 October 2000
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 Volume 9, No. 10, 36–38, 41, 43
All we know is what we “see”; astronomical spectroscopy remains the key to understanding the universe.
But it is not only an earthly phenomenon. Outer space can be thought of as one vast collection of illumination sources, burning brightly, radiating from the infrared to the ultraviolet and beyond. Elemental signatures are writ within the twinkling of the stars. Reading what has been written in various forms of radiation across the sky is the only way we have to understand the life stories of the stars. Absorption as well as emission can produce spectra. Samples can be prodded by exogenous photons to reveal their secrets. A particular spectrum is produced from the way electromagnetic radiation is absorbed or transmitted by the material involved. In space, clouds of dust and gas called nebulae are positioned directly between us and the grand concatenation of stellar illumination sources from white dwarfs to quasars. Scientists can analyze these clouds of material for their spectroscopic properties almost as easily as a sample in a cuvette stuck into a benchtop spectrophotometer illuminated by a halogen lamp. It is by examining these forms of absorption spectra that we have found the “stuff of life”, organic molecules, in great gas clouds scattered across the cosmos. Spectroscope Meets Telescope For most of the history of Western civilization, from the ancient Greeks to the mid-1600s, it was assumed that the heavens were made of materials more elegant than those of Earth. Even the laws of nature were assumed to be different above the Earth’s atmosphere, until Isaac Newton in the 17th century developed a universal theory of gravitation that explained the “falling” of the moon toward Earth in the same terms as that of an apple falling to the ground in an English country orchard. But it wasn’t until the German physicist George Kirchhoff (who developed the first spectroscope) detected the presence of sodium, calcium, magnesium, iron, and other elements in the spectrum of the Sun (without even using a telescope) that there was “proof” that the stuff of outer space was the same as that on Earth. The absorption lines in stellar and other spectra used to identify elements are thus known as “Kirchhoff” lines. The theory of the uniqueness of space was briefly revived when, in 1868, the element helium—named after the Greek sun god Helios—was discovered in the solar spectrum before it had ever been seen on Earth, and again when the element nebulium was proposed by Huggins. But a “universal” physics and chemistry proceeded apace thereafter. When Huggins turned his spectroscope toward one of the fuzzy “stars” called a nebula, an unknown absorption spectrum appeared, and the element nebulium was proposed as a unique heavenly constituent. It took several years to realize that the various nebulae were not single illumination sources, but either glowing clouds of dust and gaseous debris or distant galaxies then unknown. The so-called nebulium spectrum was actually caused by the complex and overlapping absorption pattern seen when the light of multiple stellar sources behind the particular nebula Huggins had sighted was absorbed and reflected by the gases and dust in it. Once this was realized and astronomical spectroscopy improved to the point of breaking apart the spectra of the nebula, a host of new chemical discoveries in space became possible. In 1929, however, astronomical spectroscopy truly came into its own when Edwin Hubble demonstrated the nearly universal redshift shown by the light coming from newly discovered galaxies. This so-called Hubble redshift seemed to indicate the amazing fact that all the galaxies in every direction around us were “fleeing” from our own galaxy, thereby causing a Doppler shift in the electromagnetic spectrum of their light’s wavelength toward the red as seen from Earth. Trying to explain this phenomenon led directly to the development of the “big bang” theory of the origin of our universe. The only logical rationale for everything moving away from “us” seemed to be that everything was moving not just away from us as some unique center, but from everything else equally at rapid speeds. Run the movie of this expanding universe backward to the distant past, and everything would race backward and collapse together at a single point—then explode outward. Hence, spectroscopy demonstrated the seeming necessity of a big bang. Temperature, Distance, Color, Life? Recently, astronomical spectroscopy has found tantalizing visions of not just elements but also complex organic molecules visible in the interstellar gases and distant nebulae. Although not life by any means, the European Space Agency’s infrared space telescope detected complex organic molecules of hundreds of chained carbon atoms in the clouds of gas and dust in interstellar space. In January 2000, diacetylene and triacetylene were detected in two very old stars—possible precursors of the more complex compounds. And in June 2000, for the first time, the presence of a sugar, glycolaldehyde, was found in space, some 26,000 light years from Earth, using radio wavelengths. Finding Planets Because any putative extrasolar planets are too far away to be seen directly using current telescopes, further advances in the search for extrasolar planets