Summary

• The universe includes the entirety of space, matter, energy and time.

• It is composed of normal matter, dark matter and dark energy.

• Normal matter is made up of protons, neutrons, and electrons and includes everything that we can directly observe.

• It only accounts for about 5% of the universe.

• Dark matter is an unknown form of invisible matter that is only believed to exist due to its gravitational effects.

• It accounts for about 27% of the universe.

• Dark energy is an unknown form of energy that is believed to oppose gravitational force, causing accelerated expansion of the universe.

• It accounts for about 68% of the universe.

• An astronomical body is any single, naturally occurring entity that exists within the universe, including:

• Stars – large, luminous balls of hot gas undergoing nuclear fusion.

• Planets and dwarf planets – spherical bodies that orbit stars or stellar remnants.

• Moons – bodies that orbit planets or dwarf planets.

• Asteroids – small, irregularly shaped, rocky bodies that orbit stars.

• Comets – small, irregularly shaped bodies composed of ice and dust that orbit stars and which have distinct ‘tails’.

• Meteoroids – small fragments derived from asteroids or comets.

• An astronomical object is any gravitationally bound group of astronomical bodies, including:

• Planetary systems – systems consisting of a star or stellar remnant orbited by planets and other non-stellar bodies.

• Star systems – small groups of fewer than ten stars.

• Star clusters – medium-to-large groups of tens to thousands of stars.

• Galaxies – very large or extremely large groups of millions to trillions of stars.

• The interstellar medium and the intergalactic medium contain extremely low densities of gas and dust, but account for most of the visible matter in the universe.

• Cosmic dust refers to microscopic, solid particles that are interspersed between astronomical bodies.

• Nebulae are giant clouds of interstellar gas and dust.

• They are the remnants of dying or exploding stars and can become sites where new stars form.

(Image: NASA)