@START@Cosmic Yardsticks Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. October 1994 ______________________________________________________________________________ Cosmic Yardsticks Astronomers gauge the dimensions of space by using "distance indicators" -- celestial objects with unique properties that allow for their distances to be deduced. Reliable distance measurements are a crucial factor in determining a precise value for the universe's expansion rate (called the Hubble Constant) which is needed to estimate the size and age of the universe. (To calculate the Hubble Constant, astronomers also need to know how fast a galaxy is moving away from us, measured by spectral redshift.) Measuring the distance to a faraway galaxy involves a complicated set of closely-linked steps. First, distance indicators within our galaxy are used as a stepping stone to calibrate other distance indicators in nearby galaxies, which in turn creates yet another stepping stone to calibrate distances to even more faraway galaxies. The first rung in the "distance scale ladder" can be found in our Milky Way neighborhood, in nearby open star clusters such as Hyades and the Ursa Major cluster. An open cluster is a collection of young stars with a common motion in space. Because the Hyades and the Ursa Major cluster are close to us, their distances can be derived using radial velocity (motion toward or away from us) and proper motion measurements of member stars. This allows astronomers to obtain the intrinsic brightness, or luminosity, of different types of stars in these open clusters. Astronomers then measure the brightness of stars with similar properties in more distant open clusters. By assuming that these stars would have the same intrinsic brightness as their nearby counterparts, a distance to the remote open clusters is calculated by comparing the apparent and intrinsic brightness of their member stars. To obtain distances to nearby galaxies, astronomers use "primary distance indicators." These are objects that can be observed within our galaxy or have characteristics that can be theoretically modeled. Examples include Cepheid variable stars, novae, supernovae, and RR Lyrae stars. Two well-defined primary distance indicators, or "standard candles," are the Cepheids and fainter RR Lyrae stars. They have a regular variation in brightness, and the period of this pulsation is closely linked to the star's intrinsic brightness. So, if the pulsation period of a star is known, its true brightness can be deduced. The distance to the star can then be calculated by comparing its true brightness with its apparent brightness. Cepheid variable stars are often used as distance calibrators for nearby galaxies. They are very luminous yellow giant or supergiant stars, regularly varying in brightness with periods ranging from 1 to 70 days. This type of star is in a late evolutionary stage, pulsating due to an imbalance between its inward gravitational pull and outward pressure. Cepheids are found in remote open clusters whose distances are known from comparison with nearby open clusters. It is, therefore, possible to calibrate these Cepheids with an independently obtained ruler ot yardstick. In the past, the best ground-based observations have detected Cepheids in nearby galaxies within 12 million light-years. However, all galaxies in this region have motions due to gravitational attraction of neighboring galaxies. In order to study the overall expansion of the universe, it is necessary to reach out to Cepheids in galaxies at least 30 million light-years away. Until the recent Hubble Space Telescope observations of Cepheids in M100, there were no well-calibrated standard candles observable over this distance. Therefore, astronomers have been using other kinds of objects, called "secondary distance indicators," to probe even deeper into the universe. Secondary distance indicators, such as planetary nebulae, supernovae, and the brightest stars are used in galaxies that are so remote that only prominent objects can be discerned. (These secondary indicators are calibrated in nearer galaxies, where distances are known from resident primary distance indicators, before being applied to more remote galaxies.) The galaxies themselves can also be used as secondary distance indicators. One widely-used strategy, the Tully-Fisher method, uses a correlation between the internal motions within galaxies (from radio observations of cold interstellar gas) with their luminosities. Another method, the Faber-Jackson relation, looks at the random motions of stars in a galaxy obtained from spectroscopic measurements. These relationships are based on the fact that a more massive galaxy would be more luminous, and would rotate faster than a less massive galaxy. @START@Dark Matter Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. November 1994 ___________________________________________________________________________ DARK MATTER All visible celestial objects known today account for only 10% of the mass in the universe. The rest of this "missing mass," also known as "dark matter," is presumably invisible because it does not emit or reflect visible light or other forms of electromagnetic radiation. Or perhaps its light is so feeble that current astronomical instruments are unable to detect it. However, dark matter can be indirectly detected due to its gravitational influence on other nearby visible objects. The presence of dark matter was first discovered in 1932 by astronomer Jan Oort, who measured the perpendicular motions of nearby stars relative to the disk of our Milky Way. He studied the gravitational influence of the galactic disk on these stars, and so, was able to measure the mass of the disk (just as the mass of Earth can be calculated from the acceleration of a falling object). To his surprise, this calculated mass was twice the amount of mass seen as stars and nebulae. A year later, Fritz Zwicky examined the dynamics of clusters of galaxies, and also came to the startling conclusion that the observed galaxies only accounted for 10% of the mass needed to gravitationally bind the galaxies in the cluster. One widely-used method to deduce the amount of missing mass involves measuring the rotation speed of a spiral galaxy. Spectroscopic and radio observations have obtained the rotation velocities of hundreds of spiral galaxies. These experiments have revealed that, in most cases, a galaxy's mass continues to increase toward the edge of its visible disk of stars. This implies that spiral galaxies are surrounded in haloes of matter that cannot be seen. Observations of elliptical galaxies, groups, and clusters of galaxies also indicate