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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.
January 4, 2001
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)