"Polar Ring" Galaxy NGC 4650A

The unusual disk-ring structure of polar ring galaxies like NGC 4650A is not yet understood fully. One possibility is that polar rings are the remnants of galaxy collisions in the distant past, probably at least 1 billion years ago. What is left of one galaxy has become the rotating inner disk of old red stars in the center. Meanwhile, gas from the second, likely smaller, galaxy would have been stripped off and captured by the larger one, forming a new ring of dust, gas, and stars, which orbit around the inner galaxy almost at right angles to it. This is the polar ring, which we see almost edge-on in the Hubble Space Telescope's image of NGC 4560A.