What is a red giant?

Stars like the Sun spend most of their lives fusing hydrogen into helium in their cores. When the core hydrogen runs low, the core contracts and heats up, while the outer layers expand and cool, turning the star red and making it hundreds of times wider. The Sun will become a red giant in roughly 5 billion years, likely swallowing Mercury and Venus and possibly Earth. Betelgeuse in Orion is a red supergiant, a larger cousin, and Aldebaran in Taurus is a well-known red giant. Eventually a Sun-like red giant sheds its outer layers, leaving a white dwarf.