1. What makes a star a star?

Which is ...

... when two atomic nuclei are 'fused' together.

This can only happen when ...

... material is extremely hot and dense.

Why?

Because nuclei have the same electric charge, and so they repel each other, unless pushed close enough together.

Then, suddenly the nuclei are suddenly strongly attracted.

- electric charge pushes them apart

(long-range, electro-magnetic force)

- 'strong' force pulls them together

(a short-range force)

(for a summary of forces, see lecture 5)

If density and temperature (pressure) are high enough, nuclei get 'squeezed' close enough together for the 'strong' force to win out over the electro-magnetic force.

H nuclei (single protons) are easiest to fuse because the have the smallest electric charge of all nuclei (the repellent!)

The fusion releases energy star!

Except for small rocky cores, all the gas giants are primarily made up of H and He, just like the sun

So why aren't they stars?

NO fusion

How come?

Radial profile of a gas giant:

- outer layers: cool, diffuse -- molecular H and He (gas)

- outer intermediate layers: warmer, denser -- liquid H and He

- inner intermediate layers: very warm and dense -- 'metallic' H and He

- core: hot, rocky (molten) core

What would happen if we added more mass?

Internal temperature would go up

Internal density and pressure would go up

Eventually, density and pressure would be sufficient to squeeze H nuclei (each nucleus a single proton) together to fuse.

This releases energy star

Mass is the key

Why do Jupiter, Saturn, and Neptune each give off 2-3 x as much energy as they absorb from the Sun?

(We think we know why for Jupiter and Saturn)

2. Tidal forces

Due to differential gravitational forces acting on a body, i.e. the gravitational force is not the same ( different!) at each position

Remember:

F = G m m / r²

When r changes, F changes.

F r^-2

Some consequences:

Ocean tides on Earth

- due to Moon's differential gravitational force

- mostly effects water, not the planet core, because the force is relatively weak

Io's volcanism

- due to Jupiter's differential gravitational force

- this is so strong, it 'melts' the core of the planet (just as bending a wire back and forth heats it up)

Saturn's rings

- ice particles, perhaps from a satellite ripped apart by tidal forces

all planets' rings ...

... are inside the Roche limit

Roche limit = tidal stability limit

- Earth and moon are both far outside this limit

- Io is outside this limit, but not by much

But ...

... apparently, there were a lot of other moon-sized objects orbiting around the Jovian planets that were over the Roche limit

tidal break-up

rings!

3. Rings

Ephemeral - perhaps lasting only a mere 1x10⁸ years

What are they made of?

How do we know?

Saturn - a case study

- rings discovered: 1675

- deemed not solid (unstable if so): 1857

(Why? Kepler's 3rd law states: P² a³)

- Dopler shift measurements prove rings have differential rotation (inner spins faster than outer edge): 1895

Example:

inner B ring -- 8^h period

outer A ring -- 14^h period

(Obeys Kepler's 3rd law)

- Particle sizes:

Radar images and Voyager images: 1970-1981

range from mm to 10's of meters

- Composition:

infrared observations water ice

- Ring size:

stellar occultation

(a) gravitational energy, transformed into thermal energy

(b) nuclear fusion

(c) nuclear fission

(d) cold fusion

(e) methane combustion