Motion in the Universe
Astrophysics
The hierarchy from Earth to Universe
Everything in space sits inside a tidy nested structure. Working from biggest to smallest:
- The Universe is everything that exists. All the matter, energy, space and time, taken together. The Universe contains roughly 100 billion galaxies
- A galaxy is a vast collection of billions of held together by their mutual gravity. Our galaxy is the Milky Way, and it contains around 200 billion stars
- A star is a huge ball of hot gas (mostly hydrogen) powered by nuclear fusion in its core. The Sun is one such star, fairly average in size and brightness
- A planetary system is a star together with everything that orbits it. Our planetary system is the
- A planet is a roughly spherical body that orbits a star, big enough for its own gravity to pull it into a sphere, but not big enough for fusion. The Earth is the third of the eight in the Solar System
- A moon is a smaller body that orbits a planet. Earth has one moon; Jupiter has 95 known moons
Common exam question
Size order of the Universe, galaxies and solar systems
Question: Which of a galaxy, the Solar System, the Sun, the Moon and the Universe has the largest or the smallest diameter, or which of them is a large collection of billions of galaxies? (1 mark)
Asked in 2 of the 24 papers, each time as multiple choice. The Universe is the largest: it contains billions of galaxies, and each galaxy contains billions of stars with their solar systems, so the Universe is also the "large collection of billions of galaxies". The Milky Way is the name of one galaxy, so it is a distractor there, and a nebula is a cloud of dust and gas rather than a level in the hierarchy. At the small end, a moon is smaller than a star, a solar system or a galaxy, because a moon orbits a planet, which in turn orbits a star.
Other objects in the Solar System
- Asteroids are small rocky bodies, mostly orbiting the Sun in a band between Mars and Jupiter called the asteroid belt
- Comets are small icy bodies that orbit the Sun on highly stretched paths, swinging in from the outer Solar System
- Artificial satellites are human-built objects placed in orbit around the Earth or another planet, for communications, weather, GPS, science, and spying
Where everything orbits what
| Orbiting body | What it orbits |
|---|---|
| Planet | The Sun |
| Moon | A planet |
| Comet | The Sun |
| Asteroid | The Sun |
| Artificial satellite | The Earth (or another planet) |
- Smaller bodies always orbit larger bodies. The bigger gravitational pull belongs to the larger object, so the smaller one is the one that swings around
Common exam question
Knowing what orbits what
Question: Which object do planets orbit, which object also orbits the Earth, or what type of object has a circular or an elliptical orbit around a star? (1 mark)
Asked in 4 of the 24 papers. Planets orbit a star: satellites orbit planets and comets orbit stars, so neither can be the thing a planet goes around. The bodies that orbit the Earth are the Moon and artificial satellites such as a space station; a comet is rejected (comets orbit the Sun, not the Earth). An object on a circular orbit around a star is a planet and one on an elliptical orbit is a comet (that scheme also accepted "planet", whose orbit is a slight ellipse).