# Physics (A-level)/Modelling the universe
## Measures of distance

There several measures of distance that are used when describing the universe. These are used because the SI unit, meter, is inefficient when talking about such large distances. You will need to understand how each distance is determined.

#### Astronomical Unit (AU)

An astronomical unit is the average distance between the Earth and the Sun. It measures to approximately 1.5x10$^{11}$m This unit is not often used when discussing any interstellar objects due to it's relatively small magnitude.

#### Light-year (ly)

A light-year is the distance traveled by an electromagnetic wave through free space in one year. It measures to approximately 1x10$^{16}$m.
It can be worked out based from the equation distance=speed*time, using the values of the speed of light in a vacuum and the amount of seconds in a year.

#### Parsec (pc)

A parsec is the distance from from which 1AU would subtend to 1 second of arc. It measures to approximately 3.3 ly.

## Structure of the Universe
### Formation of a star

You will need to know the steps that happen in the formation of a star
* All stars start as a cloud of dust and gas consisting of mostly hydrogen. This is called a nebula. Over time the force of gravity attracts these particles together to form progressively larger chunks. These chunks gain a stronger gravitational field strength and attract more matter to itself.
* As this process continues it eventually forms a protostar which is the very early beginnings of a star and continues to pull matter towards it.
* As more matter is attracted to the protostar and joins onto it, the matter loses gravitational E$_p$ which causes a rise in temperature. This in turn will initiate fusion of the hydrogen atoms, releasing energy.
* The fusion reaction sends energy outwards which prevents further gravitational collapse. At this point, it is said to now be a Main Sequence Star

### Probable evolution of a star

There are multiple possible fates of a star, generally depending on the star's mass.
For a star the size of our sun:
* Once all the hydrogen has been used up in the fusion reaction at the star's core, fusion stops. This causes the core to collapse due to the gravitational forces.
* This transfer of E$_p$ heats up the hydrogen on outer layers causing it to fuse to helium.
* The outer part of the star expands which also cools it to create a Red Giant
* When the core reaches about 10$^8$K, helium starts to fuse to create carbon, releasing energy.
* Eventually the outer layers drift away from the stars core and it is left as a dense White Dwarf. The core of this white dwarf is so dense that matter becomes degenerate. At this point fusion has stopped and the white dwarf is left to cool for ever.
For a star much larger than our sun:
* The temperature and pressure in the core is high enough to fuse hydrogen via the CNO cycle, which is a fusion process in which carbon, nitrogen and oxygen take part to form helium.
* A more massive star will use up all it's hydrogen faster and so will have a shorter lifespan.
* If the star is around 10 times the size of our sun, it will become a Super Red Giant once the hydrogen has been used up. The core is now hot and dense enough to start the fusion of carbon.
* Once the carbon is used up, the core collapses and starts the next fusion process. This continues until the core consists entirely of iron
* After this, the core will collapse again and cause a huge explosion called a supernova. All of the material surrounding the core is shot away by the huge shock wave. At this point, energy levels are so high that neutrons combine with other nuclei to form varying elements, some larger than iron
* After the supernova, a small core remains. This is called a Neutron Star. A neutron star has a density similar to nuclear matter.
* If the star is more than twice the mass of our sun, then a black hole will be produced. This happens when the density is so great that light cannot escape it's gravitational pull.

### Olber's paradox

Olber's paradox discusses that if the universe is of infinite size and static, then the sky would be completely white with light. This would be due to an infinite amount of stars in every direction.