Why does an expanding star like the Sun appear reddish during its late stage?
Answer
Expansion causes the surface gas to cool down significantly as it moves away from the hot core
As the star swells dramatically due to the powerful outward push from the newly active hydrogen shell burning, its outer layers of gas expand substantially. This expansion moves the surface material much farther away from the central hot core. The increased distance results in significant cooling of the surface gas. Since cooler surfaces emit light at longer wavelengths, a star that was previously yellow or white (like the Sun) will exhibit a reddish hue, characterizing it as a Red Giant or Supergiant, despite having a much higher overall luminosity due to its enormous size.
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