When a star like our Sun runs out of hydrogen fuel to fuse, it begins to evolve off the main sequence and into the red giant branch. As a star evolves along the red giant branch, its outer layers expand and cool, causing the star to become larger and redder. By the time a star reaches the tip of the red giant branch, it can have a radius that is several hundred times larger than that of the Sun. For example, the red giant star Betelgeuse in the constellation Orion has a radius that is about 1000 times larger than that of the Sun.