- NGC 6822 Barnard's Galaxy
NGC 6822 is a barred irregular galaxy in the constellation Sagittarius. It is a member of the Milky Way's Local Group of galaxies, located 1.6 million LY away. It was discovered by the American astronomer E. E. Barnard in 1884. Edwin Hubble published a detailed study of it in 1925, in which he identified 11 Cepheid variable stars. These stars allowed an accurate estimate of the galaxy's distance to be made. This was the first time that the distance to an extragalactic object beyond the Magellanic Clouds was accurately determined.
NGC 6822 contains numerous star clusters and nebulas. Two of the more interesting of these objects are just visible along the upper right edge of the galaxy. These are two large nebular structures designated as HIII (commonly called the Ring Nebula, not to be confused with M 57 in Lyra) and HI (commonly called the Bubble Nebula, not to be confused with NGC 7635 in Cassiopeia). The Ring Nebula was extensively studied by C. A. Clayton. The ring is actually a shell of ionized hydrogen glowing red in Hα emissions. The shell surrounds a tight cluster of blue-white giant stars that provide the ionizing energy for the gas shell. The shell is huge, with a diameter of around 390 LY. There is still debate as to the nature of this shell— is it a supernova remnant (like the Veil Nebula in Cygnus) or a bubble blown out in the surrounding hydrogen gas cloud by the stellar winds from the young stars at its center (like NGC 7635).
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