Why has the star Rigel moved back and forth on the HR-diagram?

Question

Have a lot of questions. Has it done this more than once? And if it has, has the mass of the star shifted with the fluctuations?

Has Rigel already been a red supergiant?

How big does the star have to be to become a black hole? (Have read a lot of different information, some say 40 solar masses, others say 20. I have also read that stars with over 2,5 solar masses become black holes, but I am guessing this is at the end of the life cycle?) And which mass is it then you have to look at? The mass at the beginning, middle, end?

Lastly, would Rigels mass keep sinking or would it grow before it becomes a black hole? (If it becomes a black hole)

Answer 1

The wiki page for Rigel is helpful here.

Rigel, when observed telescopically, is actually 4 stars. The primary star is actually a blue super giant. This star is of a variable type called an Alpha Cygni variable, meaning, amongst other things, its magnitude shifts regularly between a maximum and minimum brightness. Because the H-R diagram uses absolute magnitude, Rigel's place on it would vary (for Rigel, about every 2 days). Rigel's mass is very gradually declining because it has very strong stellar winds.

Your questions about black holes is interesting. The 2.5 solar masses you've read about is the minimum mass left over after a supernova needed to form a black hole; smaller than that, neutron matter is capable of countering the gravitational pressure, thus instead forming a neutron star. The larger masses you've read about are various speculations on what the star's mass needs to be before it goes supernova, as that violent event would cause a star to eject most of its mass.