Betelgeuse is a red supergiant of spectral type M1-2Ia-Iab. The luminosity of the star is about 10,000 times that of the sun.
The diameter of the star is about 650 times the diameter of the sun. According to Burnham the star fluctuates irregularly in brightness over a period of years and has been seen to vary by up to a factor of two in luminosity. Betelgeuse is also pulsates irregularly in size. Burnham suggests that the diameter of the star may vary as much as 60%.
If the sun were replaced by Betelgeuse, the surface of the star might stretch out beyond the asteroid belt. The planets out to the orbit of Mars would be engulfed by the star.
Betelgeuse is a massive star as well. It may contain as much as 20 times the mass of the sun. Because of its large mass, Betelgeuse is a star that will run through its life history fairly quickly. The star has already consumed the bulk of its hydrogen fuel in the core and is on the way to stellar graveyard. Betelgeuse is losing mass at reasonably rapid rate. It is estimated that within a few hundred thousand years Betelgeuse will eject about one solar mass of material.
Betelgeuse is the first star to be directly imaged by a telescope. The image above was obtained by the Hubble Space Telescope. You can find more information on this image here.