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Our Sun is one of at least 100 billion stars in the Milky Way, a spiral galaxy about 100,000 light-years across. The stars are arranged in a pinwheel pattern with four major arms, and we live in one of them, about two-thirds of the way outward from the center. Most of the stars in our galaxy are thought to host their own families of planets. Thousands of these extrasolar planets (or exoplanets) have been discovered so far, with thousands more candidates detected and awaiting confirmation. Many of these newly discovered planetary systems are quite different from our own.
All of the stars in the Milky Way orbit a supermassive black hole at the galaxy's center, which is estimated to be four million times as massive as our Sun. Fortunately, it is a safe distance from Earth, at around 28,000 light-years away. Our galaxy is one of countless billions in the universe, each having millions, or more frequently billions, of stars of its own.
We call our galaxy the Milky Way because it appeared to ancient observers to be a milky band of light – like a cosmic roadway – stretching across the dark sky.
Size and Distance
All of the stars in the Milky Way orbit a supermassive black hole at the galaxy's center, which is estimated to be some four million times as massive as our Sun. Fortunately, it is a safe distance of around 28,000 light years away from Earth. The Milky Way zips along a galactic orbit at an average speed of about 514,000 mph (828,000 km/hr). It takes about 230 million years for our solar system to make one revolution around the galactic center.
The Milky Way is part of the Local Group, a neighborhood about 10 million light years across, consisting of more than 30 galaxies that are gravitationally bound to each other. Aside from our galaxy, the most massive one in this group is Andromeda, which appears to be on course to collide with the Milky Way in about four billion years.
Scientists studying galaxies observed that the stars in the outer parts are orbiting the galactic centers just as quickly as the stars further in, a violation of Newton's well-established laws of gravitation. They deduced that something other than the stars and clouds of gas and dust known to comprise galaxies was providing extra gravity—lots of it. They calculated that there must be five times as much of this mysterious dark matter, detectable only by its gravitational pull, as there is of the matter we already knew about.
The Local Group is only one of many, many clusters of galaxies, and they are all moving away from each other as more and more space comes into being between them. This means the universe, itself, is expanding. That discovery is what led to the Big Bang theory of the origin of the Universe.
Scientists expected that the gravitational attraction of everything in the universe would put the brakes on the rate of expansion, and eventually the expansion would stop or even reverse. But in the 1990s, scientists discovered that the expansion is actually getting faster. The force responsible for this surprising acceleration was dubbed dark energy. No one is sure what it is, but one possibility is that it is energy contained within the very vacuum of space.
Since matter and energy are equivalent (as expressed in Einstein's famous equation, E=MC2) scientists have been able to calculate that whatever dark energy is, it comprises about 68 percent of everything in the universe. Dark matter accounts for another 27 percent, leaving only five percent for protons, neutrons, electrons and photons – in other words, everything we see and understand.
Scientists calculate that there are at least 100 billion galaxies in the observable universe, each one brimming with stars. On a very large scale, they form a bubbly structure, in which vast sheets and filaments of galaxies surround gargantuan voids.