Bright pink nebulae almost completely encircle
a spiral galaxy in this NASA/ESA Hubble Space Telescope
image of NGC 922
The ring structure and the galaxy's distorted shape
results from a smaller galaxy scoring a cosmic bullseye
hitting the centre of NGC 922 some 330 million years ago
Episode 60: Galaxy hits the bullseye
Hubble's image of NGC 922 shows that the galaxy is far from being an ordinary spiral
Its arms are disrupted; a stream of stars extends out into space
and a bright ring of nebulae encircles the core
Observing NGC 922 with NASA's Chandra satellite reveals more chaos
in the form of ultraluminous X-ray sources dotted around the galaxy
NGC 922's unusual form today has its origins hundreds of millions of years ago
At that time, a smaller galaxy plunged right through the heart of NGC 922
Although it's outside Hubble's field of view now
images from a survey telescope on the ground
show the interloper shooting away from the scene of the crash
As the small galaxy passed through the middle of NGC 922
it sent out ripples that disrupted the clouds of gas
and triggered the formation of new stars
The radiation from these new stars in turn lit up the gas
The bright pink colour of the resulting nebulae
is a characteristic sign of this process, and it is caused by excited hydrogen gas
the dominant element in interstellar gas clouds
It's a similar process to that used in neon signs here on Earth
In theory, if two galaxies are aligned in just the right way
with the smaller galaxy passing right through the centre of the larger one
the ring of nebulae will form a perfect circle
But more often, the two galaxies are slightly offset
forming a circle like this one, which is noticeably brighter on one side than on the other
These objects, called collisional ring galaxies, are relatively rare in our cosmic neighbourhood
Although mergers and collisions are quite common
The precise alignment and ratio of sizes
necessary to form a ring is not
Only a few are known in our cosmic neighbourhood and NGC 922
and the Cartwheel Galaxy, also snapped by Hubble, are the best known examples
Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany
The Hubble mission is a project of international cooperation between NASA and the European Space Agency
www.spacetelescope.org
Transcribed by ESA/Hubble. Translation: ----