What physical characteristic defines a Red Giant's appearance, relating to its surface temperature and spectral type?
Answer
Low surface temperature around $5,000$ K or less, giving a yellowish-orange color.
Despite their enormous physical size, which grants them high overall luminosity due to the large surface area, Red Giants are characterized by relatively cool surface temperatures. These temperatures often hover around $5,000$ Kelvin or slightly lower. This low temperature causes the peak emission of light to shift towards the longer wavelengths, resulting in the star displaying a characteristic reddish or yellowish-orange hue, corresponding generally to spectral types K and M.
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