the presence of dark matter interacting gravitationally with the visible objects. The nature of dark matter, and its abundance, are among the most important questions in modern cosmology today. What is it made of? Some astronomers believe that dark matter is composed of protons and neutrons, called baryonic or simply "normal" matter. Baryonic dark matter candidates include extra-solar planets, remnants of stellar evolution such as comets, objects not massive enough to ignite hydrogen fusion called brown dwarfs, dying embers of stars such as cold white dwarfs and neutron stars, as well as interstellar and intergalactic gases. Non-baryonic dark matter, on the other hand, could be elementary particles that do not interact strongly with normal matter. Except for the neutrino particle, many such elementary particles are still in the realm of theory and have not been detected. Since all visible matter is only a small fraction of the total mass in the universe, the amount of dark mass that is present will determine the evolutionary future of the universe. If there is not enough dark matter to gravitationally bind the universe together, it could continue expanding forever. If there is enough mass in the universe to gravitationally hold it together, the universe may slow down its expansion, come to a halt, and begin to contract and eventually collapse. The temperature of dark matter in the early universe also may have determined the early evolution of the universe. Not long after the Big Bang and prior to the formation of galaxies, matter began to aggregate under the influence of gravity. Dark matter might have provided the "seeds," a lumpy background in which ordinary matter could congregate to form galaxies and stars. If this "cold dark matter" were present, where particles had a negligible random motion, galaxy formation would begin on small scales. Matter would gather in sizes comparable to current galaxies or smaller, and eventually build to become clusters and superclusters due to the gravitational attraction of the galaxies. If, however, "warm dark matter" was present, it would erase the small galaxy-sized "seeds" that initially formed. Instead, enormous gaseous pancake-like structures as large as superclusters and clusters, are created, subsequently condensing into individual galaxies. @START@Stellar Disks and Jets SCIENCE BACKGROUND ___________________________________________________________________________ STELLAR DISKS AND JETS Stellar jets are analogous to giant lawn sprinklers. Whether a sprinkler whirls, pulses or oscillates, it offers insights into how its tiny mechanism works. Likewise stellar jets, billions or trillions of miles long offer some clues to what's happening close into the star at scales of only millions of miles, which are below even Hubble's ability to resolve detail. Hubble's new findings address a number of outstanding questions: Where Are Jets Made? Hubble shows that a jet comes from close into a star rather than the surrounding disk of material. Material either at or near the star is heated and blasted into space, where it travels for billions of miles before colliding with interstellar material. Why Are Jets So Narrow? The Hubble pictures increase the mystery as to how jets are confined into a thin beam. The pictures tend to rule out the earlier notion that a disk was needed to form a nozzle for collimating the jets, much like a garden hose nozzle squeezes water to a narrow stream. One theoretical possibility is that magnetic fields in the disk might focus the gas into narrow beams, but there is as yet no direct observational evidence that magnetic fields are important. What Causes a Jet's Beaded Structure? Hubble is solving the puzzle of a unique beaded structure in the jets, first detected from the ground but never fully understood. "Before the Hubble observations the emission knots were a mystery," said Jeff Hester. "Many astronomers thought that the knots were the result of interactions of the jet with the gas that the jet is passing through, while others thought that the knots were due to 'sputtering' of the central engine. We now know that the knots are the result of sputtering." Hester bases this conclusion on Hubble images which show the beads are real clumps of gas plowing through space like a string of motor boats. Competing theories, now disproved by Hubble, suggested a hydrodynamic effect such as shock-diamond patterns seen in the exhaust of a jet fighter. What Do Jets Tell Us about Star Birth? "The jet's clumpy structure is like a stockbroker's ticker tape; they represent a recorded history of events that occurred close to the star," said Jon Morse. "The spacing of the clumps in the jet reveals that variations are occurring on several time scales close to the star where the jet originates. Like a "put-put" motor, variations every 20 to 30 years create the strings of blobs we see," Morse concluded. "However, every few hundred years or so, a large amplitude variation generates a 'whopper' of a knot, which evolves into one of the major bow-shaped shock waves." Other Hubble views by Chris Burrows reveal new blobs may be ejected every few months. "If the circumstellar disk drives the jet then the clumpiness of the jet provides an indirect measure of irregularities in the disk." Why Are Jets "Kinky"? The Hubble pictures also show clear evidence that jets have unusual kinks along their path of motion. This might be evidence for a stellar companion or planetary system that pulls on the central star, causing it to wobble, which in turn causes the jet to change directions, like shaking a garden hose. The jet blast clears out material around the star, and perhaps determines how much gas finally collapses onto the star. Star Formation A star forms through the gravitational collapse of a vast cloud of interstellar hydrogen. According to theory, and confirmed by previous Hubble pictures, a dusty disk forms around the newborn star. As material falls onto the star, some of it can be heated and ejected along the star's spin axis as opposing jets. These jets of hot gas blaze for a relatively short period of the star's life, less than 100,000 years. However, that brief activity can predestine the star's evolution, since the final mass of a star determines its longevity, temperature, and ultimate fate. The jet might carry away a significant fraction of the material falling in toward the star, and, like a hose's water stream plowing into sand, sweeps out a cavity around the star that prevents additional gas from falling onto the circumstellar disk. Historical Background In the early 1950's, American astronomer George Herbig and Mexican astronomer Guillermo Haro independently