December 12, 2023 – asteroid Leona will cover the star Betelgeuse

Unprecedented universal audacity

Friends, this is a somewhat unexpected phenomenon for me personally. I found out about it from my friend on the FB social network – Star Estrella – thank him very much. So we would have missed everything. But the event is unusual and very interesting – for discussion, first of all.

However, the likelihood that any of us will be able to see something is extremely low. But this does not detract from the interestingness of the conversation itself.

First things first

Small planets – asteroids – are full participants in the solar system. They are as old as most of its other inhabitants – the large planets, their satellites and comets. It was once believed that they were formed during the destruction of the mythical planet Phaethon – in any case, such a hypothesis was expressed. Then it turned out that the entire total mass of already discovered and still undiscovered (science can also estimate it quite accurately) asteroids is not enough for even a small part of the Moon. According to modern estimates, the entire mass of asteroid and meteoroid material in the Main Asteroid Belt does not exceed 4% of the mass of the Moon or 0.05% of the mass of the Earth – this does not make a good Phaeton. And there was no way this substance could disappear somewhere—disappear without a trace, escape from the solar system. It turns out that we have exactly what we have – several more or less large spheroidal bodies, such as Ceres, Pallas, Vesta and Hygiea… and the rest is basically a scattering of cosmic boulders from a couple of hundred kilometers in diameter to kilometers, meters, centimeters… the vast majority of them are so small that their appearance cannot be seen in any telescope, their size cannot be determined.

Fortunately, the Universe sometimes gives us surprises, and astronomers – extremely inventive people – try to use every coincidence of circumstances for the benefit of science.

What helps scientists study asteroids?

The fact that sometimes asteroids obscure the stars.

Yes – this happens. Not to say it’s rare. There are many stars in the sky. Asteroids seem to be moving among them – at least as seen from Earth. In fact, asteroids are much closer – the distance to them is most often somewhere between the distance to Mars and Jupiter – it is the orbits of these planets that limit the Main Asteroid Belt.

(Someone probably noticed that since there is a Main One, then there must be some other – not main – asteroid belt. And as we know, for one Main One there are most often several non-main ones – this is exactly the case among people. Asteroids are about the same – there is also the Kuiper belt, Trojans, Greeks, Apollos, Cupids… The world of asteroids is very interesting and diverse… but let’s talk about the rest later…)

If asteroids cover stars, then most often it is such a weak star that you cannot see with every telescope (like an asteroid, you cannot see with every telescope – they are usually very faint). But still, astronomers do not miss the chance to learn something from such an event.

What can you learn?

First of all, make sure that the calculations for the upcoming coverage are done correctly. It only seems that science is capable of accurately predicting the movements of celestial bodies. To one degree or another, this is true for the Major Planets. But the smaller the celestial body, the less stable its orbit – more massive bodies tend to change it, but a small asteroid has to obey it. And it is not always possible to take this into account, because we most likely do not know the mass of any asteroid.

Just imagine, you are looking through a telescope at a faint star, and there is nothing other than it in your field of view. And suddenly the star went out for a split second and lit up again?

What does this mean?

This means that you are lucky – you managed, by chance, to get into a coverage strip, which on our globe is only a couple of kilometers wide – and this is good if there is so much, and sometimes it can be even less (if the size of the asteroid is smaller, and these are the majority). Imagine, everyone around didn’t see anything like that – from their locations the star was still shining and still shining – not blinking. And the asteroid is generally so faint that it is not visible in your telescope, and is not visible in their telescope… and only your evidence that the light of the star was screened for a split second by some celestial body speaks in favor of its existence.

It should be mentioned here that for such observations, astronomers often undertake special expeditions, because it is not a fact that the coverage strip will cover any of the observatories with a large and sharp-sighted instrument – you have to go to the place with a smaller telescope, which you can drag with you.

And look, you didn’t even measure anything, didn’t detect anything, but simply witnessed the coverage, but you know the geographic coordinates of the observation point – this is now determined by any phone using GPS, and you have already greatly helped to clarify the orbit of the small space rock.

And even if you didn’t see anything like that, you also helped clarify its orbit, by the evidence that the coverage was not observed in your location, which means its band passed by – this is also important.

This is so important that it is taken into account for forecasts of asteroid collisions with the Earth. And when scientists do not have enough data, they turn to amateurs for data, fortunately, the amateur network is now very extensive and active.

And then you can try to measure the time at which the star died out.

This is a direct path to determining the size of an asteroid. A significant part of the estimates of the physical sizes of small planets was made precisely from the results of observations of occultations. The characteristic orbital speed of the asteroid is 15-20 km/second. And if the diameter of the asteroid is 1 km, then the coverage will last about 1/20 of a second – it is difficult to measure this by eye. That’s why astronomers and amateurs use electronics. In particular, video sensors that capture a large number of frames per second are very useful. But if the duration of coverage can be measured, then the math is simple. We multiply the orbital speed by the duration of the occultation and get the length of that part of the asteroid with which it covered the star. It could easily turn out that in your location the asteroid only touched the star with its edge. And somewhere – right at the maximum – a central eclipse occurred.

