H-R Diagrams

H-R Diagrams

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

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

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

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

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

Summary of Differences

Questions

1.1. What is the relationship between What is the relationship between temperature and brightness for a temperature and brightness for a main sequence star?main sequence star?

2.2. What kinds of stars are the hottest?What kinds of stars are the hottest?

3.3. What kinds of stars are the What kinds of stars are the brightest? brightest?

4.4. Why do White dwarves have a low Why do White dwarves have a low brightness when they are so hot?brightness when they are so hot?

Questions

5. Why do Giants have such high Why do Giants have such high brightness when they are not brightness when they are not especially hot?especially hot?

6. What is the temperature of the 6. What is the temperature of the sun?sun?

7. What is the absolute magnitude of 7. What is the absolute magnitude of the sun?the sun?