Supernova (def): A star that is exploding at the end of its life. A supernova is an exploding star. Supernovas are extremely bright. They can form from a star dying after it has cooled to a point where the pressure, normally created by intense heat, drops allowing gravity to win, causing it to collapse. Stars cool as the nuclear core that generates heat, depletes over the course of its lifetime.
Red supergiants are ‘cool’ stars. Compare these to the blue supergiants in the diagram below showing the differences along a scale of temperature.
Due to the brightness of their spectacular explosion, it’s not surprising supernovas can be seen in the daytime from Earth. The last one seen like this was 400 years ago (in 1572).
The next one is the red supergiant, Betelgeuse, but no one knows when. It could be tomorrow or up to a millions years away. Its expected visible magnitude of −12.40 is about half the brightness of our Sun, but more than twice that of Venus. This indicates, that from Earth, people will spot it with their naked eyes even in the daytime.
It can also form from two close stars colliding, with at least one star being a white dwarf.
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