The Hertzsprung-Russell Diagram

What is the HR Diagram? Tool that shows relationships and differences between stars. Relative measure of luminosity & temperature. Shows stars of different ages/stages at the same time.

The Basics… Each star is represented by a dot. Vertical axis shows luminosity/absolute magnitude. How much energy a star radiates in 1 sec. How bright/dim a star appears, compared to Sun Horizontal axis shows surface temperature (in K.) NOTICE: Goes from high temps at left side to lower temps at right side!

Types of Stars Stars that fit the expected pattern – cool and dim; hot and bright – are called main sequence stars Most stars fall in the region called main sequence. Curved line sloping from top left to lower right of HR diagram.

Why is there a “main sequence”? Usually there is a predictable relationship between brightness and temp. Hotter things are brighter Hotter temp = more energy is radiated. Bigger stars are brighter. Bigger surface area = more energy radiated.

Exception to the rule! Some stars are cool but if they are massive enough, they can have high luminosity  red giants or supergiants

Another Exception to the Rule Some stars are very hot and shine intensely but are small so they don’t radiate much energy (appear dim)  white dwarf stars