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Let's say that, hypothetically, our sun uses up all of it's hydrogen tonight, and tomorrow morning is the first day of its red giant phase. How long would it take for the star to turn red? How long before we start noticing adverse climate conditions and runaway heating of the Earth?

For a second question, let's say that using magical future technology, we are able to widen the Earth's orbit bit by bit, to keep it inside the goldilocks zone. How would the sky look on this new red-giant-orbiting Earth. How would plants begin to evolve and change color in order to adapt?

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    $\begingroup$ It is somewhat hard to answer too hypothetical questions, since the answer depends a lot on the assumptions. If the sun's core just turned into helium instantly without a corresponding temperature change it would implode into a weird bounce not normally seen in red giants. If the core just turned into the kind of core we will get in 5 billion years, then the envelope around it (the visible sun) would have to be bigger and cooler instantly to make the pressure work out. But the thermal timescales are long, thousands to millions of years. It is the pressure ones that are fast (minutes). $\endgroup$
    – Anders Sandberg
    Jan 12, 2020 at 18:53
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    $\begingroup$ I'm voting to close this question as off-topic because this is just a bit too hypothetical, you can't just "use up the hydrogen", and the second question is biology. $\endgroup$
    – James K
    Jan 12, 2020 at 19:07
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    $\begingroup$ FWIW, the Sun will be too hot for life on Earth in about a billion years, long before it's due to become a red giant. $\endgroup$
    – PM 2Ring
    Jan 13, 2020 at 12:06
  • $\begingroup$ Also, red giant systems are probably not good places to live in. The luminosity of red giants tends to be quite variable, and red giants spew out a lot of gas and dust. $\endgroup$
    – PM 2Ring
    Jan 13, 2020 at 12:09

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