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A Supernova in Orion

A Supernova in Orion!

The constellation of Orion, the Great Hunter, a collection of stars that is instantly recognisable. Orion’s belt comprising of three stars with his sword hanging by his side is an asterism that many of us are familiar with, particularly at this time of the year when Orion is high in the night’s sky from about six o’clock. But what is with the stories reporting that Betelgeuse, a bright orange star found in the left shoulder of Orion, is about to explode in a supernova?

Firstly, Betelgeuse is a super red giant located about 640 light years away from earth, 640 light years means it will take light 640 years to reach earth and so when we look at the star we are seeing it 640 years in the past. Similarly, when we look at our own star, the sun, we see it 8 minutes in the past. Betelgeuse is a massive star; if it was in the place of our sun it would take up the space up to the orbit of Jupiter. Betelgeuse is classified as alpha Orion or a Orion, this means it is the brightest star in the constellation Orion. It is also a ‘runaway star’ A star that is travelling faster than the interstellar medium it is sitting in, for Betelgeuse to be travelling faster than the matter that’s surrounding it something must have either pulled it or pushed it away from its position. This could be gravity; something with mass pulling it, like a galaxy or other body. Or it could be an explosion that is pushing it away - a supernova, one of its neighbouring stars exploding.

Super massive stars burn through their fuel (hydrogen and helium and their isotopes; helium and hydrogen but with different numbers of neutrons) incredibly quickly. In the prime of its life, Betelgeuse would have been fusing hydrogen and making heavier elements whilst emitting immense amounts of energy. Because of its huge mass the gravitational force trying to squash it down would have been battling the thermonuclear force trying to blast the star apart. As the fuel in its core is exhausted the force of attraction, due to gravity, begins to win this battle compacting the star. This causes the temperature to skyrocket and the hydrogen and helium in the outer core and any heavier elements in the core fuse to release incredible amounts of energy. This tips the battle of gravity and thermonuclear force in the other direction, causing the star to swell. The side effect of this is that the star cools and takes on a red colour. When we see Betelgeuse now, it has a orange, red appearance to it, a Super Red Giant. The colour change of a star relates to its temperature and gives us an idea of what is happening in the core of a star and where it is in its life cycle. Our own sun is in its Main Sequence.

Eventually the force generated from the nuclear reactions in the star’s core will not be able to hold up gravity. At that moment the core will instantaneously collapse. The shockwave will obliterate the outer layers of the star; this is a supernova - and what will be left?

The Crab Nebula - A remnant of a Supernova.

The core, a body of immense density. Our sun will meet a similarly sticky end; only the shock wave will not be anywhere near as impressive! The core of Betelgeuse will have two pathways. The matter will be squished together creating a body made of neutrons. These neutrons, according to uncertainty and something called ‘neutron degeneracy’ should not be able to get any closer together, but if the core has a mass of 3.4 times that of the sun at this point then the neutrons won’t be able to hold up gravity. The result? The phantom of astrophysics…the singularity or BLACK HOLE!!

Betelgeuse is a variable star with a regular pattern in its dimming. The cycle has a period of just under 6 years. There is also a faster cycle with slight changes in its brightness of around 550 days. The current dimming of Betelgeuse is greater than previously observed but it could be that these two cycles of dimming have overlapped and as a result we are seeing a more obvious change in brightness than what we usually see. It is also very unusual for a star to undergo a dimming stage prior to a supernova. Betelgeuse will supernova and when it does it will outshine a full moon and be visible during the day. In astronomical times this will happen very soon, but unfortunately for us, very soon is 100,000 years away!

Find Orion and Betelgeuse by facing east after the sun has set and its dark. The Belt is the easiest thing to find. Three stars in a line, around the belt there appears to be four stars making up a rectangular box. Betelgeuse is the top left. Check out events and workshops page to find more wonders of the night’s sky and join us for an astronomy workshops night walk!