# 9-1 Physics/Solar system
## (A Brief History of our) Solar System

Within our solar system there is one star, the Sun, which eight planets orbit around along with smaller objects called dwarf planets. Satellites orbit around planets, so moons are thought of as being natural satellites. We are a very very (very) small part of the Milky way galaxy.
our Sun was formed by a nebula, cloud of dust and cloud, being pulled together by gravitational attraction. This is the start of a star's life cycle, when as a result of the gravitational attraction, fusion reactions begin in the star which keeps prevents the star completely falling in on itself under gravity. As energy is produced by the fusion reactions, the star expands but is also at the same time collapsing into itself. This creates a beautiful equilibrium that keeps the star, very literally in shape.

## The life cycle of a star

Whilst star's aren't actually alive they have, like most objects in the universe, a series of stages, similar to life, which they progress through. The stage of a star's life cycle is determined by the star's size.
All star's start off as a nebula, a cloud of gas and dust, brought together by gravity. Then a protostar, and as the temperature rises to be incredibly high in the protostar due to higher density, hydrogen fusion begins, releasing huge amounts of energy and turning hydrogen into helium (thus star's are very bright and very hot). Eventually the protostar becomes a main sequence star when the equilibrium between expansion and gravity pulling the star down balance, so our sun is currently a main sequence star.
${\text{Hydrogen}}\rightarrow {\text{Helium}}+{\text{Energy}}$
Then there is a junction between what happens next as the supply of hydrogen begins to run out. Star's with a size about the same as our sun, fairly small for a star, become a red giant whilst Star's much bigger than our sun become super red giants. Fusion of heavier elements is now used to keep the star burning bright (from helium to Iron). Also the outer surface now becomes red since the outer surface of the star is cooler.
A super red giant star will then become a supernova (the brightest star's in the Universe) which then either become a neutron star (very dense) or, the biggest of big stars become a black hole.
On the other hand, red giants become white dwarfs which then become black dwarfs, each time they emit less and less energy.
[Diagram of a life cycle of a star]
Fusion processes in star's produce all of the naturally occurring elements. However, elements heavier than iron can only be produced in a supernova. When a massive star such as a supernova explodes, elements are distributed throughout the Universe.

## Orbit

Gravity provides the force that allows planets and satellites (both natural (such as a moon) and artificial (such as satelitte built by NASA)) to maintain their circular orbits.
Because velocity is a vector (has a magnitude and direction), the velocity of a satelitte in orbit is constantly changing while the speed, a scalar, remains the same. Also, the acceleration is constantly changing and occurring towards the object of mass, under gravity, whilst the instantenous velocity (the velocity at a specific point in time) is pointing where the object is currently traving towards.
For a stable orbit, the radius must change if the speed changes (a longer radius would be slower, a shorter radius would be faster).

## Further reading

* ↑ Rest In Peace Stephen Hawking.