Sky watching has always been one of the most fascinating activities for mankind. And one of the most beloved. One of those that can tickle as much the fancy as the scientific curiosity, and that has led us to study the stars, planets and of course the Milky Way. We are talking about what is in fact our Galaxy, which has its name in mythology, and which has committed the time and resources of scholars to be understood in its entirety.
Today we know that, in reality, the strip of sky that we look at in the favor of darkness is a set of stars. Not only that, thanks to the advancement of technological progress, we know its spiral shape, the so-called central bar, and the black hole that surrounds it. However, not everyone knows that this "piece of sky" still hides many mysteries, which exert a very special attraction on the experts.
What is the Milky Way: between science and mythology
By definition, the Milky Way, from the Latin Via Lactea, is the galaxy to which our solar system belongs. For science, it is "the galaxy" par excellence: the name in fact derives from the greek galaxias, related to the word milk, used in Greek times to designate it. According to the most recent studies, it seems that from the morphological point of view it is a barred spiral galaxy, that is a galaxy that consists of a nucleus that in turn is crossed by a bar-shaped structure from which the spiral arms depart following a logarithmic trend. To make it even simpler, we can say that every celestial body we see in the sky, with the exception of the diffuse spots corresponding to the Andromeda galaxy and the Magellanic Clouds, belongs to our galaxy, the Milky Way.
Referring to the Hellenic mythology, in one of his many escapades, Zeus conceived a son with Alcmena, a son who should have been so strong to prevent the predicted extermination of gods and men. Alcmena gave birth to the famous Hercules, and then abandoned him in a meadow fearing the reaction of Hera, legitimate wife of the god of lightning. Hera found the child in the meadow, and decided to take care of it and breastfeed it. The baby was attached to the breast of the goddess with such a force to push Hera to retreat: a splash of its milk went into the sky, creating a furrow from side to side. The demigod remained immortal, while that mythological splash of milk in the shape of a strip remained visible in our sky, and was called the Milky Way.
Wanting to push the "technicalities", and referring to observational astronomy, the term indicates the weak whitish band of light with a milky appearance that crosses the celestial sphere diagonally. We know that it is formed by the stars and nebulosity located in the galactic disk itself. The Milky Way appears much brighter when observed in the direction of the constellation Sagittarius, where the galactic center is located. A place, this one, unfortunately not visible due to the absorption of light by the dense dust present in that direction.
It is not unusual to refer to our galaxy with the name Galaxy, with a capital G. The motivation is to be found in the etymology of the term itself, taking into account that, as specified before, in Greek Galaxia means milky. In the course of history, many myths and legends have brushed pages and minds in the attempt to explain the origin of the Milky Way. From the aforementioned milk of Hera to the ethereal Ganges of India, passing through the images described by Democritus and Arab astronomers, who were captivated by that same trail of distant stars recognized as such by Galileo Galilei and, only later, by scholars and philosophers such as Immanuel Kant, William Herschel and Lord Rosse.
Not forgetting also the vision of the Chinese people, who interpreted the celestial body as a river in which the stars were fish escaping from the Celestial Rod, that is the crescent moon. The inhabitants of Siberia, on the other hand, were convinced that the sky was broken in two and that the brightest stripe was the weld that was useful to keep it together.
With modern science, however interesting, these perspectives have been improved, if not even surpassed, with the term Milky Way now referring exclusively to the bright trail observable in the night sky. In the scientific field, especially in countries of Anglo-Saxon culture, to indicate the galaxy as a whole would be preferable to use the term Milky Way Galaxy, or alternatively Galaxy, with a capital letter. In any case, the name Milky Way remains the most popular and commonly accepted to indicate the entire galactic area.
Discovering the Milky Way: size, age and composition
The Milky Way, if we take into account its stellar disk, shows a diameter of about one hundred thousand light years, for a thickness equal to one thousand light years. If you've ever wondered how many stars in the Milky Way actually exist, know that their number has never been definitively established. Some scholars speak of about 200 billion, while according to other sources would go to the impressive figure of 400 billion.
Not only that, thanks to fairly recent observations, it has come to assume that the gaseous disc of the Milky Way has a thickness of twelve thousand light years, a value exactly twelve times higher than what was previously estimated by the scientific community. Much more difficult is instead to establish the age of our galaxy. The age estimated from that of the oldest known star in the Galaxy is about 13.7 billion years, an age not very different from that of the Universe itself.
Moving on to the level of its composition and structure, as already mentioned in previous paragraphs the Milky Way is a barred spiral galaxy, formed by a central core, called bulge, which is constantly crossed by a bar-like structure. This is composed of evolved stars surrounded mainly by gas and dust, and from it branch what seems to be four logarithmic spiral structures, with an inclination of about 12 degrees, on which the youngest star formations are arranged.
In addition, nowadays we estimate that the shape of the entire structure of the Galaxy is a flattened disk, called by experts galactic equator or galactic plane, with a very obvious central bulge, which characterizes the images that have come down to us. As if it were a sort of pinwheel. There are therefore two major arms, namely the Perseus Arm and the Shield-Cross Arm, and two complementary arms, namely the Swan Arm and the Sagittarius Arm, all originating at the center of the Galaxy, with some secondary arms, which depart instead from the larger ones.
How to see the Milky Way
Considering that the gigantic Milky Way, filling the sky with thousands of stars, is widely visible to the naked eye, we will now find out together how to best enjoy this incredible spectacle of nature. First of all, it is important to be in a rather dark and isolated place: if we are in the northern hemisphere, we must look south; if we are in the southern hemisphere, we will look straight above our heads. Scholars recommend then to try to see the Galaxy par excellence between the months of June and August, the best ones to appreciate all the nuances, the play of colors and variations in brightness. Not only, it is preferable to start observing the sky at least two hours after sunset and no later than two hours before sunrise, to avoid the sky is too clear, while it remains essential, as in the case of shooting stars, choose as a stage of your nighttime excursion a place without light pollution. Artificial light coming from buildings, cars and roads could, in fact, prevent us from seeing well the stars that make up the Galaxy.
As a consequence, areas far from inhabited centers are preferable, especially from the biggest ones!, maybe some country spots, but also nature reserves, deserts and other uninhabited areas, not crossed by main roads. Since the Milky Way always appears in the southern sky, we should head south of any large city to prevent artificial light from interfering with our observations of the night sky. There are other tricks to ensure perfect viewing as well. First of all, we should choose a night not only free of clouds, but also when the Moon is not present, new or crescent should do the trick.
The "reach" of our eyes should not be underestimated either, as we should first get them used to seeing in the dark. To do this, it is necessary to let about 20 minutes pass without using any light-generating tool, such as a flashlight or our smartphone. To close, to locate the nucleus of the Galaxy, that is, the most dense agglomeration of stars in it, an effective method is to focus with the gaze on the horizon, until the most extreme limit.