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The Wikipedia page for G-type main-sequence stars lists the expected mass and temperature for stars of each sub-classification:

$$\begin{array}{c|c|c|} & \text{Mass ($M_\odot$) } & \text{Temperature (K)} \\ \hline \text{G0V} & 1.15 & 5980 \\ \hline \text{G1V} & 1.10 & 5900 \\ \hline \text{G2V} & 1.07 & 5800 \\ \hline \text{G3V} & 1.04 & 5710 \\ \hline \text{G4V} & 1.00 & 5690 \\ \hline \text{G5V} & 0.98 & 5620 \\ \hline \text{G6V} & 0.93 & 5570 \\ \hline \text{G7V} & 0.90 & 5500 \\ \hline \text{G8V} & 0.87 & 5450 \\ \hline \text{G9V} & 0.84 & 5370 \\ \hline \end{array}$$

Spectral type is only determined through a star's temperature, and thus the Sun with a temperature of $\text{5778 K}$ is unambiguously a G2V star.

However the Sun is noticeably less massive than other G2V stars are. Why is that? Or, another way of asking, why is the Sun hotter than other stars of the same mass (G4V)?

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  • $\begingroup$ From that Wikipedia page: The Sun is not in the G4V class because even though it corresponds to the same mass, the Sun is slightly hotter than the typical temperature for a G4V star (at 5,778 K), so it is a G2V star, which is normally slightly more massive than the Sun $\endgroup$ – Rory Alsop Dec 8 '20 at 14:02
  • $\begingroup$ Thank you for re-asking my question for those less fortunate in the reading comprehension department. Why is the Sun hotter is what I'm asking. $\endgroup$ – user177107 Dec 8 '20 at 14:10
  • $\begingroup$ And where does wikipedia get this table from? $\endgroup$ – ProfRob Dec 8 '20 at 16:34
  • $\begingroup$ @user177107 - sorry, no I was just highlighting that "noticeably" was a bit of a stretch - it is "slightly" hotter, according to Wikipedia $\endgroup$ – Rory Alsop Dec 8 '20 at 17:09
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It isn't. You've just got dodgy table from wikipedia.

A more modern (and well-used) version is here.

It lists

G1V 1.07 5880

G2V 1.02 5770

G3V 1.00 5720

This is an average relationship. The closest and most consistent relationship will be between spectral type and effective temperature and indeed the Sun is normally attributed a spectral class of G2V and $T_{\rm eff} \simeq 5780$ K.

The reason for any small discrepancy at the hundredth of a solar mass level with the mass in the table could be because the Sun has a slightly different composition to the average star that defines this table. The age of the Sun is unlikely to be a major factor. According to most models, the temperature of the Sun increases by just 100 K between a billion years old and around 8 billion years old and then cools a little bit. On the other hand, stars like the Sun do lose a little mass as they get older, mainly through a stellar wind. The Sun though is probably at around the average age for an early G star in our Galactic neighbourhood.

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This is yet another example of "that's Wikipedia for you."

The authors of that Wikipedia article chose to pull a table from a book that in turn has pulled data from an obscure article. I wrote "obscure" because the article has been referenced once every two or three years since its publication three decades ago. Even worse, the correct title of the Wikipedia table should be "Thermal parameters of late-type main sequence stars." Our Sun is middle-aged. It is not yet a late-type main sequence star.

The late-type is key. Two main sequence G-class stars of the same mass can vary in luminosity by a factor of three, depending on age. The main sequence is not a line. It is instead a band. Our Sun is estimated to have been 30% less luminous than it is now when it first emerged on the main sequence, and is estimated to be over twice as luminous as it is now when it finally exits the main sequence. G-type stars get more luminous as they age, but they also get larger. The apparent temperature decreases as G-type stars age because the increase in size overwhelms the increase in luminosity.

Bottom line: Take everything you read in Wikipedia with a large grain of salt.

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    $\begingroup$ Err, late-type just means a cool star, it doesn't have anything to do with age. The Sun was indeed less luminous in the past, but its effective temperature will have been almost constant. So I would stick with your first argument, it's just a dodgy table. $\endgroup$ – ProfRob Dec 8 '20 at 16:36
  • $\begingroup$ Wikipedia is supposed to be a "living" information source. Is somebody updating the information? Hopefully, one day Wikipedia will truly be a reliable resource. As it is, I have depended on it for general information. $\endgroup$ – Jack R. Woods Dec 21 '20 at 4:39

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