1. Goal: To understand the births and deaths of stars and how they depend on mass. Objectives: 1)To lean about the births of stars 2)To understand what Stars like the sun will do at the end of their lifetimes 3)To understand how Stars somewhat bigger than the sun will have different ends 4)To understand how and why Stars quite a bit bigger than the sun will end their lives 5)To examine the deaths of Stars a whole lot bigger than the sun

2. In the beginning All you have is a large cloud of dust and gas. This cloud is very large and very cold. They are called Giant Molecular Clouds. Somehow (more on this later on in the lecture) the cloud collapses.

3. Then… A small part of the gas cloud collapses to form the starting solar nebula.

4. The initial cloud Is made of mostly Hydrogen (~90% by weight). Most of the rest is Helium (9%) The cloud has some spin. What will that do?

5. Spin city The small amount of spin acts like a merry-go-round. Much like on a merry-go-round, this spinning motion pushes things outward. However, nothing stops the collapse in the vertical direction, so the cloud collapses to a disk. The gas in the disk is literally in orbit around the center of the gas cloud.

6. And then, something special happens In the core of this bit of cloud, there are a lot of particles falling into the center (everything not lucky enough to start orbiting). This creates heat from the infalling materials. At some point the central object starts to radiate this newly acquired heat and becomes a protostar.

7. Multiple star systems Are very common. At least 50% of all star systems are multiple star systems (2 or more stars). So, our sun is actually the exception to the rule.

8. Stars like our sun. What will our sun become when it dies? A) nothing, will blow itself to bits in a fiery supernova that destroys everything B) a white dwarf C) a neutron star D) a black hole

9. Stars like our sun. What will our sun become when it dies? B) a white dwarf

10. So, how does the sun get to there? Lets back up a bit. The sun is currently 4.5 billion years old. In about 5 billion more years the sun is going to start to run out of fuel in its core. This leads to trouble.

11. Hydrostatic Equilibrium Currently the core of our sun is in balance Gravity tries to collapse it Gas pressure and radiation pressure push outwards

12. The beginning of the end! With its supply of energy from fusion dwindling, the core of the sun starts to contract (gravity is winning). This heats up the core. Meanwhile the outer parts of the sun expand. In fact they expand by a factor of 100! The sun balloons up to the size of the orbit of the earth!

13. In the core The core temperature goes up from tens of millions of degrees to hundreds. However we just have Helium and bigger. The protons are all gone. The solution might look simple, get the temperature high enough and 2 Helium atoms will collide. What happens if we do that?

14. Triple alpha process An “alpha” particle is just the nucleus of a Helium atom (2 protons and 2 neutrons). Imagine that during our atomic hug a 3 rd Helium atom came in. We could then create Carbon! So, 3 Helium atoms crashing into each other at almost the same time creates Carbon.

15. Helium Flash When helium starts to fuse in the core it is a very explosive event! The fusion heats the core. This causes more the reactions to happen a lot faster! The sun undergoes a very rapid change here.

16. Post helium flash With time the core will shrink again and the outer layers will expand (cooling the star but making it brighter). After this the sun will expand back to its previous size and temperature as what is called an Asymptotic Red Giant. Eventually the Helium will run out (well fairly quickly – it is radiating energy 1000 times faster now after all). So, what happens when the Helium starts to run out?

17. Well… Once you get a good Carbon and Helium and the core gets a bit hotter you can get some carbon to fuse with Helium to get Oxygen and maybe some Oxygen with Helium to get Neon. However, it won’t get past that. You need 600 million degrees to fuse carbon with carbon reliably. So, what will happen to the sun at this point?

18. Core continues to collapse The core continues to collapse. This makes the outer layers expand. However, the sun can no longer hold onto these layers, so they get ejected. The sun will loose half of its mass during this period. Will anything stop its collapse?

