The Hertzsprung-Russel diagram is a chart of the relationship between absolute magnitude and the surface temperature of stars. Rigel is a super giant star that shines blue or blue white with an average surface temperature nearing 20000 degrees Celsius. Sirius is a main sequence star with an average surface temp of over 10000 degrees Celsius. Sirius is similar in appearance to Rigel but is brighter in the night sky from earth.