When we look at stars, we notice that some appear brighter than others. Sirius looks much brighter than Rigel, but is it really
brighter? Or is Sirius much closer to Earth, giving it the appearance of being brighter? Actually, Sirius is about one hundred times closer to Earth than Rigel. If the two stars were the same distance from Earth, Rigel would appear much brighter. -We use two terms to reference the brightness of a star: absolute magnitude and apparent magnitude. Absolute magnitude is the measure of the light given off by a star, while apparent magnitude refers to the amount of light received on Earth. The color of a star indicates its temperature. Blue-white stars are very hot, while orange-red stars are relatively cool. In the early 1900's Ejnar Hertzsprung and Henry Russell noticed that the higher the temperature of a star, the greater the star’s absolute magnitude. They developed a graph that depicts this relationship called the H-R diagram. Most known stars fit into a diagonal band that runs from the upper left to the lower right of the diagram. This diagonal band is called the main sequence. About 90% of all stars are main sequence stars. Most stars are currently classified using the letters O, B, A, F, G, K, and M, where O stars are the hottest and the letter sequence indicates successively cooler stars down to the coolest M class. Betelgeuse is in the red spectral class. It is one of the largest of stars with extremely high luminosity although it has a relatively low surface temperature. How would Betelgeuse be classified? A. Giant
B. Supergiant
C. White Dwarf
D. Main Sequence
