Physics
Stars & light years
Every star in the night sky is a distant sun, and its light is a message from the past.
What you need first
On a clear night far from city lights you can see thousands of sparkling dots. What you are looking at is grander than it seems: every one of those dots is a sun, often bigger and brighter than our own, just unimaginably far away. And every time you look up, you are looking into the past.
Stars are distant suns
Our Sun is a star like billions of others, it just sits right on our doorstep. Stars are gigantic balls of glowing gas. Deep inside them, hydrogen nuclei fuse into helium, and that releases so much energy that a star can shine for billions of years. The Sun looks like a blinding disc while other stars look like tiny dots for one reason only: distance. The nearest star after the Sun is about 270,000 times further away than the Sun.
The light year: a tape measure for space
For distances like these, kilometres are too small. So we measure with the fastest thing there is: light races along at about 300,000 km per second. A light year is the distance light travels in one year, roughly 9.5 trillion kilometres. Careful, a popular trap: a light year is a distance, not a time. The nearest star is a good 4 light years away, and our Milky Way spans more than 100,000 light years.
Each step on the scale means ten times bigger.
Looking into the past
Because light needs time, you never see what a star looks like now, only what it looked like when its light set off. Sunlight travels for a good 8 minutes, so you always see the Sun 8 minutes old. The light of the star Vega travels for about 25 years: you see that star as it looked 25 years ago. For distant it is millions or billions of years: telescopes are genuine time machines for looking back.
Star colours reveal temperature
Look closely: stars have colours. And those colours reveal how hot their surface is, though the other way round from what everyday life suggests. Reddish stars are the coolest at around 3,000 degrees Celsius, yellowish ones like our Sun sit at about 5,500 degrees, and bluish-white stars are the hottest at over 10,000 degrees. You know this from heating metal: first it glows red, then yellow, then white. That makes our Sun a fairly average yellow star, neither especially big nor especially hot, but just right for us.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What is a light year?
Which star has the hottest surface?
The Sun's light travels to us for about 8 minutes. Roughly how many seconds is that?
Put these cosmic distances in order from shortest to longest.
- 1From the Sun to the nearest star, a good 4 light years
- 2Right across the Milky Way, over 100,000 light years
- 3From Earth to the Sun, light takes 8 minutes
Light covers about 300,000 km in one second. How many kilometres does it manage in 10 seconds?
Match each star colour to its surface temperature.
Where this leads