Main Question or Discussion Point
Let's say we have a star of 0.7 solar masses on the main sequence. Can somebody describe what happens to it when it comes off the main sequence?
Very likely the case. The Lifetime proportional to mass^3.5 approximation gives a main sequence lifetime of ~24 billion years for a star of .7 solar masses.It is entirely possible, however, the universe is not yet old enough for a star of this size to have died of old age.
Wikipedia said:Once a main sequence star consumes the hydrogen at its core, the loss of energy generation causes gravitational collapse to resume. For stars with less than 0.23 solar masses, they are predicted to become white dwarfs once energy generation by nuclear fusion comes to a halt. For stars above this threshold with up to 10 solar masses, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell. In consequence of this change, the outer envelope of the star expands and decreases in temperature, turning it into a red giant. At this point the star is evolving off the main sequence and entering the giant branch.
I flinch at the .23 solar mass limit. I see no way helium fusion of any significance can occur in a star with such low mass.
Wikipedia said:For stars above this threshold with up to 10 solar masses, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell.
At 0.23 solar masses it does not burn helium, the helium remains inert within a purely radiative core and burns a hydrogen shell surrounding the radiative helium core resulting in a red giant star.ref. 1 said:Upon emerging from its parent cloud core, the lowest mass star capable of burning hydrogen (M∗ ≈ 0.08M⊙).
A star with a slightly larger mass, M∗ = 0.23 M⊙, experiences the onset of a radiative core when the hydrogen mass fraction dips below 50%. The composition gradients which ensue are sufficient to briefly drive the star to lower effective temperature as the luminosity increases. In this sense, stars with mass M∗ = 0.23M⊙ represent the lowest mass objects that can become conventional “Red Giants”. At these low masses, however, the full giant phase is not completed. Stars with initial mass M∗ < 0.5 M⊙ will be unable to generate the high central temperatures (Tc ∼ 10^8 K) required for the helium flash; these stars abort their ascent up the giant branch by veering to the left in the H-R diagram.