Brown Dwarfs are the lowest mass products of the star formation process. They are often called “failed stars” as they don’t have enough mass to ignite nuclear fusion at their core. But that label implies that the stars are the objects that celestial objects should aspire to be when no such preference really exists. As a counter to that connotation they are sometimes referred to as “overexcited planets”.
By definition, brown dwarfs are objects with a degenerate core, or one where you can’t push the electrons close enough together to get Hydrogen to start fusing into Helium. The upper mass range on when the core becomes degenerate is around 75 times the mass of Jupiter so you can think of brown dwarfs as objects that are less massive than that value. In this article we will review five fast facts about these exotic objects.
Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History and Editor at dailygalaxy.com. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. With over 100 peer-reviewed articles in astrophysical journals, Faherty has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognizes scientists who have made an impact in the field of dynamical astronomy, and the 2021 Robert H Goddard Award for science accomplishments.
Hydrogen is the most abundant element in the Universe. As you look across the cosmos you will find Hydrogen in every direction. Molecular Hydrogen will build up into what Astronomers call giant clouds and these become the seed area for new stars. At some point either the weight of the cloud itself or possibly a shock wave from a nearby supernova will cause the cloud to compress. When that happens, star formation is underway! As the gas gets pushed tightly onto itself it gets extremely hot and fragments into pieces. There are large pieces that break off — those are your big stars– and small pieces that break off. The smallest fragments of this process are the brown dwarfs at masses less than 75 times the mass of Jupiter. We don’t have a good idea what the lowest bound is on the fragments. It could be as a single Jupiter mass or lower but those would probably be very rare. A secondary process we think might form brown dwarfs is in a disk around a newly formed star. This process is how we think planets (like the Earth!) formed and is probably not very efficient at making big brown dwarfs. But we can’t rule it out.
Brown dwarfs are cold objects compared to stars. At their surface, the highest temperatures brown dwarfs reach is ~3000 K. Think of these temperatures as the highest cook settings on a pottery kiln. The sun for comparison has a surface temperature almost twice that. The lowest temperatures we’ve discovered them at is ~250 K. That’s a cold day at the north pole! As a result of being cooler objects, brown dwarfs do not give off a lot of light in the optical. Instead, they glow brightest in the infrared. NASA has sent two major observatories into space that have studied brown dwarfs in detail in the infrared. The Wide Field Infrared Survey Explorer (WISE) and the Spitzer space telescope have both been instrumental at characterizing brown dwarfs.
Brown dwarfs are elusive celestial objects. Given that they do not have fusion ongoing at their core, they lack the “engine” which keeps their appearance constant. Consequently, brown dwarfs spend their lives cooling as they lose that gravitational energy from their formation. As they age, they can morph from looking like a low mass star, to looking like Jupiter. When they are young and hot, a more massive brown dwarf will have an atmosphere with lots of oxides (Titanium Oxide, Vanadium Oxide, Carbon Monoxide). Then as they cool, the new temperatures re-arrange the chemistry and we start to see methane gas and ammonia taking over. The same mass object will look very different depending on when you catch it in its evolutionary stage.
Another consequence of brown dwarfs lacking nuclear fusion is that they have a calm, sustained existence on the cosmological timeframe. Unlike stars that have a given fuel tank of Hydrogen at their core that can run out, brown dwarfs just slowly dim down. They don’t have a exit strategy from the Universe that stars do. There is no giant phase for brown dwarfs and no evolved stage of white dwarfs, neutron stars, or black holes. Every brown dwarf that has ever existed is still around today. The ones that are as old as the oldest stars, will have very little in their atmospheres since there were very few elements around when the first stars (and brown dwarfs) formed.
One of the most intriguing aspects of studying brown dwarfs is their strange and exotic weather patterns. The temperatures of brown dwarfs lead to conditions in the atmosphere where clouds of liquid iron, and thick silicates can form. Famously, brown dwarfs may have cloud decks of conundrum which — if given the right impurity — might be found in the form of rare gems such as rubies and sapphires. Astronomers have carefully stared at brown dwarfs as they rotate and have seen their light dip and change indicating that there are likely storms raging similar to how we see the great red spot on Jupiter.
Image credit: NASA/JPL-Caltech