Brown dwarfs are made of the same elements of their “siblings” – stars – but do not hold enough mass to start nuclear fusion. Because of this they are often referred by scientists as failed stars.
Failure of formation
The brown dwarfs start just like the other main-sequence stars – from a cloud of dust and gas, gravity then pulls them tightly and a protostar is born.
The gravity pushes the main-sequence stars inward until they start nuclear fusion. However brown dwarfs don’t reach this stage because their temperatures are not hot enough to start the fusion, and then the closely packed materials reaches a stable state and becomes a brown dwarf.
“Brown dwarfs are the missing link between gas giant planets like Jupiter and small stars like red dwarfs,” said Ian McLean, an astronomer at the University of California.
Characteristics and classifications
Brown dwarfs range from 13 to 90 times the mass of Jupiter.
They are mostly classified as M type stars – most of this type are red dwarfs and a few are brown dwarfs. The L and T dwarfs are classified by the elements of their spectra.
The Y dwarfs are the coolest of them all – their temperature ranges from 35 to 200 degrees Celsius.
Because of this, brown dwarfs give very little energy and light and were theoretical objects until the late 80s when astronomers equipment was improved enough to detect them.
They were originally called black dwarfs, but this term now stand for the final evolution stage of a star, after the white dwarf one.
Their low mass makes them easily confused for massive planets. Much like planets they can have storms and auroras, and they can host their own planets.
Is it a planet or a brown dwarf?
To tell which is which, we must measure the light they give. Like other stars, brown dwarfs emit light, mostly in the infrared spectrum which scientists can measure.
“Brown dwarfs are so elusive, so hard to find” McLean said. “They can be detected best in the infrared, and even within the infrared, they are very difficult to detect. We detect the heat glow from these faint objects in the infrared. Typically, they have to be relatively close by, within 100 light-years, for us to even detect the heat signature.”
Even considering this, its hard to find out which is a brown dwarf and which is a planet.
The International Astronomical Union says that any object massive enough to fuse deuterium is a brown dwarf, while objects less massive than 13 times Jupiter are considered planets.
“Astronomers are always looking for colder and colder free-floating, starlike objects,” said astronomer Trent Dupuy. “One key reason for this is that their atmospheres have similar temperatures to many of the gas giant planets that have been discovered orbiting stars other than the sun. So they are like little laboratories where you can study atmospheric physics relevant to extrasolar planets, but without the glare of their host star.”