Brown Dwarf

Astronomers have recently discovered a previously unknown star only 7.2 light years away. You might wonder how they could miss a star that close to us. Well, Wise J085510.83-071442.5 is not a regular star. It is a brown dwarf, a star that doesn’t actually shine. Here is the story according to the Daily Mail

A brown dwarf star that appears to be the coldest of its kind – as frosty as Earth’s North Pole – has been spotted by an American astronomer.

The discovery was made using Nasa’s Wide-field Infrared Survey Explorer (Wise) and the Spitzer Space Telescope.

Images from the space telescopes also pinpointed the object’s distance at 7.2 light years away, making it the fourth closest system to our sun.

‘It is very exciting to discover a new neighbour of our solar system that is so close,’ said Kevin Luhman, an associate professor of astronomy and astrophysics at Penn State and a researcher in the Penn State Center for Exoplanets and Habitable Worlds.

‘In addition, its extreme temperature should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.’

Brown dwarfs start their lives like stars, as collapsing balls of gas, but they lack the mass to burn nuclear fuel and radiate starlight.

The newly-found brown dwarf, named Wise J085510.83-071442.5, is thought to have a chilly temperature between -48°C to -13°C (-54°F to 9°F).

Previous record holders for coldest brown dwarfs, also found by Wise and Spitzer, were about room temperature.

To understand what a brown dwarf actually is, you must realize that like people, stars are born, they live, and eventually they die. A star’s lifespan is measured in eons rather than years and they take little notice of such microscopic mayflies such as we are. A star is born when a massive cloud of gas collapses in on itself. If you recall your high school physics class, you may remember that when a gas is compressed, in this case by the protostar’s gravity, it will heat up. The energy used in compressing the gas is converted into heat. If the protostar’s mass is greater than about .08 solar masses (8% of our Sun), the center of the cloud will become dense enough and hot enough to begin fusing hydrogen into helium. A new star is born. If the mass is less than .08 solar masses than fusion will never begin and the star is stillborn. It becomes a brown dwarf.

Some of the more massive brown dwarves, around 13 to 65 Jupiter masses may fuse deuterium and lithium. They emit mostly infrared radiation. Interestingly, brown dwarves are about the same volume as Jupiter, regardless of their masses. Gravity compresses them to about the same volume. In a way, Jupiter, Saturn, and Neptune could be considered to be very low mass brown dwarves. They emit more energy than they receive from the sun. Perhaps the precise line between a massive planet and a very low mass star is not so easy to define.

General size comparison between a low mass sta...

 (Photo credit: Wikipedia)

A brown dwarf should not be confused with a red dwarf or a white dwarf. A red dwarf is a real star with a mass from about .08 to .5 solar masses. They are small and dim but they do fuse hydrogen in their core and they emit visible light. A white dwarf is a dead star. It has run out of fuel and has collapsed under its own mass until only electron degeneracy, the pressure of electrons crammed together, prevents it from collapsing any further.

But stellar corpses are another topic which I will have to write about another time.

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