9.1 Astrophysics - The solar system

Our Solar System consists of the Sun (a star) and all the objects that orbit it.

An orbit is the path an object takes as it travels around another object, held in place by the force of gravity.

The main objects in our Solar System are:

• The Sun: A star at the centre.
• Planets: Eight large bodies that orbit the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
• Dwarf Planets: Smaller bodies like Pluto that also orbit the Sun.
• Moons: Natural satellites that orbit planets (e.g., our Moon).
• Asteroids: Lumps of rock and metal (mostly found in the asteroid belt).
• Comets: Lumps of ice and dust with very elliptical orbits.

Source: Wikimedia Commons

For an object to stay in a stable orbit, it must move at the correct speed. If it moves too slow, it will fall into the object it's orbiting. If it moves too fast, it will fly off into space.

Stars are "born" from giant clouds of gas and dust called nebulae. Gravity pulls the dust and gas together.

1. Protostar: As the gas collapses, it gets hotter and denser, forming a protostar.

2. Main Sequence Star: When the core gets hot and dense enough, nuclear fusion begins. This is the star's long, stable life period, where it fuses hydrogen into helium. Our Sun is a main sequence star.

3. The End of a Star: This depends on its mass.

Sun-like Stars (Low Mass):

• The star runs out of hydrogen and swells into a Red Giant.
• It then puffs off its outer layers, forming a Planetary Nebula.
• The hot, dense core that is left behind is called a White Dwarf, which cools over billions of years.

Massive Stars (High Mass):

• The star swells into a much larger Red Supergiant.
• It collapses and then explodes in a massive Supernova. This explosion creates and scatters all the heavy elements (like gold and uranium) into space.
• The core that is left behind becomes either a Neutron Star or, if the star was truly massive, a Black Hole.