Brown Dwarfs: What Are They?

A possible look at what a brown dwarf might be like.
A brown dwarf is a substellar object that is too cool to be a star but too warm to be a planet. This is an artist's conception of what one might look like.


Brown Dwarfs: a Sub-Stellar Object With a Difference

There are many different kinds of stars out there. You have red giants and blue giants, stars like the Sun, and at the other end of the age spectrum — the slowly cooling white dwarfs. The range of objects that we call "stars" bottoms out at something called a "brown dwarf". These are what astronomers like to call "sub-stellar objects". It simply means that they are not massive or hot enough to be actual stars (which fuse hydrogen in their cores). But, they're still part of the hierarchy of stellar objects. Another way to think of them is: too hot to be planets, too cool to be stars.

There are brown dwarfs throughout our galaxy, and most of them were born with too little mass to start the fusion process in their cores. The Hubble Space Telescope has spotted dozens of them in the nearby Orion Nebula. Since they glow in the infrared, the Spitzer Space Telescope and other infrared-sensitive instruments can study these things as well.

What do We Know About Brown Dwarfs?

Astronomers know that these objects are cool — not as cool as a glacier, or an iceberg — but cool for a "star". Their atmospheres are more like a gas giant's, such as Jupiter's. But, they're nothing at all like a gas giant planet otherwise. Their temperatures are well below the Sun's, ranging up to 3600 K (about 3300 C, or 6000F). For comparison, the Sun's temperature is 5800, or about 5526 C, or nearly 10,000 F. They're also smaller than the Sun, and nearly all are around the size of Jupiter.

Their low temperatures and sizes make spotting brown dwarfs more difficult than looking at their brighter, more massive stellar siblings. That's why infrared-enabled technology is so important in searching out these objects.

Why Study Brown Dwarfs?

There are many reasons, but mainly understanding how they form and in what numbers they exist tells astronomers something about the process of star formation in nebulae. For example, if you have a mass of gas and dust in a star-forming region, once you start making stars, you'll get a number of high-mass stars that eat up most of the star birth material. The rest forms the middle-mass and smaller-mass stars. And, the brown dwarfs also take up some of that material. Whether they're the leftovers from the whole process, or form from the same cloud but under some other conditions is something that astronomers are working to understand.

There are many sizes and masses of brown dwarfs, each with their own atmospheric compositions and activity rates. There have been some interesting findings suggesting that brown dwarfs could support planets. At least two objects have been discovered that look like they could be planets, but astronomers caution that they could also be sub-brown dwarfs, objects still too hot to be planets but too cool to be stars, and even smaller than the small brown dwarfs they orbit. But, given that brown dwarfs have been found with disks around them, and that disks are the places where planets form, it's not a huge stretch to imagine that someday we'll see one with planets. And, that will raise the question of whether or not those worlds could be habitable.

A Stellar Cannibal and a Brown Dwarf

It turns out there is another way to make a brown dwarf: by turning something that used to be a star into a brown dwarf. It requires a very hungry nearby white dwarf star. Astronomers discovered such an animal in 2016, called J1433. It's relatively near our solar system, at a distance of 730 light-years. It's actually pair or objects &nmdash; a binary system containing a white dwarf and its tiny brown dwarf companion. The companion orbits the white dwarf once every 78 minutes! Because they're so close together, the white dwarf has actually stripped away much of the material form its companion — at least 90 percent of its mass. That has turned what was once a star into a cool, low-mass brown dwarf. The process took billions of years to accomplish.

So, if it happened at J1433, could it happen elsewhere? That's possible if conditions are just right. So, now astronomers will have more than one reason to study and understand brown dwarfs. Not only do they tell us something about the star formation in a given region, but if they happen to be part of binary systems, such sub-stellar objects can reveal secrets of aging stars that cannibalize their companions.