Astrophoto: The Cocoon Nebula by Dan Kowall
Written by R. Jay Gabany
Like shards of shattered glass caught in a spotlight, the stars appear deceptively passive in the night sky. Yet, each one is an object of extraordinary ferocity. Stellar surface temperatures can reach 50,000 degrees Celsius- over ten times hotter than our Sun – and on a few it can reach over one million degrees! The heat within a star reaches even higher levels that typically exceed several million degrees – enough to tear apart atomic nuclei and transform them into new types of matter. Our casual glances upward not only fails to reveal these extreme conditions but it only hints at the enormous variety of stars that exist. Stars are arranged in pairs, triplets and quartets. Some are smaller than Earth while others are larger than our entire solar system. However, since even the nearest star is 26 trillion miles distant, almost everything we know about them, including those in the accompanying picture, has been gleaned only from their light.
Our technology, today, is still wildly incapable of sending a person or a robot to even the nearest star within a round-trip transit time spanning less than several thousand years. Therefore, the stars remain physically inaccessible now and for many years to come without an unprecedented breakthrough in space propulsion. However, even though it's not practical to visit the mountain it has been possible to study parts of the mountain that have been sent to us in the form of starlight. Almost everything we know about the stars is based on a technique known as spectroscopy- the analysis of light and other forms of radiation.
Spectroscopy's beginnings stem from Isaac Newton, the seventeenth century English mathematician and scientist. Newton was intrigued by the then strange notion, proposed by earlier thinkers such as Rene Descartes, that white light holds all the colors of the rainbow. In 1666, Newton experimented with a glass prism, a little hole in one of his window shutters and the white wall of the room. As the light from the hole passed through the prism, it was dispersed, as if by magic, into an array of slightly overlapping colors: from red to violet. He was the first to describe this as a spectrum, which is the Latin word for apparition.
Astronomy did not immediately incorporate Newton's discovery. Well into the eighteenth century, astronomers thought the stars were just a backdrop for the motion of the planets. Part of this was based on the widespread disbelief that science could ever understand the true physical nature of the stars due to their remote distance. However, all of that was changed by a German optician named Joseph Fraunhofer.
Five years after joining a Munich optical firm, Fraunhofer, then at the age of 24, was made a partner due to his skill at glass making, lens grinding and design. His pursuit for ideal lenses used in telescopes and other instruments led him to experiment with spectroscopy. In 1814 he set up a surveying telescope, mounted a prism between it and a small slit of sunlight then looked through the eyepiece to observe the spectrum that resulted. He observed a spread of colors, as he had expected, but he saw something else- an almost countless number of strong and weak vertical lines that were darker than the rest of the colors and some appeared almost black. These dark lines would later become familiar to every student of physics as the Fraunhofer absorption lines. Newton had not seen them, possibly, because the hole used in his experiment was larger than the Fraunhofer's slit.
Fascinated by these lines and certain they were not artifacts of his instrument, Fraunhofer studied them intently. Over time he mapped over 600 lines (today, there are about 20,000), then turned his attention to the Moon and closest planets. He found the lines were identical and concluded this was because the moon and planets reflected sunlight. Next he studied Sirius but found the star's spectrum had a different pattern. Every star he observed, thereafter, had a unique set of dark vertical lines that set each one apart from the others like a fingerprint. During this process, he vastly improved a device known as a diffraction grating that could be used in place of a prism. His improved grating yielded far more detailed spectra than a prism and made it possible for him to create maps of the dark lines.
Fraunhofer tested his spectroscopes- a term coined later- by observing the light of a gas flame and identifying the spectral lines that appeared. These lines, however, were not dark- they were bright because they resulted from a material that had been heated to incandescence. Fraunhofer noted the coincidence between the positions of a pair of dark lines in the solar spectrum with a pair of bright lines from his lab flames and speculated that the dark lines may be caused by the absence of a particular light as if the Sun (and the other stars) had robbed their spectra of narrow stripes of color.
The mystery of the dark lines was not solved until around 1859, when Gustav Kirchhoff and Robert Bunsen conducted experiments to identify chemical materials by their color when burned. Kirchhoff suggested that Bunsen use a spectroscope as the clearest method for making a distinction and it soon became obvious that each chemical element had a unique spectrum. For example, Sodium produced the lines first spotted by Fraunhofer several years earlier.
Kirchhoff went on to correctly understand the dark lines in the solar and stellar spectra: light from the Sun or a star passes through a surrounding atmosphere of cooler gasses. These gasses, such as sodium vapor, absorb their characteristic wavelength from the light and produce the dark lines first spotted by Fraunhofer earlier that century. This unlocked the code of cosmic chemistry.
Kirchoff later deciphered the composition of the solar atmosphere by identifying not only sodium but iron, calcium, magnesium, nickel and chromium. A few years later, in 1895, astronomers viewing a solar eclipse would confirm the spectral lines of an element that had not yet been discovered on earth- helium.
As the detective work continued, astronomers discovered that the radiation they were studying through spectroscopes extended beyond the familiar visible colors into electromagnetic regions that our eyes cannot perceive. Today, much of the work that holds the attention of professional astronomers is not with the visual characteristics of deep space objects but with the nature of their spectra. Virtually all of the newly found extra solar planets, for example, have been discovered by analyzing stellar spectrum shifts that are introduced as they orbit around their parent star.
The enormous telescopes that dot the globe in extremely remote locations are rarely used with an eyepiece and seldom take photographs like the one included with this discussion. Some of these instruments have mirror diameters in excess of 30 feet and others, still in design and funding stages, may have light collecting surfaces that exceed 100 meters! By and large, all of them, those that exist and those on the drawing board, are optimized to gather and dissect the light they collect using sophisticated spectroscopes.
Currently, many of the most beautiful deep space images, like the one featured here, are produced by gifted amateur astronomers who are drawn to the beauty of objects that drift throughout deep space. Armed with sensitive digital cameras and remarkably precise but modest-sized optical instruments, they continue to be a source of inspiration to people around the world who share their passion.
The colorful picture in the upper right was produced by Dan Kowal from his private observatory during August of this year. It presents a scene located in the direction of the northern constellation Cygnus. This complex mass of molecular hydrogen and dust is about 4,000 light years from Earth. Much of the light seen in the main part of this nebula is generated by the massive bright star near it's center. Wide angle, long exposure photographs reveals the nebula to be very extensive- essentially a vast river of interstellar dust.
This picture was produced with a six-inch apochromatic refractor and a 3.5 mega-pixel astronomical camera. The image represents almost 13 hours of exposure.
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
Filed under: Astrophotos
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March 7th, 2008 at 6:19 pm
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