Although brown dwarfs are similar in size to Jupiter, they are much more dense and produce their own light whereas Jupiter shines with reflected light from the Sun. The energy emitted in the brown dwarf flare was comparable to a small solar flare, and was a billion times greater than observed X-ray flares from Jupiter. The flaring energy is believed to come from a twisted magnetic field. The grainy appearance of the image on the right is due to a shorter exposure time. LP 944-20 is about 500 million years old and has a mass about 60 times that of Jupiter, or 6 percent of that of the Sun.
Brown dwarfs have too little mass to sustain significant nuclear reactions in their cores.
Their primary source of energy is the release of gravitational energy as they slowly contract. The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and Pennsylvania State University, University Park. NASA's Marshall Space Flight Center in Huntsville, AL, manages the Chandra program. TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft.