An Asteroid Will Occult Betelgeuse on December 12th

I cannot for the life of me remember when it was or what it was but a fair few years ago I remember positioning a telescope to observe an asteroid as it silently and perhaps slightly eerily drifted between us and the Moon. I say eerily as this asteroid had the ability to cause widespread damage had it hit but of course we knew it posed no threat.  I remember at the time thinking it was mind blowing that even today, we still use mathematics with roots (pardon the pun) centuries old to calculate the position of objects in our Solar System. We get to see evidence of this again on 12th December when something rare happens!

I have rather hinted to what I am referring, on 12th December, asteroid 319 Leona will pass directly in front of Betelgeuse, the red giant in the constellation of Orion whose name amusingly translates to armpit of the giant – now there’s a fact to amuse and astound your friends. To be able to calculate that a rock approximately 60 km across is going to pass directly in front of a star that is just over 650 light years away is really quite staggering.   

Perhaps more excitingly if you live along a corridor from central Asia and southern Europe to Florida and Mexico then at around 01:17 UTC you have a chance – clouds permitting – to see it for yourself and you don’t need any telescope or equipment, just your eyes.  

Betelgeuse is the second brightest star in the constellation Orion and is a familiar favourite with its piercing red colour. It hit the news back in 2020 when it unexpectedly dimmed in the sky due to the star itself ejecting a cloud of dust. Next month it will fade for a few seconds due to the passage of Leona right in front of it. An event like this is quite rare where the light from a bright star is blocked (or occulted) by an asteroid happening every few decades at most.  It will be fascinating to watch but is also scientifically useful giving us a chance to learn more about Betelgeuse and how it’s large convection cells behave, and to learn more about the orbit and shape of the asteroid too. 

The Constellation of Orion showing Betelgeuse at upper left (Credit : Till Credner)

Anyone out there wishing to observe the event needs to be warned though, the predictions are just that, there are a few uncertainties. The size and shape of the asteroid itself is still subject to debate. Typically we tend to assume asteroids are spherical unless we know otherwise but a previous occultation of Leona in September 2023 determined that it was more of ellipsoidal in shape measuring 80km by 55km. At its distance from Earth that means it will cover an area of sky 46 x 41 milliarcseconds which is a little more than the approximate 40 milliarcseconds for Betelgeuse.  Taking this into account suggests Betelgeuse will be completely blocked from view and therefore blink out for a few seconds. 

Betelgeuse’s somewhat diffuse outer atmosphere may mean its apparent size is more like 50 milliarcseconds so it just fades instead.   Until the event happens we will not know exactly how it will appear in the sky or exactly when.   It’s a great opportunity to learn more about these two fascinating objets so head outside on 12th December around 01:00 UTC, wait and watch and hopefully you can witness one of natures rather more rare events. 

Source : OW Cloud Data

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