catalogued several enigmatic "clots" of nebulosity near stars near the Orion nebula that have since been called Herbig-Haro objects. It is only in the last 20 years, however, that the true nature of these objects, and their role in the star formation process, has been revealed. Careful study showed that many of the Herbig-Haro objects represent portions of high-speed jets streaming away from nascent stars. Now there are nearly 300 Herbig-Haro objects identified by astronomers around the world, and the list is growing as new technologies and techniques are developed to probe the dusty depths of nearby stellar nurseries. @START@Galaxy Formation Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. December 1994 GALAXY FORMATION Although astronomers have uncovered many of the details revealing the life cycles of individual stars, they still do not completely understand how galaxies, like our Milky Way, begin and end their lives. The problem is that, although stars within the Milky Way may be seen in a variety of evolutionary stages, few examples of young galaxies are known, and their images come to us from the most distant edge of our visible universe. At these vast, multi-billion-light-year distances, it becomes increasingly difficult to determine what role environment plays in the formation of a galaxy. Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas -- the primordial elements forged in the Big Bang. They also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies. Matter either was clumped into vast collections more than a million times the mass of the Milky Way, or into small clumps one million times smaller than the mass of our Milky Way. Superclusters of galaxies may have evolved from the former. Globular clusters -- spherical collections of very old, densely packed stars usually found in orbit around galaxies, like the Milky Way -- may have evolved from the latter. Could these globular clusters be the meager leftovers of an ancient, once-common population of small clumps as predicted by theory? This possibility now seems increasingly more likely. So the question then arises: what formed the vast majority of the galaxies? Detailed ground-based and space-based images of distant galaxies are beginning to turn up some interesting insights into galaxy formation. First, as we look deeper into the universe, galaxies appear to emit more of their light in the blue part of the visible spectrum. From studies of nearby galaxies, blue light is a sign that very young, massive and luminous stars are forming. Since we see these galaxies as they were between 5 and 10 billion years ago, we appear to be witnessing events that occurred within a few billion years after these galaxies were formed. Astronomers also have noticed that as they examine the images of these distant blue galaxies, the images are frequently distorted or contain what appear to be multiple nuclei. The Milky Way seen at a similar great distance would look like a uniformed flattened disk, with a single bright nucleus -- the galactic center. Nearby "multiple-nuclei" galaxies that have been studied show the cores of individual galaxies colliding and merging into one single system of stars and gas. These collisions are violent, and take millions of years to play out. But in at least some instances, such as NGC 1275, recently observed with the Hubble Space Telescope, galaxy collisions can actually trigger the formation of massive stars. In the depths of space, we may be witnessing collisions between smaller galaxies triggering the formation of massive luminous stars. The images, rich in blue light, gives tantalizing evidence that "environment" may have been more important than cosmic "genetics." Galactic cannibalism was far more common in the ancient past. Galaxies may have grown to their current size by consuming their neighbors. The ultimate building blocks may indeed have been the paltry million-solar-mass clumps that theoreticians believe were abundant before the universe was a few million years old. @START@Grown-Up Galaxies in an Infant Universe Hubble Uncovers New Clues to Galaxy Formation National Aeronautics and Space Administration NASA Headquarters Washington, D.C. December 1994 SCIENCE BACKGROUND The Paradox: Grown-Up Galaxies in an Infant Universe Hubble Space Telescope's recent observations identify fully formed elliptical galaxies in a pair of primordial galaxy clusters that have been surveyed by teams lead by Mark Dickinson of the Space Telescope Science Institute and Duccio Macchetto of the European Space Agency and the Space Telescope Science Institute. Although the clusters were first thought to be extremely distant because of independent ground- based observations, the Hubble images provide sharp enough details to confirm what was only suspected previously. The surprise is that elliptical galaxies appeared remarkably "normal" when the universe was a fraction of its current age, meaning that they must have formed a short time after the Big Bang. Dickinson, in studying a cluster that existed when the universe was nearly one-third its current age, finds that its red galaxies resemble ordinary elliptical galaxies, the red color coming from a population of older stars. This has immediate cosmological implications, since the universe must have been old enough to accommodate them. Cosmologies with high values for the rate of expansion of space (called the Hubble Constant, which is needed for calculating the age of the universe) leave little time for these galaxies to form and evolve to the maturity we're seeing in the Hubble image," Dickinson emphasizes. Macchetto's observation of a galaxy that existed 12 billion years ago, or nearly one-tenth the universe's present age, also finds a light distribution remarkably similar to today's elliptical galaxies. "This seems to show that elliptical galaxies reach their 'mature' shape very quickly, during a robust burst of star formation, and then evolve passively," says Mauro Giavalisco of the Space Telescope Science Institute. "Astronomers suspected that this was the case for at least some ellipticals. Now, Hubble has found direct evidence for it." To produce such a shape in a galaxy requires one billion years for the gas to settle into the center of the galaxy's gravitational field. Therefore, these galaxies, which we observe as they were less than two billion years after the Big Bang, were beginning to form less than one billion years after the Big Bang! says Macchetto. "Elliptical galaxies are exceptional laboratories for studying stellar dynamics and evolution," adds Giavalisco, "and the explanation of their origin is still controversial. This new observational evidence