* Stellar Evolution: As stars age, they consume their hydrogen fuel, causing their core to contract and heat up. This heat causes the outer layers to expand and cool, making them appear redder.

* Spectral Class: Astronomers classify stars by their temperature and color using a system called the spectral classification. Red stars belong to the spectral class M, which is the coolest and most common type of star.

* Red Giants: Many old stars become red giants. This is a phase where they expand significantly and their surface cools, making them even redder.

Note: While the vast majority of old stars are red, there are exceptions. Some old stars, like white dwarfs, have already exhausted their fuel and have cooled down to a very faint white or even blue hue.