As you might guess, observation alone is not enough here – you need to have data from different points of the coverage strip. And only then will we find out a more or less plausible size. And in the case of one observation, we will have to deal with an estimate of the minimum size. That is, if an asteroid with an orbital speed of 20 kilometers per second eclipses a star for 1 second, its size is at least 20 kilometers. But it can be more – even 10 times. After all, we do not know exactly which edge it eclipsed the star in a particular location, if there are no other observations. In fact, in this case, only the length of this edge is measured.

Maybe one of the attentive readers remembered that knowing the orbital speed of an asteroid for this task is not enough, because the Earth also does not stand still – it moves in orbit, and even rotates around an axis (and at different latitudes the linear speed of axial rotation The Earth is different, but still influences the result), and even the orbits of the Earth and the asteroid do not lie in the same plane…

Oh, yes, the star is also moving somewhere!

How to take all this into account?

The good news is that most of the time the star’s motion can be neglected. In space, a star can be very fast. But she’s far away. And the angular speed of its movement is negligible in comparison with the angular speed of the asteroid.

But the rest will have to be kept in mind.

People with a school-like attitude towards geometry cannot solve such a problem. This requires fluency in stereometry and spherical geometry – this is not taught in school. To simplify understanding, I will say that in fact only the relative tangential velocity of the asteroid is needed, and not its full value in the heliocentric reference frame. This is the speed with which the asteroid moves across the observer’s line of sight, and relative to the observer himself. And we must admit that in these calculations we ourselves unwittingly descend to the Ptolemaic geocentricity. But what to do…

What if it suddenly turns out that the star blinked not once, but twice?

This means that our dear asteroid has suddenly discovered a satellite.

Now many asteroids have been discovered to have satellites, and some even have more than one satellite. And some satellites of asteroids were discovered precisely in transit – this is when an asteroid passes in front of a star.

This, of course, is not news at all. And this is roughly how the rings of the planet Uranus were discovered at one time – Uranus eclipsed the star, but upon observation it was discovered that the star faded slightly several times before the occultation and shortly after it. And a few years later, the automatic interplanetary station Voyager 2 only confirmed this explanation of the behavior of the star’s photometry.

And in the same way, rings around the asteroid Chariklo (10199) were discovered. It turns out that asteroids can also have rings. But once upon a time, no one believed in the existence of satellites on asteroids. And the results of the occultation of stars by asteroids made us believe in both.

True, Chariklo orbits the Sun not in the Main Asteroid Belt, but outside of any belts – between the orbits of Saturn and Uranus. This is the so-called “Centaur” – there is such a classification among the small bodies of the Solar System. It is quite large – 260 kilometers. But it is always very far from us – beyond the orbit of Saturn it is impossible to see these 260 kilometers, and it would be impossible to notice any other rings. But that same coincidence helped – the covering of a 12th magnitude star by an asteroid of 19th magnitude. More than 30 observing stations in South America (Argentina, Chile, Brazil and Uruguay) were used for these observations. Representatives from 12 countries and more than 30 scientific organizations participated in the study. It would seem that just some asteroid will cover a barely noticeable star, and such an honor to all this!

But we must also take into account luck – the famous ESO La Silla observatory magically fell into the coverage band. And here is the result: Open rings on the asteroid.

And now some other asteroids are also suspected of having rings. But the method is still the same – observing the occultation of stars by asteroids.

What else?

What if, in the process of comparing different observations within the coverage band, there is some inconsistency in the data on the duration of the phenomenon? Of course, there may be errors in measurements. Astronomers are people too, and they can make mistakes. That is why all measurements and observations are most often duplicated and carefully double-checked. This is science, after all, and not fortune-telling. And if scientists undertake to answer for their data, they also undertake to explain why this or that inconsistency suddenly crept into them, and look for ways to explain it.

For example: in the middle of the width of the coverage strip, but, let’s say, at its beginning, one duration of coverage was observed, and in another part of the strip – closer to the end – a duration of the same coverage noticeably different from the first. It is unlikely that the asteroid suddenly slowed down, or the star in the sky suddenly shifted. Most likely, the asteroid rotates quite quickly around its axis, and in the second case it covered the star with a larger or smaller diameter. this also happens. And the observation of occultations also provides data to clarify the rotational behavior of the asteroid.