is suggesting that at least some ellipticals formed via processes such as 'violent relaxation', where a large grouping of stars will rapidly contract into a dense cluster. Well known from a theoretical point of view, these mechanisms of galaxy formation appear to have been confirmed by the images taken with the Hubble." A Cosmic Zoo of Bizarre Galaxies Contrary to the gravitationally "relaxed" and normal looking primordial elliptical galaxies, the same set of Hubble images tells a remarkable story of the creation -- and destruction -- of spiral galaxies in large clusters. In one of the longest exposures taken to date with Hubble, representing 18 hours of continuous observing, Dickinson has uncovered a "celestial zoo" of faint, compact objects that might be the primordial building blocks from which spiral galaxies such as our Milky Way formed. These irregular bluish fragments, dating back nine billion years, may ultimately have coalesced into spiral galaxies, he reports. "We see a bewildering range of galaxy shapes. The Hubble image is like looking at a drop of pond water under a microscope, where we see a menagerie of strange creatures." Though Dickinson does not have a direct measurement of distance, he suspects these objects are also remote cluster members since they group closely around a distant radio galaxy (a class of energetic galaxy with a precisely measured distance) and do not resemble anything seen in the present universe. Very few of the bluish objects are recognizable as normal spirals, although some elongated members might be edge-on disks, Dickinson concludes. Among this zoo are "tadpole-like'' objects, disturbed and apparently merging systems dubbed "train-wrecks", a multitude of tiny shards and fragments, faint dwarf galaxies or possibly an unknown population of objects. However, Dickinson cautions that the bright blue light of star formation can dramatically affect apparent galaxy shapes at great distances (where ultraviolet light is redshifted to visible wavelengths due to the uniform expansion of space). "Nevertheless, it is difficult to escape the impression that evolutionary processes are shaping or disrupting disk galaxies." The Violent History of Spiral Galaxies While Dickinson sees the birth of spiral galaxies, Alan Dressler's Hubble images of several rich clusters chronicle the demise of spirals inhabiting large clusters. "It seems that almost as soon as nature builds spiral galaxies in clusters, it begins tearing them apart," he says. "The cause of this disappearance of spirals from clusters, from four billion years ago to the present, is unsettled and vigorously debated. Just the fact that the form of entire galaxies could be altered in so short a time is important in our attempts to find out how galaxies formed in the first place," Dressler concludes. The evidence provided by Hubble shows that this large-scale galactic "demolition derby" could explain why there were so many more spiral galaxies in rich clusters long ago than there are today. Apparently, many spiral galaxies have since been destroyed or disappeared. Hubble observations also reveal many unusual objects within the clusters that can be considered fragments of galaxies. "When we look back in time to these clusters, we see many distorted galaxies -- they appear to have been disturbed or disrupted in one way or another," says Dressler. "There are so many little shreds of galaxies -- it almost looks like galactic debris -- flying around in these clusters. Perhaps this is a result of tidal encounters, but at this point we really don't understand what's happening. However, the Hubble pictures make it pretty clear that it had taken a long time for these star systems to organize and that in their younger forms they were still easily perturbed." Hubble shows that spiral galaxies could not easily survive in the dynamic environment of a dense galaxy cluster. Detailed Hubble images show that these "fragile" disk galaxies were prone to being warped from their pancake shape. Analysis of the pictures has inspired several alternate mechanisms for explaining the galaxy distortion. One possibility is that the galaxies were disrupted by mergers and tidal interaction caused by close encounters between galaxies in the dense cluster. Also, there is evidence from nearby clusters of galaxies that the hot, high pressure gas residing in a cluster can work to remove the gas in the disks of individual spiral galaxies. Finally, disk galaxies might have been stripped of their mantles of "dark matter" (unknown material that is probably not made up of stars but accounts for a significant fraction of a Galaxy's mass) as they plunge through the cluster. Dressler points out that computer models of galaxies show that a spherical halo of material is important to stabilizing a thin disk, so loss of this material could result in the disk warping or fracturing, diminishing the galaxy's chance of survival as a spiral. Thankfully, galaxy "bumper cars" took place only in large clusters, containing hundreds or even thousands of galaxies. Our Milky Way, one of the largest members of a Local Group of nearly two dozen galaxies, presumably evolved in a far less crowded region of the universe. Finding Primeval Galaxy Clusters "We have very likely identified the long-sought population of primeval galaxies," Macchetto reports. Until the Hubble results, astronomers had searched unsuccessfully for several decades for truly primeval galaxies, which are hard to find when they are in their very early phase of existence. "If you can find the primeval galaxies at the cosmic epoch when they started to form and understand their shape, mass, color and brightness, then chances are that you will develop a better understanding of cosmology," comments Giavalisco. Macchetto and his team used quasars (bright cores of distant active galaxies) as beacons to look for the "shadowing effect" of galaxies between Earth and the quasar. Their search strategy is based on the theory that the first galaxies to appear in the universe were highly clumped in space. Therefore, if a quasar's light is modified by an intervening galaxy, it more than likely belongs to a primeval cluster. "All you have to do is to look around the quasar using a specially developed optical filter, fine-tuned to observe galaxies at the distance suggested by the change in the quasar's light," Macchetto says. Using this novel technique with ground-based telescopes, the team looked at the field around quasar Q0000-263 in the constellation Sculptor and found the farthest "normal" galaxy ever observed, at a distance of 12 billion years. This observation led Macchetto and Giavalisco to identify a whole cluster