Note the difference between NGC 7814 and NGC 4631. The former galaxy has an almost razor-thin, almost perfectly straight central dust lane. The galaxy looks perfectly symmetrical and undisturbed, and its colors are yellow. This is a perfect example of a galaxy that contains no star formation and no massive stars.
NGC 4631, by contrast, contains so much star formation and so many "angry" massive stars that the entire galaxy looks contorted.The central dust lane has been separated into a million little pieces. And the color of the galaxy is mostly blue.
If there is a supernova in a galaxy like NGC 7814, our jaws would drop to the floor if it was a core-collapse supernova. Because such a supernova requires a massive progenitor star, like, say, Betelgeuse or Rigel, and where do you find such a star in NGC 7814?
So you can tell at a glance, by looking at the galaxy, that this has to be a supernova type Ia, whose progenitor star is a white dwarf. Because of course there are white dwarfs in NGC 7814. There are white dwarfs in all and every galaxy, since they are the remnants of moderate-mass stars, and you can't find a galaxy anywhere that hasn't had a relatively abundant population of stars like Sirius and even the Sun.
A type Ia supernova occurs when a massive white dwarf, which is balancing precariously close to its Chandrasekhar limit, beyond which its electron pressure can no longer support it, receives an unwelcome helping of additional mass from a companion star that completely unsettles it and makes it go bang.
A core-collapse supernova, by contrast, happens when a massive star has exhausted all its ways of producing energy in its core to hold itself up and prevent itself from collapsing. When such a star has run through all its various ways of fusion and core shrinking and can go no further because it has an iron core, it collapses and goes out with a bang.
M65 with supernova.png
Supernova SN 2013am in M65.
Fascinatingly, there is a Messier galaxy that contains very few massive stars, but it has nevertheless produced a core-collapse supernova. That galaxy is M65. In the picture at left, you can see a tiny bluish spot to the lower right of the galaxy's center. That is (probably) a cluster of massive blue stars (or possibly a foreground binary blue star). To the lower left of that blue spot there are even fainter hints of blue. This bluish color comes from scattered massive stars.
On March 21, a supernova appeared in M65. The supernova got the designation SN 2013am, and it turned out to be a core-collapse supernova.
But normally a galaxy like M65 would produce a supernova of type Ia, not a core-collapse supernova. Well, unexpected things happen!
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