1. The Sun is a mass of Incandescent Gas A gigantic nuclear furnace

2. It’s reasonable to assume the sun is on Fire Until you do the math, which shows it would burn out in a few million years. By 1850, the Earth already was starting to look A LOT older than that – billions of years old

3. Hans Bethe worked out the details around 1930 When H + H fuses into He under tremendous heat caused by the squeeze of gravity, a tiny amount of mass is converted into an enormous amount of energy, enough to last a LONG time. The actual reactions are numerous and more complex

4. 1 H + 1 H --> 2 H + antielectron + neutrino 1 H + 1 H --> 2 H + antielectron + neutrino electron + antielectron --> photon + photon electron + antielectron --> photon + photon 2 H + 1 H --> 3 He + photon 2 H + 1 H --> 3 He + photon 3 He + 3 He --> 4 He + 1 H+ 1 H 3 He + 4 He --> 7 Be + photon – 7 Be = (4 p + 3 n) 7 Be + electron --> 7 Li + neutrino – 7 Li = (3 p + 4 n) 7 Li + 1 H --> 2 4 He alternatively – 7 Be + 1 H --> 8 B + photon 8 B = (5 p + 3 n) – 8 B --> 2 4 He + antielectron + neutrino start with 12 C – 12 C = (6 p + 6 n) 12 C + 1 H --> 13 N + photon – 13 N = (7 p + 6 n) 13 N --> 13 C + antielectron + neutrino – 13 C = (6 p + 7 n) 13 C + 1 H --> 14 N + photon – 14 N = (7 p + 7 n) 14 N + 1 H --> 15 O + photon – 15 O = (8 p + 7 n) 15 O --> 15 N + antielectron + neutrino – 15 N = (7 p + 8 n) 15 N + 1 H --> 12 C + 4 He

5. Stars are born in a region of high density Nebula, and condenses into a huge globule of gas and dust and contracts under its own gravity.

6. A region of condensing matter will begin to heat up and start to glow forming Protostars. If a protostar contains enough matter the central temperature reaches 15 million degrees C.

7. At this temperature, nuclear reactions in which hydrogen fuses to form helium can start. The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star.

8. Gravity Radiation Energy Equilibrium

9. The Sun and other stars are really only roughly in equilibrium. The Sun is extremely dynamic, and has storms larger than the Earth.

10. Some of these storms reach as far as the Earth and inrushing energetic protons interact with the Earth’s magnetic field and atmosphere to cause the Aurora. The sun, then, is a source of low energy Cosmic Rays.

11. A small star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium. The helium core now starts to contract further and reactions begin to occur in a shell around the core. The core is hot enough for the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a Red Giant.

12. In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. This is the factory where all elements heavier than Lithium are made, including the atoms important for life: Oxygen, Iron, Carbon, etc. EVERY Iron atom in your body was made in a SUPERNOVA

13. The helium core runs out, and the outer layers drift of away from the core as a gaseous shell. This gas that surrounds the core is called a Planetary Nebula.

14. The remaining core (80% of the original star) is now in its final stages. The core becomes a White Dwarf. The star eventually cools and dims. When it stops shining, the now dead star is called a Black Dwarf.

15. Massive Stars: more than 10 Solar Masses Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun! Massive stars evolve in a similar way to a small stars until it reaches its main sequence stage (see small stars, stages 1-4). The stars shine steadily until the hydrogen has fused to form helium ( it takes billions of years in a small star, but only millions in a massive star).

16. The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas

17. Eventually, Fusion reactions run out of fuel, and gravity overcomes radiation. The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than an entire galaxy for a short time).

18. These explosions are what distributes heavier atoms across the universe. Atoms, small fragments of those explosions, reach the Earth constantly, and are generically called High Energy COSMIC RAYS. Shockwaves from these explosions compress gas clouds, giving rise to the gravitational nucleus of new stars.

19. Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.

20. The famous Hertzsprung- Russell diagram compares a star’s mass and luminosity, and is used to classify stars and determine their future paths.