Illustrated Astronomy/Other types of eclipses:Transits and occultations

by Juan Carlos Beamin

 

V

OTHER TYPES
OF ECLIPSES: TRANSISTS
AND OCCULTATIONS

We have already said that there are other types of eclipses not related to the Moon and the Sun, but distant stars. 50 % of the stars are binary systems or more stars included.

The Sun, however, doesn’t have a stellar neighbor, but only planets. The binary stars can have different separations or orbits, and these are oriented randomly in the sky. Since there are many stars in our Galaxy, some binary stars orbit a plane very aligned with the Earth, so then, in a particular region of its orbit, a star passes in front of the other, blocking the brightness of the one in the back. These double stars are classified as eclipsing binary stars. A historical example is the star Algol, the second brighter star of the Perseus constellation, which name comes from the Arabic, and it means possessed star (from Ras Al-gul). When the Arabic astronomers saw the periodical change in the star brightness due to its eclipses, they understood that it was because of devilish powers given their world view of an immutable sky^[1].

The importance of eclipsing binary stars is when they are studied in detail; they allow us, for example, to calculate the radius of the stars and their brightness and, combining other special measurement instruments, we can measure their masses. All of this to understand better how they are formed and evolve.

DID YOU KNOW THAT…

...multiple systems with more than three stars are less common, but instead exist two systems of seven stars? They are Nu Scorpii and Ar Cassiopeiae.

in spite of the eclipses of distant stars, there are two phenomena similar to an eclipse: occultations and transits.

OCCULTATIONS

In our Solar System, besides the planets, there are thousands of asteroids, comets, and distant objects beyond the orbit of Neptune called trans Neptunians objects.

The best example and largest one of these objects are Pluto and its moons but there are many others in what is known as the Kuiper Belt.

On some occasions, these objects pass between a distant star and us and block the light of the star.

This phenomenon is called occultation, and it is the best way to study the Trans-Neptunian object’s shape that is hiding the starlight. In the case of planets, it has no advantages to carry a more extensive study since they are a lot, and we can measure their sizes using telescopes and probes, whereas for small bodies, such as some asteroids or trans-Neptunian objects from the Kuiper belt, is very useful.

To study the trans-Neptunian objects, we have to know very well its orbits, this is the only way we have to predict when they are going to pass between us and a star. With the calculations made, we are going to determine from which parts of the Earth they can be observed (as they are small, the occultation is not visible from every part of the Earth). Generally, when this phenomenon is recognized, an astronomical alarm is set for observatories and amateur observers to follow the event.

Having said that, as the object is small, the observations carried out in different regions of the Earth have different perspectives and the duration of the occultation changes depending on our point of view. To better describe the shape of the object and its size, we get all observations together, including the position of observers, and we analyze the process case by case.

It seems to be easy at first. However, we have to consider that occultations last only a few minutes or even seconds, so there are needed telescopes with cameras set up to take many images in a short period.

Among the most exciting findings achieved due to the observations of these occultations, it stands out the discovery of binary trans-Neptunian objects or asteroids with rings. A mini version of Saturn!

Chariklo representation, asteroid-like object which is also known as Centaurus. Illustration by Lucie Maquet.

TRANSITS

The eight planets of the Solar System orbit around the Sun practically in the same plane, with a few differences between them, orbiting in the same direction. Also, out of the eight planets, six of them rotate on its axis in the same direction of its orbit (Venus and Uranus are the exceptions). All of this would make sense if the planets were formed in a protoplanetary disk 4,500 million years ago.

The Moon is “almost” aligned in this same plane, but there is a minor inclination of only 5° regarding the plane where the Earth transit the Sun (called ecliptic). It may seem not much, but it is enough so that instead of having a solar and lunar eclipse every month, we have a total solar eclipse every eighteen months on average, as we have already seen.

There are times when Venus or Mercury pass in front of the Sun, and we can see it with the right telescopes. They can’t eclipse the Sun entirely due to the considerable distance that separates the Earth from these two planets, blocking a small part of the sunlight, which are called transits.

The outer planets (from the Earth outwards), which means, Mars, Jupiter, Saturn, Uranus, and Neptune, are never between the Earth and the Sun, so they never produce a transit.

There are also other very interesting transits. Today, we know that it is very usual that stars have planets, which are known as exoplanets or extrasolar planets.

Such is the case that there should be more planets than stars in our galaxy.

The same as eclipsing binary stars, some of these exoplanetary systems may have an orbit aligned with the Earth.

In these cases, planets partially block the light coming from the orbiting star. Unlike binary stars, in which the change in the light we receive is profound, in the case of planetary transits, the variations can be one hundredth if it is a planet like Jupiter and a few millionths in the case of a planet like Earth.

Even though it is not an easy thing to perceive, since we are talking about detecting a firefly next to a stadium spotlight located hundreds of kilometers away, we can do it thanks to telescopes and cameras specially designed to measure these changes in brightness. Actually, most of the planets have been found by this technique, either from telescopes on Earth or space ones.

The Kepler space telescope, which is currently on the TESS mission and, in the near future the PLATO mission, have been and will be responsible for discovering new planets, such as the Earth, orbiting other stars, and bringing humanity one step closer to the search for another possible Earth where life may exist because, despite the progress achieved, the most crucial question remains unanswered: Is there life beyond our planet?

1. Paradoxically, western astronomers, with a similar vision of the sky, observed changes in brightness in the star Mira (not due to eclipses but to changes in its temperature and size). The name Mira comes from Latin, and it means wonderful. A curious difference in the way both cultures perceived and called an astronomical phenomenon.