In the universe there are many different stars and planets, but there’s often been some contention as to what the difference is between a very high mass planet and a very low mass star. Brown dwarfs, supposedly failed stars, are sometimes referred to as this missing link.
A brown dwarf is often considered in the same class as regular stars because it’s born in the same way, but the former doesn’t gain enough mass for a hydrogen fusion engine in its core to ignite and thus becomes a so-called ‘sub-stellar’ object. This presents a conundrum that’s racked the ﬁeld of astronomy for the last 30 years – is a brown dwarf star really a star at all? Since they don’t have the mass to initiate nuclear fusion like a normal star during its formation, they’re often referred to as ‘failed stars’. With masses that range from just a few times larger than our solar system’s gas giant Jupiter, to around 75 times its size, brown dwarfs are often considered to be the missing link between gas giant planets and red dwarf stars – the smallest known ‘true stars’.
Measuring or even discovering the presence of a brown dwarf star is notoriously difficult because they’re so cool and small, so scientists use the presence of lithium as a determining factor. The presence of lithium is actually common in all young stars, but is usually burnt up in the ﬁrst 100 million years of its life. Since the core of a brown dwarf isn’t hot enough to get rid of the lithium it’s a very useful indicator in labelling low-mass stellar objects ‘brown dwarf stars’.
Image courtesy of NASA.