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Earth In the Universe. Where Are We?

IN SHORT - One the best introduction to astronomy is to posit the scales and distances in the Universe. As the Earth is one of the planets which orbit around the Sun, the Sun, in turn, is just another star in the Milky Way Galaxy. The Milky Way

Galaxy is one of those billion galaxies which populate the Universe, as the latter is now well thought like a filamentary web of vast filaments of hydrogen

Earth (7,830 miles in diameter) is part of the solar system. The solar system is the ensemble of planets orbiting the Sun. Earth is the third planet from Sun after Mercury and Venus. Beyond Earth are still found Mars, the gas giants -Jupiter, Saturn, Uranus, Neptune- and Pluto. Between Mars and Jupiter's orbits is found the asteroid belt. Asteroids are boulders or small planetoids. Beyond Neptune, between 2.8 billion and 28.6 billion miles (4.5 billion and 46 billion kilometers) -300 Astronomical Units (AU) -300 times the distance Sun-Earth-, lies the Kuiper Belt, a disk-shaped reservoir of comets and diverse leftovers of solar system's formation; further beyond, at the solar system boundaries another comets' reservoir exists, the Oort Cloud. This cloud begins where Kuiper Belt ends. Solar system is protected by heliosphere. This Sun's magnetic field is separating solar system from interstellar medium

Earth is 150 million kilometers (93 million miles) away from Sun. Pluton, farthest planet, is 6 billion kilometers away. Oort Cloud stretches to nearly 1 or maybe 2 light-years (1 light-year is 5,880 billion miles). Beyond solar system boundaries interstellar medium is beginning. Distances become so important that usual terrestrial measure units are not used anymore, but specific units instead. Light-year is distance which light travels in one year at speed of 300,000 km per second (671 million miles per hour), i.e. 5,880 billion miles (9,000 billion kilometers). Beginning at faraway limits of solar system, Astronomical Unit (AU) is used. It is Sun-Earth distance -93 million miles (150,000,000 km). Light-year is at the same time location of an objet in space, and in time: a star located at 500 light-years is at 500 times 5,880 billion miles as its light took 500 years to reach us. This star is seen as it was 500 years ago, about 1500 AD

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Sun is a star. Like billion of other stars it belongs to Milky Way. Milky Way is a galaxy. A galaxy is an ensemble of hundreds of billion stars. Our Milky Way Galaxy, might we seen it from the outside, would look like those spiral galaxies popularized by terrestrial and space observatories: a central bulge with spiraling arms. Our Galaxy is 100,000 light-years wide. Would the Sun be the size of the dot of an 'i' on this page, the Milky Way would stretch from Los Angeles to New-York. Sun is located 28,000 light-years away from center and it is orbiting the Galaxy in 250 million years (143 miles per second; 230 km/s), as it's about 20 light-years above the center of the plane of the disk. Locally, Sun is situated in a zone of thin density (0.3 particles per cm³). In this zone, the mean distance between stars is about 5 to 6 light-years. A black hole is found in Milky Way center as in other galaxies. A black hole is a place of huge gravity swallowing matter and from which even light cannot come out. Milky Way is containing 400 billion stars. Our Galaxy outer reaches are marked by a globular clusters halo

Milky Way in turn is part of a group of galaxies wich is called the Local Group. The Local Group is 10 million light-year-wide. It contains 3 large galaxies (of them the famous Andromeda Galaxy, M31) and more than 30 smaller ones. In the Local Group, Milky Way is second in size after M31. Closer to our Milky Way, in a 1 million light-years wide inner circle, satellites or companions galaxies are found, like the Large and Small Magellanic Clouds (179,000 and 210,000 light-years distant respectively)

The Local Group itself, through successive memberships, is taking place in other groups. These clusters of galaxies eventually line up into a filamentary web which the Universe was recently discovered to look like. Originating at Universe very beginnings, this network of vast filaments is shaping the Universe. Galaxies are found at its nodes. The Universe is 28 billion light-year-wide. It is flat, that is that two parallel lines never meet, and that it's expanding. It has no other boundaries than the ones its draws as it's unfolding. A recent estimate shows that there are about 70 thousand billion of billion stars in Universe (7 sextillion or 7 and 22 zeros), as much as grain sands found on all Earth's beaches. Recentest advanced theories state that this Universe itself would be only a minute part of a more giant cosmos still. Our Universe would be a kind of bubble of trillions light-year-wide. And this bubble would be only one out of many