This page is suitable for students studying for AQA GCSE Physics

Notes and Ramped Questions on Our Solar System

The star at the centre of our solar system is the Sun.
Our solar system is part of the Milky Way galaxy.
A natural satellite is an object, such as a moon, that orbits a planet.
A planet orbits a star, while a moon orbits a planet.
A nebula is made from dust and gas.
Gravity pulls the dust and gas in the nebula together.
The centre becomes hotter and denser because more particles are pulled closer together by gravity.
In nuclear fusion, hydrogen nuclei join together to form helium and release energy.
Fusion releases energy that creates an outward pressure, which helps stop the Sun collapsing inwards.
The Sun is stable because the inward pull of gravity is balanced by the outward pressure from fusion energy.

All stars begin in a cloud of gas and dust called a nebula.
Gravity pulls the gas and dust in a nebula together.
The temperature and density increase as the material is pulled closer together.
A protostar is a young forming star made from gas and dust pulled together by gravity.
Nuclear fusion must start inside the protostar for it to become a main sequence star.
Hydrogen is fused to form helium during the main sequence stage.
A main sequence star stays stable because gravity pulling inwards is balanced by fusion energy pushing outwards.
A star about the same size as the Sun becomes a red giant, then a white dwarf, then eventually a black dwarf.
A much more massive star becomes a red super giant, then explodes as a supernova, leaving a neutron star or black hole.
Fusion makes new elements in stars, and supernovae make elements heavier than iron and spread them through space.

Gravity is the force that keeps planets, moons and satellites moving in orbit.
The planets in our solar system orbit the Sun because of the Sun’s gravitational pull.
A natural satellite is a natural object that orbits a planet, such as the Moon orbiting the Earth.
A natural satellite is not made by humans, while an artificial satellite is a machine launched into space by humans.
Artificial satellites can be used for GPS, weather forecasting, communication, TV signals and taking images of Earth.
Without gravity, there would be no force pulling the object inwards, so it would continue moving in a straight line.
Gravity pulls the satellite towards the planet, constantly changing its direction so it follows a curved orbit instead of flying away.
The satellite’s speed can stay the same, but its velocity changes because velocity includes direction and the direction is constantly changing.
An object in a circular orbit is accelerating because its velocity is changing, even though its speed may stay constant.
For a stable orbit, the speed and radius must match; if the satellite moves faster, it needs a larger orbit, and if it moves slower, it needs a smaller orbit.

The wavelength of light from most distant galaxies increases, which means the light has been stretched.
Red-shift is the increase in wavelength of light from a galaxy, moving it towards the red end of the spectrum.
Red-shift shows that most distant galaxies are moving away from us.
The further away a galaxy is, the faster it is moving away from us.
Light becomes red-shifted because the galaxy is moving away, causing the light waves to stretch out and become longer.
The link between distance and red-shift suggests that space itself is expanding.
Space expanding means the gaps between galaxies are getting bigger, rather than galaxies just moving through empty space.
Red-shift supports the Big Bang theory because if galaxies are moving apart now, the universe must have been smaller, hotter and denser in the past.
Scientists develop theories by collecting observations, looking for patterns, and using evidence to explain what they see.
Observations of supernovae suggest the universe’s expansion is speeding up, but scientists still do not fully understand dark matter or dark energy.