of primeval galaxies in that region of the sky. Remarkably, the Hubble has shown that the cluster members are characterized by a compact shape, supporting the idea that they all underwent a similar mechanism of formation. "The very presence of the cluster shows that these large structures already existed two billion years after the Big Bang. This is unexpected and counter to many theories of cluster and galaxy formation," says Macchetto. "Although nothing conclusive can be stated with only one cluster, now that we know how to search for them we will be able to strongly constrain these theories." Dickinson selected a candidate cluster for Hubble's sharp vision as a result of a ground-based infrared survey of the environments of distant radio galaxies. Based on the color and the statistical distribution of the galaxies, Dickinson concluded that a cluster is at the same distance as the radio galaxy 3C 324, located nine billion light-years away in the constellation Serpens. The cluster appeared to have a population of very red galaxies similar in color to present-day elliptical galaxies. Hubble's 18-hour long exposure reveals thousands of faint galaxies near the limit of what Hubble can detect (29th magnitude). "Though many are presumably closer or farther than the cluster, since Hubble is peering across a tremendous volume of the universe to reach 3C 324, the galaxies concentrated around 3C 324 are most likely cluster members, he reports. The Birth of Galaxies Island cities of hundreds of billions of stars each, galaxies allow astronomers to trace the evolution of matter and structure since the beginning of the universe in the Big Bang. Scientists have sought to understand this evolution ever since American astronomer Edwin Hubble sorted nearby galaxies into three fundamental shapes: spiral or disk-shaped, elliptical, and irregular. As the Big Bang theory gained acceptance in the 1950s, astronomers realized that galaxies simply weren't made the way they appear today but must evolve over time. This notion was reinforced by two dramatic discoveries in the 1960s: the confirmation of the Big Bang by detection of the cosmic microwave background and the discovery of quasars. Quasars are theorized to be the active cores of extremely distant galaxies. Their abundance at great distances clearly shows that galaxies were at a different evolutionary stage billions of years ago. However, the fainter "normal" population of early galaxies has been elusive, because the tiny images of distant galaxies smear into faint blurs when viewed through Earth's atmosphere. In the late 1970s, astronomers found the first evidence that the stellar populations of galaxies had changed dramatically, even over a relatively small fraction of the time back toward the Big Bang. Astronomers also were puzzled by a specie of blue galaxies in distant clusters, which disappeared in our current epoch. Now, Hubble Space Telescope's sharp view at last provides for detailed studies of the properties of early galaxies. Hubble's initial results show that the mysterious blue cluster galaxies are mostly spirals, often with signs of disturbance that may provide clues about their disappearance by the present epoch. Paradoxically, elliptical galaxies appear normal throughout most of the history of the universe, with little evidence for dramatic changes in their stellar population or shape. @START@Galaxy Shapes Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. December 1994 GALAXY SHAPES Galaxies come in three major classes distinguished by their appearance: spirals, like the Milky Way, are shaped like pinwheels; irregulars have no discernible shape at all; and ellipticals are round- or oval-shaped objects. Spirals and irregulars are typically sites of ongoing star-formation and therefore contain young stars. Ellipticals, having finished their supply of fresh gas, cannot form stars any more and contain mostly very old stars. Spiral galaxies are a composite of stars and gas in a disk surrounding a central bulge, which is rather similar to an elliptical galaxy, just smaller. Waves in the disk form the spiral arms and cause the gas to collapse and form new stars. Therefore, the disk is rich in young stars. Older stars are typically found in the bulge. Elliptical galaxies and the bulges of spirals have been the subject of several decades of observational and theoretical work. For decades, astronomers thought that the rotation rate of these spherical star systems determined whether they would be round or oval shaped, with the more rapidly rotating ellipticals being the flattest. Detailed studies of thousands of ellipticals over the years now suggest an entirely different picture. Ellipticals and bulges are supported against their self-gravity, which would cause them to shrink, by the random velocities of the stars, pretty much like the motion of molecules in a hot gas. The distribution of stellar motion determines the final shape of the galaxy, that is whether it is spherical, oblate, or very flattened. In recent years, astronomers also have discovered that apparently simple galaxy shapes hide the complex, violent events that occurred in these galaxies long ago. Some contain dense cores in which millions of stars move in orbits completely different than stars farther out from the galaxy's center. In many ways, the cores of some resemble isolated populations transplanted from outside the galaxy. Astronomers are beginning to believe that these cores are the remains of companion galaxies that were consumed when they wandered too close to these elliptical galaxies many millions of years ago. When galaxies collide, the rapidly changing gravitational fields also can synchronize the stellar orbits, creating great rings of stars which surround some ellipticals like haloes. Elliptical galaxies also contain some of the oldest stars in the universe. While spirals and irregulars continue to produce new stars even to the present day, most ellipticals stopped forming stars more than 10 billion years ago in what must have been one great star-forming epoch. Ellipticals contain little or no gas and dust of their own, apparently having consumed what they had when their stars were born long ago. Those ellipticals that contain higher concentrations of gas and dust apparently accumulated the material because they cannibalized their companion galaxies. The material accumulated from these cannibalizations collides as it sinks farther and farther into the galaxy's core, and in many instances, creates new generations of massive, luminous stars. Eventually over the course of millions of years, the gas reaches the center of the galaxy where supermassive black holes may