* Globular clusters are tightly packed, spherical collections of hundreds of thousands to millions of stars.

* They are very old, dating back to the early universe.

* As stars age, they become cooler and redder. This is due to the process of stellar evolution, where stars exhaust their hydrogen fuel and begin fusing heavier elements.

* The red color of the stars in globular clusters is a direct result of their age and the fact that they are mostly composed of cooler, redder stars.

Other options might include:

* Elliptical galaxies: These galaxies are also known for containing older stars, but they are more diffuse and less densely packed than globular clusters.

* Red giant stars: These are individual stars in a late stage of their evolution, and are indeed red. However, they don't represent a whole collection of stars like globular clusters.