Tuesday, September 7, 2021

Everyone thought that the Betelgeuse was going to explode – but it was just gases!

 A drop in brightness seemed to indicate that the shoulder of the Orion constellation was about to die in a violent phenomenon called a supernova.  But there's a new, far more prosaic explanation: she blew up a cloud.

 Remember the drama of the star Betelgeuse?  Not?  Yeah, it's a kind of distant drama.  More precisely, some 600 light-years away – in kilometers, that's 56 followed by 14 zeros, a number so big it doesn't even mean anything.

 It's easier to understand by taking it literally: 600 light years means that even light, which is the fastest thing in the Universe, takes 600 years to get from Betelgeuse to here.  In comparison, sunlight takes 8 minutes to reach Earth.

 Betelgeuse is a very chubby and shiny star that corresponds to the left shoulder of the constellation Orion the hunter.  It's easy to spot in the Brazilian sky: first, look for the Três Marias, which form the Belt of Orion.  Now find the orange dot next to them (the photo above is a great reference).  You can also accompany the description with a sky map:

 Betelgeuse made the news in 2020, when everyone thought the dick was going to explode in a cataclysmic phenomenon known as a supernova.  This only happens with high-mass stars, and Betelgeuse is a sumo wrestling type: it is 14 times (by some estimates, up to 20 times) the mass of our humble Sun.

 “Blow up” is not the most accurate term.  Let's explain better.  Every star is born as a gigantic hydrogen cloud.  This cloud, with so much gas, ends up compressing under the influence of its own gravity, until it forms a compact ball.  The ball is so compact, it reaches a very high pressure inside.

 When the pressure exceeds a certain threshold, it forces the hydrogen atoms to fuse into heavier helium atoms.  This fusion releases energy – which makes the star light up and heat up.  This energy is also responsible for counteracting gravity, preventing the star from collapsing under its own weight.

 Astronomers call the period a star spends fusing hydrogen the main sequence.  Every star spends most of its life in the main sequence.  Hydrogen is the rice and beans of the stars, the basic fuel.

 When a high-mass star reaches the end of usable hydrogen stores (the bigger the star, the faster the tank empties), it appeals to Plan B, which is to fuse helium into carbon.  Then it starts to fuse the carbon itself.  It's like when you haven't been to the grocery store in a while and start eating whatever pops out of the closet.  Withered strawberry waffle expired in 2019, you know.

 (The food metaphor works in an unexpected way: the star, as it ages, also gets fatter. Betelgeuse, so swollen, would reach the orbit of Jupiter if placed at the center of the Solar System. It is very large.)

 One hour, from house to house on the periodic table, only iron atoms are left.  And fusing iron atoms does not work: this is an endothermic process, which absorbs more energy than it releases.  Iron does not work as star fuel.

 At that time, without the energy to keep everything together, gravity wins the tug of war and the star collapses.  Its outermost layers sag towards the core with a Homeric force, slamming back and forth – being expelled into open space in a huge cloud, hot and lethal to anything around it.  As in the drawing down here.

 Betelgeuse is at the end of its useful life and meets all the requirements to collapse into a spectacular supernova.  Because of its relative proximity to Earth, the boom would spend weeks visible in broad daylight.  There has never been an opportunity in modern astronomy to collect data on this phenomenon so closely (the last really cool-looking supernova here rolled in 1604 and was described by Johannes Kepler himself).

 So, as you can imagine, everyone is rooting for the worst for the best.

 Betelgeuse's brightness waxes and wanes in well-known 400-day cycles – something that's been known since 1836 – but an exceptional blackout occurred around the turn of 2019 to 2020 (Super broke the news on December 27, 2019), made the astronomers thought the end was near.  Well, "close" by cosmic standards, of course: the explosion was predicted to come within a 100,000-year window.  If it happened tomorrow, great, if it happened a thousand centuries from now, well… Whoever waited for the covid vaccine can expect anything.

 Now came the disappointment.  A new analysis led by astronomer Miguel Montargès at the Paris Observatory and published in the journal Nature provides good evidence that Betelgeuse's sudden darkening was actually something trivial rather than a harbinger of fireworks.  The team used the Very Large Telescope (VLT, literally “very large telescope”) from the European Southern Observatory in Chile – where the aridity of the Atacama provides a crystal clear atmosphere for astronomy.

 Stars have colder and hotter areas, which form or dissipate according to the behavior of the plasma flows inside the star (plasma is ionized gas, whose electrons detach from their atoms, and is the main ingredient of stars).  The researchers thought that perhaps the drop in brightness was driven by the formation of a particularly cold region in the southern hemisphere of Betelgeuse.

 Another hypothesis was even simpler: that a cloud of gas in front of the star was blocking its brightness from our perspective.  Scientists like simplicity.  It's a principle called Occam's Razor: before proposing a rambunctious explanation for a phenomenon, you always need to rule out the most obvious possibilities, in order of obviousness.

 It turned out that the correct explanation was a little bit of each thing:

 The star had ejected a certain amount of material.  Something normal – nowhere near the violent ejection of a supernova.  All this gas was close to the cold region.  Therefore, a condensed cloud was formed in front of the star.  The cloud blocked some of the glow and heat (which were already reduced) and voilà: we have a brief ice age on the red giant.  For now, no explosive death.

 Betelgeuse is dying, yes – but everything in the cosmos happens very slowly: she still has a good few hundred centuries to go.  As much as astronomers, and Super, root against it.  After all, who doesn't want to watch a supernova here from the Terran cabin?


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