As a general rule, brown dwarf stars can be described as sub-stellar objects that range from between 13 and 90 Jupiter masses. Consequently, brown dwarfs are considered a missing link between gas giant planets and small stars, with those objects below the lower limit defined as sub-brown dwarfs, and those above the upper limit including the least massive red dwarf stars.
As the range of masses mentioned means brown dwarfs are unable to sustain nuclear fusion at their core like regular stars, they have been referred to as “failed stars.” Being small and faint, brown dwarfs are difficult to find, with most efforts to locate them hinging upon whether or not the spectrum of a candidate brown dwarf shows the presence of lithium, or whether or not the star is fully convective. Below are 10 more interesting facts about brown dwarf stars you may not have known.
Brown dwarf stars are not brown
All stars are classified or categorized according to their spectral classes, but despite their name, brown dwarf stars are not brown. Brown dwarf stars have been divided into four spectral classes, i.e., types M, L, T, and Y, and occur in a wide variety of colors (mostly invisible to human eyes) within these classifications. However, brown dwarfs that emit light that can be seen by human vision would most likely appear magenta or dark orange/red.
Brown dwarf stars cannot sustain hydrogen fusion
Unlike main sequence stars that fuse hydrogen into helium for several billion years, brown dwarf stars are not massive enough to trigger the process of nuclear synthesis. However, brown dwarf stars are thought to fuse deuterium (2H) if their mass is more than 13 times that of Jupiter, and lithium (7Li) if they have a mass of more than 65 Jupiters. Note that these mass limits are still the subject of some debate, and may be revised in the future.
The first brown dwarf was only confirmed in 1994
Although brown dwarf stars were suspected to exist from as long ago as the early 1960s, the first possible candidate brown dwarf was only discovered in 1988 during an infrared search for white dwarf stars. However, the first true brown dwarf star, now designated Teide 1 (located in the Pleiades open cluster), was only positively identified in 1995, based on the presence of lithium in its spectrum at a wavelength of 670.8 nm.
It may rain molten iron on some brown dwarf stars
Investigations into the luminosity variations of brown dwarf stars have shown that at least 50% of observed brown dwarfs sustain Earth-sized storm systems in their atmospheres. However, unlike the Earth where clouds consist primarily of water, the storm clouds on brown dwarfs consist of iron and various silicates in a gaseous state. When conditions permit, the gaseous iron and silicates condense, and fall toward the surface as liquid iron and sand, which gives a whole new meaning to the term “heavy weather.”
Brown dwarf stars are all about the same size
Since massive brown dwarfs (those that range in mass between 60 and 90 times that of Jupiter) are supported against gravitational collapse by electron-degeneracy pressure (similar to white dwarfs), and those at the low end that weigh about 10 times as much as Jupiter, are supported by Coulomb pressure (as planets are), it turns out that brown dwarf stars are all about as big as the planet Jupiter.
In fact, the diameters of brown dwarf stars differ by only 10-15% over their entire range of possible masses, which makes distinguishing between a brown dwarf star and a large planet exceptionally difficult.
At least one brown dwarf has survived being engulfed by a red giant
Designated WD 0137-349 B, this little brown dwarf located about 300 light years away is known to have survived its binary companions’ red giant phase, and as a result, it now orbits a white dwarf which used to be a red giant star. Because the brown dwarf was relatively massive, the act of engulfing the brown dwarf is thought to have reduced the primary star’s red giant phase from about 100 million years to as short as a few decades, with the massive brown dwarf assisting in rapidly expelling large amounts of the red giants’ matter.
The coldest brown dwarf star is colder than water ice
Discovered in April 2014, the brown dwarf star designated WISE 0855-0714, was shown to have a surface temperature of between 225K to 260K (-48 to -13 °C or -55 to 8 °F). Compare this to the upper end of what we know as “room temperature,” which is 298K (25°C or 77°F). This particular brown dwarf is located only 7.2 light years away and emits no visible light.
Many brown dwarf stars are X-ray sources
Despite their low temperatures, many brown dwarfs are known to be strong X-ray sources. Most investigators believe that the energy source for observed X-ray emissions is strong magnetic fields that originate in the boiling, highly convective state of the star’s sub-surface regions. Essentially, sub-surface flares are thought to break through the surface, which causes electrical currents that are discharged in the manner of terrestrial lightning, which in turn, causes strong X-ray emissions.
Some brown dwarf stars have planets
While the gas giant planets 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b are known to orbit brown dwarf parent stars, the brown dwarf star designated Cha 110913-773444, may be in the process of hosting the formation of a miniature “solar” system 500 light-years away in the constellation Chameleon. Investigators from Pennsylvania State University have discovered a dense disc of gas and dust around the diminutive star, which is thought to be analogous, if not similar to the disc out of which our solar system had formed. If these observations turn out to be accurate, the star Cha 110913-773444 will be the smallest star known to host a family of planets, seeing that the star is only about eight times as massive as Jupiter.
Life will likely not exist around brown dwarf stars
Even though brown dwarf stars are as likely to host Earth-type planets as any other type, computer modeling has shown that life will likely not have sufficient time to take hold on rocky planets around brown dwarf stars. Since brown dwarf stars cool down fairly rapidly, the habitable zone around such stars will be very narrow and decrease over time due to the cooling of the star. Moreover, planets in the habitable zone have to have nearly circular orbits with eccentricities of less than 10−6 to avoid the tidal forces and resulting greenhouse effects that result from tidal interactions with the host star.