lie in wait for a new supply of fuel. @START@Globular Star Clusters Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. November 1994 ___________________________________________________________________________ GLOBULAR STAR CLUSTERS Globular star clusters are among the oldest objects in our galaxy. Their beauty is easily discerned through amateur telescopes that resolve tightly-packed swarms of glistening stars, suspended in the night sky like Christmas ornaments. More than 150 globular star clusters are known to be associated with the Milky Way Galaxy. Each cluster contains hundreds of thousands to a million stars within a volume of 10 to 30 light-years across. In 1918, Harlow Shapley recognized the existence and structure of globular clusters. By studying the clusters' distribution in the sky and measuring their distances, he was able to deduce the location of the center of the Milky Way Galaxy and the Sun's distance from it. In the 1930s, Edwin P. Hubble discovered globular clusters in the neighboring Andromeda Galaxy, and since then globular star clusters have been found surrounding many other galaxies. Globular clusters reside within a spherical volume of space called the "galactic halo," which surrounds the disk of our galaxy. The clusters orbit around the galactic center, taking millions of year to complete their highly elongated, randomly oriented orbits. Most globular clusters wander as far as 90 to 120 thousand light-years from the galactic center, and some extend as far as 300 thousand light-years out. The motions of these distant objects, influenced by the gravitational pull of the entire galaxy, allows astronomers to calculate the amount of mass in the galaxy. Some recent estimates reveal that the galaxy is 500 billion times the mass of the Sun. This estimate is significantly higher than the mass contributed by visible stars and nubulae alone, indicating that there is a great amount of unseen dark matter in the galaxy. When compared to the Sun and other stars of the galactic disk, globular cluster stars appear to be deficient in heavy elements. This indicates that they are ancient objects, made from the pristine gas that condensed to form the galaxy long ago. However, about 20% of globular clusters are slightly richer in heavy elements compared to their counterparts, and are, therefore, presumably younger. Although chemical composition differs from one cluster to the next, all member stars within a given cluster have a similar composition, indicating that they were born from the same cloud. This provides a unique opportunity for the study of stellar evolution. Yet each star began life with a different mass. By observing the luminosity and temperatures of their current states, astronomers are learning a great deal about the life cycles of stars. Globular clusters contain mostly low-mass stars that are so tightly packed together that the density of stars near the center is about 2 stars per cubic light-year. In comparison, our solar neighborhood has about one star per 300 cubic light-years. If you were looking into the sky from a hypothetical planet in the middle of a globular cluster, like 47 Tucanae, you would be surrounded in a perpetual twilight cast by the light of thousands of nearby stars. @START@The Hubble Constant Space Science Short National Aeronautics and Space Administration NASA Headquarters Washington, D.C. October 1994 ______________________________________________________________________________ The Hubble Constant The Hubble Constant (Ho) is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. This long-sought number indicates the rate at which the universe is expanding, from the primordial "Big Bang." The Hubble Constant can be used to determine the intrinsic brightness and masses of stars in nearby galaxies, examine those same properties in more distant galaxies and galaxy clusters, deduce the amount of dark matter present in the universe, obtain the scale size of faraway galaxy clusters, and serve as a test for theoretical cosmological models. In 1929, American astronomer Edwin Hubble announced his discovery that galaxies, from all directions, appeared to be moving away from us. This phenomenon was observed as a displacement of known spectral lines towards the red-end of a galaxy's spectrum (when compared to the same spectral lines from a source on Earth). This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble Constant can be stated as a simple mathematical expression, Ho = v/d, where v is the galaxy's radial outward velocity (in other words, motion along our line-of-sight), d is the galaxy's distance from earth, and Ho is the current value of the Hubble Constant. However, obtaining a true value for Ho is very complicated. Astronomers need two measurements. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity. The second measurement, the most difficult value to determine, is the galaxy's precise distance from earth. Reliable "distance indicators," such as variable stars and supernovae, must be found in galaxies. The value of Ho itself must be cautiously derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small. The units of the Hubble Constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. (A megaparsec is 3.26 million light-years.) For example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s. The value of the Hubble Constant initially obtained by Edwin Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances. For the past three decades, there have been two major lines of investigation into the Hubble Constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for Ho around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate Ho to be around 100 km/s/Mpc. A long-term, key program for HST is to refine the value of the Hubble Constant. @START@The Search for the Kuiper Belt SCIENCE BACKGROUND THE SEARCH FOR THE KUIPER BELT In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is. A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond Neptune. In fact, he argued, it would be unusual not to find such a continuum of particles since this would imply the primordial solar system has a discrete "edge." This notion was reinforced by the realization that there is a separate population of comets, called the Jupiter family, that behave strikingly different than those coming from the far reaches of the Oort cloud. Besides orbiting the Sun in less than 20 years (as opposed to 200 million years for an Oort member), the comets are unique because their orbits lie near the plane of the Earth's orbit around the Sun. In addition, all these comets go around the Sun in the same direction as the planets. Kuiper's hypothesis was reinforced in the early 