Spectroscopy is the study of the absorption and emission of light by atoms and molecules, and how this relates to the wavelengths of light. It is a science of the spectrum, a set of energy bands of varying wavelengths (colours) produced by electromagnetic radiation. It is typified for a visually oriented species like humans by the rainbow band of colours that emerges when you pass white light through a prism (or sunlight through raindrops). This property of light became the domain of Isaac Newton in the late 17th century. In his 1704 masterpiece of theoretical dualism, Opticks, Newton defined many of the ground rules 200 years before spectroscopy was first seriously applied.
The latter half of the 19th century saw the development of two cornerstones of the use of light in astronomy: Astronomical spectroscopy, and photography. It would be hard to exaggerate the significance of spectroscopy to astronomy and its kindred studies; combined with photography, though, it rose to become the foundation of astrophysics. How so? Let’s take a look at a spectrum.
In 1825, French philosopher Auguste Comte made what appeared then to be a very safe prediction. He wrote that the chemical composition of stars was one aspect of the universe that would remain forever beyond the reach of man. How wrong he was! What he didn’t know was that the stars were sending precisely that information, and much more besides, right to us here on Earth. It’s amazing to think of the clues we have been given on a plate to help us unravel these mysteries. For thousands of years, human beings must have considered rainbows to be mere decorations in the sky, but they are so much more than that. Rainbows are spectra, perfectly displayed for all to see, and it took the scientific curiosity of some enterprising individual to make the association. These curved bands of light, always the same colours in the same order, occur when sunlight shines through moisture-laden air. Substitute pieces of glass for drops of water, and the magic dances for you!
Isaac Newton revealed the chromatic nature of light in 1666, but for 150 years progress was slow. It was not until 1814 that Joseph Fraunhofer made his startling discovery. By passing sunlight through a slit and then through a prism, Fraunhofer was able to reveal hundreds of lines crossing the spectrum. Instinctively, he knew he had stumbled onto something momentous, and examined the lines very closely. Each one appeared to be a replicated image of the slit, and occurred at very precise wavelengths. They have become known as Fraunhofer lines, and their significance to the advancement of science is immeasurable. Each line is the result of absorption by a specific chemical element. Fraunhofer had indeed unlocked a secret that put Comte’s prediction in its place, and gave astronomy the tool it needed to systematically analyse the chemistry of the cosmos.
These laws are the key to understanding the spectra of light that arrive here from outer space. Superimposed on the spectra are dark absorption lines, and in certain cases also interspersed with light emission lines, giving them the distinct appearance of universal product barcodes found on consumer goods around the world, and prompting the title cosmic barcode. Just like the codes in your local supermarket, there is a unique pattern of lines for each and every element and compound, and like the market’s checkout scanner, the spectrometer can read and pass on a wealth of information about the source of the light. By analysing spectra we can deduce the chemical composition of the source, its temperature, its relative motion and velocity, and the extent of electrical, magnetic, and gravitational fields in its vicinity. Further analysis can reveal much more, including the mass of the celestial body, its angular momentum (orbital motion and spin), and lots of similar information about the system in which it orbits. Fortunately or unfortunately, light is affected significantly by what it passes on its way here, so that has to be factored out to arrive at data for the source object specifically.
Before we could tackle astronomical spectra, we needed to improve the quality of light we were collecting from the stars. Isaac Newton made a considerable improvement to the design of the telescope in 1671, seven years before he published his monumental Principia. Instead of a linear system of glass lenses, Newton used a curved mirror at the back end of the telescope to gather the light, and send it forward to a secondary mirror that diverted the light sideways to the eyepiece. The reflecting telescope is the design most used in optical astronomy to this day, although astronomers have subsequently developed various configurations of the way that it focuses the image. It was a vastly more efficient instrument than the refracting telescope, and progress was rapid from there on. By the mid 19th century, astronomers were using the parallax effect to calculate the distance of stars. Astronomy was galloping along, but it would be another hundred years before the optical telescope finally got what it desperately needed: A platform for observation unencumbered by atmospheric distortion.
