Galaxies, light-years and redshift
Watch the whole lecture free
Make a free account to watch every lecture to the end: 1 worked example, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
That was the teaching part. The rest of this lecture is free with an account: 1 worked example, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Looking back in time
- Galaxies
- The light-year
Read the transcript of the teaching part
Last time
Before we start: last time, in The Solar System, stars and the Universe, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Looking back in time
When you look at the Sun, you see it as it was eight minutes ago. When you look at the nearest star, you see it as it was four years ago. Light takes time to arrive, and the stars are so far away that we measure their distances in the time light takes to reach us.
Galaxies
A galaxy is a huge group of billions of stars, held together by gravity, with gas and dust. The Sun is one star in the Milky Way, a spiral galaxy of about a hundred billion stars. The Universe contains billions of galaxies.
The light-year
A light-year is the distance light travels in one year. It is a unit of distance, not of time. One light-year is 3.0 times 10 to the 8 metres per second, times 365 days, times 24 hours, times 3600 seconds. That is 9.5 times 10 to the 15 metres.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Dispersion of light · Galaxies and the Universe.
The whole chapter: Light, Stars and the Universe, Light.
6.1–6.2 Earth, stars and the Universe
- Gravitational fields, orbits and planetary data 6 min
- Galaxies, light-years and redshift 5 min
- The Sun and the life cycle of a star 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.