Astronomers recently stumbled upon a teeny star called 2MASS J0523-1403 located just 40 light years away. It's not only the smallest star discovered so far - it may also represent the smallest possible star. By studying stars such as this, scientists are starting to be able to answer the question: where do stars end and brown dwarfs begin?
Stars are burning balls of gas held together by gravity that are fuelled by the fusion of hydrogen atoms to helium in their cores. Stars come in a variety of sizes; the smallest stars, known as red dwarfs, can possess as little as 10% of the mass of our Sun, whereas the biggest stars (hypergiants) can be over 100 times as massive as the Sun. But just how small can an object be and still be defined as a star? This has mystified astronomers for years. All that was previously known is that objects below this limit don’t have enough mass to ignite the fusion of hydrogen in their cores. These objects are known as brown dwarfs.
Brown dwarfs are elusive objects that are thought to be the missing link between gas giants and low-mass stars such as red dwarfs. They’re generally around the size of Jupiter, but they don’t have enough mass to become a star. Unlike stars, brown dwarfs have no internal energy source.
There exists another strange difference between brown dwarfs and stars; they have opposite relationships between mass and size. The more material you add to a star, in the form of hydrogen, the bigger the radius of the star. I.e. stars increase in size as mass increases. Brown dwarfs, on the other hand, actually shrink in size with increasing mass because of something called electron degeneracy pressure.
So how do we find the limit that dictates whether an object is a star or a brown dwarf? Astronomers scanned the skies and located objects that were thought to lie around the stellar/brown dwarf border. They then calculated the luminosity, temperatures and radii of all of these objects and plotted them. Temperature is dependent on mass but it’s easier to measure. They found that as temperature decreased, so did radius; this is the expected trend for stellar objects. However, they found that after temperatures of around 2100K (1826oC [3320 oF]) there was a break until radius starts to increase with decreasing temperature; this is the trend that would be expected for brown dwarfs.
Thanks to this data, scientists can now pinpoint the specific temperature, luminosity and radius at which the main sequence ends. The main sequence is a relationship between luminosity and temperature (and luminosity and radius) that is obeyed by stars throughout the majority of their lives. 2MASS J0523-1403 is located around this boundary, but toward the stellar side. This star actually has a temperature of 2074K, which is the lowest described temperature so far for a main sequence star. It’s also the smallest and the least massive; if it had less mass then it would be a brown dwarf. This star has therefore been identified as a representative of the smallest possible star. However, it is theoretically possible that a star with a slightly smaller mass than 2MASS J0523-1403 could exist, but we haven’t found it yet.
Scientists believe that information such as this could help us in our search for life on other planets. Brown dwarfs cool much quicker than stars, so their surrounding planets are likely not very habitable. Therefore knowing the temperatures of objects around the star/brown dwarf boundary will assist astronomers in their search for candidates that could support habitable planets.