April 21, 2003
The universe has been described as a symphony of light. And just as any piece of music is comprised of individual notes up and down the musical scale, the dazzling display of light from celestial sources is made up of photons, or "packets" of light, scaled by energy along the electromagnetic spectrum. The sound of the musical A note is distinguishable from that of a D, and photons of varied energy appear as different "colors" of light -- some in visible forms such as the colors of the rainbow, others invisible like X-rays and radio waves.
Analyzing just a few notes in a musical composition can reveal much about the piece as a whole, and, arguably, even hint at the personality of the composer. In similar fashion astronomers can gain many insights about the nature of astronomical objects and processes by analyzing the light they emit. In fact, light carries with it information about an object's distance, motion, temperature, brightness and chemical composition.
Objects emit light at many different wavelengths. The shortest wavelengths of light have the highest energies and are called gamma rays and X-rays. Ultraviolet light has longer wavelengths and less energy. As we continue across the electromagnetic spectrum through visible light, infrared light, and finally radio waves, wavelengths get longer and energy decreases. The human eye can only detect visible light. For humans to 'see' most of these other forms of exotic light, we must rely on electronic eyes, or detectors.
Hotter objects emit more light at all wavelengths than cooler objects. Moreover, hot objects emit more of their light at shorter wavelengths, or higher energies. This trend can be illustrated by looking within the color spectrum of visible light alone. Imagine placing a metal rod in a campfire. It begins to glow red, and as its temperature rises it begins to appear orange, then green, and soon the rod turns a bluish purple as it gets ever hotter. The same is true for stars -- blue stars are hotter than red stars.
Cataclysmic space events like supernova explosions generate a lot of heat, and thereby release extremely energetic light in forms such as gamma rays and X-rays. While cooler cosmic objects like interstellar clouds of dust and gas or lonely space rocks emit and reflect light at longer wavelengths, or lower energies. Accordingly, these cool objects appear most robustly in infrared light and radio waves.
Different types of light interact with matter in different ways. For example, long-wavelength (low-energy) radio signals will bounce off a slab of metal that short-wavelength X-rays or gamma rays zip right through. And Earth's atmosphere scatters or absorbs ultraviolet light, X-rays and gamma rays -- but is virtually transparent to visible light and radio waves. The interplay of various forms of light with matter has dictated telescope design, and has given astronomers a better grasp of which types of light help to best explain a given astronomical phenomenon.
Only by scrutinizing light at many different energies will astronomers solve the most vexing of astronomical puzzles. NASA's Great Observatories program reflects this belief. The Space Infrared Telescope Facility, launching this year, is the last observatory in this grand undertaking. It will make follow-on infrared observations of some of the cosmic mysteries that the Hubble Space Telescope, Chandra X-ray Observatory and Compton Gamma-ray Observatory have uncovered and explored.
NASA will soon add yet another band of wavelength coverage to this suite of telescopes. The Galaxy Evolution Explorer telescope, with a planned launch on April 28, will soon capitalize on a new generation of super-sensitive ultraviolet detectors, a large field of view, and its orbit above Earth's ultraviolet-absorbing atmosphere to observe how galaxies have changed and evolved across 10 billion years of cosmic history.
Observing the universe with such instruments, astronomers are destined to hit some high notes in scientific discovery.
Contact: JPL/Paul Morledge (818) 354-0850