Stellar Certainties

There may still be no consensus on what makes a planet a planet, but at least there is agreement on when a star is a star, as opposed to being a ‘failed star’ or ‘brown dwarf’. This boundary occurs at around 80 times the mass of Jupiter, beneath which there is insufficient gravity to start off the fusion reactions that make stars what they are, and Hubble images of some of the dimmest stars in an ancient star cluster seem to bear out this long held theory.