E.2.1 - 2.6

E.2.1:
Fusion takes place in all types of stars, and it is this process which releases energy which we perceive as light and heat. Fusion is NOT a chemical reaction, rather it is a physical process. High temperatures and pressures are needed. This is because the high temperature, in other words a lot of heat energy, is required to overcome the electrostatic repulsion between the protons being fused, and therefore to get them close enough such that the strong force become prominent relative to the electrostatic force. High pressure is needed because you need a large density of the protons in order to increase the likelihood of the protons colliding with each other. It is also important to note that there is a mass defect between the 4H being fused to 1He. This mass of the 4H is greater than the mass of the 1He (note: only two of the protons actually make up the helium nucleus, the other two become positrons). This excess mass is where the light and heat energy which we see from stars comes from.

E.2.2:
When the fusion takes place, there is a lot of heat energy releases, and it is known that gases expand when subjected to higher temperatures. As a result, the star (which is essentially a ball of hot gases) will expand and will take shape. However because space is a vacuum, there is nothing pushing down on it externally to stop this expansion, yet it is still occupies a fixed volume. This is due to the fact that the gases themselves all have mass, and we know that gravity is associated to all mass, therefore the gravitational attraction between the particles is equal and opposite to the pressure which is causing the gas to expand and occupy a larger volume. This state is known as the equilibrium between radiation pressure and gravitational pressure.

E.2.3:
Luminosity is defined as the power radiated by a star. Therefore the unit for Luminosity is the Watt. The power received per unit area is known as the apparent brightness.


E.2.4:

 Apparent brightness is how an observer perceives the luminosity of a planet over a given distance. Since light is attenuated following the inverse square law, the equation for the apparent brightness of an object is: 

b = L/4πr² 

Apparent brightness is measured in Wm^-2

E.2.5:

Astrophysics. Yum.

A stellar (globular) cluster is a group of stars which are close together in the universe due to gravity, as a result of them arising from the collapse of the same gas cloud. These clusters can contain from 100,000 to 1,0000,000 stars, ranging within distances of 30-100 light years across.

A light year is defined as the distance which light travels in a year. This is therefore equal to 300,000,000m/s (speed of light) multiplied (365 x 24 x 60 x 60) = 9.46 x 10¹⁵m.

Planets In Our Solar System:

Relative Distances: