What does it mean for a comet to be "in outburst"?
Also, is that a galaxy to the left of the comet? It looks like there might be a lot of galaxies in the picture.
From an observational standpoint the initial phase of a cometary "outburst" is revealed by the appearance of a new bright star-like nucleus at the heart of the comet's coma accompanied by a distinct rise in the comet's apparent total brightness/magnitude (largely just the result of the newly formed bright nucleus). In subsequent days the star-like nucleus becomes increasing more diffuse in appearance, transitioning into a small fairly sharp disk-like feature which expands day by day until it diffuses away. In the end the comet may, or may not, retain its newly increased brightness, awaiting the next outburst, if any.
Brightenings of 29P always start out with the star-like phase as illustrated in the OP's image. In days the comet, due to the rotation of the nucleus, or perhaps our viewing angle of it, either the disk phase will evolve, or take on the appearance of a one-armed spiral galaxy. I would add that outbursts of 29P are typically among the largest displayed by any comet, often amounting to 8 to 10 magnitudes!
I believe that I was the first to call attention to the fact that outbursts of 29P were far more common than long believed (thought to be only once or twice a year, according to Dr. E. Roemer, USNO) when I began observing outbursts coming at the rate of nearly one a month back in the 1970's or early 80's. Since that time the comet has been frequently observed in outburst via monitoring by other advanced amateur astronomers.
Edited by BrooksObs, 21 November 2020 - 11:04 AM.