The Stellar Population

1. The Stellar Population

2. Last Time • The most important property of a star is its mass • More massive stars have stronger gravity, which increases pressure and temperature to the point that fusion can start

3. Last Time • A higher mass and temperature leads to a higher luminosity • A higher mass also needs more support against gravity • This support comes from increased energy generation from fusion • This is another way to see why massive stars are more luminous

4. Last Time • More luminous stars use up more energy in shorter period of time • Although they have more fuel, they use it so quickly that massive stars don’t live very long

5. Last Time • Magnitudes • Old habits die hard • Use logarithms • This is called apparent magnitude

6. Last Time • Apparent magnitudes depend on our observation, but don’t tell us about the true nature of the star • For this we use absolute magnitude (M)

7. Last Time • Stars all have a very similar composition • 70% Hydrogen 28% Helium 2% Everything else • How do we know all this? • Composition from line spectra

8. Last Time • We looked at other possible sources of energy besides fusion • Chemical burning and gravitational collapse cannot produce enough energy • We don’t have the right materials for fission and antimatter power

9. Last Time • There was a problem with fusion • Not detecting enough neutrinos • After a lot of work, the problem was explained • Did not understand neutrinos • Great example of how science works

10. Last Time • Began to talk about stellar populations • The Sun is not typical or common, but it also doesn’t really stand out • Most stars are low mass, cool, and long-lived • Massive, hot stars are very rare

11. Last Time • Spectral Classes O B A F G K MOh Be A Fine Girl/Guy Kiss Me

12. This Time • Demo Day! • Temperature and pressure relationship • Lenses and mirrors • Photo-electric effect • Polarization • After this, we will talk more about stellar populations

13. Pressure Temperature Relationship

14. The Energy of The Sun

15. The Energy of the Sun

16. Photoelectric Effect

17. Polarization

18. Back To Stellar Populations • Let’s take a quick look at some spectra

19. Comparing Spectra

20. Stellar Classification • Now that we have our groups, we want to try to understand why they are the way they are

21. 17 of 18 What do you think is responsible for the different spectra? • Different composition • Different environments • Different temperature

22. Temperature • The main difference between different classes is the temperature of the star • It all has to do with the blackbody curve and energy • Our old applet friend

23. Temperature • Each type of absorption line that we saw has a certain energy at which it is most efficiently produced • Some ions and molecules are destroyed in the intense radiation of the hottest stars

24. Other Observables • Now astronomers had a way of measuring temperature, though it was not well calibrated • They could also measure apparent brightness • But this is not intrinsic • How can we measure intrinsic brightness

25. Luminosity • Two possibilities • Measure the distance to the stars (hard) • Measure apparent brightness for stars at a fixed distance (not as hard)

26. Star Clusters • Astronomers knew of objects whose stars were all at a fixed distance

27. The Hertzprung-Russell Diagram • The next thing to do is to combine this information • Luminosity • Temperature • What do you think we will see?

28. The Hertzprung-Russell Diagram

29. The Hertzprung-Russell Diagram • A high luminosity and low temp indicate a very big star • The main line of stars are called the main sequence • Temperature and luminosity increase steadily (as does radius) • A high temp and low luminosity indicate a very small star

30. The H-R Diagram

31. The H-R Diagram • But is this a special circumstance, or is it true of all populations of stars • Need to measure the distance to stars

32. Parallax • Hold your thumb, and switch between one eye being opened and the other closed • What happens? • Why?

33. Parallax • The same thing happens for stars, but the are so far away that the change is small • Hard to detect

34. Parallax

35. Measuring Parallax • To measure parallax, we need to know positions very accurately • This was a big project here at UVa • This is also where we get the unit of parsecs from • At a distance of 1 parsec, a star has a parallax of one arcsecond

36. Back To The H-R Diagram • The same trend is seen • This is a real phenomenon • Stars come in a wide range of temperatures, sizes, and masses • As a dramatic example of this, here is a short video

37. Consider It… • With nothing but spectral lines and luminosity, we have discovered something amazing • There is a rich variety of stars in our galaxy • The next question is, why?

38. Alien Observer • If an alien came to Earth, it would see: • Young, tiny humans that constantly poop • Larger, rambunctious creatures that fear cooties • Larger still beings that are obsessed with bad music • Lots of large beings that work all day • Smaller, frailer beings that complain about all the other ones

39. Alien Observer • This alien could conclude two things • Humans are created in an amazing variety of sizes and they all are annoying • Humans grow and go through a lifecycle, and are annoying at each stage • Either way, the end result is the same…

41. 17 of 18 Which is it for stars? • They are born with this variety • They go through a life-cycle

42. The Lives of Stars

43. The Lives of Stars • It is best to begin at the beginning, with the birth of a new star • As we will see, stars spend most of their lives doing relatively nothing • But when things do get interesting, they get really interesting