HOAG'S OBJECT: A WHEEL WITHIN A WHEEL
A nearly perfect ring of hot,
blue stars pinwheels about the yellow nucleus
of an unusual galaxy known as Hoag's Object.
This image from NASA's Hubble Space Telescope
captures a face-on view of the galaxy's ring
of stars, revealing more detail than any existing
photo of this object. The image may help astronomers
unravel clues on how such strange objects form.
The entire galaxy is about 120,000
light-years wide, which is slightly larger than
our Milky Way Galaxy. The blue ring, which is
dominated by clusters of young, massive stars,
contrasts sharply with the yellow nucleus of
mostly older stars. What appears to be a "gap"
separating the two stellar populations may actually
contain some star clusters that are almost too
faint to see. Curiously, an object that bears
an uncanny resemblance to Hoag's Object can
be seen in the gap at the one o'clock position.
The object is probably a background ring galaxy.
Ring-shaped galaxies can form in several different
ways. One possible scenario is through a collision
with another galaxy. Sometimes the second galaxy
speeds through the first, leaving a "splash"
of star formation. But in Hoag's Object there
is no sign of the second galaxy, which leads
to the suspicion that the blue ring of stars
may be the shredded remains of a galaxy that
passed nearby. Some astronomers estimate that
the encounter occurred about 2 to 3 billion
This unusual galaxy was discovered
in 1950 by astronomer Art Hoag. Hoag thought
the smoke-ring-like object resembled a planetary
nebula, the glowing remains of a Sun-like star.
But he quickly discounted that possibility,
suggesting that the mysterious object was most
likely a galaxy. Observations in the 1970s confirmed
this prediction, though many of the details
of Hoag's galaxy remain a mystery.
The galaxy is 600 million light-years
away in the constellation Serpens. The Wide
Field and Planetary Camera 2 took this image
on July 9, 2001.
Credit: NASA and the Hubble
Heritage Team (STScI/AURA)
Acknowledgment: R. Lucas(STScI/AURA)