The following figure, which is a composite of two images in blue and near-infrared light taken with the Wide-Field and Planetary Camera (WFPC2) of the Hubble Space Telescope, suggests that two galaxies have actually collided with each other, with a violent wave of star-forming material ejected as a result (Source). The Cartwheel Galaxy is a member of a group of galaxies about 500 million light years away in the constellation Sculptor. The remarkable ring-like structure seen in the right side of the image is about 150,000 light years across (larger than the Milky Way Galaxy).
The ring is a wave of star formation traveling outwards at about 200,000 miles per hour that would not be there if the galaxy had not collided with another. It is a ripple of energy deposited in the center of the galaxy by the passage of another galaxy of the group through it. As the wave passes outward it compresses and heats the matter that it passes through, triggering the star formation.
The top left inset shows a magnified portion of the ring. The bright blue regions are clusters of new star formation and the loops and bubbles result from supernovae produced by massive star formation. The loop extending to the left near the center of this inset appears to be a wave of new secondary star formation on the leading edge of an expanding bubble that is a supernova remnant. It is estimated that the wave of star formation in the ring represents 2 billion new stars.
The lower left inset shows the nucleus and inner part of the galaxy, which is just beginning to recover its spiral structure. The center contains large amounts of dust and clusters of new star formation (the bright points). We may guess that this galaxy was a spiral much like our own Milky Way before this collision took place. Click here for an explanation of what you are seeing.
I'm assuming the T is time but not sure what the Gyr is.
