The California Nebula is a glowing cloud of hydrogen gas. It is lit by the hot blue star Menkib (xi Persei), visible in this image as the very bright star above the nebula. Menkib is one of the hottest and intrinsically brightest stars visible to the naked eye. It can be found in the Orion arm of the Milky Way, the same arm in which our Sun lies.

Menkib's surface temperature is about 37,000 Kelvin (about 66,000 degrees Fahrenheit, or more than six times hotter than the Sun). It has about 40 times the mass of the Sun and gives off 330,000 times the amount of light. Menkib is a runaway star and the fast stellar wind it blows is piling up in front of it to create a shock wave. This shock wave is heating up dust. Menkib was born from the California Nebula only a few million years ago, along with some other hot blue stars. It lights up the nebula, as well as heating and ionising it. In visible light, the ionised gas glows red, while in infrared light we see the heated dust.

This nebula cannot be seen by the naked eye because of its low surface brightness. It can only be viewed with a wide-field telescope under a dark sky.