The Sun is a mass of Incandescent Gas

A gigantic nuclear furnace

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

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

• 1H + 1H --> 2H + antielectron + neutrino1H + 1H --> 2H + antielectron + neutrinoelectron + antielectron --> photon + photonelectron + antielectron --> photon + photon2H + 1H --> 3He + photon2H + 1H --> 3He + photon3He + 3He --> 4He + 1H+ 1H

• 3He + 4He --> 7Be + photon – 7Be = (4 p + 3 n)

• 7Be + electron --> 7Li + neutrino – 7Li = (3 p + 4 n)

• 7Li + 1H --> 2 4He • alternatively

– 7Be + 1H --> 8B + photon • 8B = (5 p + 3 n)

– 8B --> 2 4He + antielectron + neutrino

• start with 12C – 12C = (6 p + 6 n)

• 12C + 1H --> 13N + photon – 13N = (7 p + 6 n)

• 13N --> 13C + antielectron + neutrino – 13C = (6 p + 7 n)

• 13C + 1H --> 14N + photon – 14N = (7 p + 7 n)

• 14N + 1H --> 15O + photon – 15O = (8 p + 7 n)

• 15O --> 15N + antielectron + neutrino – 15N = (7 p + 8 n)

• 15N + 1H --> 12C + 4He

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.

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.

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.

GravityGravity

Gravity

Gravity

Gravity

Gravity

Gravity

Gravity

Gravity

RadiationEnergy

Equilibrium

The Sun and other stars are really only

roughly in equilibrium. The Sun is extremely dynamic, and has storms larger than

the Earth.

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.

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.

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

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.

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.

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).

The massive star then

becomes a Red

Supergiant and starts of with a helium

core surrounded by

a shell of cooling,

expanding gas

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).

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 explosionscompress gas clouds, giving rise to the

gravitational nucleus of new stars.

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.

The famous Hertzsprung-

Russell diagram

compares a star’s mass and luminosity, and

is used to classify stars

and determine their future

paths.