When you wish upon a star

Have you ever heard someone say “when you look up at the night sky, you’re actually looking into the past”. Is it true? Well technically, yes. As we know, light travels at a speed of roughly 300,000 kilometres a second. Even the brightest star in our sky (Sirius A) is 8.6 light years away. This means that if it was to suddenly die and create a supernova, we wouldn’t be able to know about it for nearly a decade! This has led to the popular and quite satirical phase being thrown around: 

When looking at stars, you’re actually looking into the past. Many of the stars we see at night have already died. Like your dreams.

Supernovae are essentially colossal explosions due to an imbalance within the star’s core, which leads to some pretty spectacular visual displays. Once they have occurred, a nebula is left behind in its place – an assortment of dust and remnants shaped by shockwaves. There are two ways in which a supernova can form:

  • When two stars orbit on the same axis, one star (a white dwarf) can often gain matter from its counterpart. Eventually, the star will gain too much mass to support itself and will subsequently explode. 
  • At the end of a massive star’s lifecycle. This occurs when there is insufficient nuclear fuel to power the core. Eventually some of the outer mass is pulled toward the core by gravity as the inner mass drifts upwards in the star structure. The gravity and pressure in the star can no longer balance and it explodes. 

But will we actually ever see a supernova in our lifetime? In 1987 a group of scientists identified a supernova in a dwarf galaxy just outside the Milky Way (only 168,000 light years away!). This was the most recent and closest-observed supernova in recorded history. However, there are a few possible stars up for further contention, one of which being the Red Giant star Betelgeuse in the constellation “Orion.” This star is roughly 650 light years away, so could still be active but may have formed a supernova some centuries ago without us knowing. Statistics shows that on average, one or two stars per galaxy go supernova every century. One of the clearest galaxies to the naked eye is called Andromeda, which contains somewhere between 200 and 400 billion stars. Therefore, we can be almost certain that during the human existence on the Earth (100,000 years), there will have been a good few visible supernovas in the sky. With a telescope, one is able to look at more distant galaxies, increasing the chance to identify a supernova. 

But what about our Sun? How long will it last? Most small stars (around the size of the Sun) have a lifespan of roughly 10 billion years, with smaller stars lasting longer than the giants. The star in our solar system is 4.6 billion years old, therefore by that reasoning, our solar system is safe for another 6 billion years or so! In fact, our star is still getting brighter – 4 billion years ago the Sun was 75% dimmer than it is today! So, when the Sun finally decides to pop, it will firstly expand into a red giant. A 2008 study by astronomers Schröder and Connon-Smith estimated that the Sun will get so large that its surface layers will reach about 108 million miles out, covering Mercury, Venus and Earth. When this happens, the outlying planets will become too hot to be hospitable. The next planet to be suitable to live would be Pluto and the other dwarf objects in the Kuiper belt. The whole process of turning into a red giant should take about 5 million years, a relative blip in the sun’s lifetime. Just to put this timescale into perspective; if the age of the Earth was compressed into a 24-hour day, then humans would occupy the last second of the day. Hopefully by the time that our Sun becomes a supernova, we will have colonised a different planet/solar system – that is assuming that we get past the next few decades!


James Deed


Image: Sankrit & W.Blair