Betelgeuse Versus the Asteroid… What Happened?

A rare occultation of the bright star Betelgeuse by asteroid 319 Leona turned up mixed results.

In science and astronomy, sometimes a negative or subtle result can be as interesting as a positive one. That’s just what occultation-chasers where confronted with this past Monday evening on the night of December 11th/12th, when asteroid 319 Leona occulted (passed in front of) the +0.5 magnitude star Betelgeuse.

The Event

This was the most anticipated asteroid occultation of a bright star since 163 Erigone occulted Regulus on over the northeastern United States in 2014. Much like that event, Monday night’s occultation was a hit-or-miss affair along the 142 kilometer-wide path in terms of clouds. The track of the ‘asteroid’s shadow’ across the Earth crossed over the southern tip of Florida, the Atlantic and central Europe.

Leona v. Betelgeuse
Asteroid Leona (circled) approaches Betelgeuse. Credit: Tom Wildoner

A Subtle Celestial Affair

Nabbing an asteroid occultation usually means traveling to the path, and Monday night’s event was no exception. It was hoped that observers would see Betelgeuse wink out for a dozen-odd seconds. A study of the resulting light curve could not only refine the shape and size of the asteroid, but help to say something about the diameter of Betelgeuse itself.

Plans were laid, and cameras and telescopes were at the ready for the best asteroid occultation of a bright star for 2023. But when the key moment came, observers saw nary a blip in brightness.

Well, almost nothing. If you watch video recordings of the event, Betelgeuse did seem to quiver a bit at the key time.

So, what happened?

Betelgeuse (Betelgeuse, Betelgeuse)

I think the key to the mystery lies in the nature of both of the objects involved. Betelgeuse is a variable star, and at roughly 550 light-years distant, one of the closest candidates to our solar system with a potential to go supernova. Remember the excitement back in late 2019 when the star conspicuously dimmed? At the time, there was hope that this was a harbinger for a cataclysmic event. Although Betelgeuse is crowned Alpha Orionis (the brightest star in Orion), it often swaps places with the star Rigel as the lucida for the constellation Orion.

An image of the surface of Betelgeuse, constructed using interferometry. Credit: Xavier Haubois (Paris Observatory).

Now, Red Giant Betelgeuse is massive enough and close enough to show a tiny but discernible angular diameter. In fact, Betelgeuse was the first star to have its apparent angular diameter efectively measured. Astronomers Albert Michelson and Francis Pease completed this feat in 1920. They did this using a six-meter interferometer mounted on the front of the 2.5-meter telescope at Mount Wilson. They came up with a value of 47 mas (milli-arcseconds, or 1/1,000th of an arcsecond). Today, the accepted value of the diameter of Betelgeuse is 45-55 mas. This makes Betelgeuse the second largest star in the sky in terms of apparent angular diameter, behind R Doradus at 57 mas.

319 Leona
Asteroid 319 Leona, constructed from an occultation in September 2023. Credit: J.L. Ortitiz/MNRAS Letters

Asteroid 319 Leona was discovered by astronomer Auguste Charlois from the Nice Observatory on the night of October 8th, 1891. Orbiting the Sun once every 6.3 years, 319 Leona is an estimated 80 by 55 kilometers across, yielding an angular size of about 46 mas in diameter. The asteroid was 2.763 Astronomical Units (AU) distant at the time of the event.

Occultation Obfuscation

Of course, 319 Leona, like most small asteroids, is oblong-shaped. I’m thinking what happened Monday night was a partial or annular occultation, which failed to dim mighty Betelgeuse entirely. Think of the strange potato-shaped eclipses that the rovers on Mars sometimes witness. These occur when the misshapen moons Phobos and Deimos pass in front of the Sun.

Phobos v Sun
Perseverance sees Phobos pass in front of the Sun. Credit: NASA

“At least there was a very obvious dimming,” Andreas Dill who recorded the event from Spain told Universe Today. “Whether this was because it was an annular or a partial eclipse remains to be analyzed. What I am eager to see, and what is the main goal of observations: will it be possible to reconstruct the surface of Betelgeuse from all the obtained light curves?”

An early analysis of the occultation event, along with the extrapolated light curve. Credit: Domenico Licchelli

Sure, maybe the occultation wasn’t a clean cut event. Still, those guttering flickers of the star could yield some interesting results. Maybe our understanding of the true size and nature of Betelgeuse is due for a slight revision.

And someday, tonight or thousands of years from now, Betelgeuse will indeed go supernova. It will then put on a fine show, giving astronomers a chance to study such a spectacle up close. We can only hope this occurs in a month such as December, putting Betelgeuse in full view as a ‘Christmas Star’ like no other (June would put it almost directly behind the Sun!). After its final show is over, Betelgeuse will then ‘wink out’ for good, and Orion the Hunter will never truly look the same again.