If you look up at the night sky, you’ll see thousands of stars. Some are brighter than others, some larger, and some twinkling. You may have heard of the nursery rhyme, “Twinkle Twinkle Little Star” where the stars are described as “twinkling” and moving in the sky. Hate to break the news to you all, but this is actually not true: these stars aren’t moving nor are they twinkling. If you go into space and see these stars, you’ll find they’re stationary; so why do they twinkle on Earth?
The light rays coming in from these stars have to pass through Earth’s atmosphere. It deflects these light rays as they enter, making it seem as though the star is moving. This twinkling depends on the density and temperature of the air it enters so if you are at the Earth’s horizon, the star you see appears to be twinkling more than a star viewing from the north or south pole.
To imagine this better, picture the famous example of a pencil in a cup of water. When viewed from the side, the pencil looks much different in the water, it seems that a different pencil is there; why is this the case? The light rays that form the image of the pencil pass through two mediums: air and water. Air and water have different densities, with water being the densest. When a light wave travels through a less dense material (air) to a more dense material (water), it bends more towards something called the normal (shown in the diagram below). In case you’re really confused, don’t worry too much about it; all you need to know is that the light rays travelling from these stars get bent in many angles and on top of that, the air in the earth’s atmosphere is moving, leading to the “twinkling” of stars.
The fun doesn’t stop there as these stars you are viewing are probably not how they really look at this time. You could be looking at the state of a star from a hundred, thousand, or a million years ago, but how? This is where our trusty, mischievous light rays come into play again.
Light travels at a definite speed of three hundred million meters per second (3x10^8 m/s). The light rays from the water bottle in front of you, the computer, and the tree outside all travelled at this speed to reach your eye to form its image. The stars’ light rays travel at this speed every second to reach our eyes. Remember, these stars aren’t close to us; the closest one to us is 40,208,000,000,000 km, that’s pretty far! Walking there would take 950 million years! Light has to travel this distance to reach our eyes which explains why we cannot see the star in its current state. So if this is the case, in what period are we seeing these stars? This is where lightyears come in. A light year is a measurement of how far light travels in a year. In a year, light travels 9.46 trillion kilometres. Doing some math, you’d find that the nearest star to us is 4.24 light years away, meaning it takes over four years for the light from this star to reach us so we are technically viewing this star as it was four years ago. Weird, huh?
Light is quite strange and has been something physicists have been grappling with for centuries. Many have been trying to figure out whether light is composed of waves or photons and there has been a lot of research done on it over the years which I won’t go into here otherwise your brains will probably start to hurt and this blog would be far too long! If this is something that interests you, I highly recommend doing some more research!