Supernovae – The Universe’s fireworks
- 6th Aug 2024
- Author: Joseph Walters
A supernova is one of the most beautiful sights in the universe. They can unleash the same amount of energy during the explosion as our Sun will during its entire 10 billion year lifespan! Given how powerful and explosive a supernova is, it is worth asking: are we humans in any danger from them?
What is a Supernova?
To discover if we are in danger from a supernova, first it must be understood what they are. A supernova is a transition period for a star, in which it changes from a red supergiant to either a neutron star or a black hole. When a huge star reaches the end of its life and halts its nuclear fusion (how stars make their light and heat energy), gravity takes over and crushes the star in. The pressure then gets so high in the core of the star that the outer layers are blown apart in an explosion known as a supernova, kind of like opening up a shaken can of coke.
Sometimes these supernovae are close enough to Earth that we can see them. The last seen supernova was in 1987. Whilst that may not seem all too long ago, the one before that was in 1604. This shows just how rare they are for humans to see, but if one were to begin in our own Milky Way galaxy, we would be sure to know about it.
The next star that scientists believe will go supernova in our galaxy is called Betelgeuse and is located approximately 700 light-years away (the distance travelled by light over the duration of one year), or four thousand million million miles away. That is the same as the distance from Earth to the Sun 43 million times! This means that Betelgeuse could have already exploded up to 700 years ago and the light just has not reached us yet. Whilst this may seem like an incredible distance, in the context of the universe, that is not as far as you would expect. Our galaxy is approximately 100,000 light-years across! But much closer to us (only 8 light minutes) is our very own star known as the Sun, and that will have a very different life to Betelgeuse.
Will Our Sun Become a Supernova?
If our Sun were to explode in a supernova, it would certainly lead to the end of life on Earth. Fortunately for us, physicists have determined the mass of stars required for a supernova explosion and it is now known that our Sun is at least eight times too small and light for there to be any risk of a supernova.
This is because of the nuclear fusion that occurs in stars. Nuclear fusion is the process where small atoms (such as hydrogen) are forced together under extremely high temperatures and pressures to become heavier elements, and the supernova occurs when the outward pressure from fusion reactions stops being able to counteract the inward pull of gravity. With our Sun, it will never be able to fuse elements together heavier than oxygen, and stars that go supernova can use fusion to sustainably create elements all the way up to iron.
We also know that our Sun is only about halfway through its life as a main sequence star, meaning that we would have around five billion years until it becomes a red giant, so even if it were to become a supernova, we would be safe for many billions of years. Because of its size, when our Sun reaches the end of its life as a red giant it will not become a black hole or neutron star, instead it will become a white dwarf (a very small star that has run out of fuel and is just emitting the last of its energy).
How Common are Supernovae?
The more frequent supernovae are, one could assume the higher the risk for us on Earth. However, it has been decades since the last observed supernova, and centuries since the one before that.
Even though a supernova at its peak can out-shine entire galaxies, and with there being an estimated 200 billion trillion stars in the observable universe, supernovae are not as common as perhaps expected.
Due to the size of the stars needed to go supernova (at least eight solar masses), only a small fraction of known stars will ever explode like this. That combined with a cosmic timeline, where the universe is approximately 13.7 billion years old, means that the universe is in no rush to let humans see them again.
Are We in Danger from Any Supernova Explosions?
Whilst it is true that on a cosmic scale Betelgeuse is close to Earth (it lies in the Orion constellation), it is not close enough to interfere with us.
The closest star to us that is believed to go supernova is called IK Pegasi B and is 150 light-years away. This is also too far away for Earth to experience anything other than seeing its brightness, and it will take place approximately 2 billion years in the future.
Given this, and the fact that our Sun will not become a supernova, it is the view of scientists that Earth is not in danger from any of supernova explosions.
Almost 1,000 years ago, a large star blew up in a supernova which caused the crab nebula. This was visible from the Earth for weeks and left the people who saw it full of awe and wonder as they did not know what it could be.
Even though we now know what a supernova is, and that we humans do not need to fear supernovae, we can still feel the same sense of awe if we were to see one, so be sure to look up at the sky because if Betelgeuse (or any other huge star) does explode, you may well get to see it!
Full references / credits:
(Banner) NASA's Hubble Space Telescope has unveiled in detail a section of the Veil Nebula. Credit: NASA/ESA/Hubble Heritage Team
(1) The SN 1987A Supernova, taken by the James Webb Space Telescope's NIRCam instrument. Credit: NASA, ESA, CSA, M. Matsuura (Cardiff University), R. Arendt (NASA’s Goddard Spaceflight Center & University of Maryland, Baltimore County), C. Fransson (Stockholm University), J. Larsson (KTH Royal Institute of Technology), A. Pagan (STScI)
(2) The different layers of a star. Credit: Pablo Carlos Budassi CC BY-SA 4.0 (https://upload.wikimedia.org/wikipedia/commons/a/ac/Layers_of_an_evolved_star.png)
(3) The Milky Way Galaxy as seen from Earth. Credit: Public Domain
(4) Diagram of the Orion Constellation. Credit: Anirban Nandi CC BY 3.0 (https://upload.wikimedia.org/wikipedia/commons/9/91/Orion_constellation_with_star_labels.jpg)
