There’s a famous star that I’m sure you’ve seen in the sky. Its name is Betelgeuse and you can find it in the constellation Orion, where it marks Orion’s right shoulder. If you want to call it “Beetlejuice,” I’m fine with that, as long as you don’t say it three times.
But something is happening up there. This red supergiant has dimmed repeatedly in recent years, which could mean it’s ready to go full supernova very soon (and by “soon” we mean within the next 10,000 years). In fact, since it is about 500 light years away, it is possible that already It exploded and we just don’t know yet. It could appear tomorrow.
One thing is certain: if Betelgeuse explodes, it will be the brightest supernova ever witnessed by humans. How brilliant are we talking? Could you see it during the day? It would be dangerous? I’m going to show you how to solve all of this with just some very basic physics.
What is a supernova?
In most stars, the core is composed of hydrogen and helium, the two lightest elements, but only the positively charged nuclei of those atoms, since it is too hot for the electrons to stay still. Under immense gravity and temperatures, these nuclei can fuse into heavier elements, releasing enormous amounts of energy in the process. (This nuclear fusion is where our sun gets its energy from.)
For a stable star like our sun, there is a balance between two opposing forces. The mass of all the matter in the star produces a gravitational force that tends to collapse the star. However, this is counteracted by the force pushing outward from the core, so the star remains fairly constant in size, even though it is not a solid object like a planet.
But as a star ages, it gradually uses up its hydrogen and helium and begins to produce heavier elements such as carbon, oxygen, silicon, and eventually iron. And that’s it: fusing elements heavier than iron. you accept energy instead of creating it, so the star essentially runs out of fuel and collapses in on itself.
In some cases, this collapse can be very severe, so severe that the pressure and temperature in the star’s core rapidly increases. The star then explodes. Big boomGood, big silent boom, as explosions make no sound in the vacuum of space.
But this produces a LOT of light energy. In comparison, our sun has a luminosity, or power output, of 3.8 x 1026 watts. TO Supernova observed in 2015 (ASASSN-15h) had a maximum luminosity of about 2 x 1038 watts. That’s more power output than 500. billion Suns. It’s crazy. Oh, you didn’t see that one? Yes, because it was in a different galaxy. Betelgeuse is in our backyard, astronomically speaking.
Brightness and Luminosity
Long ago, a Greek philosopher named Hipparchus classified stars into six groups, based on their brightness in the night sky. From there, we have developed an “apparent magnitude” classification scheme, such that a magnitude 1 star appears very bright, while a magnitude 6 star is likely not even visible through light pollution. Betelgeuse is in the first group.
To be clear, this isn’t about a star’s actual luminosity, but rather how bright it appears from Earth, which depends on (1) how much light it produces and (2) how far away it is. Oh, and (3) magnitude is also based on how the human eye sees objects, and it’s not linear. A magnitude 1 object has a light intensity (in watts per square meter) that’s 100 times greater than a magnitude 6 object.