Galaxies and the universe
Describe the scale of the universe, including the structure of galaxies, the position of the Sun in the Milky Way, and the use of light years as a unit of distance, and appreciate why space exploration requires enormous timescales
What a learner can do afterwards
- Defines a light year as the distance light travels in one year
- Describes the structure of a galaxy and states that the Sun is a star in the Milky Way galaxy
- Compares the distances within the solar system with the distances between stars and between galaxies, expressing them in appropriate units
The lesson
Zoom out from Earth and you pass through layers of space, each far bigger than the last. Earth orbits the Sun. The Sun and its planets make up the solar system. The solar system is just one tiny part of a much bigger galaxy called the Milky Way. And the Milky Way is only one of billions of galaxies in the universe.
Distances between stars are too huge to measure in kilometers, so scientists use a light year instead. Light travels about 300,000 kilometers every second, faster than anything else we know. A light year is the distance light travels in one whole year, about 9.5 trillion kilometers. It measures distance, not time, even though the word year is inside it.
Light from the Sun takes about 8 minutes to reach Earth, so the Sun is 8 light-minutes away. Light from Proxima Centauri, the closest star beyond the Sun, takes about 4 years to reach us, so that star is 4 light years away. The Milky Way itself stretches about 100,000 light years from edge to edge.
This is why space exploration takes such enormous timescales. Spacecraft are far slower than light, so a trip that takes light only 4 years would take a spacecraft tens of thousands of years. That is why we study distant stars with telescopes instead of visiting them.
The Sun is one star among billions in the Milky Way, and light years measure the huge distances between stars and galaxies.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.