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The average distance from the Earth to the Sun, approximately 149.6 million kilometers.

The distance that light travels in a vacuum in one Julian year (365.25 days), roughly 9.46 trillion kilometers.

A unit of distance equal to 3.26 light-years, used to express distances to stars and galaxies, defined as the distance at which one astronomical unit subtends an angle of one arcsecond.

A standard unit of mass in astronomy equal to the mass of the Sun, approximately $2 \times 10^{30}$ kilograms.

A unit of distance used to express the size of stars in astronomy, equal to the current radius of the Sun, approximately 696,340 kilometers.

A unit of luminous power used in astronomy, equal to the luminosity of the Sun, approximately $3.828 \times 10^{26}$ watts.

A non-SI unit of spectral flux density, used in radio astronomy, equal to $10^{-26}$ watts per square meter per hertz.

A measure of how much the wavelength of the light from an object has been stretched by the expansion of the universe. It is dimensionless.

A measure of the brightness of a celestial object as seen from Earth. The scale is logarithmic and inversely proportional: the brighter the object, the lower the number.

A measure of the intrinsic brightness of a celestial object. It is defined as the apparent magnitude the object would have if it were 10 parsecs away from the observer.

The rate of expansion of the universe, given in kilometers per second per megaparsec (km/s/Mpc). Current estimates place it around 70 km/s/Mpc.

A measure of the total luminosity of a celestial object over all wavelengths of light, not just visible light.

A timekeeping system used by astronomers to keep track of the direction to observe stars in the night sky. It is based on Earth's rate of rotation measured relative to fixed stars.

Radiation that is a remnant from the early stages of the universe, also known as 'relic radiation', important in cosmology.