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Although it appears to be a single star to the naked eye, it is actually a star system of four stars in two binary pairs. The first pair consists of two bright, type-G giant stars, designated Capella Aa and Capella Ab, in a very tight circular orbit some 0.76 AU apart and a derived orbital period of 104 days. Capella Aa is the brighter of the two at spectral class G8III (G8 Giant) whereas Ab is slightly smaller and of spectral class G0III (G0 Giant). Aa has a calculated mass of 3.05 times that of the Sun and Ab some 2.57 times that of the Sun. These two stars have both exhausted their core hydrogen fuel and become giant stars, though it is unclear exactly what stage they are on the stellar evolutionary pathway.