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A rare and spectacular head-on collision between two galaxies
appears in this NASA Hubble Space Telescope true-color image of the
Cartwheel Galaxy, located 500 million light-years away in the
constellation Sculptor. The new details of star birth resolved by
Hubble provide an opportunity to study how extremely massive stars are
born in large fragmented gas clouds.
The striking ring-like feature is a direct result of a smaller intruder
galaxy -- possibly one of two objects to the right of the ring -- that
careened through the core of the host galaxy. Like a rock tossed into
a lake, the collision sent a ripple of energy into space, plowing gas
and dust in front of it. Expanding at 200,000 miles per hour, this
cosmic tsunami leaves in its wake a firestorm of new star creation.
Hubble resolves bright blue knots that are gigantic clusters of newborn
stars and immense loops and bubbles blown into space by exploding stars
(supernovae) going off like a string of firecrackers.
The Cartwheel Galaxy presumably was a normal spiral galaxy like our
Milky Way before the collision. This spiral structure is beginning to
re-emerge, as seen in the faint arms or spokes between the outer ring
and bulls-eye shaped nucleus. The ring contains at least several
billion new stars that would not normally have been created in such a
short time span and is so large (150,000 light-years across) our entire
Milky Way Galaxy would fit inside.
Hubble's new view does not solve the mystery as to which of the two
small galaxies might have been the intruder. The blue galaxy is
disrupted and has new star formation which strongly suggests it is the
interloper. However, the smoother-looking companion has no gas, which
is consistent with the idea that gas was stripped out of it during
passage through the Cartwheel Galaxy.
The picture was taken with the Wide Field Planetary Camera-2 on October
16, 1994. It is a combination of two images, taken in blue and
January 10, 1995
Credit: Kirk Borne (ST ScI), and NASA