required the use of more elaborate spectral techniques. In 1995, Michel Major and Didier Queloz at the Geneva Observatory in Switzerland found the first evidence of a Jupiter-sized planet orbiting the sunlike star 51 Pegasi. Finding the planet involved examining the same type of Doppler effect on light spectra that led to our understanding of the expanding nature of the universe. Because of its gravitational drag or pull, depending on which part of its orbit the planet is in, the planet speeds up or slows down the star with respect to its general movement as seen from Earth. This creates a varying effect on the observed Doppler shift, forcing it either more to the blue or more to the red and back again in a periodic fashion over time. Careful comparative spectral measurements allow for the calculation of the unknown affecting the planet’s presumed size and distance from the star. The planet around 51 Pegasi would be only the first of many purported giant extrasolar planets found orbiting stars—much more closely than Jupiter orbits our own sun. These presumed planets are, in fact, so close to their suns that it is difficult to explain the discrepancy with current theories of the evolution of solar systems. Some scientists have speculated that rather than being Jupiter-sized planets, they are actually dim “brown dwarf” stars linked in a binary fashion to their visible partners. But currently, most astronomers think of them as “hot Jupiters”, and nearly 1% of nearby stars have them. Recently, rather than using Doppler shifts as with the original discoveries, the simple dimming of candidate stars as the planets pass in front of them is used as a rapid criterion for their presence. By the late 1990s, evidence of Jupiter-sized planets orbiting where they were supposed to be—at the appropriate theoretical distance from their stars—was discovered. This made the planetary nature of these unseen objects seem more plausible, prompting the goal of developing space telescopes capable of visualizing Earth-sized planets directly in the visible spectrum (if they exist) by the mid-21st century. The Microwave Sky According to the big bang theory, roughly a million years into the expansion of the newborn universe, light became capable of escaping the decreasingly dense gravity of the rapidly thinning ball of mass and radiated outward. Such light was predicted to be dimmed by the struggle against the still intense gravitational fields and was postulated to exist as a cosmic microwave background. In 1989, the Cosmic Background Explorer (COBE) satellite was launched to examine that microwave spectrum. As predicted, a generalized microwave background was discovered. It is distributed uniformly across the known universe and serves to raise the temperature of space some 3 degrees above absolute zero, exactly as the big bang model predicted. In the 1990s, more subtle examinations of the microwave universe showed that the radiation existed in subtle patterns across the sky. These were indicative of bubblelike asymmetries in the structure of the universe and showed that matter (in the form of galaxies and gases) was not uniformly distributed but had at some point been launched as gigantic clusters, probably caused by quantum fluctuations developing from the big bang period. (A crude analogy might be bubbles bursting forth randomly in boiling custard.) The Other-Wavelength Skies UV spectroscopy is of particular interest to astronomy for its ability to detect the strong transitions of the elements H, D, He, C, N, O, Mg, Si, S, and Fe. A number of these are of obvious interest in studying the development in space of the molecules, including organics and water, crucial to the origins of life. By observing the X-ray spectrum, the Chandra X-ray Observatory Satellite has yielded data on the apparent existence and behavior of those most bizarre entities—black holes. X-ray studies by the ROSAT (ROentgen SATellite) have presented strong evidence for the existence of vast amounts of “dark matter” in the universe, about whose nature we can only speculate. Nine years of study by the now defunct Compton Gamma-ray Observatory satellite revealed a gamma-burst universe, a place of unparalleled explosions, of “blazars”—particularly active quasars that eject huge jets of matter at almost the speed of light. Ultimately, full knowledge of any space object is not deemed complete until it has been examined carefully across all wavelengths in which it is “visible”. Toward the Future Ultimately, astronomical spectroscopy is perhaps one of the most amazing success stories in science. Bunsen and Kirchhoff’s element-identifying trick using an earthly flame, a piece of amazingly useful but ultimately simple technology, became the basis for nearly all of our understanding of the nature of the universe—cosmology born of chemistry rather than the myths, fables, and childhood stories of a distant past. Further Reading
Copyright © 2000 American Chemical Society.
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Typically, chemists think of spectroscopy in “earthly” terms. Samples of this or that element, inorganic compound, or biochemical are burned or energized in such a fashion as to create light. Whether in a laboratory mass spectrometer or in the burning of a treated log in a mountain chalet, light can be produced in a unique signature when a material is energized.
