1. H-R Diagrams

2. An H-R Diagram is… A graph of stars’ BRIGHTNESS and TEMPERATURE –It also shows color since color is related to temperature –It was made by two astronomers who plotted the data for thousands of stars and noticed some trends. –It stands for Hertzsprung-Russell Diagram

4. Notice….. Temperature DECREASES as you move to the Right on the X-axis Absolute Magnitude INCREASES as you move up on the Y-axis. NEGATIVE absolute magnitude values are BRIGHTER than POSITIVE absolute magnitude values

6. A star… Starts off in the lower right hand corner in MAIN SEQUENCE when it first becomes a star. As it gets hotter and brighter, it moves up the main sequence. Then it moves to upper right section when it changes to Giants/Supergiants. It ends in the lower left section when it changes to a white dwarf

10. Arrival on the Main Sequence The mass of the protostar determines: –how long the protostar phase will last –where the new-born star will show up on the Main Sequence –i.e., what spectral type the star will have while on the main sequence Limits –Upper ~ 100 M  –Lower = 0.08 M 

11. Supergiants on the HR Diagram As the shells of fusion around the core increase in number: –thermal pressure overbalances the lower gravity in the outer layers –the surface of the star expands –the surface of the star cools The star moves toward the upper right of HR Diagram –it becomes a red supergiant –example: Betelgeuse For the most massive stars: –the core evolves too quickly for the outer layers to respond –they explode before even becoming a red supergiant

12. Summary of Differences

13. Questions 1.What is the relationship between temperature and brightness for a main sequence star? 2.What kinds of stars are the hottest? 3.What kinds of stars are the brightest? 4.Why do White dwarves have a low brightness when they are so hot?

15. Questions Why do Giants have such high brightness when they are not especially hot? 5. Why do Giants have such high brightness when they are not especially hot? 6. What is the temperature of the sun? 7. What is the absolute magnitude of the sun?