formation of the Universe

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Most astronomers think that the Universe was formed during an event called the Big Bang. This was a giant explosion which occurred between 10 and 20 billion years ago. During the Big Bang, all of the space, time, matter, and energy in the Universe was created. This giant explosion hurled matter in all directions and caused space itself to expand. As the Universe cooled, the material in it combined to form galaxies, stars, and planets. When this dust cloud collapsed, it formed a solar nebula—a spinning, swirling disk of material. At the center, gravity pulled more and more material in. Eventually the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. Matter farther out in the disk was also clumping together. These clumps smashed into one another, forming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets and large moons. In other cases, planets did not form. The asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroids, comets, meteoroids, and small, irregular moons. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own, and the Milky Way is but one of perhaps 100 billion galaxies in the universe. The planetary system we call home is located in an outer spiral arm of the Milky Way galaxy. Our solar system was formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. With that, our Sun was born, and it eventually amassed more than 99 percent of the available matter. Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula—a spinning, swirling disk of material. Our solar system consists of our star, the Sun, and everything bound to it by gravity — the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, dwarf planets such as Pluto, dozens of moons and millions of asteroids, comets and meteoroids. Beyond our own solar system, there are more planets than stars in night sky. So far, we have discovered thousands of planetary systems orbiting other stars in the Milky Way, with more planets being found all the time. The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest the Sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets—Mercury, Venus, Earth and Mars—are terrestrial planets. They're small with solid, rocky surfaces. Meanwhile, materials we are used to seeing as ice, liquid or gas settled in the outer regions of the young solar system. Gravity pulled these materials together, and that is where we find gas giants Jupiter and Saturn and ice giants Uranus and Neptune. Interstellar Space? Five spacecraft have achieved enough velocity to eventually travel beyond the boundaries of our solar system. Two of them reached the unexplored space between the stars after several decades in space. Voyager 1 went interstellar in 2012 and Voyager 2 joined it in 2018. Both spacecraft are still in communication with Earth. Both spacecraft launched in 1977. NASA's New Horizons spacecraft—currently exploring the an icy region beyond Neptune called the Kuiper Belt—eventually will leave our solar system. Pioneer 10 and Pioneer 11 also will ultimately travel silently among the stars. The spacecraft used up their power supplies decades ago.

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