We just enjoyed the shortest night of the year in the northern hemisphere a few days ago. Now that the hours of darkness are increasing as summer progresses, some of you might like to enjoy the clear skies typical of this time of year, and take advantage of the cooler temperatures by looking up at the sky and fantasizing about distant worlds or corners of the universe remote in space and time. What can we see in the sky this summer without any instruments, just with our eyes?
Let’s start with the most common and easy to observe. With the naked eye, and in a place that is not too dark, such as the outskirts of a town or a beach in a city that minimally respects the limits of light pollution (and the law), you can see stars that are around 200 light years away. Considering that our galaxy is about 100,000 light years from end to end, it is clear that our vision is quite short. Of all the stars that we can see at that time, the most famous are those that form the so-called summer triangle, which also overlooks the Camino de Santiago; what better way to see the Milky Way than from the north of Spain.
The stars of the summer triangle are Altair, the soaring eagle in Arabic, which is the brightest star in the constellation of Aquila, known by that name for at least 25 centuries according to the Greek astronomer Eudoxus. Although the Greeks surely took the name of the constellation from the Babylonians, and perhaps from there (or from the rescue of Greek books) the link with the Arabic name of the brightest star in the constellation would be established (forgive me historians, I am just surprised by the similarities between Greco-Roman, Babylonian, Chinese constellations, …).
The second star in the triangle is Deneb, which means “the tail of the swan” in Arabic, which is its constellation, known as a bird even in Polynesia. Seeing Deneb in the Swan is nice, but wouldn’t it be more impressive and extraordinary to be able to “see” in that constellation the first black hole recognized as such, the so-called Cygnus X-1? In Cygnus X-1, there actually cohabits a blue supergiant star about 30 times more massive than the Sun and an object that concentrates a mass about 20 times that of the Sun in less than 300 kilometers. Neither of these components is visible with the naked eye, and the black hole is only visible in X-rays when it swallows some material from its companion.
The third star in the summer triangle is Vega. Vega is special, the brightness of all the stars has been referred to it for centuries, it was called “the most important star in the sky after the Sun”. It is like the meter kept in Paris and which served as a standard measure to calculate distances, in the case of Vega, apparent brightness. Again, Vega seems to come from Arabic and would mean the fallen eagle. If you have binoculars at hand, I recommend that you observe the closest star to Vega within the constellation of Lyra, the so-called epsilon Lyra. It turns out that what the eye sees as one star, binoculars turn into two. And with a small telescope, each of that pair becomes two others, for a total of four stars where the eye only saw one. It is the Double Double star, I already talked about it, and it was the first object I saw with a telescope and it left its mark on me. For me it is extraordinary.
I have yet to introduce you to the exceptional thing that we can see in the sky on these summer nights. I would describe the Perseid meteor shower as exceptional, although it happens every year; this year the best will be on August 11 and 12. We are eating up the comet 109P/Swift-Tuttle (the trail it leaves behind in its orbit around the Sun) at a rate of several tons per year, so the meteor shower is never the same, every year an exceptionally beautiful meteor can fall.
Another exceptional event is the passage of comet 13P/Olbers, which was discovered by the famous astronomer of the paradox, and also studied by such famous mathematicians as Gauss and Bessel. Just on the last day of June it reached its closest point to the Sun, its brightest, after having been crossing the solar system for 69 years. To see it you need binoculars, close to the horizon, a once-in-a-lifetime view, truly exceptional.
Perhaps not as exceptional, but equally spectacular, will be the Moon’s coincidences in the sky with Mars, Jupiter, and Uranus on July 30 (you’ll need binoculars to see Uranus) and with the first two again on August 27. That will be just a day after the Moon also appears near the Pleiades, the Seven Sisters star cluster (see if you can spot all 7 stars) that has been named in the Bible, the Iliad, and Don Quixote, among many books, and seems to have been among the favorite stars of American, Asian, and European civilizations, with an interesting variety in the number each civilization saw: 6, 7, or “many”!
As an astrophysicist who studies galaxies, I cannot recommend enough that you observe the most distant astronomical object that we can see with our naked eyes without the aid of any instrument: Andromeda, our twin galaxy. To see this galaxy you do not need exceptional eyesight, but you will need a lot of patience. If you have very sharp eyesight, try observing two other galaxies, M33 and M81, which are even more distant than Andromeda. I have never been able to see them without a telescope. If you want to know what was happening 2.5 million years ago, how long it takes for their light to travel to us, in a distant galaxy (I won’t say very very distant because it is the closest galaxy of a size similar to the Milky Way), be sure to observe Andromeda, which in August and September is visible near the zenith of the sky for most of the night. It is not easy to see, because a galaxy is a strange thing in the sky, a “little cloud” as the Persian astronomer Abd al-Raḥmān al-Ṣūfī described it 1060 years ago. It is not a little point of light like a star or a planet, so you have to get used to seeing something diffuse. In fact, Andromeda is brighter than the Double Double star we mentioned earlier, but with its light spread over an area 36 times larger than the Moon.
I hope you enjoy your holidays, those of you who are lucky enough to have them. But above all, may the heavens be kind to you by day and by night!
Cosmic Void is a section that presents our knowledge about the universe in a qualitative and quantitative way. It aims to explain the importance of understanding the cosmos not only from a scientific point of view, but also from a philosophical, social and economic one. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than one atom per cubic meter, even though in our environment, paradoxically, there are quintillions of atoms per cubic meter, which invites a reflection on our existence and the presence of life in the universe. The section is made up of Pablo G. Perez Gonzalezresearcher at the Center for Astrobiology, and Eva VillaverDirector of the Space and Society Office of the Spanish Space Agency, and Research Professor at the Institute of Astrophysics of the Canary Islands.
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