The Dimming Effect of Betelgeuse

Betelgeuse is one of the most famous stars, located on the shoulder of the Orion constellation. It’s the 10th brightest star as seen through the naked eye. However, during 2020, Betelgeuse portrayed some unusual behavior which dimmed it down to ⅓ rd of its actual brightness; now demoting it to the 24th brightest star in the night sky.

Betelgeuse is far younger than our sun. Estimation concludes that it was born roughly 10 million years ago which means that terrestrial planets might not have formed there yet or never will since it’s on the edge of its death. It has already transformed into its giant phase. Born with 15 times the mass of our Sun, it spews out 100,000 times more power than our Sun. Its short lifespan might well define this statement, “The candle that burns twice as hot lasts half as long”. Stars that burn 10,000 times as bright last 1000 less long. These sorts of stars are increasingly rare in our universe.

Before learning what was actually happening with Betelgeuse, let’s first learn about the anatomy of a star. A star is essentially made up of a hydrogen-burning core ( where mainly fusion occurs) enveloped by hot gases of hydrogen. Inside the core, hydrogen fuses into helium which gradually creates an inert core of helium. Helium being a denser element makes the core shrink in size. At this point, the internal pressure and temperature become extreme so that Helium further fuses into carbon increasing the energy output of the core. This will cause the outer envelope of non-fusing hydrogen to expand making it humongous in size. In the core, temperature and pressure will continue to increase causing further fusion of carbon and other heavier elements.

Massive stars terminate in an incredible event known as a “supernova”. Previously, we talked about the fusion of heavier elements. Fusion will continue till the iron is produced in the core. Fusing much heavier elements will start up an endothermic reaction. This reaction will drain out energy from the core. A gravitational collapse will occur since there are no forces to resist.

The star may explode in a bang or depart its life in a whimper.

It may happen that as soon as the outer envelope of hydrogen gas collapses, the core will resist the infalling wave and it shall bounce off, outside. This can be the situation or another scenario may occur. The core may not be able to resist the collapse and simply get crushed down.

Stars that are on the lighter part of the massive star spectrum, i.e 10 times the mass of our sun won’t be able to generate enough pressure to collapse the inner core. It will be compressed into a super-dense neutron star and bounce off the neutronic interior. The bounce back will lead to a giant shock wave that propagates out into space releasing tremendous amounts of energy and matter.

But what if the star is 10 times heavier than our sun?

In this case, a lot more material will be falling down onto the inner core; which will overwhelm the resistive strength of the neutronic matter, crushing the core. It will fall all the way into a singular point of infinite density creating a blackhole. There will be no bounce back, no supernova, no bang.

Since we aren’t well aware of the mass of Betelgeuse, it might disappear one day without a bang.

Betelgeuse is a variable star. So the change in its brightness is not surprising since it has been changing from time to time for decades. The alteration in its brightness was around 10%. But now, there has been a fairly constant and steep decline in its brightness – that is ⅓ rd of its usual brightness which grabs our attention. Differences in its appearance have also been detected.

Well, now we can conclude that this star will go supernova since there is no clear prediction which states that stars undergo dimming before going supernova. These reported dimming in the star’s brightness has been in the visible light part of the spectrum. But like all stars, Betelgeuse produces all kinds of radiation. So, in order to calculate luminosity, we need to add up all the brightness across the entire spectrum.

The dimming effect was only observed for visible light. To obtain certainty, observations in other parts of the spectrum had to be made. Precise measurement at the O’Brien Observatory in Minnesota showed that the star has the same brightness in the infrared as it did 50 years ago.

So, we could deduce that the star’s overall power output did not change. The dimming was only limited to the visible part of the spectrum.