(However, to identify and study the rotation of asteroids, there are still many alternative methods. It is not necessary to wait for occultations for this. Useful data is provided by photometry (this is when an asteroid periodically changes its brightness, like a variable star – let the polarization measurement be included here), spectroscopy (the spectrum of an object in astronomy in general is like his passport, or personal file – it will tell you more about the object being studied than anything else) and radar.)

Well, a completely unexpected aspect of observing occultations opens up for astronomers the opportunity to study relativistic effects in the movement of celestial bodies in the Solar System.

If anyone remembers that the secular shift of the perihelion of Mercury (the fastest and closest planet to the Sun) was explained only at the beginning of the 20th century using Albert Einstein’s General Theory of Relativity. But similar effects are observed in some asteroids, which, in their elongated orbits, dive into the vicinity of the Sun deeper than the orbit of Venus and even Mercury. In particular, the asteroid Phaeton (this Phaeton again!… fortunately, “this is different”) – a potentially dangerous celestial object approaching the Earth, which is something between an asteroid and a comet, also demonstrates relativistic effects in the evolution of its own orbit. And for us, the people of Earth, it is absolutely important to understand what is happening with the orbits of dangerous rock blocks. The phaeton, by the way, is a very impressive block – 6 kilometers across. Once upon a time, such a block was quite enough for dinosaurs, and we should be prudent in relation to everything like that. And tracking the evolution of the orbit of Phaethon (as well as similar Apollos, Atons and Atyrs – these are all classifications of asteroids falling out of the Main Belt) is largely helped by occultations of stars, simultaneously providing food for the study of relativistic effects in the Solar system.

Well, now let’s turn our attention to the hero of today’s celebration:

Asteroid #319 Leona

This small planet was discovered in 1891 by the French astronomer Auguste Charlois, and received its name from him. No one can now say for sure in whose honor the asteroid got its name. But we can assume that it was a woman.

Leona is unusual in that it revolves around the Sun in a stable resonance with Jupiter – it makes exactly 7 revolutions in 4 Jupiterian years. 4/7 is one of the common orbital resonances. And there is a whole family of small planets whose orbits are concentrated at the farthest edge of the Main Asteroid Belt (the semi-major axes of their orbits range from 3.3 to 3.7 astronomical units). It is called the Family of Cybele. Asteroid Cybele is the largest object of this family – 240 km across. And Leona is a so-so pebble – 50 x 80 km – it’s not even round at all…

But how did they manage to measure this Leona?

It is known how – it has already covered the star. True, quite weak. Then a team of Spanish astronomers succeeded in their research. But beyond this, the asteroid was actively studied.

What else is known about Leon?

This is a very dark, rocky celestial body, brownish-red in hue, reflecting about 2% of sunlight – just like a lump of brown coal, and it seems surprising that something like this could be discovered between the orbits of Jupiter and Mars. Spectral analysis indicates that the body consists of silicates and carbon compounds, possibly even organic (this is not uncommon in the Universe). The presence of water ice cannot be ruled out – somewhere inside the object, and it may even be the ice that holds together fragments of the object, if it was formed from several smaller bodies through freezing – frozen water, even in space, is quite plastic.

Leona rotates around its axis, albeit extremely slowly – one revolution every 430 hours. One day on Leon lasts 18 Earth days. Just don’t confuse this with relativistic time dilation. Leona is simply one of the slowest rotators among asteroids. But at the same time, Leona precesses very strongly, as if it is tumbling along another axis of rotation, but with a period of about 1100 hours. This complex rotation is difficult to explain. And it is possible that Leona is not alone in her orbital journey – it is possible that she also has a satellite, although she is somewhat small for this.

The force of gravity on Leon is negligible – it is 500 times weaker than Earth’s gravity. To leave this asteroid forever, an astronaut only needs to reach the speed of Usain Bolt in the 100-meter dash. It will be very difficult to do this on your own in a spacesuit, but a jetpack can definitely handle it.

Well, Leona makes one revolution around the Sun in 6.28 Earth years. It is always very faint – the apparent magnitude does not exceed 13m. Now it is even weaker: 14.2m.

And this cosmic block attempted to cover one of the brightest stars in the sky, namely the star Betelgeuse – alpha Orion, and the most likely supernova of the near future.

Of course, Betelgeuse will never know about such audacity, and certainly will not explode prematurely due to such a coincidence. But astronomers suddenly had a chance to explore both in one fell swoop—a distant, little-studied asteroid and one of the most interesting stars in our Galaxy.

Up to this point, we have only talked about how asteroid occultations of stars help study asteroids. But then we were talking exclusively about covering faint stars.

When it comes to bright stars, things change dramatically.

If the star is bright, it is probably either close or quite large in size. Why else would she be bright?

