Sun's Stellar Class at Birth? The Sun currently has a stellar class of G2V. However, since its birth, the Sun has brightened significantly. What would the Sun's class have been when it was born ~4.6 gyr ago?
Spectral type is determined by (and is an indicator of) photospheric temperature and surface gravity. The classification of G2V applies to a star with a photospheric temperature of about 5800K and a surface gravity of about 100-1000 m/s$^{2}$.

The Sun's temperature and gravity have not changed greatly since it first appeared on the "zero age main sequence" - it has got a little bigger and more luminous and a little bit hotter (by about 170 K- Schroder &amp; Smith 2008) - so maybe it was a G3V-G4V star at the zero age main sequence (Mamajek 2018).

However, there were much bigger changes during the 20 million years or so before the Sun reached the ZAMS. It was cooler and briefly much larger. This would be reflected in a spectral classification that was something like K5IV- M0IV; indicating a subgiant-like gravity and a temperature of about 4000K. As it aged, it would then shrink and become hotter, passing through the G5V-K5V spectral types before reaching the main sequence.

If you can synthesise these two diagrams in your head, you can see what I mean.

Pre main sequence evolution in the Hertzsprung-Russell diagram


Spectral types on the Hertzsprung-Russell diagram