1. Introducing the Stars

2. There’s No Shortage!

3. What We Would Like to Know Distances (Critical for many purposes!) Distances (Critical for many purposes!) Masses Masses Sizes/shapes Sizes/shapes Densities (Hints of structure.) Densities (Hints of structure.)

4. …and more Temperature (surface and interior) Temperature (surface and interior) Internal structure and composition Internal structure and composition Ages (and potential lifetimes) Ages (and potential lifetimes) Correlations among these! (What do we learn?) This is how science is done! Correlations among these! (What do we learn?) This is how science is done!

5. …and yet more Sources of energy (Are all stars like the sun? What happens as they run short of fuel?) Sources of energy (Are all stars like the sun? What happens as they run short of fuel?) Potential longevity with various fuels Potential longevity with various fuels Stellar variability and activity Stellar variability and activity Systems of stars (do they come in groups?) Systems of stars (do they come in groups?)

6. …plus Stellar formation Stellar formation Stellar death (What do stellar remnants look like?) Stellar death (What do stellar remnants look like?)

7. Let’s Get Started!

8. To Start, a Simple Question: Why Is the Sun So Bright?

9. Two Possibilities 1. Perhaps it is fundamentally different from the stars – bigger, brighter, special, maybe of different nature or composition. 2. Perhaps it is a run-of-the-mill star, like many others, - just particularly nearby, hence apparently bright.

10. Let’s Turn the Argument Around Let’s Turn the Argument Around Assume the sun is an average star! This implies that the other stars look faint because of their remoteness How far away must they be to look so faint?

11. Darkness at Night We see only a few thousand stars at night! Even collectively, they don’t provide much illumination! So they are least 1000x fainter than the Sun!

12. A Very Rough Guess: Use Holes in the Wall

13. The Approximate Answer The ‘average star’ looks about one trillionth as bright as the sun That’s 1 / 1,000,000,000,000 or 1 / 10 12 or 1 / 10 12

14. So: How Far Away is the ‘Average Star’? (Hint: use the ‘Inverse Square’ Law)

15. Deduction Remember that one trillion = one million x one million Consequently, the average visible star must be about one million times as far away as the sun! This is a distance of a few light years. In fact, that’s roughly correct. [The sun is actually a run-of-the-mill star!] [The sun is actually a run-of-the-mill star!]

16. In Fact The nearest known star is ~4 light years away. That is, the light reaching us now from that star left it ~4 years ago. Astronomers are always looking ‘into the past.’ For nearby stars, this is not enough of a ‘time lag’ to make any difference in our studies. (But for remote galaxies, it matters!)

17. Still, That’s Amazingly Remote! The nearest star is four hundred million times as far away as the moon!

18. Are Stars ‘Close Neighbours’?

19. Consider Our Cosy Solar System

20. How Big is It? Distance to Neptune ~ 30 A.U. = 4.5 billion km (Call this the size of the Solar System) (Call this the size of the Solar System) Within this distance there is a busy swarm of planets, moons, asteroids, etc (and of course there is also a bigger surrounding ‘halo’ of tiny objects in the Oort Cloud)

21. BUT… The nearest star lies 4.2 light years away = about 40 trillion km This is ~ 11,000 “solar system sizes”

22. In Other Words, There’s a Lot of Empty Space!

23. It’s Mostly Void! Space is predominantly empty! (and even more so between the galaxies!) We live in very atypical surroundings  on a rock!  close to a star!

24. On the Other Hand, Stars Move… Do they ever come close to each other? Interact? Even collide?

25. Consider Some Numbers Our Sun is typical, moving at about 30 km/sec relative to its stellar neighbours

26. The Implication At a speed of 30 km/sec, it would take about one trillion seconds to cover the distance to the nearest star. One trillion seconds is about 30,000 years. Does this mean that the sun comes close to other stars as frequently as that? Were we close to another star some tens of thousands of years ago?

27. Is That Perhaps Even How the Solar System Formed? We considered this possibility in ASTR 101.

28. It’s Not That Simple! All the stars are moving in random directions.

29. Analogy: Do Bullets Collide?

30. Yes – But Very Rarely!

31. But It’s Very Unlikely Mostly they miss – by a long way, since they are all on the move in various directions Likewise the stars: collisions, or even close passages, are very rare indeed. (This is good for life on Earth!)

32. One Important Qualification As noted, the stars do not often collide, or even come close. [And that’s not how the Solar System formed!] But the stars don’t completely ignore each other in the way that speeding bullets do Instead, each star moves under the combined gravitational influence of all the material in the Milky Way galaxy – the other stars, gas, dust and dark matter.

33. Moreover, Star Clusters Exist (in these close confines, stars can interact more directly)

34. There are Even ‘Close Binaries’ These are stars literally in contact, in a mutual gravitational orbit. (More on binary stars later.)