The star Betelgeuse, in the constellation Orion, is 500 times bigger and emits 16,000 times more visible light than the Sun. This is why the star appears so bright on the sky, despite its considerable distance. Betelgeuse is redder than our Sun, which implies that it is cooler. Its outer layers radiate at 5000 degrees Fahrenheit. Because of its color and size, Betelgeuse is called a red giant.

Most stars in the Universe appear to us as points of light. But Betelgeuse is so large that it reveals a disk when observed with the best telescopes. The visible light and radio images show a slightly non-circular appearance. This is probably due to activity in the outer layers of its atmosphere, similar to the Sun's corona. But even this giant star's x-rays are too weak to detect from Earth.

A star becomes a red giant late in its life, when nuclear fusion has transformed all the hydrogen in the core into helium. The resulting increases in size and brightness mark the beginning of the end for the star. Heavy stars turn into red giants more quickly than light stars. Betelgeuse is some 12 times heavier than the Sun. It is already a red giant, despite being only some 7 million years old. By contrast, the Sun will not become a red giant until its age reaches 10 billion years, another 5 billion years from now. Red giants that are at least twice as heavy as Betelgeuse later become black holes after a supernova explosion.