THE UNIVERSE
The image below is a photo of the exhibit poster. A text transcript follows.
Approximate dimensions: H:50 in W:32 in
NATIONAL GEOGRAPHIC poster
Text by Stephen P. Maran; Art by Kenneth Eward; Planetary alignment, January 1, 2000
As far as we can see with our ever improving telescopes, there are at least a hundred billion galaxies arrayed throughout the universe. Each, like the Milky Way, is an "island universe" containing billions of stars. Nearly all galaxies are members of groups or clusters, which are part of even larger structures called superclusters. All of these large concentrations are connected by filaments or sheets of galaxies, which enclose huge, bubble-like volumes of empty spaces, the cosmic voids.
The Great unifier of the cosmos is gravity. It holds the stars of a galaxy, and the galaxies of a cluster, together. But clusters, groups, and isolated individual galaxies are all flying away from each other, a continuing aftermath of the big bang, an explosion of space itself that astronomers believe formed the universe 11 to 15 billion years ago.
EVOLUTION OF THE UNIVERSE
Combining evidence from microwave observations of deep space with supercomputer models, scientists have theorized about the structure of the universe from the big bang to the present. From the time that radiation and matter separated -- 300,000 years or so after the big bang -- gravity began drawing matter first into clumps, then into stars. Perhaps a billion years later the first galaxies formed. As clusters of matter accumulated, the filament-and-void structure of today's universe unfolded.
1) SIZE OF THE UNIVERSE
So vast is space that just to find our solar system we must make five leaps of scale. In the background image on this sheet we see a mere sliver of the sky -- roughly one percent of the dimater of the observable universe -- yet even the smallest dots represent not stars or galaxies but great congregations of glaxies. Scattered clumps of dark matter and glalaxies appearl as bright colors in the image, which is based on a supercomputer simulation. Within this sliver lies our supercluster (right), mapped using the actual positions of its celestial elements.
2) OUR SUPERCLUSTER
Some 150 milion light-years across, our supercluster is a great aggregation of clusters of galaxies. The supercluster is centered on the Virgo cluster, which itself contains thousands of galaxies. Among them is M87, which astronomers now know surrounds a gigantic black hole. Virgo's gravity affects the movement of its neighbors, including the Local Group.
Virgo, the Ursa Major cluster, and other clusters float in our supercluster, last outposts before a space traveler would enter a nearly galaxy-free region called a cosmic void. Not that the region within our supercluster is teeming with galaxies: Although the supercluster has a mass equaling some thousand trillion suns, virtually all its volume is empty. Empty, that is, except for a certain density of dark matter, the hidden mass of the universe that helps hold galaxies and clusters together.
Recent observations reveal that early in the universe's history galaxies collided and merged more often than they do now. That indicates that they were more numerous in the past and that many have grown larger over billions of years.
3) THE LOCAL GROUP
Galaxies stretch in every direction beyond the Milky Way, but gravity keeps a family of some 30 galaxies, including our own, loosely bound.
This Local Group of galaxies extends some four million light-years across. Most galaxies in the group are considered dwarfs, but the two largest -- our own Milky Way and the Andromeda galaxy -- are giant spirals. Andromeda is at the center of a small subgroup, which includes two elliptical galaxies, M32 and NGC 205, where star formation has ceased. Even though it is more than two million light-years away from Earth, the Andromeda galaxy can be seen readily with the naked eye. With even a small telescope, its bright bulge is unmistakable.
It is possible that astronomers have not found some of our group's smaller galaxies, which may be hidden behind dust clouds in the Milky Way.
All three main types of galaxies are represented in the Local Group: spirals, ellipticals, and irregulars. Nevertheless, the group has relatively few members, a hundredth of that of some large galaxy clusters.
The galaxies of the Local Group are traveling together through space. Measurements show that the Milky Way is falling toward Andromeda. No collision is expected, however: Like race cars on opposite straightaways, the two are most likely at opposite ends of a highly elongated orbit around the group's center of mass.
4) OUR GALACTIC REALM
Until the early 20th century the best guess of astronomers was that the Milky Way galaxy was the entire universe. We now know that ours is only one of at least a hundred billion galaxies.
The Milky Way is a spiral galaxy, and our solar system is located in what is called spiral's Orion arm, about 25,000 light-years from the center. Our sun orbits the galactic center about once every 225 million years and has made the circuit about 20 times. In the spiral arms, new stars form as clouds of gas and dust condense. The stars' energy ionizes nearby parts of the clouds, causing them to glow.
The central bulge of the galaxy glows with the light of older, redder stars. More ancient stars, orbiting the galaxy in a diffuse halo, formed more than ten billion years ago. Several statellite galaxies cluster around the Milky Way. Two of them, the Large and the Small Magellanic Clouds, are visible on Earth only from the Southern Hemisphere. The closest galaxy is a small spheroid called the Sagittarius Dwarf.
5) OUR SUN'S NEIGHBORHOOD
The stars reaching 20 light-years in all directions form our sun make up the solar neighborhood. Each light-year measures 5.9 trillion miles, yet the neighborhood is a tiny part of the Milky Way. Most of the stars shown here are too dim to be seen with the naked eye, but a few, such as Sirius and Procyon -- greatly exaggerated here and each actually two stars -- are beacons in the sky.
In cosmic terms the sun is an ordinary yellow star, average in size and temperature. The hottest stars glow blue, the coolest shine red.
Many stars are binary or multiple. Our nearest neighbor is the Alpha Centauri triple system, 4.3 light-years away. Closest of the three is Proxima Centauri (Alpha Centauri C), a red dwarf with 1/10 the mass and 1/17,000 the brightness of the sun.
In recent years astronomers have discovered evidence of planets, similar to Jupiter in mass, orbiting stars. The closest example is GI 876.
6) OUR SOLAR SYSTEM
A tiny dot on the scale of the universe, the solar system -- our home in the galaxy -- measures some 4.6 billion miles from the sun to the farthest extent of Pluto's orbit. Sunlight reaches Earth in about 8 minutes and Jupiter in 43 minutes, but it takes nearly 7 hours to pass the orbit of Pluto.
Beyond Neptune are icy bodies smaller than planets. These are Kuiper belt objects, over a hundred of which have been observed by telescope. Some scientists suggest that Pluto is Kuiper belt object, rather than a planet. Spread in a great sphere around the entire solar system is the Oort cloud, trillions of comets, a few of which make spectactular visits to the sun.