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12

The luminosity mass relation is inexact. The luminosity also depends on the composition of the star, particularly in and around the nuclear burning regions. The composition changes during the main sequence lifetime as hydrogen gets turned into helium. The average mass per particle goes up and the number of electrons per mass unit goes down. The former means ...

4

If the Sun collided with another star about the same mass, then its mass would be slightly less than 2 solar masses, as some material would be ejected away from the merger. This would result in an A-type star, as the merger's mass is about 2 solar masses. A good example of a 2 solar mass star is Fomalhaut A, which is an A3V star. Therefore, this merger ...

3

The evolution of a star is irreversible process, so strictly speaking, it is impossible to return the star back to its earlier phase (it is impossible to collect all energy it has radiated to space and use it for conversion of helium back to hydrogen). But it is possible for a star plotted on HR diagram to return to a point corresponding to main sequence ...

2

This diagram from the Wikipedia article on supernovas mostly answers your question I think. There is in fact an interval that is expected to not produce a remnant, but it is not at the lower end of the mass range, interestingly. Instead it's a result of the particular mechanism that triggers the supernova for these heavy stars, which blows the core apart ...

2

The website on Main sequence stars fromr the Austalian national telescope facility lists star mass, temperature and life span: Mass/MSun Luminosity/LSun T=Effective Temperature/K Radius/RSun t=Main sequence lifespan/yrs 0.10 $3×10^{-3}$ 2,900 0.16 $2×10^{12}$ 0.50 0.03 3,800 0.6 $2×10^{11}$ 0.75 0.3 5,000 0.8 $3×10^{10}$ 1.0 1 6,000 1.0 $1×10^{10}$ 1.5 ...

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