Asteroid to Cover Betelgeuse on Night of December 11-12 (Updated: Livestream Link)

Imagine your favorite constellation without one of its brightest stars. For a few seconds on the night of December 11-12 this may happen to Orion, at least for viewers along a narrow path from central Asia and southern Europe to Florida and Mexico. Around 1:17 UTC December 12th (8:17 p.m. EST December 11th), the main-belt asteroid 319 Leona will pass in front of Betelgeuse, the orange-red star marking Orion’s shoulder, and block at least some of its light for up to several seconds.

An occultation of a 1st-magnitude star is rare — such an event is visible from Earth only every few decades. But recording one can give astronomers valuable scientific information. By precisely timing the duration of the occultation from many sites simultaneously, they can refine the size and shape of the asteroid. This time they may even be able to map Betelgeuse’s strangely large convective cells, by which the star brightens and darkens for months at a time. Betelgeuse is the 10th brightest star in the sky at magnitude +0.5, so observers need only modest equipment to make a valuable recording.

But you'll have to be in the right place.

How to Observe the Occultation

The Betelgeuse event was a hot topic last September in Armagh, Northern Ireland, at the annual European Symposium for Occultation Projects (ESOP). Organized by the International Occultation Timing Association’s (IOTA) European Section, amateur astronomers and professionals discussed how to observe and exploit this rare opportunity.

As Bernd Gährken (IOTA) explained his talk at the conference, the easiest way to capture the event is to use a simple DSLR camera on a tripod. With the camera in video mode, observers can record Betelgeuse’s magnitude drop and time its exact duration. To be of use for later analysis, video frames must have a short (few-millisecond) exposure time so that the star is not overexposed. A softening filter, mounted before the lens, may help.

An interesting experiment will be to employ a red or blue filter, as Betelgeuse’s diameter is different at different wavelengths. For your images to be compared with others taken at the same instant from other locations, you'll need to record the precise latitude and longitude of your observing site. Your phone's GPS is good enough for this.

Other types of high-speed video equipment, like that used in planetary imaging, would work too.

Millisecond accuracy timing is crucial: Dedicated observers use GPS-controlled time inserters that imprint a time stamp directly on each video frame. Some video software used for planetary imaging can do that, too, but unknown time delays during USB transmission may cause inaccuracies. If you chose the latter method, make sure your computer’s internal clock is constantly kept in sync via the network time protocol. IOTA has tutorials on how to properly record a video observation and sync your computer clock.

Of course, there’s an app for everything! An ideal one for timestamping occultation observations is called Occult Flash Tag (for Android; see tutorial here) or AstroFlashTimer (iPhone). These apps fire the camera flash of the phone at a certain programmed time in order to timestamp a video or sequence of images down to the nearest millisecond. The apps produce accurate results, comparable to professional equipment, but require some practice ahead of time.

Unique View of a Red Giant Star

Even at about 550 light-years from Earth, Betelgeuse is one of the brightest stars in the sky because it’s a supergiant. Its diameter is about 760 times the Sun’s, so it appears as a disk 50 milliarcseconds across on the sky, a much greater apparent diameter than most stars. Therefore, unlike most other occultation events, the beginning and end of Betelgeuse’s occultation will not be instantaneous.

As the asteroid moves across the star's face, it will cover up large convection cells that are brighter than the rest of the star’s surface. Thus, measuring the brightness of the star throughout the occultation should prove revealing. “We might even obtain information on the distribution of these cells and see if these can explain Betelgeuse’s mass-loss observed with large telescopes,” says Miguel Montargès (Paris Observatory), who presented at the ESOP conference.

Unlike the thousands of small, short-lived convection cells on our Sun which come and go in minutes, Betelgeuse’s few, huge cells can last months or even years, contributing to the changing overall brightness as the bloated star slowly rotates in a 30-year period. 

“This result will be unprecedented since there is no visible-light interferometer allowing such an observation to be made,” Montargès adds. The occultation observations will complement data collected by professional infrared interferometers. For equipped amateurs and professionals, he suggests photometric observations taken through R, G, B or H-alpha band filters, as well as spectrometry to reveal possible changes in velocity between the different cells.

How Well Do We Know Leona?

For these observations to be fruitful, planners must know Leona’s shape as precisely as possible. Until recently, the asteroid’s girth was known only to be roughly 60 kilometers. It was assumed to be spherical for lack of better information.

That changed three months ago on September 13, 2023: In advance of the December event, Leona occulted another object, this time a 12th-magnitude star. Observers at different locations delivered 17 chords, based on timings of the occultation at 17 different sites, reported Carles Schnabel (IOTA) at the ESOP conference. Using these measurements, José Ortiz (Institute de Astrophysics of Andalucía, Spain) and colleagues were able to determine that the asteroid is slightly elliptical.  Utilizing a second faint-star occultation on September 16th and an estimate of the rotation based on the asteroid’s light curve, Josef Ďurech (Charles University, Czech Republic) and his team created a preliminary 3D model of Leona, above.

The model establishes the asteroid’s slightly elliptical shape and confirms its size is about 80 by 55 kilometers. This can be used to predict Leona’s likely silhouette on December 12th as well as how the occultation is expected to proceed. Ortiz and colleagues believe that during the occultation, Leona will appear as a silhouette of about 46 by 41 milliarcseconds on the sky:

That’s about the same size as Betelgeuse, whose angular size on the sky is also about 40 milliarcseconds. Due to its diffuse outer atmosphere, though, the star could appear even larger, more like 50 milliarcseconds.

If Betelgeuse appears larger than Leona, even a central occultation will end up resembling an annular solar eclipse: The star won’t be completely blocked even at the centerline of the shadow’s path. If Betelgeuse is more on the smaller side though, and the Leona shape model proves to be correct, there will be a narrow strip of “totality” just a few kilometers wide around the center line, where the star completely disappears for a few seconds. (Leona itself will only be visible in large telescopes, as its brightness is 14.3 mag). The farther you are from the center line, the lesser the magnitude drop you'll see and the briefer the whole event.

Try this interactive simulator to see what the occultation will look like, depending on your distance from the centerline and on the sizes of Leona and Betelgeuse.

Exactly where the asteroid’s shadow will pass is still a bit uncertain, due to uncertainty in Betelgeuse’s position in the sky. The European Space Agency’s Gaia satellite has greatly improved most stellar positions as well as asteroid orbital data and made asteroidal occultation predictions much more reliable. But its instruments have trouble with bright stars.

Although both IOTA’s prediction for the shadow’s expected path and the one by the Lucky Star Project currently agree within just 1.5 km, there’s still room for surprises  — and last-minute changes. Only a few years ago, asteroid-occultation predictions could be off by hundreds of kilometers, making a successful observation a matter of luck. While this is no longer the case, it might be worth trying for an observation even if you’re located a little outside the shadow’s path.

Or, if clouds, location, or other circumstances don't allow direct observation, make sure to tune in to the Virtual Telescope's live feed of the event.