By Michael Williams, UNE Discovery Voyager

As nights grow shorter and the sun approaches equinox the signs of seasonal change are hard to miss. Perhaps images of flowers in bloom, fledgling birds and outdoor adventures fill your mind? Yet, for those familiar with the night skies signs of seasonal change are equally hard to miss, as the winter constellations start to set and summer constellations begin to rise.

The constellation Orion, setting from UNE Kirby Observatory (April 2025). The red star Betelgeuse may be seen just up and left from the centre of the frame. Image Credit: Michael Williams

One constellation synonymous with our southern summer is ‘Orion’, the hunter. Looking up at Orion, the star ‘Betelgeuse’ stands out for its red hue. Betelgeuse is red because it’s a very big old star, destined to go out with a bang as a supernova. Not only that, but it is expected to do so ‘soon’, i.e. sometime in the next few hundred thousand years! Betelgeuse is also close to Earth (in cosmological terms), so it promises a spectacular show when it does go supernova. But when will that happen? Let’s talk stellar evolution.

Stars are big hot nuclear reactors. Stars are mostly Hydrogen, hot dense balls of Hydrogen. Being so dense and hot, the Hydrogen is colliding at great speed. The hot fast Hydrogen doesn’t simply fly off into space because the stars gravitational force is strong enough to prevent it. If the colliding Hydrogen ‘stick together’ via nuclear fusion they create Helium and energy is released. A lot of energy. This constant supply of energy keeps the star from collapsing with the force of its own gravity.

In time, the Hydrogen gets used up. With no Hydrogen to produce energy via nuclear fusion, the star begins to collapse. As it collapses, it heats up. The star gets hot enough to fuse Helium, which releases more energy than hydrogen fusion. The energy from the Helium expands the star once again, hotter and bigger. This process continues with heavier elements, outwardly the star expands and contracts with each new fuel fused, changing in colour and brightness.

Our sun will eventually get too big and hot to for its gravity to keep holding it together. It will then somewhat putter out with a huff. Bigger stars, such as Betelgeuse, explode as a supernova. Rapid changes in brightness may indicate a star is about to go supernova.

Images of Betelgeuse before and during ‘the great dimming of 2019’ as observed by the SPHERE instrument on ESO’s Very Large Telescope. Image from ‘A dusty veil shading Betelgeuse during its Great Dimming’ by Montargès, M., Cannon, E., Lagadec, E. et al., 2021, Nature, vol. 594.

It’s 2019, suddenly Betelgeuse gets a LOT dimmer. Speculation abounded, was a supernova about to happen in our cosmological backyard, soon? Not cosmological time soon either, we may get to see it soon. At the time of writing its 2026, Betelgeuse has not yet gone supernova. Should we stay affixed to the edges of our seats?

Even before ‘the great dimming’ of 2019 the brightness of Betelgeuse was known to fluctuate. That fluctuation was periodic, repeating every 6 years. This was indicative of Betelgeuse not being alone. It is now known that Betelgeuse has a smaller companion star. The distance between Betelgeuse and its companion is less than the distance between Jupiter and our sun. At that distance, tidal forces between the two stars tugging on each other are many times stronger than those causing ocean tides on Earth.

Observations by the Hubble Space telescope discerned the great dimming was caused by a truly epic belch. Disrupted by its companion star, Betelgeuse expelled a whole lot of material, which cooled and condensed as dust. Like looking through a dusty window, the light wasn’t really getting through making Betelgeuse appear dimmer. Not a supernova, this time.