1. Physical Properties Review
Temperature
Composition
Mass
Radius
Distance
Luminosity
Velocity
Planets
Age

2. Stellar evolution
How can we ever know how stars will evolve if they take millions and billions of year to do so?

3. Discussion
Aliens visit Earth in order to determine the life cycle of humans. But to avoid detection they only gather 1 seconds worth of data. What clues can they get and how from such a short visit?

4. The main sequence
90% of stars are main sequence stars. Thus, if star formation is relatively constant, stars must spend about 90% of their lives on the main sequence.

5. Pleiades
100 million years old

6. M 80
12 billion years old

8. Orion nebula

9. Discussion
In the previous pictures, why does some of the gas appear blue, some red and some dark?

10. Emission nebula
Hot stars ionize the hydrogen in the interstellar gas. When the hydrogen recombines with an electron, it emits Balmer photons, primarily in the red in photographs.

11. Reflection nebula
Stars are not hot enough to ionize the hydrogen gas or gas is too thin to recombine with electrons. Instead the light we see is scattered star light, and because shorter wavelengths are more easily scatter, reflection nebulae appear blue.

12. Dark nebula
Dust and gas that is thick enough to block light from passing through it.

16. Discussion
How can astronomers tell that the dark lanes in the previous picture are actually caused by opaque gas and dust blocking the light from behind it and not just dark empty space devoid of stars and gas? What observations could you make to prove it?

19. Discussion
Many open star clusters are embedded in a cloud of interstellar gas and dust. What do you think is going to happen to that gas and dust over time?

22. NGC 3603
Carina

24. NGC 2264
NGC 2264

26. 2 assumptions
Assume all the stars in the cluster are at the same distance.
Assume that all the stars in the cluster formed at the same time.

27. NGC 2246 in Monoceros

28. NGC 2246

30. Pleiades

31. Discussion
In these two pictures, which do you think has the younger stars? Why?

32. Discussion
What are the differences between the previous two H-R diagrams?

33. NGC 2246
Pleiades

34. M 55

35. M55 in Sagittarius
M55 Sagittarius
12.5 Gyr +- 1 Gyr

36. Discussion
Why do you think the hottest stars are not on the main sequence in some of the H-R diagrams?

37. Main Sequence Lifetimes