On December 12, the asteroid 319 Leona will pass directly in front of the red giant Betelgeuse in the constellation Orion. Astronomers can now calculate that a celestial body about 60 kilometers in diameter will pass directly in front of a star more than 650 light-years away from us.
Perhaps even more exciting is that if you live in the observing corridor from central Asia and southern Europe to Florida and Mexico, then around 01:17 UTC time, clouds permitting, you have the chance to see it firsthand, without any telescope or equipment, just your eyes.
Betelgeuse is the second brightest star in the constellation Orion and is best known for its distinct red color. Back in 2020, the star made news when it unexpectedly dimmed in brightness in the sky due to clouds of dust ejected by itself. Next month, the star Leona will dim for a few seconds as it passes directly in front of it. This occurrence of a bright star's light being blocked (or obscured) by an asteroid is very rare, occurring about once every few decades. This phenomenon is not only fascinating but also scientifically useful, giving us the opportunity to learn more about how Betelgeuse and its large convective cells behave, while also allowing us to learn more about the asteroid's orbit and shape.
However, those who hope to observe this event need to be reminded that predictions are just predictions and there are still some uncertainties. The size and shape of the asteroid itself is still debated. Normally, we tend to assume that asteroids are spherical unless we know otherwise, but a previous occultation of Leona in September 2023 determined that it was more oval-shaped, measuring 80 kilometers by 55 kilometers. Based on its distance from Earth, this means it will cover a 46x41 femtosecond area of the sky, which is a little more than Betelgeuse's roughly 40 femtoseconds. With this in mind, Betelgeuse will be completely obscured and therefore will flicker for a few seconds.
Betelgeuse's outer atmosphere is somewhat diffusive, which could mean its apparent size is more like 50 nanoseconds, so it simply fades away. Until this event occurs, we won't know exactly how it will appear in the sky, nor exactly when. This is a great opportunity to get to know these two fascinating celestial bodies, so get outside around 01:00 UTC on December 12, wait and watch, and hopefully you'll witness one of nature's rare events.