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What are the main characteristics of red giant stars?

I'm learning stellar evolution and can follow the theoretical track on an HR diagram, but I want to connect that to what we actually observe. When we classify a star as a red giant, what specific signatures in its spectrum, luminosity, color, and behavior are we looking for, and what internal changes are driving these observable features?

 

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By Prajwal Sharma Answered 3 years ago

When I observe a red giant, several features immediately stand out. Spectroscopically, they show strong absorption lines from molecules like TiO (titanium oxide), which only form in cool atmospheres, confirming low surface temperatures (~3,500-5,000 K). Photometrically, they are very bright in absolute terms but appear red in color indices (e.g., a large B-V value). The physical driver is the end of core hydrogen burning. The core contracts and heats up, igniting hydrogen in a shell around it. This massive energy output causes the outer envelope to expand by a factor of 100 or more, dropping its temperature and boosting its luminosity, placing it on the right side of the HR diagram.

 

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