We live in a universe filled with galaxies. Galaxies are vast gravitationally bound aggregations of hydrogen gas clouds, stars that are produced when part of a cloud collapses under its own enormous weight, atoms that have been ionized by stellar radiation and dust formed from the remnants of previous stars that have either exploded or thrown off their outer layers during old age. Of these, the largest directly observable constituents are the hydrogen gas billows. Older terms survive within the astronomical lexicon. Any extended object in the sky (other than the Sun, Moon, planets and comets) has at one time or another been called a nebula. The root meaning, however, is cloud and it’s now most often used to reference places that contain gas and dust such as the view provided by the image accompanying this article.
The term dust is also broadly applied astronomically- it’s not your household variety but grains of material that are only fractions of a micron in diameter. Other more exotic material is also suspected within galaxies- often referenced as dark matter due to our inability, thus far, to observe it directly.
The great gas clouds that fill our galaxy, the Milky Way, are organized into a persistent spiral pattern similar to the arms that are wound about the center of other galaxies observed throughout the cosmos. Piercing these clouds are great tendrils of light absorbing dust that impart fantastic, at times familiar, shapes to the clouds when viewed from a relatively close distance such as the outline of the North American continent seen on the left side of this picture.
Our galaxy has the relative proportions of two CD’s stacked on top of each other. The disc is so broad that it takes light about 100,000 years to travel from edge to edge and about two thousand years to traverse top to bottom except near the center. The central area has a large, slightly flattened, oblong bulge about 7,000 light years thick, at its greatest, that also displays a curious bar shaped pattern– something only recently discovered. Four arms made of gas, dust and stars slowly wind outward more or less continuously from the central area. These are punctuated by one (and maybe more) fragmented arms, about mid-way across the disc. Our Sun, with its system of planets and smaller bodies in tow, currently resides inside a fragment. Ours is known as the Local or Orion Arm.
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Most of the bright stars that form our familiar constellations exist within the same arm fragment with us- at least all those within about 1,500 light years, more or less. One prominent feature observed in spiral galaxies are the dark lanes of dust which often outline the edges of their spiral pattern. We are close to one and you can see it by looking towards the northern summer constellation named Cygnus. It’s called the Cygnus Rift or the Northern Coalsack and it’s a cloud of light absorbing dust that lines our Local Arm. It can be spotted with the naked eye from a dark site because it blocks the glow seen from the vast and more distant Cygnus Star Cloud that runs the length of this constellation.
The Cygnus Star Cloud is composed of the combined light from countless stars stacked up behind each other along our line of sight and along the length of the Local Arm. Much closer to us hangs the North American and Pelican Nebulae, pictured here. The Pelican Nebula is depicted on the right side of the image. They are situated near the star Deneb, the brightest star in Cygnus and are about 1,800 light years from the Sun. Though they have the appearance of being separate both are part of the same nebula- light absorbing dust tendrils hang in front, intervene and seem to divide the gas cloud thus giving an illusion that there are two objects. The entire nebula, as seen here, is over 100 light years wide.
The ultraviolet light from a single star illuminates this nebula. The energy thrown off by this star is bright enough to ionize the material within the cloud. Ionization occurs when electrons are temporarily ejected from atoms and when they recombine a photon of light is released. Special filters can be placed in front of cameras that only pass the glow emitted by specific ionized atoms. This picture used that technique and assigned a unique color to each element. Hydrogen atoms are tinted green, sulfur is colored red and the hue for oxygen is blue. Therefore, the image not only displays the nebula’s physical appearance but also provides information about its chemical makeup.
Astronomer Don Goldman produced this intense and beautiful image on July 8, 2006 from his suburban Sacramento, California back yard. It required a 3.5 hour exposure through a seven-inch telescope with an 11 megapixel astronomical camera.
Do you have photos you’d like to share? Post them to the Universe Today astrophotography forum or email them, and we might feature one in Universe Today.
Written by R. Jay GaBany