Brown dwarfs

¡SkyCaramba! Weekly astronomy blog for the week ending May 24, 2014

Ever since the invention of the telescope, astronomers have known there is much more to the universe than meets the naked eye. Today we understand the search for knowledge of the cosmos isn’t just about magnifying distant and dim objects until the naked eye can see them. Some of the closest and most common objects may not even emit visible light.

Stars produce heat and light through nuclear fusion. Innumerable atoms are pulled closer and closer together by each other’s gravity until there’s no distinction anymore between where one atom ends and another begins. Two hydrogen nuclei become one helium nucleus. Other hydrogen nuclei are pulled apart and become deuterium, an isotope of hydrogen. Other heavier atoms are created too. Bits of light energy called photons emerge.

It may take thousands of years for photons to escape the star’s center and millions or more to reach us. Light travel time from the sun’s surface to the earth is a little more than eight minutes. Light from the farthest known galaxy started heading our way 13.4 billion years ago.

The Hubble Space Telescope has collected quite a lot of that ancient light, but even very close objects can be hard to see. Just six light years away, a red giant shines so dimly, astronomers didn’t know it existed until it showed up in photographs in 1888. It’s now called Barnard’s star, after Edward Emerson Barnard, the first astronomer to give it serious study in 1916. He’s considered to have discovered the star, since nobody else paid attention to it.

Barnard’s star would actually be a fairly bright star if we could see the sky in infrared through Earth’s atmosphere. Orbiting telescopes with infrared sensors do that. Some of them have been used to find star-like objects with too little density and gravity for any fusion to happen at all. Those objects have been named brown dwarfs.

Brown dwarfs are somewhat like the gas giant planets in our solar system. They’re a lot bigger, just not nearly as big as stars. In them, atoms pull together through gravitational attraction but not enough to merge into heavier atoms. There’s enough activity to generate heat which is seen in infrared. In some cases, a brown dwarf could have once been an actual shining star that now has some leftover heat.

We’re not talking about a lot of heat. The surface of the hottest brown dwarf could be a comfortable room temperature for a human being. Most are much colder. A brown dwarf pair just seven light years from us is colder than Earth’s polar regions. The two were discovered in 2013 using images taken two and three years previous.

Scientists believe quite a lot of the universe’s mass is dark matter incapable of producing its own light but having undeniable gravitational effects. Brown dwarfs are part of the yet emerging picture.