The Cartwheel Galaxy (aka ESO 350-40) is a ring galaxy about 500 million light-years away in the constellation Sculptor. It is about 150,000 light-years diameter. 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. When the other galaxy passed through the Cartwheel Galaxy, the collision caused a powerful shock wave. Moving at high speed, the shock wave swept up gas and dust, creating a starburst around the galaxy’s center portion forming the bluish ring around the central brighter portion. The galaxy appears to be retaking the form a spiral galaxy with thin arms beginning to spread from its central core. The Cartwheel contains an exceptionally large number of black hole binary X-ray sources because many massive stars formed in the ring.
Image Credit: ESA / NASA