Welcome to the first of a 3-part blog series that aims to help secondary school students become more interested in astronomy simply by standing out in their own gardens and appreciating that what they see in the night sky can open up a huge amount of curiosity and wonder by understanding how fast light travels, what can be seen with the unaided eye and how photographing the night sky can show you even more than you imagined.

When you look up at the night sky, everything you see with your unaided eyes (i.e. without the use of a telescope) is in our own Galaxy – the Milky Way – where our sun is just one of around 400 billion stars. Some of the bright objects you see like the moon and the planets are in our own Solar System, but everything else is trillions of miles away.

Of course, with telescopes we can see outside of our galaxy and observe other distant galaxies, but for now, we’ll just focus on what we can see with the unaided eye.

“All the stars we see appear to be fixed at the same distance as if they were all painted on an invisible canvas in the sky.”

Looking up on a dark evening, all the stars we see appear to be fixed at the same distance as if they were all painted on an invisible canvas in the sky. However, each of these stars are at significantly different distances from the Earth and that the brighter/bigger stars don’t necessarily mean they are closer to us. Some stars exist much bigger than others in size and mass, so they burn much hotter and brighter. Therefore, a big/hot/bright star that is further away can appear brighter to us than a small, cooler star that is much closer to us.

Firstly, we need to understand that the distance of stars to Earth is so massive that we don’t talk about miles or kilometres, we have to talk ‘light years’. This unit of measure is also used for the distance of other things too like the size of a galaxy, the distance between galaxies and also the size of our universe…because they are so big!

“In one second, light travels around the Earth’s equator seven and a half times.”

Our nearest star – the Sun – is around 93 million miles away (relatively close in cosmic terms!) and with light travelling at the speed of light (186,000 miles per second) that means it takes roughly 8 minutes for the sun’s light to reach Earth. So, when we look at the sun, we see it as it existed 8 minutes ago, as that light has had to travel for 8 minutes before it reaches our eyes here on Earth. We call this eight ‘light minutes’.

But for all the other stars we see, because of their distance from us, ‘light minutes’ becomes ‘light years’. After our sun, let’s take the nearest star to us which is called Proxima Centauri. The distance is so great that it takes light travelling at its constant speed of 186,000 miles per second over 4 years to reach us! That’s 4 light years…and that’s our Solar System’s closest neighbour! For context, Proxima Centauri is a low mass star, so it doesn’t shine bright enough for us to see the star in the night sky with our unaided eye; which reinforces the earlier point about stars closer to us not always appearing brighter.

We can try and get a sense of how fast light travels by trying to imagine this fictional scenario: if you stood on the Equator of the Earth and flashed a torch on and off – in just one second, that light would have travelled around the Equator seven and a half times!

“The bright star, Betelgeuse, in the constellation Orion is 642 light years away!”

The brightest star in the Northern sky that we can see with the unaided eye is called Sirius. This star can be best viewed in the Autumn/Winter months very low to the horizon just to the left of the constellation, Orion (more on constellations in the next blog). This star exists 8 light years away. Another popular bright star, Betelgeuse (in Orion) is 642 light years away! These are just two examples of the varying distances of stars that we can see. See if you can imagine how far away Betelgeuse is by using the torch scenario above to then travel at that speed in your mind for 642 years!

So, if light takes a long time to travel to us from distant stars, we’re seeing them as they existed in the past. So yes, the whole night sky is showing you stars as they existed years ago, sometimes, hundreds or thousands of years ago – you are therefore time travelling with just your eyes! Ok, other than perhaps a slight shift in colour and size, all the stars we see with our unaided eye look the same, but hopefully this blog will make you look at the stars differently on the next clear night now you have the knowledge that light has been travelling from these stars for decades and even centuries.

Recommended videos:

• What if you could travel at the speed of light? https://www.youtube.com/watch?v=jfPZWiTOugg
• How far are we looking back in time? https://www.youtube.com/watch?v=3Id1ygiwY4k

In the next blog post, I’ll be talking about what are the easiest and most exciting things (in my opinion) that a beginner can see in the night sky with the unaided eye or a basic pair of binoculars and how you can find these. A sneak preview – one of these things is the International Space Station!

About the Author:

Jordan Mawbey is a senior marketing professional day-to-day, but his hobby outside of work is Astronomy, Cosmology and Astrophotography. Jordan enjoys sharing his knowledge on these subjects to others to help them become more enthusiastic about looking at the night sky, understanding what is out there, and how you can photograph these objects in more detail. Follow his Instagram photography page at https://www.instagram.com/jjm_astro/