Big Body Briefly Blocks Betelgeuse

December 11, 2023   Issue #654

Subscribers never block the stars 

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

On the evening of December 11/12 — tonight! — an extremely rare astronomical event will unfold: the occultation of Betelgeuse by an asteroid. An occultation is when an object passes directly in front of another, and in this case the occulter will be the asteroid 319 Leona.

The event occurs a few minutes after 01:00 UTC that evening (8:00 p.m. Eastern US time), though the exact time depends on the location of an observer. The geometry means it will be visible in a small slice of Florida and southern Europe. Technical details are available at AsteroidOccultation.com, and there’s a map as well (shown above). Those folks along the path (which is only about 140 km wide though many thousands of km long) will see Betelgeuse dim to invisibility — maybe — for about 11 seconds maximum.

Why “maybe”? Because this occultation is weird. Normally, when a star is blocked by asteroid, the light from the star winks out in a fraction of a second: blip! And it’s gone.

But this is different. Betelgeuse is a swollen, bloated monster of a gas bag, a thousand times wider than the Sun. Even at a distance of 650 light-years, we see it not as a zero-dimensional point but an actual disk in the sky. A small disk, sure, but big enough that it might take the asteroid a moment or two to completely block it. And this is where things get interesting.

We use angular measures to measure size on the sky; there are 360° in a circle and a degree is 3600 arcseconds. The Moon, for example, is 0.5° in size, so about 1,800 arcseconds. Betelgeuse is about 0.05 arcseconds in size in the sky. That’s small, but big enough to see as a disk in some of the largest telescopes on the planet. That means the occultation won’t be instantaneous but take a small amount of time for the asteroid to cover the star.

But it gets even better. Leona is about 70 km wide, and will be about 270 million km from Earth at the time of the event. That means it will be… let’s see, take the size and divide by the distance, then take the inverse tangent of that using the small angle formula… 0.05 arcseconds in size.

Hey! By a cosmic coincidence, the two objects are both about the same size as seen from Earth! More or less. We don’t know how big Leona is, exactly, so it might appear to be a bit smaller or larger than Betelgeuse. Either way, this makes things more difficult. I mean fun!

Like I said, the path to see this event on Earth is narrow, and if you’re too far north or south, then parallax means you’ll miss it. But people very close to the line might see a partial occultation, depending on the exact size of the asteroid. If enough people watch this event carefully, and time it accurately, we might be able to determine a better measurement for the asteroid’s diameter. We might even get its shape from this, and whether it has a moon. This has happened with occultations before.

If you know an astronomy dork who likes this sort of thing, feel free to share this article with them!

If the asteroid does block the star entirely, Betelgeuse will drop in brightness by a factor of about 400,000 (all you’ll see is the asteroid, which is about 14 magnitudes fainter than the star), so to the naked eye it will vanish. If Leona doesn’t eclipse Betelgeuse entirely, you’ll see the star fade but then rise in brightness rapidly. That would be amazing.

So if you’re an amateur astronomer with a telescope and digital camera, I urge you to observe this and get images! You can also report your results. And if you don’t have any equipment, all you need are your eyes and clear skies. If you’re on the path, just watch! It’ll be pretty cool.

I’m extremely jealous of folks who will see this. But don’t despair if you’re not on the path: Gianluca Masi of the Virtual Telescope Project will be observing it and will have a live feed set up starting at 01:00. I’ll be watching!

News Roundup

Who can keep up with everything these days?

• Some good climate news, kinda: We’re making a lot of progress on combating climate change. That’s good news, but as the linked article (by my pal and climate scientist Dana Nuccitelli) says, not many people are really aware of it. Huge strides have been made in just the past few years in producing more energy via solar and wind, and electric vehicle sales are way up. We’re still not up to the goals set in the Paris climate agreement in 2015, but we’re making headway. Climate deniers both in media and the government still make a lot of noise, but we need to cut through that and save the planet despite them. We’re still emitting gigatons of CO2 every year (though that may peak this year, incredibly and happily), and the planet is still heating up — 2023 is gearing up to be the hottest on record — but we are not doomed. We just need to take this existential threat seriously, vote out the deniers, and work harder. We’ve done it before, and we can do it again.

• This article is about a small (15 cm!) orbiting telescope that was used to follow up on an observation of an exoplanet, and how it was critical in determining the orbital period of the planet. It’s a good story [link to research paper], about the need to use smaller telescopes to characterize some planets that are hard to observe with big telescopes, but the real reason I’m mentioning it here is because the project it was part of is called Giant Outer Transiting Exoplanet Mass Survey, or GOT ‘EM. I am not one for making tortured acronyms for names, but this one made me laugh. It’s even appropriate!

Et alia

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