Astronomical Units
5 Minutes
Astronomical units explained for students: learn how astronomical units, light years, parsecs, and solar mass measure space. An English science lesson.
- Astronomical units and planetary distances for Venus and Mars
- Light years and parsecs for measuring nearby stars and galaxies
- Solar mass for celestial bodies and atomic mass units for atoms
What this lesson teaches
Astronomical units are specialized standards of measurement used to express the enormous distances and masses of celestial bodies using manageable numbers. When measuring planets, stars, and galaxies, standard meters and kilograms produce values that are far too large to handle easily, so scientists define units based on natural cosmic scales.
Distances inside the solar system
Distances within our planetary neighborhood are measured using the astronomical unit, which represents the exact distance from Earth to the Sun. One astronomical unit equals 149,500,000 kilometers, an amount roughly equivalent to circling Earth 4,000 times. By using this baseline, planetary distances can be expressed in simple values, such as Venus being 0.723 astronomical units from the Sun and Mars being 1.524 astronomical units away.
Distances to stars and galaxies
Stars outside the solar system require larger units, such as the light year, which is the distance light travels in one year. One light year equals 9.461 multiplied by 10 to the power of 12 kilometers, which is equivalent to circling Earth 240,000 times. Proxima Centauri, the nearest star, is 4.22 light years away, and the bright night star Sirius is 8.6 light years away. For even more distant galaxies, scientists use the parsec, which equals 3.26 light years, placing the Andromeda Galaxy at 0.78 multiplied by 10 to the power of 6 parsecs and the Virgo Cluster Galaxy at 16.5 multiplied by 10 to the power of 6 parsecs.
Measuring mass from stars to atoms
Mass in space is measured using the solar mass, which equals the mass of our Sun at 2 multiplied by 10 to the power of 30 kilograms, or the mass of 333,000 Earths combined. The black hole at the center of the Milky Way galaxy is 4 million times larger than the Sun, giving it a mass of 4 million solar masses. On the microscopic scale, tiny particles are measured using the atomic mass unit, defined as one piece of a carbon atom divided into twelve parts, equal to 1.66 multiplied by 10 to the power of minus 27 kilograms. Using this microscopic standard, a single water molecule has a mass of 18.02 atomic mass units.