1. The Hertzsprung-Russell Diagram

2. 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.

3. 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!

4. 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.

5. 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.

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

7. 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