Evolution
The galaxy was once a normal spiral galaxy before it apparently underwent a head-on collision with a smaller companion approximately 200 million years ago (i.e., 200 million years prior to the image). When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful shock wave through the galaxy, like a rock being tossed into a sandbed. Moving at high speed, the shock wave swept up gas and dust, creating a starburst around the galaxy's center portion that were unscathed. This explains the bluish ring around the center, brighter portion. It can be seen that the galaxy is beginning to retake the form of a normal spiral galaxy, with arms spreading out from a central core.
Alternatively, a model based on the gravitational Jeans instability of both axisymmetric (radial) and nonaxisymmetric (spiral) small-amplitude gravity perturbations allows an association between growing clumps of matter and the gravitationally unstable axisymmetric and nonaxisymmetric waves which take on the appearance of a ring and spokes.
Read more about this topic: Cartwheel Galaxy
Famous quotes containing the word evolution:
“Historians will have to face the fact that natural selection determined the evolution of cultures in the same manner as it did that of species.”
—Konrad Lorenz (1903–1989)
“The evolution of humans can not only be seen as the grand total of their wars, it is also defined by the evolution of the human mind and the development of the human consciousness.”
—Friedrich Dürrenmatt (1921–1990)
“The more specific idea of evolution now reached is—a change from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity, accompanying the dissipation of motion and integration of matter.”
—Herbert Spencer (1820–1903)