What would be the consequences for the Earth if Betelgeuse exploded in the form of a supernova? Let’s first see what a supernova is to get an idea of what it’s all about.
A constant nuclear fusion reaction takes place in the core of stars. So why don’t stars explode? Well, because the weight of the remaining mass of the star, which presses on the core from all directions due to the effect of gravity, keeps the explosion confined to its center. That’s why stars have a spherical shape.
Early in their lives, stars glow by fusing hydrogen in their cores into helium. This process releases a tremendous amount of energy that raises the temperature of the rest of the star’s mass into a ball of glowing gas (plasma, for the picky ones) that is so hot it glows with its own light, just like a piece of iron after being heated by a blowtorch.
But hydrogen reserves are limited and, when it runs out, only helium remains in the core. Medium-sized stars, like our sun, are unable to fuse helium atoms into heavier elements to produce energy so, for an instant, the nuclear fusion reaction that energizes the star stops and, without the resistance offered by the shock wave generated in the core, the rest of the star rushes towards the helium core with all its weight, compressing it. As it compresses, the core heats up and transfers enough heat to its surroundings that hydrogen fusion can begin again in the surrounding shell.
This layer around the inert helium nucleus has a larger volume than the original nucleus, so it produces more energy and, therefore, a higher temperature. When something is heated, it expands, so this increase in the amount of heat produced is translated into the expansion of the star, which begins to grow until it reaches a diameter up to 400 times its original size. For this reason, even though the nucleus is producing more energy than usual, it has to be distributed over a larger surface area. The surface of the star begins to cool down to a reddish or orange hue. Hence these stars are called red giants.
But since it cannot generate matter out of nothing, the star becomes less dense as it swells and its outer layers spread freely through space. The expansion of the star will continue until, finally, there will come a point when all its mass is scattered through space and all that remains of the star are the remnants of the compact, whitish core. Such bodies were given the name of white dwarfs.
And that’s a supernova? Some kind of silent fart slowly spreading across the room?
No, no, this is what happens with stars of a similar size to the sun. It is the largest stars, with a mass at least 5 to 10 times that of the sun, that end their lives with a glorious cosmic explosion.
And why tell me about it if it’s not going to be on the exam?!
Star self-destruction in the form of a supernova
The gravity of very massive stars is so intense that the nuclear fusion reactions taking place in their core are forced to proceed at a much faster rate. For a medium-sized star like the sun, the fuel can last about 10 billion years. A giant star like Betelgeuse, on the other hand, runs out of fuel in 10 million years.
At the beginning of their lives, very massive stars quickly exhaust their hydrogen reserves but, thanks to the pressure to which their core is subjected, when the hydrogen is exhausted they are able to start fusing helium to produce carbon.
Not only that, but the extreme conditions that govern the core of a massive star force the new heavier elements that form in it to fuse into heavier and heavier elements:
Until, in the end, nickel begins to accumulate in the core. The problem with nickel is that its fusion stops releasing energy: it only absorbs it. Therefore, when the nucleus accumulates enough nickel, the nuclear fusion reaction stops. And with it the shock wave that keeps the rest of the star’s mass at bay.
With nothing to slow it down. The entire mass of the star rushes at once toward the core at speeds up to 23% of the speed of light. The tremendous pressure generated throughout the star triggers nuclear fusion reactions throughout its mass that, with nothing to contain it, cause an explosion of such magnitude that it can outshine the entire galaxy that contains it.
And that, ladies and gentlemen, is a supernova. A thermonuclear bomb several million kilometers in diameter.
Supernovae and the Effect They May Have on Earth
The following image shows a supernova in the galaxy M82 that occurred at a distance of 11 to 12 million light-years. To put this in perspective, remember that one light year is about 10 trillion kilometers, the distance light travels in one year (at a speed of 300,000 km/s).
On the left side you see the image of the galaxy before the explosion and on the right side as it is happening. Remember that this galaxy contains about 50 billion stars, judging by the brightness emanating from the single star that just exploded.
