The saying "X" marks the spot holds true in this NASA Hubble Space Telescope (HST) image where Hubble-X marks the location of a dramatic burst of star formation, very much like the Orion Nebula in our Milky Way galaxy, but on a vastly greater scale.
Hubble-X is a glowing gas cloud, one of the most active star-forming regions within galaxy NGC 6822. The name Hubble-X does not refer to the shape of the gas cloud, but rather is derived from a catalog of objects in this particular galaxy. The "X" is actually a Roman numeral designation. The galaxy lies in the constellation Sagittarius at a distance of only 1,630,000 light-years and is one of the Milky Way's closest neighbors. The intense star formation in Hubble-X occurred only about 4 million years ago, a small fraction of the approximate 10 billion year age of the universe.
Giant gas clouds in NGC 6822 have held a special attraction for astronomers since their discovery by the visual observer E. E. Barnard in 1881. Edwin P. Hubble, after whom the HST is named, used the then-new 100-inch telescope at Mount Wilson Observatory in 1925 to make the first detailed photographic investigation of NGC 6822. The Hubble image reveals details too fine to be resolved from telescopes on the ground.
Stars form in groups from enormous clouds of gas and dust called giant molecular clouds. Once star formation begins in a molecular cloud, its rate accelerates until the process is stopped when one or more very massive hot stars are formed. At that point the clouds change from near darkness into the brightly glowing objects such as seen in Hubble-X. It is the intense ultraviolet radiation from the massive stars that causes the residual gas to glow. Radiation and gas outflows, called stellar winds, then cause the gas to disperse, bringing further star formation to an abrupt end.
The Hubble-X image was taken with Hubble's Wide Field Planetary Camera 2 (WFPC2) in September 1997, by astronomers C. Robert O'Dell of Vanderbilt University, Paul W. Hodge of the University of Washington, and R. C. Kennicutt, Jr. of Steward Observatory at the University of Arizona.
The image shows a nearly circular bright cloud at the core of Hubble-X. The cloud's diameter is about 110 light-years, and contains many thousands of newly formed stars in a central cluster. The brightest of these young stars are easily visible in the Hubble image, where they appear as numerous bright white dots.
Hubble-X is many times brighter and larger than the Orion Nebula, the brightest nearby star formation region in our own Milky Way galaxy. In fact, the tiny cloud just below Hubble-X, barely resolved even by HST, has about the same size and brightness as the Orion Nebula.