3. 1. Where Are We?? We live in the Milky Way Galaxy, which is one of at least 1 2 5 billion (maybe up to 4 0 0 !) galaxies in the universe. The Milky Way is a barred spiral galaxy, which is much more complex than the earliest galaxies. There are at least 4 0 0 billion stars and nebulae in the Milky Way (maybe as many as 500 billion). Our solar system is on the Orion Arm of the Milky Way.

4. Nebulae are large clouds of gas and dust Nebulae

5. Stellar nurseries born Stars are born in gaseous bright nebulae…

6. and in dark nebulae. Stellar nurseries

7. The birth of a Protostar Within a nebula, a ball of hydrogen gas collapses under the force of gravity and begins to heat up – a protostar is born!

8. Basic Star Formation…. A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

9. Our Sun Our Sun is a small to medium size yellow star

10. Our Sun 1) A star is a huge nuclear furnace, turning 4 0 0 million tons of hydrogen into helium every second!!

11. Hydrogen Fusion Inside the sun, 4 Hydrogen nuclei fuse to form 1 Helium nucleus, releasing starlight, heat and radiation.

12. Hydrogen Fusion Note: Positrons (anti-electrons) and electrons annihilate each other to create gamma rays & photons of light energy (starlight)

13. Proton-proton chain reaction Step 1: 2 1 H 1  1 H 2 + (neutrino) + positron (e + )

14. Proton-proton chain reaction Step 2: 1 H 2 + 1 H 1  2 He 3 +  (gamma ray)

15. Proton-proton chain reaction Step 3: 2 He 3 + 2 He 3  2 He 4 + 2 1 H 1

16. Proton-proton chain reaction Net Reaction: 4 1 H 1  2 He 4

17. Two Opposing Forces 1) The life of a star is marked by 2 opposing forces (see next page): a) The pressure created by thermonuclear reactions in the core of a star pushing outwards ( ) and...

18. Two Opposing Forces 2) The crushing force of gravity pulling the star’s surface inwards. ( )

19. Two Opposing Forces Pressure from Nuclear Reactions pushing outward Gravity pulling inward

20. Two Opposing Forces 3) At first, gravity dominates and causes the core of the star to collapse inward.

21. Two Opposing Forces 4) As the core starts to shrink, the core temperature rises to 40 million C and Hydrogen fuses to make Helium.

22. Two Opposing Forces 5) The outward flow of energy increases, causing the star to expand outward.

23. Two Opposing Forces 6) When a star runs out of hydrogen, its nuclear “fuel”, the force of gravity takes over and the core of the star collapses.

24. Two Opposing Forces 7)This time, the compression caused by the gravitational collapse heats the star’s core even more & starts a whole new chain of thermonuclear reactions with Helium replacing Hydrogen as the nuclear fuel.

25. Two Opposing Forces 8) When stars finish “burning up” both their hydrogen and helium nuclear fuel, they begin to die out again and, once again, they collapse inward. What happens next depends on the size of the star… (to be continued...)

27. The mass of a star determines its fate Red Giant Supergiant Supernova White Dwarf Neutron Star Black Hole Supergiant Brown Dwarf Tiny Stars Sun-Sized Stars 3 – 10 M Stars 20 – 30 M Stars