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Infrared, Ultraviolet, and X-ray Astronomy

Infrared, Ultraviolet and X-ray Astronomy

Infrared astronomy deals with electromagnetic radiation at frequencies somewhat lower than visible light, but as low as radio waves. While radio waves are largely unaffected by atmospheric moisture, most infrared radiation is absorbed by it. Therefore, infrared detectors, except those operating in the “near” infrared spectrum close to the frequency of visible light, must be lifted above most of the Earth’s atmosphere, whether by rocket, balloon or spacecraft. An additional problem with infrared devices is that infrared radiation is essentially thermal, or heat, radiation. Therefore, all bodies, including the telescope and detector, emit infrared radiation. The only way to keep the infrared emission of the telescope from swamping out the radiation received from space is to cool the telescope to near-zero temperatures using liquid helium or liquid nitrogen. When these instruments are successfully sent into space, they enable astronomers to see through the clouds of dust and gases that completely obscure their view of many areas.

At the other end of the visible light spectrum lies ultraviolet radiation, which emits frequencies higher than visible light. Our atmosphere is basically opaque to these frequencies, and so, as in the case of infrared devices, the ultraviolet detectors must be lifted above the Earth’s atmosphere. Celestial objects that emit strongly in the ultraviolet spectrum are typically hotter objects. Stars and galaxies, for example, which are young, tend to be hotter and emit more ultraviolet radiation. So, observing the skies in the ultraviolet spectrum has given astronomers special insights into the early history and evolution of stars and galaxies.

At higher frequencies beyond the ultraviolet spectrum lie X-rays and gamma rays. Instrument design for X-ray telescopes is fundamentally different from the design for lower frequency observations. X-rays that strike material surfaces are absorbed rather than deflected. However, X-rays that graze a surface at a flat angle may be deflected rather than absorbed. Therefore, X-ray telescopes have a unique design of nested “mirrors” that gradually deflect X-rays many times, bringing them to a focal point and creating an image. Like the ultraviolet telescopes, X-ray telescopes must be placed above Earth’s atmosphere to function. Celestial X-ray sources are typically gases that have extremely high energy levels and temperatures. One type of object that often emits abundant X-rays is the black hole, which may heat gases to extraordinary temperatures as they are drawn into the black hole’s immense gravitational field.

Gamma ray detectors produce the highest-frequency and, therefore, highest-energy, observations. These instruments don’t produce an image in the conventional sense of the term, but simply count photons in the gamma ray spectrum arriving from some point in the sky. As with infrared, ultraviolet and X-ray detectors, gamma ray detectors must also be placed above the Earth’s atmosphere. Gamma ray instruments have detected a fascinating phenomenon – brief gamma ray bursts from various directions in the universe that last from a fraction of a second to several minutes and then fade away. The source of these bursts is still unknown, but one hypothesis states that they are caused by large supernova explosions that result in the formation of black holes rather than neutron stars.

Lesson Resources

The Copernican Model: A Sun-Centered Solar System (n.d.). Retrieved July 14, 2008, from University of Tennessee, Knoxville Physics Department Web site:

Lesson Glossary

  • Astrolabe: a mechanical device used in ancient times and the Middle Ages to locate and predict the positions of the Sun, moon, planets and stars; to tell local time; and as an aid to navigation and surveying
  • Astrology: an ancient “science” that presumed to analyze the influence that the Sun, moon and planets had on human affairs and personalities
  • Absorption lines: lines in the spectrum of light emitted or reflected from a material that indicate the composition of that material
  • Blueshift: the tendency for the perceived spectrum of light emitted from a moving body to shift towards the blue or violet end of the spectrum if the body is moving towards the observer
  • Chromatic aberration: the distortion in an image produced by a lens that results from the tendency of the lens to refract light of different wavelengths (or different colors) at different angles
  • Copernican Revolution: the shift in scientific thinking from a geocentric model of the solar system to a heliocentric model
  • Doppler Effect: the shift in perceived frequency of either sound or electromagnetic radiation that results when the body emitting the sound or radiation moves towards or away from the observer
  • Empirical: pertains to knowledge that is gained from direct experience or scientific experiments
  • Frequency: the characteristic of a wave that refers to the number of wave cycles passing a given point in a given unit of time
  • Geocentric: a model of the solar system that puts Earth at the center with the Sun and planets orbiting Earth
  • Heliocentric: a model for the solar system that puts the Sun at the center with Earth and other planets orbiting the Sun
  • Inertia: the quality of matter that causes a body in motion to stay in motion in the same direction and at the same speed and which causes a body at rest to stay at rest unless these bodies are acted upon by some force
  • Opaque: not able to transmit or pass light; not transparent or translucent
  • Redshift: the tendency for the perceived spectrum of light emitted from a moving body to shift towards the red end of the spectrum if the body is moving away from the observer
  • Reflecting telescope: a telescope that gathers and concentrates light with a mirror that is ground to a parabolic curve on one side
  • Refracting telescope: a telescope that gathers and concentrates light with an objective lens that is ground to a curve on both sides
  • Resolution: the ability of a telescope to separate, or “resolve,” two objects that are very close together in the field of view
  • Seeing: the general conditions of the atmosphere in a given place and time that affect astronomical observations; the clarity and stability of the atmosphere
  • Sensitivity: the light-gathering power of a telescope
  • Solstice: the two times each year when the Sun is at its greatest distance from the celestial equator; the first day of summer and the first day of winter
  • Spectrometer: an instrument that analyzes the spectrum (various light frequencies) of radiation that a body emits
  • Spectrum: a breakdown of the various frequencies of radiation that a body emits
  • Supernova: a sudden, catastrophic explosion of a massive star when its nuclear power source is no longer able to protect it from gravitational collapse
  • Wavelength: the distance from the crest of one wave to the crest of the following wave; wavelength is inversely proportional to frequency
  • Wave Period: the period of time it takes one wave cycle to pass a given point

Internet Links

The following links can provide more information about topics discussed.


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