Comet Holmes is a periodic comet discovered in 1892. When it came around in 2007 it brightened from a magnitude 17 to 2.8 in 42 hours, a factor of about a half a million times brighter. This is the largest known comet outburst ever.
The comet is about the size of the moon (as viewed from Earth) in this image. The comet will be back again in February 2021.
Taking a picture of a comet is difficult, as I can track either the comet, or the stars. That means one or the other will be smeared unless further processing tricks are used. Carefully find the nucleus of the comet, and you’ll notice it’s a small line instead of a point, this was the comet moving during the exposure.