Uploaded August 2021 | Updated September 2026, 2 hours ago
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
First we’ll start with Astronomical Objects
A star is a large, glowing ball of gas that generates heat and light through nuclear fusion in its core. Our Sun is a star.
A planet is a moderately large object that orbits a star and shines primarily by reflecting light from its star. According to the current definition, an object can be considered a planet only if it (1) orbits a star, (2) is large enough for its own gravity to make it round, and (3) has cleared most other objects from its orbital path. An object that meets the first two criteria but has not cleared its orbital path, like Pluto, which is NOT a planet, fight me, is designated a dwarf planet.
A moon (or satellite) An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object.
An asteroid is a relatively small and rocky object that orbits a star.
A comet is a relatively small and ice-rich object that orbits a star.
A small solar system body is an asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet.
Next, let’s touch on some Collections of Astronomical Objects
The solar system is The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means “of the Sun”), it is often applied to other star systems as well.
A star system is a star (sometimes more than one star) and any planets and other materials that orbit it.
A galaxy is A great island of stars in space, all held together by gravity and orbiting a common center, with a total mass equivalent to millions, billions, or even trillions of stars.
A cluster of galaxies (or group of galaxies) is a collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters.
A supercluster is a gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe.
The universe is The sum total of all matter and energy—that is, all galaxies and everything between them.
The observable universe is the portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.
Next, we’ll review Astronomical Distance Units
An astronomical unit (AU) is the average distance between Earth and the Sun, which is about 93 million miles.
A light-year is the distance that light can travel in 1 year, which is about 9.46 trillion km
Now, here are some Terms Relating to Motion
The rotation is the spinning of an object around its axis. For example, Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.
An orbit (or revolution) is the orbital motion of one object around another due to gravity. For example, Earth orbits the Sun once each year.
And expansion (of the universe) is the increase in the average distance between galaxies as time progresses. But we’ll dive deep into that one later.
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
First we’ll start with Astronomical Objects
A star is a large, glowing ball of gas that generates heat and light through nuclear fusion in its core. Our Sun is a star.
A planet is a moderately large object that orbits a star and shines primarily by reflecting light from its star. According to the current definition, an object can be considered a planet only if it (1) orbits a star, (2) is large enough for its own gravity to make it round, and (3) has cleared most other objects from its orbital path. An object that meets the first two criteria but has not cleared its orbital path, like Pluto, which is NOT a planet, fight me, is designated a dwarf planet.
A moon (or satellite) An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object.
An asteroid is a relatively small and rocky object that orbits a star.
A comet is a relatively small and ice-rich object that orbits a star.
A small solar system body is an asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet.
Next, let’s touch on some Collections of Astronomical Objects
The solar system is The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means “of the Sun”), it is often applied to other star systems as well.
A star system is a star (sometimes more than one star) and any planets and other materials that orbit it.
A galaxy is A great island of stars in space, all held together by gravity and orbiting a common center, with a total mass equivalent to millions, billions, or even trillions of stars.
A cluster of galaxies (or group of galaxies) is a collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters.
A supercluster is a gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe.
The universe is The sum total of all matter and energy—that is, all galaxies and everything between them.
The observable universe is the portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.
Next, we’ll review Astronomical Distance Units
An astronomical unit (AU) is the average distance between Earth and the Sun, which is about 93 million miles.
A light-year is the distance that light can travel in 1 year, which is about 9.46 trillion km
Now, here are some Terms Relating to Motion
The rotation is the spinning of an object around its axis. For example, Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.
An orbit (or revolution) is the orbital motion of one object around another due to gravity. For example, Earth orbits the Sun once each year.
And expansion (of the universe) is the increase in the average distance between galaxies as time progresses. But we’ll dive deep into that one later.
