Pleiades and comet C/2016 R2

Both objects in this image are blue but very different in nature. On the right is the famous Pleiades, a cluster of extremely bright blue stars 400 light years away that can be seen relatively easily from Earth, while on the left is the bright blue tail produced by comet C/2016 R2. The comet appears to be moving towards the cluster but actually is travelling in the upwards direction, and the blue tail is produced by solar radiation, which causes the unusually high amount of ionised carbon monoxide to emit fluorescent light. One theory about the complex shape of the tail is that the comet contains a rapidly rotating nucleus. If C/2016 R2 survives its passage at the closest point near the Sun in May 2018, it will return to Earth again – but not for another 20,000 years or so.