"spectral signatures" of each element enables astronomers to identify the elements present in distant stars by analyzing their spectra. There are three kinds of spectra: continuous, absorption, and emission. The continuous spectrum appears as a continuous band of color ranging from red to violet when observed through a spectroscope. An absorption spectrum occurs when the light from a star passes through a cloud of gas, hydrogen for example, before reaching the spectroscope. As a result, some wavelengths of light are absorbed by the hydrogen atoms. This selective absorption produces a spectrum that is a broad band of color interrupted by dark lines representing certain wavelengths of light that were absorbed by the hydrogen cloud. Such a situation occurs when a star is located inside or behind a gas cloud or nebula. An emission spectrum is observed when energy is absorbed by the gas atoms in a nebula and is reradiated by those atoms at specific wavelengths. This spectrum consists of bright lines against a black background. The light from fluorescent tubes and neon lights produce emission spectra.

Stellar spectra allow astronomers to determine star temperature, chemical composition, and motion along the line of sight. This enables astronomers to classify stars into spectral categories and estimate their age, reconstruct their histories, and postulate their future evolution. When available, astronomers prefer stellar spectra collected by orbiting spacecraft over spectra collected by Earth-based telescopes since they are not affected by atmospheric filtering and are therefore more accurate. Included in the spectra collected by spacecraft are infrared, ultraviolet, x-ray, and gamma ray bands that simply do not reach ground-based spectroscopes.

Notes:
  • This spectroscope works better with a holographic diffraction grating than with standard diffraction gratings. Refer to the source for holographic gratings listed in the Projecting Spectrums activity.
  • This spectroscope can be used to analyze the wavelengths of light from many light sources. Compare incandescent light, fluorescent light, and sunlight. If you have spectrum tubes and a power supply (available from science supply houses), examine the wavelengths of light produced by the different gases in the tubes. Many high school physics departments have this equipment and it may be possible to borrow it if your school does not. Use the spectroscope to examine neon signs and street lights. Science supply houses sell spectrum flame kits consisting of various salts that are heated in the flame of a Bunsen burner. These kits are much less expensive than spectrum tubes but are more difficult to work with because the flames do not last very long.

For Further Research:

  • Compare the solar spectrum at midday and at sunset. Are there any differences? Caution: Be careful not to look directly at the Sun.
  • What do spectra tell us about the nature of stars and other objects in space?
  • Show how temperature and radiation are related by connecting a clear light bulb to a dimmer switch. Gradually increase the current passing through the filament by turning up the dimmer. Observe the color and brightness of the filament as the temperature of the filament climbs with increasing current.
  • Ask students to identify what elements are present in the object that produced the "Spectra of Unknown Composition" on page 37, and to explain their method. How does this activity relate to the way astronomers use spectra to identify the composition of a star?
Next page Teacher Resources


Last modified prior to September, 2000 by the Windows Team

The source of this material is Windows to the Universe, at http://windows2universe.org/ from the National Earth Science Teachers Association (NESTA). The Website was developed in part with the support of UCAR and NCAR, where it resided from 2000 - 2010. © 2010 National Earth Science Teachers Association. Windows to the Universe® is a registered trademark of NESTA. All Rights Reserved. Site policies and disclaimer.