1. The Sun and the Life of Stars

4. Orion Nebula

5. NEBULA:
A large ball of dust and gas.

6. All the mass in the nebula will be pulled in toward the centre.
The density of the forming star (called a protostar) will increase.

7. Since these nebula are massive (~1030 kg), the gravitational force has a strong pull towards its centre on all its matter.
This results in immense pressure in the core.
The high pressure results in high temperature (16 million oC), which allows nuclear fusion to occur.

8. Nuclear Fusion
Fusion results when two nuclei collide at very high speeds to “fuse” into a new nuclei.
The speeds have to be sufficient to overcome the strong electrical repulsion from the positive nuclei. (This is why the temperature must be high.)

9. The simplest fusion is between H nuclei.
Nuclear Fusion
The simplest fusion is between H nuclei.
Essentially, the H nuclei fuse together (in stages), to create a helium nuclei
Along with He, energy in the form of light is produced (along with invisible particles called neutrinos).

10. Nuclear Fusion
The light produced is very high energy  rays. If this light made it to Earth, we would be “fried”.
So what happens?
The light goes through a series of collisions with atoms in the radiative zone, which we call a random walk.
During each collision the light loses energy, eventually becoming UV and visible light.

11. …higher temperatures, which means…
Nuclear Fusion
Similar processes can occur to create larger nuclei, but this would require…
…higher temperatures, which means…
…higher pressure in the core, which means…
…more mass is needed, which means…
…a larger star!
But… this can happen to our star too!

12. Nuclear Fusion
As it stands currently, the gravity pushing towards the centre of the sun is balanced with the outward pressure due to fusion.
Once the fusion stops, (H runs out) gravity will force the sun to collapse, which will increase the temperature so He can fuse (to form carbon).
When it does this, the outer layers “explode” and it becomes a Red Giant star.

13. Questions
1) How do you think this process would be different for a) larger stars? b) smaller stars?
2) Our sun will be “productive” for about 10 billion years. How do you think this compares to a) larger stars? b) smaller stars?

14. Death of a Star
For small/medium stars, the outer (red) layer will just drift away.
What is left is a small dwarf star, white dwarf, which dims as it cools.
Large stars continue to collapse, and eventually explode  SUPERNOVA!
Moderately large stars will leave behind a very dense neutron star
Extremely large stars leave behind a black hole!