A rainbow rises over a misty forest. (Credit: U.S. Fish and Wildlife Service)
A spectrum is simply a chart or a graph that shows the intensity of light beingemitted over a range of energies. Have you ever seen a spectrum before? Probably. Nature makes beautiful ones we call rainbows. Sunlight sentthrough raindrops is spread out to display its various colors (thedifferent colors are just the way our eyes perceive radiation withslightly different energies).
Spectroscopy can be very useful in helping scientists understand howan object like ablack hole,neutron star, or active galaxy produces light, how fast it is moving, and whatelements it is composed of. Spectra can be produced forany energy of light, from low-energy radio waves to very high-energy gamma rays.
Each spectrum holds a wide variety of information. For instance,there are many different mechanisms by which an object, like a star, canproduce light. Each of these mechanisms has a characteristic spectrum.
The Electromagnetic Spectrum
White light (what we call visible or optical light) can be split up into its constituent colors easilyand with a familiar result: the rainbow. All we have to do is use aslit to focus a narrow beam of the light at a prism. This setup isactually a basicspectrometer.
Spectrum of white light
The resultant rainbow is really a continuous spectrum that shows usthe different energies of light (from red to blue) present in visiblelight. But the electromagnetic spectrum encompasses more thanjust optical light. It covers all energies of light, extending fromlow-energy radio waves, to microwaves, to infrared, to optical light,to ultraviolet,to very high-energy X-rays and gamma rays.
The full electromagnetic spectrum. (Credit: NASA's Imagine the Universe)
Tell Me More About the Electromagnetic Spectrum!
What Can Scientists Learn From a Spectrum?
Emission or Bright Line
Absorption or Dark Line
Three types of spectra: continuous, emission line and absorption.(Credit: NASA's Imagine the Universe)
Each element in the periodic table can appear in gaseous form and will produce aseries of bright lines unique to that element.Hydrogen will not look like helium which will not look like carbon which will not look like iron... and soon. Thus,astronomers can identify what kinds of stuff are in starsfrom the lines they find in the star's spectrum. This type of study iscalledspectroscopy.
The science of spectroscopy is quite sophisticated. From spectral linesastronomers can determine not only the element, but the temperature anddensity of that element in the star. The spectral line also can tell us aboutanymagnetic field of the star. The width of the line can tell us how fast the material ismoving. We can learn aboutwinds in stars from this. If the lines shift back and forth we can learn that the starmay be orbiting another star. We can estimate themass and size of the star from this. If the lines grow and fade in strengthwe can learn about the physical changes in the star. Spectralinformation can also tell us about material around stars. This materialmay be falling onto the star from a doughnut-shaped disk around the starcalled anaccretion disk. These disks often form around a neutron star or black hole. The light from the stuff between the stars allows astronomers to study the interstellar medium (ISM). This tells us what type of stuff fills the space between the stars. Space is not empty! There is lots of gas anddust between the stars. Spectroscopy is one of the fundamental tools whichscientists use to study the Universe.
Use Hera to analyze spectra.
Updated: August 2013
