GALAXIES
The image below is a photo of the exhibit poster.
The scientist does not study Nature because it is useful;
he studies it because he delights in it, and he delights in
it because it is beautiful. If Nature were not beautiful,
it would not be worth knowing, and if Nature were not
worth knowing, life would not be worth living.
-- Henri Poincare
Galaxies are the stellar "cities of the universe" -- the gathering places and residences of most stars. These collections of stars and clouds of gas and dust are scattered throughout space in a vast array of shapes and sizes; some galaxies have an irregular shape, some form magnificent spirals, and others are so unusual in appearance that they are referrred to as "peculiar." Like snowflakes, no two galaxies are exactly alike. Galaxies are grouped into clusters, and in turn these clusters belong to "superclusters." We live in a galaxy we call the Milky Way, and view the universe by looking beyond the stars within the Milky Way. We view the galaxies not as they are now, but as they were millions or billions of years ago, when the light from these distant giants first began its long journey through space. By harvesting light which has been so long in transit, our telescopes have become time machines, allowing us to glimpse the past of our expanding universe. Galaxies are both the hallmarks and the Rosetta Stones of the cosmos. Their shapes, sizes, compositions, motions, and ages provide clues to the nature, structure, history, and origin of the entire universe. Information about galaxies grants us insight about ourselves and our relationship with all we see around us.
ANATOMY OF A GALAXY
The Milky Way and other spiral galaxies are majestic, rotating bodies, bound together by gravity as they drift through space.
A. From inside, we look toward the center of the Milky Way Galaxy (above right) to see pink patches of bright nebulae, clusters of stars, and dark clouds of fine dust concentrated into lanes, filaments, and patches. Several of the most famous deep-sky objects can be identified in this natural-color photography.
Copyright 1981 Anglo-Australian Telescope Board. Photograph by David F. Malin.
B. Swarming around galaxies are thousands of huge "globular clusters," each cluster consisting of hundreds of thousands of stars bound tightly together by gravity. Globular clusters are among the oldest objects associated with galaxies. The globular cluster M15 (below) is located in the constellation Pegasus.
Copyright 1984 Board of Regents, University of Hawaii
C. Within the youngest areas of galaxies are illuminated regions of gas and dust called bright nebulae. Energy from nearby stars reflects from dust grains and excites gas molecules, resulting in colorful fluorescent displays. These nebulae, like the Monoceros Nebula (below), are the birth places of stars.
Copyright 1981 Anglo-Australian Telescope Board. Photograph by David F. Malin.
D. Time and gravitation transform nebulae into star clusters. Spread along the flattened disk of spiral galaxies are large numbers of relatively young "open clusters" such as NGC 3293 (below), one of the many open clusters within the Milky Way Galaxy.
Copyright 1977 Anglo-Australian Telescope Board. Photograph by David F. Malin.
E. The central regions of M31 (above) and many other galaxies are packed with so many yellow stars that the remainder of the galaxy glows only feebly in comparison. The innermost core is so densely populated with stars that the very center appears as a huge, bright, fuzzy globe. Dark shreds of gas and dust form the innermost traces of the spiral arms winding outward from the center.
Photograph courtesy U.S. Naval Observatory.
Giant, massive stars give up their energy rapidly. Some of them explode violently, producing one of the universe's most spectacular events, a supernova. Chemical elements forged inside the star explode outward to enrich the surrounding space. It was in this fashion that ancient, dying stars bequeathed to our solar system the heavy elements that comprise Earth's atmosphere; oceans, rocks, trees, animals, and human beings. The Crab Nebula (above) in the constellation Taurus is the best known example of a supernova remnant.
Copyright UC Regents, Lick Observatory photograph, UC Santa Cruz
THE WHIRLPOOL GALAXY
VISIBLE LIGHT photography (right) shows two interacting galaxies at a distance of about 20,000,000 light years. The larger spiral is connected to its smaller companion by a bridge of stars, gas, and dust. The images below illustrate how astronomers are able to learn more about galaxies by extending their range of observations throughout the electromagnetic energy spectrum.
Photograph courtesy of U.S. Naval Observatory.
