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
During the productive phase of its existence, the gravity pushing towards the centre of a star is balanced with the outward pressure from 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. The outer layers of the red giant, are now too far away to be held tightly by the star’s gravity, so they drift away.
What is left collapses inward to become a small, hot dense white dwarf
This will eventually cool and fade.

14. 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?

15. Answers: Smaller stars have a smaller gravitational force, therefore…
Less acceleration, so it takes longer to form
Less pressure, so the internal temperature is less, so fusion proceeds less rapidly, so the star lasts longer (even though it has less “fuel to burn”
Other than that, the process remains the same.

16. Larger Stars…
-If a star has more than 8 solar masses (rare), it becomes a red supergiant instead of a red giant.
-When the core collapses inward, the outer layers explode outward in what is called a supernova
APOD: January 24, Supernova 1987a Fireball Resolved
-The core continues to collapse, becoming a very dense neutron star
-If the star is more than 30 solar masses (very rare), when it collapses it becomes a black hole which is so dense nothing can escape its gravity once it gets too close.