English: This "stately" nebula resides some 1000 light years away towards the outer (Orion) arm of our galaxy. The glowing portion of the nebula is around 100 light years in extent. However, this entire region is filled with galactic gas from which many massive and luminous stars have formed. This family of young and bright stars in this spiral arm of our galaxy is called the Perseus OB2 association. The bright star to the right of the nebula in this image is Zeta Persei. It is both a member of the Perseus OB2 group and it is probably responsible to making the California nebula glow. This nebula is a well-known example of one that not only glows strongly due to H-alpha emission, but also H-beta. The emission of photons by the hydrogen atoms in this gas are caused by the elevation and subsequent drop in energy level of electrons. The electrons gain energy by collision and (most importantly in this case) by interacting with energetic photons (coming from Zeta Persei). The larger the "fall" or number of energy levels an electron drops to, the more energetic the photon that is released. In this case H-alpha photons are released when an electron falls one level (from n3 to n2 with a wavelength of 6563 angstroms). H-beta photons are released when the electron falls two levels (from n4 to n2 with a wavelength of 4861 angstroms). In most nebulae, the conditions for electrons to fall one level (H-alpha) are easier to have, so most nebulae are not strong emitters in H-beta. In addition to this nebula, the Horsehead Nebula is another well-known example that is a good H-Beta emitter.This image was taken as part of Advanced Observing Program (AOP) program at Kitt Peak Visitor Center during 2014.