1. The Lives of Stars

2. 1. Where Are We?? We live in the Milky Way Galaxy, which is one of at
least 400 billion 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 400 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.

3. Nebulae
Nebulae are large clouds of gas and dust 3

4. Stellar nurseries
Stars are born in gaseous nebulae.

5. Stellar nurseries
And Bright nebulae.

6. 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! 6

7. Basic Star Formation….
A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins. 7

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

9. Our Sun
1) Our sun is a huge nuclear furnace, turning 400 million tons of hydrogen into helium every second!!

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

11. Hydrogen Fusion
Note: Positrons (anti-electrons) and electrons annihilate each other to create photons of light energy (called starlight)

12. Proton-proton chain reaction
Step 1:
2 1H1 
1H2 + n (neutrino)
+ positron

13. Proton-proton chain reaction
Step 2:
1H2 + 1H1 
2He3 + g (gamma ray)

14. Proton-proton chain reaction
Step 3:
2He3 + 2He3 
2He H1

15. Proton-proton chain reaction
Net Reaction:
4 1H1  2He4

16. Two Opposing Forces
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...

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

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

19. Two Opposing Forces
3) When the core begins to run out of fuel, gravity becomes greater than pressure.

20. Two Opposing Forces
4) The core starts to shrink, the core temperature cools and Hydrogen fuses to make Helium.

21. 5) Energy flowing outward increases, causing the star to expand.
Two Opposing Forces
5) Energy flowing outward increases, causing the star to expand.

22. 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 .

23. Two Opposing Forces
7) However, the compression caused by the gravitational collapse reheats the star’s core & starts a whole new chain of thermonuclear reactions with Helium replacing Hydrogen as the nuclear fuel.

24. What happens next depends on the size of the star…
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...)

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