Supernova! For real this time!

Remember that image I mentioned in my last post? Well, I reduced it, and it came out pretty nice. Unfortunately, it's in gray-scale because clouds came in and covered the galaxy before I could get the color exposures, but it still shows remarkable detail.

Messier 51 with supernova SN2011dh. Companion galaxy NGC 5195 at bottom.

I'd like to draw your attention to just how remarkable this star is. Every other individual star you see in the picture is within the Milky Way Galaxy - that bright one on the right might just be visible with the naked eye (I'm not sure it's the same star, but I can just make one out in the area where M51 is). Each of these Milky Way stars is, at most, a few tens of thousands of light-years away. Supernova SN2011dh is around 25 million. And it's as bright as or brighter than many of the stars in the picture. Now factor in the fact that the brightness of an object (which is what is represented here) decreases as the square of the distance increases, and that's one seriously bright star. (For comparison, none of the other stars in M51 can be made out individually in this picture - the ones that appear to be part of it are really foreground Milky Way stars.)

In fact, when this supernova first exploded, it was emitting as much energy individually as the rest of the hundreds of billions stars in M51 combined. The reasons it doesn't appear as bright as the rest of the galaxy are 1) it was about five days old by the time I took this picture, so it had begun to fade a little. And 2) most of that energy is not in the visible portion of the spectrum. Remind me sometime when it's not so late to explain the details behind a supernova type IIa.

As an aside, you can't see them in this picture because of the exposure settings I chose, but I caught two extremely faint satellite trails in my exposures, one of which neatly passed diagonally from lower-left to upper-right between M51 and the star on the right.