Space Physics

### 1. The Earth and the Solar System

Our Solar System is a gravitationally bound system comprising the Sun and the objects that orbit it. The Sun, a star, is at the centre and contains over 99.8% of the system's total mass. Its immense gravitational field holds everything in orbit.

The largest objects orbiting the Sun are the eight planets. In order from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The paths these planets follow are called orbits, which are slightly elliptical (almost circular).

Beyond the planets, the Solar System also includes:

* Moons (Natural Satellites): Objects that orbit a planet. Earth has one moon, while Jupiter has over 80.

* Dwarf Planets: Such as Pluto, which orbit the Sun but haven't cleared their orbital path of other debris.

* Asteroids: Rocky objects, mostly found in the asteroid belt between Mars and Jupiter.

* Comets: Icy bodies that develop a glowing tail when they pass close to the Sun.

* Artificial Satellites: Man-made objects placed in orbit around the Earth or other celestial bodies for purposes like communication, navigation (GPS), and scientific observation.

### 2. The Sun as a Star and Stellar Energy

The Sun is a typical star. Stars are massive, luminous spheres of hot gas that produce their own light and heat. The energy source for the Sun and other main-sequence stars is a process called nuclear fusion occurring in their core.

Inside the core, the immense pressure and temperature (around 15 million °C) are so high that hydrogen nuclei are forced to fuse together to form helium nuclei. This process is described by the equation:

4 ¹H → ⁴He + 2e⁺ + 2ν + energy

During this fusion, a tiny amount of mass is converted into a huge amount of energy, as described by Einstein's famous equation, E = mc². This energy radiates outwards from the core, through the Sun's layers, and into space as electromagnetic radiation, including visible light and heat, sustaining life on Earth.

### 3. The Life Cycle of Stars

Stars are not eternal; they are born, live for billions of years, and eventually die. Their life cycle is determined by their initial mass.

a) Birth of a Star: Stars begin as part of a vast, cold cloud of gas and dust called a **nebula**. **Gravity** slowly pulls clumps of this material together. As a clump gets denser, it heats up and forms a **protostar**. When the core becomes sufficiently hot and dense, **nuclear fusion** begins, and a stable star is born. It enters the longest phase of its life, the **main sequence**.

b) The Main Sequence: During this phase, the outward pressure from the heat of nuclear fusion is balanced by the inward pull of gravity. Our Sun is a main sequence star and will remain so for another 5 billion years.

c) End of a Star (Low Mass): For stars with a mass similar to our Sun:

d) End of a Star (High Mass): For stars much more massive than our Sun:

### 4. The Expanding Universe and the Big Bang

A galaxy is a massive system of billions of stars, along with gas and dust, held together by gravity. Our galaxy is the Milky Way. When we observe light from distant galaxies, we notice a phenomenon called red-shift. This is an example of the Doppler effect for light. As a galaxy moves away from us, the wavelength of the light it emits is stretched, shifting it towards the red end of the electromagnetic spectrum.

The astronomer Edwin Hubble observed that the red-shift of a galaxy is proportional to its distance from us. This means the further away a galaxy is, the faster it is moving away. This is the key evidence for the expanding Universe—the idea that the fabric of space itself is stretching, carrying galaxies along with it.

This observation leads to the Big Bang Theory, the leading scientific model for how the universe began. It states that the universe started from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding and cooling ever since. Another crucial piece of evidence for this theory is the Cosmic Microwave Background (CMB) radiation, which is the faint afterglow of the Big Bang, detected in all directions of space.