Betelgeuse is not the closest star, but still not very distant – 550 light years, on a galactic scale this is right very close – this is comparable to the size of the so-called Local Bubble – our stellar abode, in which we exist comfortably, feeling as if protected from all sorts of cosmic misfortunes such as black holes, neutron stars, nebulous remnants of supernovae or themselves. Therefore, it would not be an exaggeration to designate Betelgeuse as one of the stars in our close environment.

And without a doubt, Betelgeuse is a very, very big star. Betelgeuse is almost 1000 times larger in diameter than the Sun. If it were instead of the Sun, it would be bad for everyone, including Saturn. I think that Uranus and Neptune would not like such a replacement at all.

And even from a distance of 550 light years, Betelgeuse is visible in powerful telescopes not as a dot (strictly speaking, in telescopes, distant stars are usually visible as small diffraction disks, devoid of at least some details), but as a quite noticeable round piece. The most powerful telescopes-interferometers on the surface of Betelgeuse reveal inhomogeneities, interpreted as giant spots similar to the sun, only on a Betelgeuse scale – if a dozen Earths can easily drown in a sunspot, then hundreds of stars comparable to the Sun can drown in the dark spots of the Betelgeuse photosphere.

Just imagine how much astronomers would love to get a closer look at this terrifying fiery ocean… or at least find some way to see it in better detail.

And now, an opportunity!

An absolutely amazing coincidence of circumstances. However, it was pre-calculated.

I mentioned that star occultation by asteroids is quite common. But again, this only concerned weak stars. The occultation of brights is an extremely rare astronomical event. I don’t even remember that in all the time I have been studying asteroids there has been an occultation of such a bright star. Wikipedia claims that this has never happened before. And the brightest star covered by asteroids (during the era of asteroid studies) was Eta Ophiuchi (magnitude 2.4). But Betelgeuse, one of the brightest stars in the sky, reaches zero magnitude at its maximum brightness (it is a variable star of the wrong type).

In short, we are witnessing an extremely rare and extremely important event for astronomy.

Calculations show that most likely little Leona will not be able to completely close Betelgeuse. And although the estimated duration of the eclipse is about 15 seconds, the angular size of the asteroid and star will be approximately the same.

Betelgeuse has an average angular size of 40 milliarcseconds. This is an incomprehensible quantity for many people. But imagine the entire horizon around you – it’s 360 degrees. One degree no longer seems like a big deal. But astronomers divide it into 60 slices and get one arc minute. It can also be divided. And 1/60th of an arc minute is an arc second – a very small fraction not visible (not distinguishable) by the eye and even in an amateur telescope. Betelgeuse is 25 times smaller. But nevertheless, astronomers are trying to look at something there. Sometimes they even succeed.

The angular size of the Leon asteroid at the time of occultation is expected to be about 41×46 milliseconds of arc. Literally flush with Betelgeuse.

At the moment of total eclipse, only its outer atmosphere will remain visible from Betelgeuse, or in other words – Crown of Betelgeuse.

Never before have scientists had the opportunity to observe a total eclipse of such an interesting star while its corona remained visible.

But that’s not all.

Moving toward Betelgeuse’s disk, Leon will alternately obscure and then, in the same sequence, reveal all the surface inhomogeneities of the star, allowing them to be studied discretely. This will of course last seconds. But astronomers are no strangers to short-term phenomena. Many events in the Universe last only seconds or fractions of a second. And one can hope that scientists have technologies that allow them to record in detail all the dynamics of changes in Betelgeuse’s brightness during the occultation, and photograph separately the spectrum of the corona and prominences, if Betelgeuse has any. And in general, it will be possible to understand what is happening with this star, and how soon we can expect its final collapse, followed by a supernova explosion. And in this light, the upcoming coverage begins to seem very prophetic.

Where will the coverage line of sight be?

It starts in eastern China, crosses Central Asia (Tajikistan, Turkmenistan), Transcaucasia (Azerbaijan, Armenia, Turkey), enters the Mediterranean Sea, crosses Greece, Italy, the southern tip of Spain and Portugal, goes into the Atlantic, and at the end reaches the southern tip Florida and ends in Mexico.

Of course, these are all very rough guidelines. But astronomers know more precise coordinates where they need to be in order to make successful observations. You can also find out whether the occultation of the Betelgeuse star by the Leona asteroid is visible in your location using the Stellarium program. She will also tell you the time of the phenomenon.

For a rough orientation in time, I will inform you that on the whole planet the phenomenon will last from 01:09 to 01:27 Universal Time on the night of December 11-12, 2023.

Perhaps someone will not see this amazing and interesting astronomical phenomenon. I think that most of those reading will not be able to watch this. But just to know what interesting times we live in, to get acquainted with the results of research, which I hope will soon be published in various scientific publications, is worth a lot. And I am sure that in the near future we will learn a lot of unexpected things both about the small planet Leona and about one of the most interesting stars visible to the eye even in the city – about Betelgeuse.