IGCSE Physics / Motion in the universe
The universe, galaxies and our solar system revision notes
Edexcel International GCSE Physics (4PH1), chapter 26: Motion in the universe, specification point 8.2, 8.1. Free lesson.
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The universe contains galaxies, and galaxies contain stars and their orbiting objects. Knowing how the solar system fits into this order, and which units describe motion in space, helps you answer astronomy questions.
What is in our solar system?
Our solar system is the Sun and everything that orbits it. To orbit means to travel round and round another object. The Sun is at the centre, and eight planets orbit it. Other objects orbit the Sun or, in the case of moons, a planet. Planets orbit the Sun. Earth is the third planet out. Moons orbit planets, rather than orbiting the Sun directly. Dwarf planets, such as Pluto, are smaller than planets. Asteroids are lumps of rock and metal. Most are in a belt between Mars and Jupiter. Comets are lumps of ice and dust that travel on long, stretched orbits around the…
How do planets, stars, galaxies and the universe fit together?
The objects can be put in order by size. Each larger object contains the one before it. A galaxy is a huge collection of billions of stars held together by gravity. The universe contains billions of galaxies. Smallest to largest: planet, star, solar system, galaxy, universe. The Sun is a star. It looks much bigger and brighter than other stars because it is much closer to Earth. The stars you see at night are suns too, and many have planets of their own. Our solar system is in the Milky Way galaxy. The Milky Way is one galaxy within the universe.
Which units are used for astronomy questions?
Astronomy questions use SI units, the standard units used in physics. Distances and orbital radii are measured in metres. Large distances may be written in standard form. Times must be in seconds, so convert an orbit given in days or years before using it in a calculation. Distance and orbital radius: metre (m). Time and time period: second (s). Speed, including orbital speed: metres per second (m/s). Force, including weight: newton (N). Mass: kilogram (kg). Gravitational field strength, g: newtons per kilogram (N/kg).
How do you order the objects by size?
To put these objects in order, start with the smallest and follow which object contains the previous one. A planet is part of a solar system, a solar system is within a galaxy, and a galaxy is within the universe.