Classificagtion of Stars: The H-R diagram “The stars are distant and unobtrusive, but bright and enduring as our fairest and most memorable experiences.” Henry David Thoreau (1849) Are Stars similar to our Sun? How far away are they? Where did they come from? What do they do? Do they live forever?

Reading Assignment All Chapter 9 All Chapter 9

Discussion Question How can I understand the performance of CARS P = P(Weight; Power; Overall Built) Make a plot that shows the general relationship between Weight and Horsepower of cars. -now add to your plot sports cars… -… racing cars… -… and economy models This kind of plots summarizes in a powerful way general features of most cars

Classification of Stars 1)Collect information on a large sample of stars. 2)Measure their luminosities (need the distance!) 3)Measure their surface temperatures (need their spectra)

The Hertzsprung-Russell Diagram

The Main Sequence - all main sequence stars fuse H into He in their cores - this is the defining characteristic of a main sequence star.

The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. Ex. Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

The Hertzsprung-Russell Diagram

White Dwarfs - White Dwarfs are hot but since they are so small, they are not very luminous.

The Hertzsprung-Russell Diagram Size of Star Mass of Star

The Hertzsprung-Russell Diagram Lifetime of Star Shorter Longer More mass, more fuel, very fast burning. Less mass, less fuel, slow, steady burning. How do we know the age of a star? Think SUV vs a Honda Civic

The H-R diagram  Which is the faintest? the sun, an O star, a white dwarf, or a red giant?  Which of these star is the hottest?  What are Sun-like stars (0.4 M sun < M < 8 M sun ) in common?  What about red dwarfs (0.08 M sun < M < 0.4 M sun ) ?  Where do stars spend most of their time? O What is the order of stellar evolution of a star like the Sun?

Mass-Luminosity relation Most stars appear on the Main Sequence, where stars appear to obey a Mass-Luminosity relation: L  M 3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor ~ 11. Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous. The more massive a Main Sequence star is, the hotter (bluer), and more luminous. The Main Sequence is a mass sequence!

To calculate a star's radius, you must know its 1) temperature and luminosity. 2) chemical composition and temperature. 3) color and chemical composition. 4) luminosity and surface gravity. L=4 π R 2 σ T 4

3) 4 times larger 4) the same 1) ½ times as large 2) ¼ times as large If a star is half as hot as our Sun, but has the same luminosity, how large is its radius compared to the Sun? L=4 π R 2 σ T 4

What is burning in stars? Gasoline Gasoline Nuclear fission Nuclear fission Nuclear fusion Nuclear fusion Natural gas Natural gas

Review Questions 1. What is the Hertzsprung-Russell Diagram? 2. Why are most stars seen along the so-called main sequence? 3. What makes more massive stars hotter and brighter?