Orbiting and interplanetary observatories are now commonplace, but like any great endeavour, it took blood, sweat, and tears to get them there. When the crew of the shuttle Discovery put the Hubble Space Telescope (HST) into orbit 600km above the Earth on April 25th 1990, a large number of people on Earth breathed a collective sigh of relief and broke out the champagne. Congratulations were in order. No one knows who had the dream first, but astronomers had been working hard to get an eye above the atmosphere for decades, and it started to become reality when US Congress gave the go-ahead to NASA in 1977. It was a challenging project that pushed the envelope of technology into a new era, and involved an army of over ten thousand scientists and technicians in the most adventurous project ever undertaken in astronomy.
The HST satellite is about the size of a school bus, and contains an array of sophisticated optical tools. The telescope itself has a primary mirror of 2.4 metres, and is connected to recording instruments that can detect images in visible, infrared, and ultraviolet wavelengths. Totally freed of the optically cloying terrestrial atmosphere, the HST stood ready to see things 50 times fainter and with ten times the resolution of any Earth-bound telescope then in existence. But something was horribly wrong.
Astronomers pored over the first pictures to come from the observatory in space, stopped, cleaned their spectacles, and looked again. There was no doubt about it—something had gone dramatically awry. The pictures were fuzzy and distorted, and within a month, the awful reality sank in. The main mirror was out of kilter, and would have to be replaced. It had been ground incorrectly, and the testing procedures hadn’t been properly followed. The result was a spherical aberration in the mirror, and it would not be able to focus the image. An unprecedented embarrassment swept the world of astronomy. Several well-known scientists were heard to vehemently deny their profession, claiming instead to be anything from communist spies to drug dealers preying on small children—anything but astronomers!
It was a $1,500,000,000 blunder, but all was not lost. What followed was the most daring and ultimately successful space repair mission ever undertaken. Cost and logistics dictated that replacing the main mirror—which would have entailed bringing the entire satellite back to Earth and then redeploying it to orbit once more—was not feasible. Instead, a completely new instrument complex consisting of five pairs of optical mirrors was built to refocus the light before it got to the measuring and recording equipment at the base of the telescope. Seven astronauts were chosen for the mission, and they spent a full year practicing their manoeuvres, mostly in a gigantic swimming pool to simulate weightlessness. The corrective unit, called COSTAR for Corrective Optics Space Telescope Axial Replacement, was packed into a case, loaded onto a space shuttle along with the seven astronauts, and sent out to the orbiting Hubble satellite. Between 2nd and 13th December 1993, the astronauts performed an incredible five space walks, one lasting eight daunting hours. The good news is that the repairs were an unqualified success, and overnight the erstwhile spies and drug lords became astronomers once more.
The HST commenced a photographic foray into the universe that we couldn’t have dreamt of thirty years ago. It can detect things seven times more distant than any telescope on terra firma, and the clarity of its images brings tears to my eyes. Freed of the misty scattering of the atmosphere, the HST can gaze unhindered into the deep, dark yonder. In 1995 the Hubble Deep Field Image spent ten days taking photographs of a maze of galaxies previously unseen and undreamt of. The survey was repeated in 1998, and again in 2004, the last becoming known as the Hubble Ultra Deep Field. The results form a catalogue of pictures that are beyond breathtaking. Words fail; the images speak for themselves.
On 26th October 2004, NASA launched the $160-million SWIFT satellite that has taken us further into the secret domain of gamma ray bursts than was ever hitherto possible. It gets its name from its rapid response capability—a gamma ray telescope picks up the burst and the craft re-orientates itself very quickly to point its optical and X-ray instruments at the target. If you can imagine the difficulties of spinning an orbiting platform around and locking it with great precision onto a distant target, then you will be as impressed as I am.
It sends a shiver down my spine to contemplate the exciting times we live in.
 In addition to installing COSTAR, the service mission also replaced the Wide Field Planetary Camera (WFPC) with WFPC2.