Twinkle, Twinkle, Giant Star Betelgeuse’s Inconstant Brilliance

By Matt Woods

Betelgeuse, one of the brightest stars in the night sky, has recently been exhibiting an unusual increase in brightness, leading to speculation it might soon explode in a supernova.

A star that is in its Red Giant phase, it is six hundred and fifty light-years from Earth, and can be found in the left shoulder of the constellation of Orion. It is an enormous star that has burned through all its supply of hydrogen fuel in its core and has expanded hundreds of times beyond its original size. It is now fusing helium into carbon and oxygen, so we know the star’s demise astronomically speaking will happen soon.

Since early April 2023, it has climbed from being the tenth brightest star to the seventh brightest, currently shining at over 140% of its usual brightness. Betelgeuse’s recent antics started back in late 2019, when the “Great Dimming” took place. During the Great Dimming, Betelgeuse unexpectedly dimmed beyond what had ever been seen before. It went from the tenth-brightest star to outside the top twenty bright stars. It was 2.5 times fainter than its at usual dimmest point.

This led some people to start speculating that its death might be near. If Betelgeuse were to go boom, it would be the nearest supernova explosion in more than four hundred years and the first supernova in the Milky Way galaxy since 1604. This had a lot of people hoping to see the spectacular event. The problem is, Betelgeuse is a variable star known for regular changes between brighter and dimmer periods. For more than a hundred years, astronomers have observed Betelgeuse lighten up every four hundred days, then drop to about half of its peak brightness and brighten up again.

The Great Dimming ended up being in an enormous expulsion of material from the star’s interior that created a huge dust cloud that obscured our view of the star.

Betelgeuse has since recovered its usual brightness, but the star has not been its old self since the Great Dimming. Its 400-day brightness period has halved to two hundred days and, on top of that, its continuing to brighten. So, is it dying?

The best models’ astronomers have produced, indicate that Betelgeuse still has hundreds of thousands of years left. While Betelgeuse’s regular life has now ended as it has run out of hydrogen, its current helium fusing phase should last hundreds of thousands of years. When that helium fusing phase has finished, it will sustain itself by turning that carbon and oxygen into neon and magnesium giving it another then thousand years. It will then fuse those elements into silicon which takes a further thousands of years, until eventually, the star’s core starts creating iron which in some cases only takes days or hours to happen. This is when the fireworks begin.

When a star’s core becomes completely iron, the end is nigh for the star. This is because the creation of iron in the star’s core requires an endothermic reaction to take place which takes in energy rather than give off energy. Suddenly, rather than a reaction releasing tremendous amounts of energy that does mighty battle with the star’s own gravity so it can exist, the centre of the star starts absorbing energy. At the end, the core of the star collapses on itself and that starts off what we call the core-collapse supernova or a Type II supernova.

Because most astronomers think Betelgeuse is still fusing helium, they were unconcerned by the recent unexpected brightening. The star has been this bright previously, albeit only for brief periods of time. The current thinking from astronomers is that Betelgeuse will return within the next five to 10 years to its usual ways, slowing its cycle of brightening and dimming to the normal four hundred days. Astronomers think the huge bubble of material that was responsible for the Great Dimming in 2019, caused the star’s outer layers and its interior to move in opposite directions, and, as a result, the star is now pulsating twice as fast compared to its normal cycle.

We can make these theories about Betelgeuse due to the star’s size, and position in the Milky Way galaxy. This allows astronomers to study Betelgeuse in better detail than most stars and for longer in our history as well.

We see most stars other than our sun as just points of light, but Betelgeuse is big enough that we can resolve it with the Hubble Space Telescope, and with radio telescopes like the Atacama Large Millimetre/Submillimeter Array (ALMA). Betelgeuse is such an enormous star, if we were to place it at the centre of our solar system, it would extend all the way to Jupiter.

Astronomers can see what is happening in Betelgeuse’s outer layers so well that they can measure the chemical composition of the star’s atmosphere. The images taken reveal a striking body quite unlike our Sun. Rather than a smooth sphere of superhot plasma like our Sun, Betelgeuse is a lumpy clump of boiling gas bubbles. We see huge convection cells of hot material, some as large as a small star, rise from Betelgeuse’s core to its surface, then cool down and disappear back inside its interior. It is like the sun’s cycle but on huge doses of steroids, as the convection cells on the Sun are the size of Texas or Western Australia. We also know once every few centuries, Betelgeuse burps out a bubble of material so large that a Great Dimming ensues.

If Betelgeuse were about to go supernova, we would certainly know it. When it does happen, the star will become as bright as the full moon for around two months. It will be so bright; you would be able to read a book in the light of the supernova on a moonless night and it will be visible in the daylight too. Thankfully, although close to us in astronomical terms, Betelgeuse is too far away from Earth for its explosion to be dangerous to us. Astronomers think that a giant star would have to blow up within 160 light-years from our planet for us to feel the explosion’s effect.

The last known supernova to have exploded in the Milky Way galaxy was SN 1604, also known as the Kepler supernova, named after astronomer Johannes Kepler. According to historical records, it was thirty times more distant than Betelgeuse, and it remained visible during the day for over three weeks.

From our observations of similar aged galaxies like ours, we are long overdue for a supernova to occur in our galaxy. We should have three supernovas each century, but we are in the middle of an awfully long dry spell that is 419 years and counting.

Betelgeuse has not been visible in the night sky for most of June as the Sun was in the same part of the sky as the constellation of Orion. This means astronomers are having to wait until July when it reappears in the morning sky to check on its progress. Should it continue to brighten, it gives us more of a chance to understand the Red Giant phase of a star’s life.