1980s when computer simulations of the solar system's formation predicted that a disk of debris should naturally form around the edge of the solar system. According to this scenario, planets would have agglomerated quickly in the inner region of the Sun's primordial circumstellar disk, and gravitationally swept up residual debris. However, beyond Neptune, the last of the gas giants, there should be a debris-field of icy objects that never coalesced to form planets. The Kuiper belt remained theory until the 1992 detection of a 150-mile wide body, called 1992QB1 at the distance of the suspected belt. Several similar-sized objects were discovered quickly confirming the Kuiper belt was real. The planet Pluto, discovered in 1930, is considered the largest member of this Kuiper belt region. Also, Neptune's satellites, Triton and Nereid, and Saturn's satellite, Phoebe are in unusual orbits and may be captured Kuiper belt objects. Observational Techniques To isolate and subtract the effects of cosmic ray strikes on the WFPC 2's electronic detectors, which could mimic the faint signature of a comet, thirty-four images were taken of the same piece of sky. The cosmic ray hits change from picture to picture, but real objects remain constant. However, pinpointing comets was even trickier because they drift slowly along their orbit about the Sun. Although the orbital periods of these objects are 200 years or longer, the HST has sufficient spatial resolution to see them move in just a few minutes. This means the comets change position from picture to picture, just as cosmic ray strikes would. However, cosmic ray strikes are randomly placed events while the motions of the comets are well defined. To distinguish between the comets and cosmic ray effects, the 34 images were then digitally shifted and stacked to the predicted offset to account for the expected drift rate of comets. It's like having a fixed camera on a tripod take a rapid series of snapshots of someone walking in front of the lens. The resulting snapshots could be stacked so that the person appeared stationary. The researchers tested the reliability of this approach by shifting the stacked pictures in the opposite direction of the expected comets' motion. Ideally, no comets should have appeared, but random alignments added up to 24 anomalous detections. When the team stack-shifted the pictures in the direction of the predicted comet motion, they came up with 53 objects. Assuming that 24 of these are, statistically, anomalous too, leaves a remainder of 29 objects considered "real." The shift-stack technique was further tested by dividing the images into two groups and running an automated search algorithm to look for objects that showed up in the same position on sets of exposures. @START@Mars: A Cooler, Cleaner World SCIENCE BACKGROUNDER HUBBLE MONITORS WEATHER ON NEIGHBORING PLANETS MARS: A COOLER, CLEARER WORLD Four years, (or two Mars years') worth of Hubble observations show that the Red Planet's climate has changed since the mid-1970's. "The Hubble results show us that the Viking years are not the rule, and perhaps not typical. Our early assumptions about the Martian climate were wrong," said Philip James of the University of Toledo. "There has been a global drop in temperature. The planet is cooler and the atmosphere clearer than seen before," said Steven Lee of the University of Colorado in Boulder. "This shows the need for continuous monitoring of Mars. Space probes provided a close-up look, but it's difficult to extrapolate to long-term conditions based upon these brief encounters." The researchers attribute the cooling of the Martian atmosphere to diminished dust storm activity, which was rampant when a pair of NASA Viking orbiter and lander spacecraft arrived at Mars in 1976. Two major dust storms occurred during the first year of the Viking visits, which left fine dust particles suspended in the Martian atmosphere for longer than normal. Warmed by the Sun, these dust particles (some only a micron in diameter, about the size of smoke particles) are the primary source of heat in the Martian atmosphere. "Hubble is showing that our early understanding based on these visits is wrong. We just happened to visit Mars when it was dusty, and now the dust has settled out," Lee said. "We are going to have to look at Mars for many years to truly understand the workings of the climate," said Todd Clancy, of the Space Science Institute, Boulder, Colorado. Knowledge about the Martian climate has been limited by the fact that ground-based telescopes can only see weather details when Earth and Mars are closest -- an event called opposition -- that happens only once every two years. Though Hubble has observed Mars only for four years, the observations are equivalent to 15 years of ground- based observing because Hubble can follow seasonal changes through most of Mars' orbit. Though the Mariner and Viking series of flyby, orbiter and lander spacecraft that visited Mars in the late 60's and 70's provided a close-up look at Martian weather, these were snapshots of the planet's complex climate. Hubble provides the advantage of a global view - much like the satellites that monitor Earth's weather, and can follow martian seasonal changes over many years. When Mars is closest to Earth, Hubble returns near-weather satellite resolution. MARS -- NO LACK OF OZONE Although there has been concern about a lack of ozone (a form of molecular oxygen created by the effects of sunlight on an atmosphere), dubbed the "ozone hole" over Earth's poles, there are no ozone holes on Mars. By contrast, the planet has a surplus of ozone over its northern polar cap, as first identified by the Mariner 9 spacecraft in 1971. (However the Martian atmosphere is different enough from Earth's that few parallels can be drawn about processes controlling the production and destruction of ozone.) Hubble's ultraviolet sensitivity is ideal for monitoring ozone levels on a global scale. The Martian ozone is yet another indication the planet has grown drier, because the water in the atmosphere that normally destroys ozone has frozen-out to become ice-crystal clouds. Spectroscopic observations made with the Faint Object Spectrograph (FOS) show that ozone now extends down from Mars' north pole to mid and lower latitudes. However, the Martian atmosphere is so thin, even this added ozone would offer future human explorers little protection from the Sun's harmful ultraviolet rays. SEASONS ON MARS The fourth planet from the Sun, Mars is one of the most intensely scrutinized worlds because of its Earth-like characteristics. Mars is tilted on its axis by about the same amount Earth is, hence Mars