An old star with a very large radius and a relatively low surface temperature. The colour of a star is a guide to its surface temperature – blue-white is the hottest and red is the coolest.

a large cool star; has a relatively low surface temperature and a diameter large relative to the sun

a "giant" because of its size and not because of its mass

an old star that has expanded in size

a star in a late stage of evolution, having exhausted the hydrogen fuel in its core

An old, bright star, much larger in size and cooler than the Sun.

An old, bright star, much larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of a red giant.

A red giant is a relatively old star whose diameter has swollen enormously. It's temperature has also cooled appreciably, it's contracting hydrogen core has turned to helium and eventually to carbon. Our Sun will become a red giant star in about 5 billion years.