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Friends and fellow celestial

Turning our eyes to the night sky there is no doubt that one of the first impressions that come to mind is: why the stars we see are arranged in that way in the celestial sphere? Obviously, a more precise question would require the replacement of the word "stars" with the more general term of bodies heavenly. In fact, looking in more detail the so-called stars are not only stars but also galaxies , planets and asteroids . And why are grouped close to each other? The answer is that the force of gravity brings the heavenly bodies to move closer to each other as time goes by.

Let's start looking into such "home": the first thing you notice is that the Moon , the "companion" Earth revolves around it. In turn, the Earth revolves around the Sun , located at the center of the Solar System. Around it 8 orbiting the planets (in order of distance: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), plus a vast family of smaller bodies including comets, asteroids, and some dwarf planets like Ceres and the now demoted Pluto with its companion Charon. The Earth is at a distance from the Sun about 150 million kilometers, the light reaches the airport in about 8 minutes, and takes longer than 5 hours to reach Pluto!

The bodies of the solar system, therefore, tend to be in company, and this is due to the training took place, most likely, the collapse gravitational a so-called protoplanetary cloud. If we go beyond our solar system we see that the Sun has other stars as "neighbors." The closest is Alpha Centauri, which is part of a triple star system (Proxima, Alpha Centauri A and Alpha B), at a distance of 4.3 light years: this means that they can travel at the speed of light takes 4 years and 4 months to reach it. Even Sirius, the brightest star in the night sky, it's pretty close to the Sun and is the fifth in order of distance from it, at about 8:58 light-years.

Another thing that has always intrigued those who watch the night sky is a band in the sky thickens where a large number of stars. These stars, of course with our Sun, are joined in the star system to put together a very numerous (about a few hundred billion) to which they belong, in fact, all the stars we see with the naked eye in the sky: it's our galaxy, the Milky Way. Considering our entire galaxy, we realize also that the Sun's distance to Alpha Centauri and Sirius, a few light years, is insignificant compared to the Universe on a grand scale: even the Sun is 27.000 years much light from the center of the Milky Way.

The Milky Way galaxy is a so-called "spiral" which covers about 78.500 light years, and in turn is part of a system of several galaxies called the Local Group. So we see that all the celestial objects tend to have similar as their neighbors. The Local Group is in fact made up of fifty galaxies is centered at a point between the Milky Way and the nearby Andromeda galaxy, and extends for about ten million light years. Among the members, in addition to the Milky Way and Andromeda, the galaxy include the Triangle, the Small and the Large Magellanic Cloud and a considerable number of small galaxies, elliptical and spheroidal dwarf galaxies such as Barnard's Galaxy, Leo I and II. On a still larger , the Local Group is in turn part, along with other groups of galaxies, a huge system called the Virgo Supercluster, one of the many galaxy clusters that populate the universe. We, with our solar system, the Milky Way and the Local Group, we are on the outskirts of the supercluster, and we are slowly moving toward its center.

Universe, is accompanied by celestial objects therefore to each other, they approach and move away a bit 'as we humans do with the people around us. The difference is that while human relationships can be both attractive to repulsive, the relationship of "friendship" of celestial bodies is governed by gravity and will therefore always attractive.

CARLO JUGGLER

illustration above, the planets of the solar system, Earth's closest companions, and in the background, the large number of stars, distant friends of the Sun, which together form the Milky Way ( www.pianeti.info ).
illustration below, cosmic agglomerations gradually larger from left to right and from top to bottom, the Solar System, the "neighborhood" of the Sun, with all the stars in the area, the "realm" galactic , size of the Milky Way and nearby galaxies, the Local Group and ultimately the Virgo Supercluster. The position of Earth is shown in red, and the scale of their figures are shown next to the pictures (German Space Agency - DLR / Azcolvin).