Several different time tables try to point to exactly how many billions of years will pass before the Sun reaches status as a RG (Red Giant). Most scientists who have done extensive research suggest that the Sun's attainment of Red Giant status will occur gradually over hundreds of millions of years. In your view as an expert, what are the one or two most important signs that will show that the Sun is beginning to expand and, although it is still solidly in the main sequence, is showing signs of aging?
Nancy Ann Watanabe
i) An enlarged photosphere (disc diameter measurement)
ii) A cooler photosphere (spectroscopy and a fit to a black body curve)
As these changes are in the far future, we maybe can postulate that whatever species is around may have neutrino telescopes that would reveal a hotter-burning core as the hydrogen content falls and higher elements undergo fusion.
Yeah, but by that time, we might have SUNSPIRINE to administer to the poor star and make it stay well again.
Dear Stefan
SR: when hydrogen in the core of our sun is completely "burnt" into helium, the sun will contract because of gravity, because fusion reactions have stopped, so no interior pressure is present that holds gravity. The contraction will stop when much higher density and pressure is reached in the core, so that helium burning will start. Then, the sun will blow up and become a red giant.
That's not quite right, the Sun will enter the "red giant branch" phase when hydrogen burning in the spherical core finishes and it moves to a hydrogen burning shell surrounding the core. the helium core is too cool to fuse during that stage but the temperature rises as more helium accumulates. When it gets high enough, the helium core fuses in a very short time producing a "helium flash" marking the tip of the RGB.
https://en.wikipedia.org/wiki/Red-giant_branch
Dear Nancy Ann Watanabe,
perhaps the first signs of the expansion of the Sun can be detected by an increase in its diameter according to direct measurements. But this is not very soon. According to the standard model, to date, about half of the hydrogen in the solar core has burned out, turning into helium during the reaction, mainly of the proton-proton cycle. The remaining reserves will be enough for another 4-5 billion years. This is a lot for a person, almost forever. The transformation of the Sun into a red giant will be grandiose in scale, but it will not be an explosion like a New star or Supernova. It is a great blessing for us that the Sun is namely such, that it is a single star located in such a place of the Galaxy where there are no stars of large mass, which in the process of evolution give catastrophic explosions like the Supernova. All the best, Vsevolod.
Dear Nancy,
to be precise, the Sun has long been increasing in size, but very small, by about 2 cm per year. This theoretical estimation follows from standard model of the Sun. At this rate of expansion, solar radius will increase about by 100 thousand kilometers (on ~14%) during nearest 5 billion year. But for a person, 5 billion is an eternity. No worries. Regards, Vsevolod.
Eclipses will stop being total and become partial.
The diameter of the Sun will look bigger to solar astonomers.
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