of a brown dwarf. That doesn't mean it would be a good idea for us to land a ship on one, however; the lightest brown dwarfs are far more massive than Jupiter, so the gravity would squash us flat. Even if we survived, we would be unable to take off again. Radiation and wild weather -- possibly including iron rain! -- would also be serious problems. That doesn't mean it would be impossible to live on a planet orbiting a brown dwarf, however. Some brown-dwarf planets might even be warm enough and stable enough to support indigenous life.

Compared to Planets

Strangely enough, most brown dwarfs are believed to be about the same radius as Jupiter, even though their mass can be much greater. For the least massive brown dwarfs, a property known as Coulomb pressure keeps them from shrinking farther, as it does for gas giants like Jupiter. For brown dwarfs at the upper end of the mass range, the property called electron-degeneracy pressure controls size; this also applies to the small true stars known as white dwarfs.

What this means is that brown dwarfs are all about the same size as large planets, and they don't shine brightly like main-sequence stars. These facts can make them hard to identify, but there are a couple of clues that can give them away: they glow, and some emit X-rays. To be sure, however, observers need to catch brown dwarfs before they cool off enough to look just as dark and quiet as real planets.

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