Active stars maintain hot, magnetically trapped plasmas above their photospheres.
The mechanism of heating these corone is unknown. Astrophysical plasmas show various physical conditions that laboratory experiments cannot reproduce. However, coronae of active stars provide a much larger range of magnetic phenomena due to to their different ages, their various rotation periods and spectral types. Flares on the Sun and stars are observed over a wide range of radiated energy. The whole family of flares is thought to originate from energy released by reconnection of the magnetic field.