This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
Orbital motion, gravitational fields and circular motion in the solar system tested as shorter questions.
The hierarchy from Earth to Universe
Everything in space sits inside a tidy nested structure. Working from biggest to smallest:
The 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
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
Exam tip
Largest structure in the hierarchy
What comes up: a multiple-choice question gives you Milky Way, nebula, solar system, and Universe and asks which is the largest (or "a large collection of billions of galaxies").
Write: the Universe — it is the only structure that contains all galaxies, including the Milky Way. A galaxy holds billions of stars; a solar system holds one star and its planets; a nebula is a cloud of gas and dust (not a structural level in the hierarchy).
Watch out: the Milky Way is the name of one specific galaxy, not the Universe. Choosing it is the most common wrong answer.