hottest star

The central stars of planetary nebulae are the hottest known stars. They have been detected with surface temperatures up to 250,000 K. One example of a planetary nebula with such a hot central star is NGC 2240. At such high temperatures, most of the radiation is emitted in the ultraviolet part of the spectrum so the star is often not obvious in visual images of the nebula. Planetary nebulae form when evolved stars eject their outer layers. Their central stars are what were the stellar cores and their surface temperatures continues to rise after the nebulae have formed. The maximum temperature reached is predetermined by the mass of the stellar core. Masses are thought to range between 0,55 and 1.2 times the Sun's mass. The greater the mass, the higher the maximum surface temperature achieved by the star.