Hello
Las night I decided to recap an old target I begun imaging two years ago, and gave up because I still had some trouble with my RC.
The imaged object is the globular cluster NGC6397
The surprice was today, when doing a preprocessing of the captured images of last night, that there was a missing bright star, which was replaced by a very dim red one.
I did not even have flats for the two old image, so there is a lot of dust on it.
Just going to do some thinking out aloud here. If it was a supernova then all that would be left is a supernova remnant (neutron star/black hole and nebulosity around it, think Crab Nebula) and not a red star. Given that it happened two years ago, if it was a supernova within our galaxy everyone here would know about it, would have made world news. So, it would have needed to have been in the not so distant past for that brightness that it appears. Distant supernovae are bright but not that bright.
So, I personally think we can discount supernovae. It could very well be a nova, these are periodic explosions that occur in binary star systems. If you have a white dwarf that is orbiting around its companion and either the white dwarf gets closer to its companion due to loss of orbital energy OR its companion gets bigger due to moving to another stage in its evolution, mass from the star will be sucked onto the white dwarf.
As mass accumulates on the white dwarf it basically makes a surface layer of what is mostly going to be hydrogen gas; sucked from its companion. When enough hydrogen has been sucked onto the white dwarf nuclear fusion begins in an almighty flash!
At this stage one of two things can happen. The white dwarf either adds a little more helium to itself getting marginally larger and begins sucking more hydrogen over. This is known as a recurring nova as it occurs as often as it takes for the white dwarf to suck enough hydrogen onto its surface for nuclear fusion to start.
OR
The white dwarf goes over its 1.44 solar mass critical threshold and explodes itself.
So, what I think has happened is that your red star may have gone fallen away from being a Main Sequence star (burning hydrogen into helium) and be moving towards the ABG stage (burning helium into carbon). During this stage it becomes significantly larger (imagine our sun expanding from what it is now to the orbit of the Earth in radius) where it becomes huge and red.
The white dwarf is not something that your going to be able to detect, they may be bright but they're also incredibly small (size of the Earth).
I didn't know that variable stars had such a huge range of brightness. One more reason to to be captured by this amaizing hobby, ervey day we learn something new.
pretty variable, variable star...nice work.
