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The
California Nebula (
NGC 1499/
Sh2-220) is an
emission nebula located in the constellation
Perseus. Its name comes from its resemblance to the outline of the
US State of
California in long exposure photographs. It is almost 2.5° long on the sky and, because of its very low
surface brightness, it is extremely difficult to observe visually. It can be observed with a
Hα filter (isolates the H
α line at 656 nm) or H
β filter (isolates the H
β line at 486 nm) in a rich-field
telescope under dark skies. It lies at a distance of about 1,000
light years from
Earth. Its fluorescence is due to excitation of the H
β line in the nebula by the nearby prodigiously energetic O7 star,
Xi Persei (also known as
Menkib).
The California Nebula was discovered by
E. E. Barnard in 1884.By coincidence, the California Nebula transits in the zenith in central California as the latitude matches the declination of the object. NASA selected the California Nebula as its Astronomy Picture of the Day on October, 22, 2022, based on a submission from an amateur astronomer taken from a ground-based telescope.The California Nebula stretches about 100 light years across. Even though it is large and has a visual magnitude of 6.0, the nebula is very difficult to observe visually because it has a low surface brightness. Its bright integrated magnitude is spread over a large area.The nebula requires exceptionally clear, dark skies without any light pollution and the use of a rich field telescope with an Hα (H-alpha) or Hβ (H-beta) filter. An H-alpha filter is a narrow band filter that lets the H-alpha spectral lines of emission nebulae pass while blocking almost all other wavelengths. It is essential for observing challenging visual targets that emit hydrogen alpha lines.
The California Nebula is formed of gases that are ionized and made fluorescent by high-energy ultraviolet photons emitted from a young, hot, blue star in the vicinity. The massive star in question, Menkib (Xi Persei), is one of the hottest stars visible to the unaided eye. It shines at magnitude 4.04 from a distance of about 1,200 light years.Menkib is a blue giant of the spectral type O7.5III. It has a mass 26 – 36 times that of the Sun and has expanded to a size of 14 solar radii as it evolved away from the main sequence. The star shines with 263,000 solar luminosities (bolometric) with a surface temperature of 35,000 K. It is a member of the Perseus OB2 association.Menkib is an exceptionally fast spinner, with a projected rotational velocity of 220 km/s. The star’s estimated age is only 7 million years. Menkib has evolved quickly due to its high mass and will likely meet its fiery end as a supernova in the next million years.
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