California nebula — Emission Nebula in Perseus

The California Nebula is a large emission nebula in the constellation Perseus, about 1,000 light-years from Earth. It spans roughly 2.5 degrees of sky — five full Moons end-to-end — and takes its name from its unmistakable outline, which traces the U.S. state almost exactly. The nebula's hydrogen is ionized by the nearby hot blue star Xi Persei (Menkib), one of the most luminous O-type stars within 2,000 light-years of the Sun. Its low surface brightness and specific emission wavelengths make it a famous target for imaging and narrowband observation despite its near-invisibility in conventional filters.

Visually one of the hardest showpieces in the northern sky — its surface brightness is extremely low, and without a filter it is essentially invisible even from dark sites. An H-beta filter (sometimes called the 'California filter') transforms it: in binoculars or a short, fast refractor at very low power, a ghostly U-shaped arc appears. A UHC filter also works, OIII does not. Best framed around Xi Persei at a true field of at least 2.5 degrees. Photographically trivial with a DSLR and H-alpha filter — a 30-second exposure shows the shape clearly. Best observed from October through February.

Discovered by Edward Emerson Barnard on November 3, 1884 using a 6-inch refractor. Barnard — later famous for his dark-nebula catalog and for discovering Barnard's Star — noticed the faint glow around Xi Persei while searching for comets. Its California-like shape was obvious to American observers from the start, and the name stuck. Isaac Roberts produced one of the first photographs of the nebula in the 1890s, and it has remained a staple target for deep-sky imagers ever since.