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The Big Matryoshka
Truly colossal in scale, the cosmos we can see spans roughly 28 billions light-years, or about 165 sextillion miles (that's 165 billion trillion, or 165,000,000,000,000,000,000,000). How to fathom the unfathomable? By splitting in into smaller and smaller subunits that nest within each other, just like Russian matryoshka dolls. Some 125 billion galaxies occupy the visible universe; each of them, like our home galaxy in Milky Way, is an "island universe" consisting of billions of stars. Galaxies occur in relatively small groups, in larger band bands called clusters, and in still larger, filamentary aggregations known as superclusters. They contain enormous amounts of matter, yet are arrayed at the periphery of even more voids of empty space.
KNOWN UNIVERSE
Like soap bubbles adrift in air, the visible universe's 125 billion galaxies are separated by huge voids. Wile gravity binds together the stars of a given galaxy and galaxies of a given cluster, different clusters and groups are all speeding away from each other due to the Big Band, a titanic explosion we believe took place some 12 billions years ago, spawning the enormity of the universe from a single point in space-time.
LOCAL SUPER-CLUSTER
This great aggregation of the clusters of galaxies spans more than a hundred million light-years. It is centered on the Virgo cluster and includes the Ursa Major and other clusters, each of which has thousands of separate galaxies. The total mass of this supercluster is roughly equivalent to that of a quadrillion-that is, a thousand trillion-of our sun.
LOCAL GROUP
A relatively small clusters on the outskirts of this supercluster, our local galaxy group extends over three millions light-years from Milky Way, and includes two other large spiral galaxies, Andromeda (M31) and Triangulum (M33). It is receding from Virgo as the universe continues to expand.
MILKY WAY GALAXY
About 100,000 light-years in diameter, our home galaxy contains a few hundred billion stars, concentrated in a bulging core and flat, spiral arms. Just as planets circle the sun, the Milky Way's member stars orbit the galactic core, believed to enclose a gigantic black hole.
LOCAL STARS
Positioned in the Orion arm of the Milky Way, our local stellar neighborhood includes stars that exist within 20 to 30 light-years of Earth. Most are too dim to be seen without telescopes, but a few, such as Sirius, are familiar even to casual stargazers. The nearest star to our solar system, Proxima Centauri, is red dwarf some 4.3 light-years away. It has only a tenth of mass of the sun, itself a very ordinary star in an ordinary spiral galaxy.
OUTER SOLAR SYSTEM
The five outermost planets orbiting our solar system include the four so-called gas giants-huge, cold, gaseous orbs with rings and numerous moons-and the rocky dwarf we call Pluto. Beyond the gas giants lie the flat Kuiper belt and the far larger, spherical Oort cloud, both of which harbor comets. We believe that all the planets, as well as the comets and the sun, condensed about 4.5 billion years ago from the disk-like solar nebula, a great cloud of dust and gas.
INNER SOLAR SYSTEM
Four rocky "terrestrial planets" orbit nearest the sun, accompanied by thousands of far smaller but equally rocky asteroids that occupy the asteroid belt between Jupiter and Mars. Earth, the only known abode of life, is outstanding also for its surface aggregation of liquid water as well as its volcanism, seafloor spreading, mountain building, continental drift, and other dynamic processes.
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