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

Apparent Magnitude vs. Absolute Magnitude Apparent Magnitude: The magnitude of a star as it appears to an observer on the earth. Absolute Magnitude: The magnitude of a star as it would appear to a hypothetical observer at a distance of 10 parsecs or 32.6 light-years.

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

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