APOD Robot wrote:In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946.
By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so.
Is there thought to any particular reason for this, or is this statistics at play ?
That's a very hard question to answer, if there is an answer at all. I'll speculate.
The diameter of the galaxy is approximately 40,000 light-years or just about a third of the size of the Milky Way.
To me, it is interesting that the galaxy is so small. Another interesting thing is that the galaxy is very dusty.
NGC 6946 is highly obscured by interstellar matter of the Milky Way galaxy, as it is quite close to the galactic plane.
Well, I'm going to disagree a bit with Wikipedia. I don't doubt that NGC 6946 is somewhat reddened by dust in our own galaxy, but in my opinion, most of the reddening of NGC 6946 is caused by dust of NGC 6946's own making.
Take a look at the picture at left. I apologize for the streaks seen along the image. The reason why I chose this particular image is that it highlights the tremendous blue super star cluster of NGC 6946, seen at 9 o'clock. As you can see, this star cluster appears very blue here, not very reddened at all. James D Wray made the same observation of the super star cluster of NGC 6946 in his UBV galaxy photography book The Color Atlas of Galaxies
: the very bright blue clusters of NGC 6946 don't appear very reddened at all. (There is one more bright blue cluster in NGC 6946, at 2 o'clock in the picture at left.)
Widespread recent and ongoing star formation in M101.
Photo: Isaac Newton Telescope/R. Barrena and D. López (IAC).
My conclusion is that the formation of these huge clusters has blown away much of the dust surrounding them, leaving them mostly unreddened. This also means that the reddening that we see in much of the rest of the galaxy appears to be internal, caused by dust created by the galaxy itself. The color index of NGC 6946 is 0.800, which is really red for a starforming spiral galaxy seen face on, but it's just right for a galaxy reddened by its own almost ubiquitous dust.
In the picture at right you can see M101, a giant galaxy forming stars all over its super-sized arms. The star formation in M101 is recent and ongoing, and the galaxy contains relatively little dust (some of which is shown in brown in the picture) and it contains large numbers of fresh pink emission nebulas.
I think that the ubiquitous dust in NGC 6946 might be a remnant of very widespread star formation in this galaxy in the past, with huge numbers of supernovas exploding and scattering their copious dust all over NGC 6946. All these supernovas would also have blown much of the hydrogen gas clear out of the galaxy itself, but because NGC 6946 appears to be quite isolated in the sky, the evacuated gas would not be captured by other galaxies, and at least some of the gas may now be falling back onto NGC 6946 itself, feeding more star formation.
Perhaps we are now seeing the tail end of the latest great burst of star formation in NGC 6946 and the peak of the latest bout of supernovas.
And perhaps, because NGC 6946 is both small and isolated, it will create self-propagating cycles running through it without interruption of many outside forces.
Well, that's the end of my speculation!