INFRARED is the part of the spectrum just beyond the red light visible to the human eye. This near infrared image (right) of the Whirlpool Galaxy looks quite similar to the visible light image. It shows that both galaxies have large numbers of older red stars, especially in their centers, which strongly emit infrared energy.
Photograph by Debra Elmegreen at Mt. Palomar's 48-inch Schmidt telescope. Processing by Debra and Bruce Elemegreen and Philip Seiden at the IBM J.T. Watson Research Center.
ULTRAVIOLET is beyond the blue end of the visible light spectrum. This ultraviolet image (left) of the Whirlpool shows the bright, hot, recently formed stars. The image is color-coded to reveal brightness differences. The red regions show where the hottest, brightest stars are located. These young stars are powerful emitters of ultraviolet radiation. Yellow, green, and blue typically indicate older, colder, and therefore dimmer (in ultraviolet) stars. The smaller companion galaxy does not appear as bright because it contains few, if any, of the hot young stars.
Photograph courtesy Ralph Bohlin.
RADIO ENERGY imaging (left) shows the detailed structure of the spiral arms of the Whirlpool. Different colors are used to represent different radio intensities with red being brightest, then yellow, green, and blue showing the lowest levels of radio energy. Notice that both galaxies, especially the larger one, have bright radio centers. Astronomers use radio images to study magnetic fields, motions of charged particles, and the presence of various molecules in the gases between stars within galaxies.
Photograph courtesy Kapteyn Laboratorium/Science Photo Library.
TYPES OF GALAXIES
Galaxies are found in a variety of forms. In order to compare them, they are classified into several different types.
SPIRAL galaxies, like the Milky Way, have a central elliptical region composed mostly of old stars. There is a spherical halo of old stars and globular clusters surrounding the centers of these galaxies. The outer spiral arms are composed of younger stars and regions of gas and dust where stars still form. The galaxy shown (above) is M83.
Copyright R.J. Dufour, Rice University
Variations between spiral galaxies are apparent when comparing the beautiful spiral known as M101 (above) with others on this chart. This galaxy is dominated by dark dust lanes near the nucleus and bright starry spiral arms continuing farther out. The spiral pattern is built up of branching arcs and disconnected sections rather than a symmetrical pinwheel such as we find in spirals like the Whirlpool Galaxy.
Copyright California Institute of Technology
PECULIAR galaxies do not seem to fit into any category at all. Many peculiar galaxies appear to be undergoing violent activity. Indeed, some are known as "exploding" galaxies and are powerful sources of energy throughout the electromagnetic spectrum. Some peculiar galaxies change in brightness over intervals ranging from days, weeks, and longer. Centaurus A (above) looks as if it could be a pair of galaxies, an elliptical wrapped within a spiral. This could be an instance of cosmic cannibalism; a giant elliptical galaxy gobbling up a spiral galaxy which made its way into the neighborhood.
Copyright 1980 Anglo-Australia Telescope Board. Photograph by David F. Malin.
ELLIPTICAL galaxies contain almost no young stars or gas and dust and range in size from dwarf galaxies with only a few million stars, up to giants which contain perhaps a trillion stars. Elliptical galaxies are likely the older groupings of stars on the galactic scale. A massive elliptical galaxy known as M87 (lower left) or Virgo A, is the central member of the Virgo cluster of galaxies.
Photograph courtesy National Optical Astronomy Observatories.
BARRED SPIRAL galaxies owe their name to the unusual shapes of their interior regions. The galaxy known as NGC 1300 (above) shows how the spiral arms are attached to an elongation, or "bar" extending form the nucleus. Often the arms are abruptly bent at right angles to the central bar. Compare this galaxy to other spirals shown on this chart.
Copyright 1984 Board of Regents, University of Hawaii
IRREGULAR galaxies have no particular form. Many of these, such as the Large Magellanic Cloud (above) are companions to other galaxies. The Large Magellanic Cloud, so named because it was first scientifically described by a member of Ferdinand Magellan's crew, is one of the two irregular galaxies orbiting the Milky Way Galaxy. This satellite of the Milky Way is located only about 180,000 light years away from our own galaxy.
Photograph courtesy National Optical Astronomy Observatories.
LENTICULAR, lens-shaped galaxies, also known as SO galaxies, are shaped like spirals but lack spiral arms. As NGC 1201 (below) shows, they consist of central bulges surrounded by disks of stars devoid of dust and gas.
