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Curious Question here.

How do we know the Sun is a G-Type Star?

Our sun (Classed as G2V) apparently lies around the 5,780 Kelvin. Yet NASA generalizes it as 6,000 K- and is apparently white even though it's classed as a Yellow Dwarf.

The differences in classification seem pretty small compared to how the temperature of the Sun is (often) given:

G-type Stars range from 5,000 Kelvin to 6,000 Kelvin (Yellow/white)

F-Type Stars are 6,000 kelvin to 7,000 Kelvin which is also described as Yellow/white

Thus: How sure are we about the exact classification of our Sun as G2 dwarf? Are there more obvious differences between F and G type Stars?

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    $\begingroup$ I'd recommend asking a question like this in the form of "Why is the Sun a G-Type star if it has these features?" instead of the current style, which is more "I found these features of the Sun in 5 minutes of websearching, have I proved most Astronomers wrong?" as the former is more likely to attract answers, and the latter is more likely to attract downvotes. $\endgroup$
    – notovny
    Mar 23, 2023 at 20:03
  • $\begingroup$ I never said that lol. I'm sorry you took it that way. Not what I meant. $\endgroup$ Mar 23, 2023 at 23:26
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    $\begingroup$ @Andyhasaquestion I reworded your qestion slightly to make to sound less... sceptical. If you could amend (edit) your question with e.g. links to the different sources where you find the different temperatures for the Sun, that would be awesome. Please revert and / or re-edit if you think I changed the meaning of your question. $\endgroup$ Mar 24, 2023 at 8:10
  • $\begingroup$ My apologies I wasn't trying to sound skeptical more just curious. $\endgroup$ Mar 26, 2023 at 5:36

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Formally the spectral type of a star is defined by the type of dark lines in the spectrum. G-type stars have prominent lines from singly ionised calcium in their (ultraviolet) spectrum. These are the "H and K lines". F type stars have these lines too, but they are less prominent. The hydrogen line in G type stars is less strong than in F type stars.

The presence or absence of particular lines is linked to temperature, and so indirectly to mass. The temperature is around 5780 Kelvin (the sun lacks a surface and there is variation of the temperature with depth so there can't be an exact value for this) and would appear yellow-white from space, but more-or-less-pure-white when seen through the atmosphere.

But note that, just as colours are in a continuous spectrum, the types of stars are like the difference between "Green" and "Yellow", and there is a continuum of "greeny-yellows". The hottest G0 stars are only infinitesimally cooler than the coolest F9 stars.

A temperature of 5780 Kelvin, and prominent H and K Calcium lines places the sun clearly within the G type of stars, and in particular as a G2V star. This gives us great confidence in it's classification, and therefore in its properties like lifespan. The G2 sun will shine for about 10 billion years in total.

F type stars are a little hotter, but not by much, only by a couple of hundred degrees. At G2, the sun is close to the hot end of the G type classification. There would be little visual difference between an F9 and a G0 or even a G2 star. But there are subtle differences in spectra.

Finally, of course, the sun is the most intensively studied star, we know more about it than about any other star, and so one can be very confident in the correctness of its classification. (In biologist terms, it is like asking "Are humans actually gibbons?" - I'm sure most biologists would be quite confident in the classification of humans within the great apes)

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  • $\begingroup$ Thank you, I appreciate it. They honestly seemed quite similar besides the temperature. Like I said I'm not studying the sun or anything space related topics. Just noticed some stark similarities. Kinda worried me considering our sun is 4.6 b years old and F stars only live to 2 or 4 billion years vs the 10 b years of G-Stars. So I got in my own head and concerned myself. $\endgroup$ Mar 23, 2023 at 23:44
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    $\begingroup$ Remember these are on a continum the hottest G stars last for the only infinitesimally less than the coolest F stars. But we know well about the sun and its lifespan. We are about as certain that the sun is G2 as we are that Humans aren't frogs. $\endgroup$
    – James K
    Mar 24, 2023 at 7:39

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