Key content
Physics only
Space physics is physics only content.
Our solar system
Our solar system contains:
- one star – the Sun
- the eight planets and the dwarf planets (such as Pluto) that orbit the Sun
- natural satellites – the moons that orbit planets.
Our solar system is a small part of the Milky Way galaxy.
How the Sun formed:
- The Sun formed from a cloud of dust and gas called a nebula.
- Gravity pulled the dust and gas together. As it collapsed, it became hotter and denser, forming a protostar.
- When the temperature and pressure were high enough, hydrogen nuclei began to fuse into helium, releasing energy. A main sequence star was born.
During the main sequence, the star is stable: the inward force of gravity is balanced by the outward pressure from the energy released by fusion. This equilibrium can last for billions of years.
The life cycle of a star
A star's life cycle is determined by its size (mass).
A star about the size of the Sun:
Nebula → protostar → main sequence star → red giant → white dwarf → black dwarf
A star much more massive than the Sun:
Nebula → protostar → main sequence star → red super giant → supernova → neutron star or black hole
- When a star runs low on hydrogen, it swells into a red giant (or red super giant), fusing helium and then heavier elements.
- A small star then collapses into a hot, dense white dwarf, which cools to a black dwarf.
- A massive star explodes in a supernova. The core left behind becomes a neutron star, or – if it is massive enough – a black hole.
Making new elements:
- Fusion processes in stars produce all of the naturally occurring elements. Elements up to iron are made by fusion during a star's life.
- Elements heavier than iron are produced in a supernova.
- The explosion of a supernova distributes the elements throughout the universe, where they can form new stars and planets.
Orbital motion, natural and artificial satellites
Gravity provides the force that allows planets and satellites (both natural and artificial) to maintain their circular orbits.
| Object | Orbits | Example |
|---|---|---|
| Planet | A star | Earth orbits the Sun |
| Moon (natural satellite) | A planet | The Moon orbits the Earth |
| Artificial satellite | A planet (usually Earth) | Communications and weather satellites |
Higher tier only
Higher tier:
- In a circular orbit, gravity acts towards the centre of the orbit, at right angles to the motion. It changes the direction of motion but not the speed. So the velocity changes (it is a vector) while the speed stays the same – the object is constantly accelerating towards the centre.
- For a stable orbit, the radius must change if the speed changes. A satellite in a smaller orbit (closer to the planet) must move faster, because the gravitational force is stronger there.
Common misconceptions
Common misconception
"There's no gravity in orbit." Gravity is what keeps a satellite in orbit – without it, the satellite would fly off in a straight line.
Common misconception
"All stars end as black holes." Only stars much more massive than the Sun can become black holes. The Sun will end as a white dwarf.