Copyright California Institute of Technology
LIGHT TRAVEL TIME
Light travels 300,000 kilometers (186,000 miles) per second, which is the equivalent of more than seven times around the Earth per second. This speed provides a convenient unit for measuring distances. Light travels between the Earth and Moon in just over one second, from the Sun to Earth in about eight minutes, and across the orbit of Pluto in less than 12 hours. It requires 100,000 years for light to travel across the Milky Way Galaxy, so we say that the galaxy is 100,000 light years in diameter. One light year, then, is the distance light travels in one year, about 9,460,000,000,000 kilometers (5,878,000,000,000 miles). The scales accompanying the illustrations below indicate the dimensions astronomers deal with when studying galaxies.
Image captions: 25,000 LY; 100,000 LY; 25,000,000 LY; 250,000,000 LYIllustration by Brad Greenwood
FROM EARTH INTO THE DEPTHS OF THE UNIVERSE
THE SOLAR SYSTEM consists of an average-sized star we call the Sun orbited by a family of a few large planets, several smaller planets, and millions of asteroids and tiny meteoroids. The Solar System is surrounded by a vast cloud of comets, a few of which form lengthy gas and dust tails as they approach the ruling star.
It took thousands of years of human observation to understand that the Solar System is one example of a planetary system, orbiting a star, surrounded by other stars, all part of a vast collection of stars which form the hazy band of light we call the Milky Way. Light can travel across the Solar System in about 12 hours.
THE NEARBY MILKY WAY was first mapped during the twentieth century. At the scale of this view we can begin to see that clouds of stars are grouped into bands or "arms" winding in a spiral pattern around a central region densely packed with yellowish stars. These older central stars contrast with younger blue stars forming the spiral arms. Spiral arms are also outlined by dark nebulae and studded with bright patches of glowing nebulae fluorescing under the brilliant light of newborn stars. The Solar System is lost in this view, which encompasses an area some 65,000 light years across. A beam of light completing this distance would have started its journey when mammoths and saber-toothed cats roamed the Earth.
THE MILKY WAY GALAXY was found to resemble other spiral galaxies seen in the universe. Details of the spiral structure of our galaxy emerged with the advent of radio astronomy. Observing with radio waves has allowed us to see through dust, gas, and star clouds to map the very core of the galaxy and its surrounding spiral arms. The two irregular satellite galaxies are called the Magellanic Clouds. We see our home galaxy as it would appear 400,000 light years away. The area shown is about 200,000 light years across.
NEARBY GALAXIES were found to be arranged in groups and clusters. Just as the Sun was found to be only one of many stars, the Milky Way Galaxy was found to be a single member of "the Local Group" of galaxies. This grouping in turn is an outrider of a massive swarm of galaxies known as the Virgo Cluster, so named because most of the cluster lies in the direction of the constellation Virgo as seen from Earth. The center of this cluster is dominated by the huge elliptical galaxy called Virgo A, or M87.
Looking across a region of space 75 million light years wide, the vast emptiness of the cosmos is interrupted by only a modest sprinkling of visible matter. Dinosaurs ruled the Earth when light now reaching our world began traveling that distance from the galaxies.
Illustration based on a map by Laurent Nottale.
SUPERCLUSTERS OF GALAXIES are grouped into enormous strands or filaments with immense empty areas between them. The shapes formed by these largest known physical structures have been compared to the outlines of closely packed soap bubbles. This large scale distribution of matter might contain profound secrets revealing the behavior of matter in the very early universe. We view a portion of the universe about 500 million light years across. Light now reaching the Earth from such a distance stated on its way when primitive jellyfish, worms and crustaceans inhabited the seas of Earth.
CREDITS: Funded in part by a grant from the National Science Foundation. Based on Galaxies, a panetarium program written and narrated by Timothy Ferris, produced by Hansen Planetarium. Deisnged by Brad Greenwood. Production by Kerry Meyer. Text by Von Del chamberlain and Don Davis. Illustrations by Don Davis (except where noted). Published by Hansen Planetarium, Salt Lake City, Utah, U.S.A. Copyright 1988 Hansen Planetarium.