The Hand of Orion: Betelgeuse

On a clear night during winter, if your gaze happens to be fixated on the constellation of Orion, you will find an odd red star stand out from amongst its peers. Located above the three stars forming Orion's "belt", this reddish star is found at the "hand" of Orion, which is why in Arabic it was named "Yad Al-Jauz", nowadays more popularly referred to as Betelgeuse.

Ancient and medieval astronomers were well aware that stars in the night sky came in multiple colors – some blue, some red and others yellow. Even in today's light polluted skies, Betelgeuse's characteristic red hue is quite noticeable with the unaided eye. But where former generations of astronomers were in the dark regarding this variance in colour, we have been able to demystify it in the last one hundred years. We know now that the colours of stars are in fact a measure of their temperature. A red star will tend to be "cooler" while a blue one will be warmer and a yellow one (like our Sun) will be somewhere in the middle. Now of course Betelgeuse is no hill resort, it has a surface temperature of about 3000 Kelvin, which is still extremely hot. However, when compared to the Sun's temperature of about 6000 K it is in fact relatively cool.

Another interesting feature of Betelgeuse is its size. To get some sense of this, let us consider the following question: How different would our sky look if suddenly our Sun were to be replaced by Betelgeuse? In that case the earth would be embedded in the atmosphere of Betelgeuse as a result of which the planet would cease to exist. Betelgeuse's size means that if brought into our Solar System, it would fill up the space just beyond the orbit of Mars! Imagine that! A solar system without Mercury, Venus, Earth and Mars! Perhaps then it is a relief that we have been spared from that fate, of course for now.

For why does Betelgeuse seem to be so different from our Sun? They are both stars, so why the differences in colour and size? Well for starters, Betelgeuse is a type of star known as a red supergiant, while the Sun is a yellow "dwarf". All stars start out as dwarfs and then eventually evolve to become giants or super giants. Stars "burn" hydrogen in their cores for most of their lives, but this supply of hydrogen eventually ceases, after which the star begins to contract under the influence of gravity. As it contracts, its outer layers start to heat up, which ignites hydrogen fusion within the outermost shell of material in the star. The energy released from shell fusion causes the star to expand and consequently cool, which results in the star appearing redder, due to its overall temperature decreasing. Stars like our Sun will eventually become Red giants (in about 4 billion years) but more massive stars such as Betelgeuse (20 times more massive than the Sun) turn into red supergiants instead. As stars continue to evolve, depending on how massive they are, more and heavier elements are fused in their cores to release energy.

In the case of Betelgeuse, it is believed that the star is nearing the end of its carbon burning stage, after which it is expected to end its life in a fiery explosion known as a supernova. Due to Betelgeuse’s massive size, the hold of gravity on the material in its outer layers is weak, as a result of which fast winds emanate from the stellar surface, propagating in all directions. These winds are thousands of times faster than the fiercest hurricanes that can arise here on our planet. What is more bizarre is that every year, the stellar winds of Betelgeuse expel material roughly equivalent in mass to the Earth.

Our civilisation, and its cradle’s worth of material being burped out each year by a fiery Deo, sounds almost like something from the epics and folktales of old. Betelgeuse due to its mass and size has also been found to exhibit variations in its brightness, i.e. it brightens and dims every now and then. This is believed to occur due to the heating and cooling of gas cells within the interior and on the surface of the star. Very recently though, in 2020 it underwent an unprecedented dimming that caused the star’s brightness to decrease by half. It’s an enigma that is still being discussed and debated by astronomers today. Although since then Betelgeuse has brightened but the cause of its ‘great dimming’, as it is called, remains elusive. A tantalising possibility could be in fact that Betelgeuse is nearing its explosive death and the dimming was perhaps heralding the arrival of the end times for the giant star.

As stated above, Betelgeuse is expected to go out in a blaze of glory, which according to some astronomers could be in the next few decades. Or it could be the case that the star has already gone supernova and given its distance of 640 light years we shall only know about it centuries later or sometime soon if it already exploded in the past. When this Deo in Orion does explode, it shall outshine the entire galaxy, illuminating our night sky for many weeks.