Star Clusters and Nebulae Help (page 2)
Star Clusters—Open Clusters
Some star systems contain hundreds, thousands, or even millions of individual stars, all held in each other’s vicinity by their mutual gravitation. Some huge gas-and-dust clouds condense into star clusters . There are three main types of star clusters found within our galaxy and presumably within other galaxies.
The Pleiades , also known as the “seven sisters,” are a familiar star cluster that can be seen easily with the unaided eye. On a clear night, most people can see six individual stars in this group. People with sharp vision can see seven stars; with a good pair of binoculars, it becomes apparent that there are a lot more than seven stars in this cluster. The Pleiades are an example of an open star cluster . This type of star group is also known as a galactic cluster . The stars are held together by gravity, but they are not tightly packed. They do not show a striking central concentration nor an orderly pattern or structure.
How do we know that the stars in an open cluster are associated and are not just accidentally close to each other because they all happen to fall along the same line of sight relative to Earth? One way to find out is to measure the radial speeds of the individual stars relative to us and then compare these speeds. If the stars are in a common group, held in each other’s vicinity by gravity, then we should not observe much difference in the radial speeds of the stars. If, however, we’re just seeing a coincidental lineup of stars, the individuals in the swarm should have much different radial speeds, just like stars chosen at random in the sky. It turns out that the stars in an open cluster all have radial speeds that are nearly identical.
The most spectacular star groups, besides galaxies themselves, are known as globular clusters . They get this name because of their symmetrical appearance. Some of these clusters contain more than 1 million stars, 50 percent of which lie inside a spherical shell whose radius is called the median radius . The concentration of stars is greatest near the center of the cluster and decreases uniformly in all directions away from the center, as shown in Fig. 13-6.
An excellent example of a globular cluster is M13 in the constellation Hercules. This can be seen in the summer sky from the northern hemisphere. When viewed at low magnification through a large amateur telescope, M13 is spectacular. More than 100 globular clusters have been observed in and around the Milky Way. Globular clusters, unlike open clusters, are arranged in a large spherical halo around the galaxy. There are many globular clusters that lie outside the disk-shaped main part of the Milky Way whose stars are arranged in spirals like the pattern of rainbands in a hurricane.
Globular star clusters are believed to be very old, among the oldest objects in the observable Universe. Such clusters contain large numbers of variable stars, much greater in proportion than the stars in the rest of the galaxy. The stars in these clusters are mostly metal-deficient . The term metal in this context refers to any element other than hydrogen and helium, the two most abundant and oldest elements in the Cosmos. This fact suggests that globular clusters formed from the original stuff of the Universe, the original gases hydrogen and helium, and that few or none of the stars in the clusters are the result of congealed gas and dust from previously blown-up stars.
The space between the stars is far from empty. Gas and dust, some of it left over from the primordial Universe and much of it the remnants of exploded massive stars, is strewn throughout the Milky Way. Most of this material is found in or near the plane of our galaxy.
When viewed through a good pair of binoculars or a small telescope, the glow of the Milky Way, especially in the direction of the constellation Sagittarius, is resolved in detail. Millions of stars can be seen. In time-exposure photographs, dark rifts, wisps, and tendrils appear. Some of these look like clouds (and they are, but much larger than any clouds on Earth). This motivated astronomers to call such an object a nebula , the Latin word for “cloud.” The plural is nebulae , in true academic fashion.
At first, the dark nebulae were mistaken for regions of diluted stars. However, spectral analysis of starlight shining through revealed the truth. When photographed through large telescopes, it became obvious that the dark nebulae were indeed clouds. We cannot observe their motion directly, but these clouds are blowing around in space, having been given momentum by the explosions from which they came and also because they are affected by the stellar winds, gravitation, and magnetic fields produced by stars near them. Sometimes the clouds form vortices like cosmic dust devils or newborn hurricanes. These rotating nebulae collapse into new stars and, in some cases, into planetary systems like our Solar System.
Once astronomers had access to large telescopes, sophisticated cameras, and spectroscopes, a visual world was opened to them that revealed not only dark clouds but also glowing ones. Such clouds are called emission nebulae because they appear bright against the dark background of space.
If you have a good amateur telescope measuring 20 cm (8 in) or more in diameter and can set it up for low magnification, try looking at the center star in Orion’s sword on a moonless, exceptionally clear night when Orion is high in the sky. You’ll see a fine example of an emission nebula, known as the Great Nebula in Orion . This cloud of gas and dust glows because some of its atoms are ionized by ultraviolet and x-rays from stars in its vicinity. This nebula is approximately 1,600 light-years from our Solar System and has an actual diameter of some 20 or 30 light-years.
Emission nebulae can take on almost any shape. One of the more interesting of these is the Ring Nebula in the constellation Lyra. There are many other such ring-shaped emission clouds visible in our galaxy through large telescopes. Some appear visually similar to planets and easily can be mistaken for planets by a casual observer; hence they are called planetary nebulae . The ring-shaped nebulae are actually shaped like spheres with thick shells. We see them as rings or donuts because we are looking at the most glowing material when our line of sight passes near, but not exactly at, the periphery. The objects appear less bright toward the center and dimmest at the extreme outer edge (Fig. 13-7).
Planetary nebulae always have stars at their centers, and they are always expanding, as can be determined by looking at the emission spectra. The emission lines, the signatures of chemical elements, appear shifted toward the violet end of the visual spectrum near the center of such a nebula and are shifted little or not at all near the edges. This indicates that the whole sphere is growing like a balloon being inflated, although not fast enough for us to observe directly. It is believed that these nebulae are fluorescent gas cast off by stars nearing the ends of their lives. The gas gradually disperses into the interstellar medium, where some of it finds its way into new stars, asteroids, comets, planets, and moons.
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