The HertzsprungRussell Diagram What is the HR Diagram

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!

Let’s Practice… http: //sunshine. chpc. utah. edu/Labs/Star. Life/ support/HR_init. swf

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

Let’s see how a star changes throughout its life… http: //sunshine. chpc. utah. edu/Labs/Star. Life/ support/HR_animated. swf

More Practice! http: //sunshine. chpc. utah. edu/Labs/Star. Life/ support/HR_static. swf