600 light-years faraway Betelgeuse red supergiant (destined to go supernova in probably some tens of thousands years) keeps "to gradually decrease in brightness" (less 25% since just last September 2019) (P), and astronomers don't exactly know why, but they think it could be due to "changes in the envelop-outer convection atmosphere" (P) of the star, also known as Alpha Orionis.
Well, pal, this dumb blog has a different explanation for that. See the following cartoon.
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Betelgeuse’s name is a corruption of the Arabic "yad al jauza" which means the "hand of al-jauza”, al-jauza being the name of the figure ancient Arabs were seeing in the star constellation, probably a woman. The constellation in question is the Orion one, and Betelgeuse is one of the two brightest stars of it, the other being Rigel, with the couple also called respectively Alpha and Beta Orionis.
Betelgeuse brightness is indeed between 80 thousand to 100 thousand times that of the Sun, and its radius is at least double the size of Mars' orbit but it could even be as big as Jupiter's orbit (meaning about 1000 times the size of the Sun), while its mass, at its birth, was probably 18 or 19 times that of the Sun (see).
Now, dear listener, let’s cover some very basic infos to fully get why it’s strange a star like Betelgeuse is getting dimmer and not brighter.
For starter let’s refresh our notion of what’s a light year, ok? Just so to better grasp how far Betelgeuse actually be.
A light-year is the distance light travels in a year. It’s about 9.5 thousand billion kilometers (or 59 hundreds billion miles), it is more than 60 thousand astronomical units that is more than 60 thousand times the distance between Earth and the Sun.
Basically if in a parallel reality New York were the Earth, and Beijing were the 600 light-years far Betelgeuse, well, the extent of a single step made in that parallel reality by you, dear human listener, would be enough to take you to the Moon, in this actual reality of yours.
Now, let’s go to the star itself and its mysterious dimming.
Betelgeuse is a red super-giant.
How a star gets to be a red-super-giant?
Well, y’all know how stars be giant balls of gas where, deep in their guts, nuclear fusion of hydrogen atoms occurs and therefore helium ones are formed.
The energy coming out from such deeper parts of the star is what ultimately provides the pressure needed to keep the star from collapsing under its own weight, and it’s also what makes the star a shiny thing in the space.
As long as the star uses hydrogen as fuel, thus producing helium, well, it is stable and it is classified as a Main Sequence Star, which is a star in the stable part of its life-span, that, depending on the star, usually (but not always) lasts billions of years.
But what happens when a star runs out of hydrogen in its core?
Well, then its core begins to collapse and gets hotter. So the starving star counters it’s core's collapsing by expanding its outer envelope, and therefore its overall surface. This makes the expanding star’s surface’s temperature to go pretty down, and the star’s luminosity to go pretty up, to increase, in other words it gets much brighter.
Aaaand, dear listener, this is the time when a star becomes a Red Giant (see).
Now, at this point, as mentioned, the star’s core has no more hydrogen, right, but it has helium (previously produced by the fusion of the hydrogen atoms). And, as said, the core temperature has risen, so now the core is hot enough to ignite the fusion of helium’s atoms into heavier stuff. Besides, right outside the star’s core, there’s still some hydrogen. And, at this point, surrounding the star’s hotter and hotter core there’s now a shell which is, in turn, reaching itself a temperature sufficient to ignite the fusion of the remaining hydrogen into helium. So, more helium is produced in this hot shell around the core. And this brand-new helium then falls onto the core where it becomes more fuel.
Now, dear listener, this Red Giant’s phase lasts much less then the main sequence phase, just a few million years instead of billions.
And, when even the helium runs out in the red giant’s core, well, then the process of core contraction and surface dilatation and so on repeats itself (only with helium and heavier elements instead of hydrogen) and the star expands itself becoming a Red Super-giant (see).
Betelgeuse is in this phase, at the end of it actually. It’s made, by now, of "onion layers" of heavier and heavier elements.
But it’s not able to fuse elements heavier than iron, so its core is an iron one by now.
When this iron core finally reaches a mass about 1.4 times that of the Sun, the so called Chandrasekhar Mass (by the name of the Indian-American astrophysicist who calculated it), well pal, then, the core collapses aaaaand two things happen (see).
The first is that protons and electrons are so pushed together that they form neutrons and neutrinos and, though electrically neutral neutrinos usually don’t interact with matter, at such monster-high densities they cause a gigantic outward pressure.
The second thing that happens when the iron core collapses, is that the outer layers fall inward.
So there’s an overall contraction of the star. But going on with the process, at the end of it, the core stops collapsing, ‘cause the neutrons gets stuffed too tightly, and the outer layers eventually plow into the core itself. And so: bang! And then the outer layers rebound. And this creates huuuge shock waves that run outward.
Sooo, pal, the final result of the combination of the neutrinos gigantic outward pressure and the just mentioned shock waves going outwards is that what’s out of the star’s core is blown apart in a humongous explosion 10 billion times more luminous than the Sun, as bright as entire galaxies for weeks and that takes months or even years to fade away.
Yup pal, I’m describing a supernova (well, a type II supernova, to be precise)(see).
Aaand, dear listener, to put there some more infos, the immense amount of energy released in a supernova is enough to fuse elements heavier than iron, therefore it is what creates things like gold, silver, zinc, uranium and so on which in turns spread through space and end up being part of planets like your beloved Earth(see).
And, if you’re asking yourself what happens to the core of the star after the final explosion, well, dear listener, that depends on its remaining mass.
If it’s less than 2 or 3 solar masses, then it gets to be a neutron star, but if the residual mass is more than 2 or 3 solar masses, then it becomes a mighty infamous black hole (see).
So, dear listener, to super sum up, a red super-giant like Betelgeuse should not be dimming, and astronomers and their colleagues space brains are puzzled by that.
PS: Anyway, in the lately, the playful star, apparently has regained some of its shine.