 THE
LURE OF THE RINGS
Resembling a diamond-encrusted bracelet, a ring
of brilliant blue star clusters wraps around the
yellowish nucleus of what was once a normal spiral
galaxy in this new image from NASA's Hubble Space
Telescope (HST). This image is being released to
commemorate the 14th anniversary of Hubble's launch
on April 24, 1990 and its deployment from the space
shuttle Discovery on April 25, 1990.
The sparkling blue ring is 150,000 light-years
in diameter, making it larger than our entire home
galaxy, the Milky Way. The galaxy, cataloged as
AM 0644-741, is a member of the class of so- called
"ring galaxies." It lies 300 million light-years
away in the direction of the southern constellation
Volans.
Ring galaxies are an especially striking example
of how collisions between galaxies can dramatically
change their structure, while also triggering the
formation of new stars. They arise from a particular
type of collision, in which one galaxy (the "intruder")
plunges directly through the disk of another one
(the "target"). In the case of AM 0644-741,
the galaxy that pierced through the ring galaxy
is out of the image but visible in larger-field
images. The soft spiral galaxy that is visible to
the left of the ring galaxy in the image is a coincidental
background galaxy that in the same group as the
other two, but is not interacting directly with
the ring.
The resulting gravitational shock imparted due
to the collision drastically changes the orbits
of stars and gas in the target galaxy's disk, causing
them to rush outward, somewhat like ripples in a
pond after a large rock has been thrown in. As the
ring plows outward into its surroundings, gas clouds
collide and are compressed. The clouds can then
contract under their own gravity, collapse, and
form an abundance of new stars.
The rampant star formation explains why the ring
is so blue: It is continuously forming massive,
young, hot stars, which are blue in color. Another
sign of robust star formation is the pink regions
along the ring. These are rarefied clouds of glowing
hydrogen gas, fluorescing because of the strong
ultraviolet light from the newly formed massive
stars.
Anyone who lives on planets embedded in the ring
would be treated to a view of a brilliant band of
blue stars arching across the heavens. The view
would be relatively short-lived because theoretical
studies indicate that the blue ring will not continue
to expand forever. After about 300 million years,
it will reach a maximum radius, and then begin to
disintegrate.
The Hubble Heritage Team used the Hubble Advanced Camera for
Surveys to take this image in January 2004. The team used a
combination of four separate filters that isolate blue, green,
red, and near-infrared light to create the color image.
Image Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)
Acknowledgment: J. Higdon (Cornell U.) and I. Jordan (STScI)
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