Answer :
The three stars all orange in color may fall on the same spectral type but one is on the main sequence, one is a giant and one is a supergiant star. These stars differ in luminosity and radius.
The luminosity of stars is the measure of energy output by a star or other celestial object per second. Luminosity depends on the brightness and distance of the star. Therefore luminosity of the star can be different even if both spectral type and luminosity are directly influenced by temperature and brightness because distance has a factor to play.
Also, the three stars are of different sizes and thus have different radii irrespective of them being in the same spectral type.
Know more about spectral types of stars
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