There is a famed star that you have surely seen in the sky. It is called Betelgeuse and can be found in the constellation Orion, where it marks Orion’s right arm. If you want to call it Beetlejuice, I don’t mind, as long as you don’t say it three times.
But something is happening there. This red supergiant has dimmed several times over the past few years, which could mean it’s ready to go full-blown supernova soon—and by “soon,” we mean within the next 10,000 years. In fact, since it’s about 500 light-years away, it’s possible Already it exploded and we don’t know it yet. It may show up tomorrow.
One thing is certain: if Betelgeuse explodes, it will be the brightest supernova ever seen by humans. How astute do we say? Could you see it during the day? Would it be unsafe? I’ll show you how to calculate all of this using very basic physics.
What is a supernova?
In most stars, the core is made of hydrogen and helium, the two lightest elements—but only the positively charged nuclei of these atoms, because it is too sizzling for electrons to stay in place. Under the influence of the enormous gravity and temperatures, these nuclei can fuse into heavier elements, releasing extensive amounts of energy in the process. (It is from this nuclear fusion that our sun draws its energy.)
In a stable star like our sun, there is a balance between two opposing forces. The mass of all the material in the star creates a gravitational force that tends to collapse the star. However, this is balanced by the repulsive force from the core, so the star remains fairly constant in size, even though it is not a solid object like a planet.
However, as the star ages, it gradually consumes hydrogen and helium and begins to produce heavier elements such as carbon, oxygen, silicon and finally iron. And that’s it when it comes to melting elements heavier than iron lasts energy rather than creating it, so the star essentially runs out of fuel and collapses.
In some cases, this collapse can be very severe—so severe that it dramatically increases the pressure and temperature in the star’s core. The star then goes boom. Big boom. Well, substantial peaceful boom, because explosions in the vacuum of space make no sound.
But that produces a LOT of delicate energy. For comparison, our sun has about brightness or output 3.8 x 1026 cotton wool. AND supernova observed in 2015 (ASASSN-15h) it had a maximum brightness of about 2 x 1038 watts. That’s more power output than 500 billion sun. This is crazy. Oh, haven’t you seen this? Yes, because it was in another galaxy. Betelgeuse is in our backyard, astronomically speaking.
Brightness and luminosity
Long ago, the Greek philosopher Hipparchus divided stars into six groups based on their brightness in the night sky. From this, we developed a classification scheme of “apparent brightness,” such that a 1st-magnitude star looks very brilliant, while you probably can’t even see a 6th-magnitude star due to delicate pollution. Betelgeuse belongs to the first group.
To be clear, it’s not about the star’s actual brightness – it’s about how brilliant it appears from Earth, which depends on (1) how much delicate it produces and (2) how far away it is. Oh, and (3), stellar magnitude is based on how the human eye sees objects, and is not linear. An object of size 1 has a delicate intensity (in watts per square meter) that is 100 times greater than an object of size 6.