Astronomers have observed a star before it became a nova.
Total Eclipse of the Heart
In August of last year a star briefly brightened by a factor of 5 in a single day.
Teapot Star
A new star known as a nova has appeared in the constellation Sagittarius (aka the teapot). It’s one of the few novas that is visible with the naked eye.
Not So Super Nova
Last year there was a new nova in the night sky, now officially named Nova Delphini 2013. From Earth, it looked like a fairly dim new star in the constellation Delphinus. Novae such as this one are similar to the more popular supernovae. The popular view of a supernova is that of an exploding star. A large star runs out of hydrogen to fuel, and as a result collapses upon itself. This “core collapse” causes intense nuclear reactions which rip the star apart in a huge explosion, which is why this type of supernova is known as a core-collapse supernova.
Play It Again, Sam
Novas occur when a white dwarf orbits with another star and captures some of the star’s outer material. This material forms an accretion disk around the white dwarf, which gradually falls to its surface. When material accumulates on the surface of the white dwarf, it can trigger a nuclear explosion that causes it to brighten similar to a supernova, but not nearly as intense. Since the explosion doesn’t destroy the star, it is possible for a nova to occur again after more material has accumulated.
Stella Nova
The appearance of this new star, or stella nova as it was known in Latin, was a major discovery. The prevailing view at the time was that the stars were fixed and unchanging. Planets wander through the sky and comets appear from time to time, but the stars were eternal. Tycho’s observations demonstrated this was not the case, and it spurred a new interest in making accurate measurements of the heavens.