How would the Sun evolve if it collided with another G-type main sequence star, perhaps a more massive one like Alpha Cen A? Since its mass would then be above the Chandrasekhar limit, could it eventually become a neutron star instead of a white dwarf? Or would it lose enough mass until then so that it would still become a white dwarf, just like Sirius A (whose mass is above the Chandrasekhar limit too but will develop into a white dwarf anyway due to mass loss)? Would the Sun's yellow-dwarf-to-red-giant phase be prolonged due to the additional mass? When largest, would the red giant be bigger than the currently estimated 256 solar radii?
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$\begingroup$ 2 solar masses is far too light for a star to explode in a core collapse supernova. IIRC, the progenitor of a neutron star needs over 10 $M_\odot$. $\endgroup$– PM 2RingCommented Nov 27, 2020 at 12:33
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1$\begingroup$ Accretion induced collapse (AIC) , is a means of forming neutron stars. $\endgroup$– ProfRobCommented Nov 27, 2020 at 18:03
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$\begingroup$ It's a pity PM2Ring deleted his informative comments following his first one and my question in retard. $\endgroup$– GreenhornCommented Nov 28, 2020 at 15:23
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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 would be somewhere around Fomalhaut's position on the HR diagram.
If the mass of a star is doubled, then its lifespan is decreased by eightfold, meaning that the merger would live for about $\dfrac{10}{8}= 1.25 \text{ billion years}.$ With that said, the transition to the red giant phase would not be prolonged, but shortened.
When this merger becomes a red giant, its core will have less mass than the Chandrasekhar limit, and become a white dwarf. At its largest, it will exceed 256 solar radii, up to about 400 radii, according to Wikipedia. χ Cygni is a red giant that is about 2 solar masses, which is a good reference to the aging merger. This star will become a white dwarf, so that is further proof that the Sun-merger will have a shorter lifespan as well as not have enough mass to exceed the Chandrasekhar limit.