Hmm. Whatayaknow. I would have thought it would gradually cool slightly owing to gradual diminishment of fuel, before the red giant phase, that is.

But this bit from wikipedia states just the opposite:

In fact, even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The Sun used to be fainter in the past, which is possibly the reason why life on Earth has only existed for about 1 billion years on land. The increase in solar temperatures is such that already in about a billion years, the surface of the Earth will become too hot for liquid water to exist, ending all terrestrial life.

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1986 300SDL, 362K
1984 300D, 138K