The closest supernova on record took place in 1987, 168,000 light-years away, in the Large Magellanic Cloud (a small satellite galaxy circling our own). At this distance, the supernova was visible to the naked eye from the southern hemisphere with a magnitude of 3.03, which is equivalent to the brightness of the faintest stars that can be seen at night in an urban environment.
Betelgeuse is very close in astronomical terms. As for the actual distance, it is about 642 light years. Of course: with a diameter of about 1,600 million kilometers, Betelgeuse is a gigantic star. If we substituted this star for our sun, its surface would surely engulf everything on its way to the orbit of Jupiter.
Betelgeuse is a huge star, yes, but it is not as massive as it appears. Precisely because it is a red giant, that is, a star that has begun to expand greatly with low density. Although its radius is 950 to 1200 times that of our Sun, its mass is “only” between 7.7 and 20 times greater.
But, if its mass is so spread out in space… How is it going to explode in the form of a supernova?
Good question. Although some of its mass is scattered throughout the cosmos, Betelgeuse is so large that it still contains a compact and immense core at its center. A very massive star in itself, which is the one that will trigger the supernova.
But don’t start saving up to buy a ship to flee to another solar system. Keep the following in mind.
When a star explodes, it releases a very energetic shock wave that spreads in a spherical way through space.
When a star explodes, it releases a very energetic shock wave that spreads out into space in a spherical shape. The geometry of this shock wave is important. The energy released in the supernova explosion must be distributed over a larger and larger area as the size of the sphere increases. In the case of a sphere, the energy is dissipated with the square of the distance. This means that if the shockwave sphere doubles in size, its surface area increases by four times, so the energy released must be distributed over an area four times larger.
For example, if you are at any distance from the explosion, you will receive a certain amount of energy. But if you double the distance between you and the explosion, you will no longer receive half the energy, but only a quarter. If you quadruple the distance, you will receive 16 times less energy, if you stand at a distance 8 times the original distance, you will receive 64 times less radiation than in your starting position, and…. Well, you get the point, don’t you? The energy of such phenomena, just like the light or WiFi signal, dissipates very quickly as it moves through space.
And, of course, despite the fact that supernovae release unimaginable power, the distances that separate things in space are equally tremendous. That’s why, according to astronomers’ estimates, a supernova would have to be very close to Earth (between 50 and 100 light years away) for it to pose any danger. Between us and Betelgeuse there are 642 light years of empty space in which the explosion can lose energy, so we have nothing to worry about.
So Betelgeuse isn’t going to wipe Earth off the map?
The only thing that would be triggered on Earth by Betelgeuse supernova explosion would be a nice light show.
We can calculate, more or less, how bright a supernova like Betelgeuse would be in the sky. We know that a supernova reaches a maximum luminosity of about 10 billion times that of the sun and that Betelgeuse is about 642 light-years away.
Following this method, we can see that the supernova would start to shine at magnitude -14.06 at the moment of the explosion. From that moment on, its brightness could evolve in two ways:
- If it were a type I supernova, it would shine with a brightness of -14.06, which is about three times the brightness of the full moon (-12.74), and then its brightness would decrease until it disappears.
- On the other hand, if it going to be type II, it would stabilize at a brightness of -11.55 for a few months after reaching its maximum brightness at the beginning, and it would be visible in the sky during the day and at night with a brightness three times less than that of the full moon. After that, its brightness gradually decreases until it equals that of the other stars and eventually disappears. This is the most likely scenario.
The claim that Betelgeuse could become a supernova any day is an exaggeration, for a change, made by the press and documentaries to make their content more eye-catching. In reality, the most recent studies estimate that this day is still about 100,000 years away. It could happen sooner, of course, but the probability is very low. In short, it does not pay to spend every night of your lives awake watching Betelgeuse, waiting for it to explode. Because you will surely waste your time for nothing.