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Q. Have you been able to locate our position in the Universe? Why don't the stars in the sky change? — Russ A.
A. Earth is a moist rock circling a star in the outer arm of a spiral galaxy we call the Milky Way.
We refer to our star as the Sun. We live on one of nine planets which are accompanied by moons, comets, asteroids, gas and dust in a collection of stuff circling our star. We refer to this stuff and the Sun as our Solar System.
The Sun is one star among 100 billion stars in the Milky Way, which is a flat disk-shaped spiral galaxy 100,000 lightyears in diameter with a bulge at the center, like a huge blazing pinwheel floating through space.
Stars. A star is a sphere of hot glowing gas, a single ball of fire in space, often millions of miles in diameter. Stars differ in size, brightness and color. The material in some stars is 10,000 times as thin as Earth's air at sea level. On the other hand, some stars are so dense a cupful of material would weigh tons if it could be brought down to Earth. Inside, stars have temperatures measured in millions of degrees. At their surfaces, temperatures up to 55,000 degrees are common.
Stars are a long way off, yet all stars visible from Earth, even with telescopes, are within our own Milky Way galaxy. Proxima Centauri, the star nearest Earth, is so far away it's only a pinpoint of light in the largest telescopes on Earth.
Astronomers now think many stars may be ringed by planets and other small bodies, just as our Sun has its Solar System of planets, moons, comets and asteroids. Planets reflect starlight, while stars shine with their own light.
Galaxies. A galaxy is a vast island of stars floating through the Universe -- a cloud of millions or billions of stars attracted to each other and held close by their gravity, while floating as a group through the Universe. There seem to be more galaxies strewn across the Universe than grains of sand on a beach.
Astronomers have seen galaxies of many different shapes and sizes. Most common are flat pinwheel spirals, egg-shaped ellipticals, and oddly-shaped irregulars. The galaxies most widely known are Andromeda Galaxy, Large Magellanic Cloud galaxy and our own Milky Way galaxy
Many galaxies are very, very far away. In fact, some may be at the most distant edge of the Universe. The light arriving from them takes so long to get to Earth that what we see originated very far back in time. Looking at them is like staring back into the history of the Universe. One faraway clutch of stars is estimated to be as much as15 billion lightyears away. It is known as 4C41.17 and is one of the most distant known galaxies. A lightyear is approximately 5.9 trillion miles.
Starry Sky. There are untold trillions of galaxies strewn across the Universe like grains of sand on a vast beach. From our earliest times we've looked up and wondered what's out there. From our perspective down on Earth, stars seem to hold the same positions in the sky year after year. Over many centuries they do appear to move, however, as our Solar System slowly circles the core of our own Milky Way galaxy.
While astronomers ponder the idea of clusters and even super-clusters of galaxies, stranger objects wash up from time to time on the vast beach. Supernovas, neutron stars, pulsars, black holes, quasars and more . . .
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