HUBBLE SEES THE GLOW OF STAR FORMATION
IN A NEIGHBOR GALAXY
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
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.
Credit: NASA and The Hubble Heritage Team (STScI/AURA)