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I am constantly reading that the Sun is at least 20% 'hotter', in terms of total radiation/luminosity, than it was a few million years after its formation (i.e., after the Hayashi stage...)

But what about its absolute temperature? I know that it hasn't changed as much, but has it increased (or decreased) at all?

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    $\begingroup$ Let's hope your sources say, that the sun is 20% brighter, not hotter. 20% of the temperature would be a whopping 1000K $\endgroup$ Jun 11, 2023 at 0:22
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    $\begingroup$ See astronomy.stackexchange.com/questions/14535/… $\endgroup$
    – ProfRob
    Jun 11, 2023 at 15:36
  • $\begingroup$ The sun is a near-perfect (at the scales we are talking about) black body radiator. Power per square meter is the 4th power of temperature. If the sun is brighter, it must be either larger or hotter. $\endgroup$ Jun 12, 2023 at 15:19
  • $\begingroup$ Two out of three answers were plagiarised (now deleted). Don't they have any shame? $\endgroup$ Aug 9, 2023 at 13:22

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From the Wikipedia article on the sun, it is about 150 K:

The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main sequence: since the beginning of its main sequence life, it has expanded in radius by 15% and the surface has increased in temperature from 5,620 K (5,350 °C; 9,660 °F) to 5,777 K (5,504 °C; 9,939 °F), resulting in a 48% increase in luminosity from 0.677 solar luminosities to its present-day 1.0 solar luminosity.

That Wikipedia page indirectly sources a paper from the Gaia mission observing sunlike stars which allow verification of these values by comparison with other solar mass stars of different ages: Gaia Data Release 3: A golden sample of astrophysical parameters

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  • $\begingroup$ Only covers the main sequence. $\endgroup$
    – ProfRob
    Jun 11, 2023 at 15:36
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    $\begingroup$ One should add in that the gradual changes you quote happened of a time period of 4,6 billion years, very gradually indeed. $\endgroup$
    – quarague
    Jun 12, 2023 at 6:25
  • $\begingroup$ this raise further questions. how to they know that distant stars are similar to the sun (by mass and age and radius)? how do they know that the temperature was 5620 C? how do they know that luminosity was 0.677? etc $\endgroup$ Jun 12, 2023 at 6:52
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    $\begingroup$ @BЈовић these are all perfectly fine questions to ask, but require significantly more space and time to answer than what's afforded in the couple paragraphs that fit in a SO answer. $\endgroup$
    – Cubic
    Jun 12, 2023 at 8:16
  • $\begingroup$ @BЈовић: Those are great questions; feel free to post them as top-level questions, linking back to this question, as recommended by "What is the the best way to ask follow up questions?" . $\endgroup$
    – David Cary
    Jun 12, 2023 at 18:39

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