goes through seasonal changes. However, because Mars' atmosphere is much thinner than Earth's, it is far more sensitive to minor changes in the amount of light and heat received from the Sun. This is intensified by Mars' orbit that is more elliptical than Earth's, so it's range of distance from the Sun is greater during the Martian year. Mars is now so distant, the sun is nearly 25% dimmer than average. This chills Mars' average temperature by 36 degrees Fahrenheit (20 degrees Kelvin). At these cold temperatures, water vapor at low altitudes freezes out to form ice-crystal clouds now seen in abundance by Hubble. "Clouds weren't considered to be very important to the Martian climate during the Viking visits because they were so scarce," says Clancy. "Now we can see where they may play a role in transporting water between the north and south poles during the Martian year." Seasonal winds also play a major role is transporting dust across Mars' surface, and rapidly changing the appearance of a region. This gave early astronomers the misperception that Mars' shifting surface color was evidence of vegetation following a season cycle. As clearly seen in the Hubble images, past dust storms in Mars' southern hemisphere have scoured the plains of fine light dust and transported the dust northward. This leaves behind a relatively coarser, less reflective sand in the southern hemisphere. VENUS: NO EVIDENCE FOR NEW VOLCANIC ERUPTIONS Hubble spectroscopic observations of Venus taken with the Goddard High Resolution Spectrograph provide a new opportunity to look for evidence of volcanic activity on the planet's surface. Though radar maps of the Venusian surface taken by the Magellan orbiter revealed numerous volcanoes, Magellan did not find clear cut evidence for active volcanoes. Hubble can trace atmospheric changes that might be driven by volcanism. An abundance of sulfur dioxide in the atmosphere could be a tell-tale sign of an active volcanos. Sulfur dioxide was first detected by the Venus Pioneer probe in the late 1970s and has been declining ever since. The Hubble observations show that sulfur dioxide levels continue to decline. This means there is no evidence for the recurrence of large scale volcanic eruptions in the last few years. Ejected high into Venus' murky atmosphere, this sulfur dioxide is broken apart by sunlight to make an acid rain of concentrated sulfuric acid. This is similar to what happens on Earth above coal-burning power plants - but on a much larger and more intense scale. FUTURE PLANS More Hubble observations of Mars and Venus are critical to planning visits by future space probes. In particular, both robotic and human missions to Mars will need to be targeted for times during the Martian year when there is a minimal chance of getting caught in a dust storm. Knowing whether the atmosphere is relatively hot or cold is crucial to planning aerobraking maneuvers, where spacecraft use the aerodynamic drag of an atmosphere to slow down and enter an orbit around the planet. This reduces the amount of propellant needed for the journey. "If the atmosphere is more extended than expected the added friction could burn up an aerobraking spacecraft, just as Earth's atmosphere incinerates infalling meteors," says James. Ultimately, knowing the Martian climate will be an fundamental prerequisite for any future plans to establish a permanent human outpost on the Red Planet. @START@Uranus Space Science Shorts November 1994 --------------------------------------------------------------------------- HUBBLE OBSERVES URANUS The Wide Field/Planetary Camera 2 of NASA's Hubble Space Telescope provided the first detailed view of Uranus, its satellites and the ring structure, since the 1986 fly-by of the planet by the Voyager 2 spacecraft. During the Voyager encounter ten new satellites were discovered and precise orbits were determined. Since then, none of these inner satellites has been further observed, and no detailed observations of the rings have been possible. ORBITS OF URANUS' SATELLITES Detailed measurements of the positions and motions of the inner satellites of Uranus will allow their orbits to be calculated more precisely. This in turn will allow the further investigation of the unusual resonances (ratios of their orbital periods) which have been found among the inner satellites of Uranus. With this increase in accuracy, astronomers can better probe the unusual dynamics of this complicated system. Though initial data on the positions and motions of the satellite were obtained by Voyager 2, the moons have orbited about the planet nearly 10,000 times since the 1986 fly-by. By measuring current satellite positions, the "accumulated" effects of small errors in satellite motions since the time of the Voyager 2 encounter can be determined and precise corrections can be made to orbital elements, calculated from Voyager data. COLORS AND BRIGHTNESS OF URANUS' SATELLITES Hubble can help reveal the mineralogical composition of the moons by observing the brightness of the inner satellites in four different colors. Thismay permit a more detailed understanding of the origin or source of the satellites, and how Uranus' satellite system has evolved since the planet's formation 4.5 billion years ago. THE EPSILON RING Uranus' ring system was discovered in March, 1977 from ground-based observations of stellar occultations, where the star "blinked" as it passed behind each ring. Details about the rings, including structure and brightness, were not determined until the Voyager 2 fly-by in 1986. In the HST observations, the bright outermost epsilon ring can be clearly seen and accurate measurement of brightness and color (photometry) is now feasible. Structure related to the inner rings is also visible. Accurate photometry on the epsilon ring may indicate the source, or origin, of the particles that compose the rings. CLOUD STRUCTURE IN UPPER ATMOSPHERE HST reveals a pair of bright clouds (20oS and 35oS latitude) in Uranus' southern (Sun pointing) hemisphere. Similar structures were observed by Voyager during the fly-by, though the apparent increase in contrast may be due to the different Sun illumination since the Voyager fly-by. Then, the Sun was over the "south pole" and there were no variations in the position of the Sun during a day. At present, there are seasonal illumination changes since the orbital motion has moved the pole about 35o away from the Sun. These observations may provide information on this unique system which, during some portions of the "year," has normal daily variations of sunlight and, during other seasons when the Sun "hangs" over the south pole, has no daily changes in sunlight. @END@
