θ1 Orionis C is an O type star found in the Orion Nebula. An O type star on the main sequence is 15 to 90 times more massive than the sun, and has a surface temperature between 30,000 and 50,000 K. These massive stars were not thought to have magnetic fields until one was discovered around θ1 Orionis C. Since then, it has been found that 10% of massive stars have magnetic fields. The magnetic field of a star can be measured by observing the Zeeman splitting in the spectral lines, or observing the circular polarization induced in the spectral line due to the Zeeman effect.
Fatta, Nicholas, "The Magnetic Field of Theta1 Orionis C" (2015). Aerospace, Physics, and Space Science Student Publications. 23.