Star Clusters and Nebulae Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 17, 2011

Nebulae—Dark Nebulae

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.

Dark Nebulae

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.

Emission Nebulae

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.

Planetary Nebulae

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).

Stars and Nebulae Stellar Anatomy And
Longevity Planetary Nebulae

Figure 13-7. Planetary nebulae appear donut-shaped because the amount of glowing material we see depends on where we look. This is a cross-sectional, simplified, negative view.

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.

Practice problems of this concept can be found at: Stars and Nebulae Practice Problems

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