What feature do the spectra of Bright Giants exhibit compared to main-sequence stars of the same temperature?
Answer
Narrower spectral lines due to lower surface gravity
The analysis of a star's spectrum provides crucial clues regarding its luminosity class through observations of spectral line width. Because bright giants have expanded significantly, their outer layers are less densely packed than those of a main-sequence star of equal mass, resulting in lower surface gravity. Stars experiencing this lower surface gravity exhibit spectral lines that are noticeably narrower when compared to main-sequence stars sharing the same surface temperature. This characteristic helps astronomers spectroscopically assign the star to a luminosity class like II or III.
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