Adhara

Binary star about 430 light years from Earth. The surface temperature of its brighter component, Adhara A, is 21,000 K (20,700°C/37,292°F), almost four times that of the Sun. Adhara's apparent magnitude is 1.5, so it can be regarded as the least bright of the first-magnitude stars or the brightest of the second-magnitude stars. It appears to us as the brightest source of extreme ultraviolet radiation in the sky, apart from the Sun.

Most of space contains un-ionized hydrogen in tiny amounts but Adhara is connected to our local cloud by a tunnel of almost hydrogen-free space. This means its ionizing radiation reaches our local cloud without obstruction. US astronomers concluded in 1995 that it plays a greater part in ionizing hydrogen in our region of the Galaxy than all the other 3 million stars lying closer to our local cloud.