14.11.2023

Astronomers are gearing up for an unusual celestial event: an asteroid’s occultation, or total covering, of an iconic star.

Betelgeuse shines yellow-red in the constellation Orion.
Gary Seronik / Sky & Telescope

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

An occultation of a 1st-magnitude star is rare — such an event is visible from Earth only every few decades — but watching one can give astronomers valuable scientific information. By precisely timing the duration of the occultation from many sites simultaneously, they can refine their knowledge of the size and shape of the asteroid. 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 our skies (+0.5 magnitude), so observers need only modest equipment to participate.

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 even 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. Softening filters, mounted before the lens, may help.

An interesting experiment will be to employ red or blue filters, as Betelgeuse’s diameter is different at different wavelengths. For the images to be compared with others taken from different locations, it’s also important to record the precise latitude and longitude of the observing site.

Other types of 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 the video frames. 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

Miguel Montargès and his team used the European Southern Observatory's Very Large Telescope to image Betelgeuse’s surface during the “Great Dimming” that took place between January 2019 and March 2020. The dimming, which was visible to the naked eye, was caused when Betelgeuse ejected one of its convective gas bubbles. When a patch of the surface cooled down shortly after, that temperature decrease was enough for the gas to condense into solid dust.
ESO / M. Montargès et al.

At about 550 light-years from Earth, Betelgeuse is nevertheless one of the brightest stars in the sky because it’s a giant star. Its diameter is 760 times the Sun’s, so it appears as a disk 50 milliarcseconds across in the sky, much larger than most other stars. Therefore, unlike most other occulation events, the beginning and ending of Betelgeuse’s occultation will not happen instantaneously.

As the asteroid moves across the stellar disk, it will cross over large convection cells, which are brighter than most of the the star’s visible surface. Thus, measuring the brightness of the star throughout the occultation will prove vital. “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.

This is a simulation showing three different convective patterns on Betelgeuse (left: A, B, and C). As Leona moves across the star, the different patterns on the visible surface result in different shapes of the light curve (at right), which shows the star's brightness throughout the occultation.
Miguel Montargès (Observatory of Paris) / Andrea Chiavassa (Observatory of Côte d'Azur)

Unlike the short-lived cells of our Sun, Betelgeuse’s cells can last months or even years, contribute to the changing brightness as the bloated star slowly rotates on its 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 visual 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 on September 13, 2023: In advance of the December event, Leona occulted another object, this time of a 12th-magnitude star. Observers delivered 17 chords, timings of the actual occultation, 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 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.

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 what the occultation is expected look like. Ortiz and colleagues believe that during the occultation, Leona will appear as a silhouette of about 46×41 mas in 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, the occultation will end up resembling an annular solar eclipse: The star won’t be completely blocked even at the center line of the shadow’s path. If it’s more on the smaller side though, and the Leona shape model proves to be correct, there will be a narrow strip of “totality” a few kilometers wide around the center line, where the star completely disappears for something between 5 and 15 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.

Try this interactive simulator to see what the event will look like depending on your location from the centerline, and on the size of Leona and Betelgeuse.

Exactly where the asteroid’s shadow will fall is still somewhat uncertain, due to uncertainty in Betelgeuse’s position in the sky. The European Space Agency’s Gaia satellite has greatly improved both 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, as well as 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, shadow path 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 outside the shadow’s path.

Quelle: Sky&Telescope