19. Electrons to the rescue! It is humbling that to save this large star it takes something as small as an electron to save it. At some point the density of the core gets to a MILLION times the density of water! At this point the electrons are crammed so closely that they repel each other. While seeming innocent, this gives enough outward pressure to repel gravity. And the sun is saved! This is called electron degeneracy pressure.

20. What is left? What is left is the core (the rest is ejected into space). The remains is half the mass of our current sun with a diameter of our earth (which is 1% of the diameter of the current sun). This object is called a white dwarf.

21. Butterfly Nebula (planetary)

22. Ant Nebula

23. Ring Nebula (4k light years away)

24. Eskimo Nebula

25. Cat’s Eye Nebula Binary system?

26. Stars between 4-8 time the mass of the sun These stars have a different evolution. However their evolution is not completely understood. When they reach the Helium Flash they have a chance of detonating their entire core due to the core being held together by electrons. This would completely destroy the star. However, it is not completely understood what happens in these cases.

27. Stars 8-20 times the mass of the sun. The start is the same as the sun. However, once Helium gets fused into carbon the core is able to ready 600 million degrees. At 600 million degrees Carbon fuses with Carbon to form an array of heavier elements. At a billion degrees Oxygen can fuse with Oxygen. 2.7 billion degrees to fuse Silicon. In a short period of time (a thousand years) you go from finishing the Helium burning to creating heavier and heavier elements. Where will it end?

28. Iron The end is Iron.

29. Once the core reaches Iron For the pressures in the core the “Iron” is actually Nickel. Anyhow, once you reach that point you can go no further. Since it takes energy to go higher, you are stuck. Stars are like businesses – if they don’t produce energy (money) – they don’t survive!

30. So… The core collapses. This time electrons won’t be able to save it. They don’t produce enough pressure to win out over gravity. So, the atoms themselves collapse together. The core basically becomes one giant atom (and the electrons fuse with the protons). The energy to do this (remember it takes energy to break down atoms if they are smaller than iron) comes from the gravitational collapse.

31. Also, Neutrinos are formed which fly outward. Since they have little mass and no charge they are not affected much by matter. Once the core reaches the density of matter (400 trillion times the density of water) the collapse slows. The density is now so high that neutrons try to take up the same space as other neutrons, which is not allowed to happen. This causes a neutron degeneracy pressure (the neutrons hold up the star). The core has become a neutron star!

32. Meanwhile Just outside the core, this causes a rebound to occur (sort of like a pile up of cars on the freeway when someone slams on their brakes). This causes a reversal and some material now flies outward. The rest of the star is collapsing inward at 15% of the speed of light (but the star is so big that its radius is several light minutes). The now out flowing matter hits the inward falling layers and both now move outward. A shockwave is produced which moves outward taking all of the star with it.

33. SUPERNOVA! Once this reaches the surface there is nothing to stop it, and all of the star except for the neutron star at the core flies into space at a fraction of the speed of light. This is a SUPERNOVA!!! This process only takes a few seconds. The materials from the star now shine very brightly (they are extremely hot and effectively over a large area) – up to a million times brighter than the star it leaves.

34. Supernova So, the star can actually outshine the galaxy for a few days! They are bright enough to be seen in the DAY if it occurs in our galaxy. At first you are seeing the hot gas radiate. Eventually a decay of Nickel to Iron takes over.

35. Crab Nebula

36. > 25 solar mass stars In this case the mass of the core exceeds the limit at which even neutrons can hold themselves up (which is about 1.4 times the mass of the sun). In this case the core does not hold up. It collapses even further! What stops it this time?

37. NOTHING! Nothing stops the collapse. The entire core collapses into a single point. This creates a BLACK HOLE! The rest of the star – similar to before – is blasted outward in a supernova event.

38. Conclusion The fate of a star is locked to its mass. Stars like the sun become white dwarves (and do not supernova) while ejecting a planetary nebula. Stars > 8 solar masses all supernova and become either neutron stars (8-25 solar masses) or black holes (> 25 solar masses). White dwarves are held up by electrons while neutron stars are held up by neutrons.