Electromagnetic Spectrum

Astrophysics:

Electromagnetic Spectrum

Published:
Updated: April 24, 2009, 3:18 pm
Topics: Astrophysics

Introduction

The electromagnetic spectrum spans the entire range of electromagnetic radiation from the lowest frequency/longest wavelength to the highest frequency/shortest wavelength. Various segments of the spectrum are useful in exploring the Universe, and enable us to "see" into the universe beyond what we are able to see from visible light.

Pictoral display of the electromagnetic spectrum. Pictoral display of the electromagnetic spectrum.

The electromagnetic spectrum at-a-glance. (Source: NASA.)

 

Electromagnetic Spectrum Bands (Overview)

Band

 Wavelength
 		 Frequency

 Temperature
Range		 Quantum Energy
Radio 600 m - 0.187 m 30 Hz - 1.6x106 Hz   2x10-9 - 0.6x10-5 eV
Microwave 187 mm - 1 mm 1 - 300 106 Hz   0.6x10-5 - 0.1x10-2 eV
Sub-millimeter 539 - 616 μm 487 - 556x106 Hz   2.0x10-3 - 2.3x10-3 eV
Far Infrared 40 - 350 μm 300x106 Hz - 30x1012 Hz 11.6 - 140 K 3.1x10-2 - 0.35x10-2 eV
Mid Infrared 5 - 40 μm 30 - 120x1012 Hz 140 - 740 K 3.1x10-2 - 2.5x10-1 eV
Near Infrared 1 - 5 μm 120 - 440x1012 Hz 740 - 3,000 K 2.5x10-1 - 1.2 eV
Optical 380 - 780 nm 400 - 790x1012 Hz   1.59 - 3.3 eV
Ultraviolet 10 - 400 nm 750x1012 Hz - 30x1015 Hz   3.1 - 124 eV
X-ray 10 - 0.01 nm 30x1015 Hz - 100x1018 Hz  106 - 108 K 124 - 1.24x105 eV
Gamma-ray 0.01 - 0.000006 nm 100x1018 Hz - 3,862x1021 Hz   1.24x105 - 2.07x108 eV
"Cosmic-ray" 10 - 0.000006 nm 30x1015 Hz - 3,862x1021 Hz   124 - 2.07x108 eV


Please Note: The table above lists the full range of the electromagnetic spectrum. Not all bands are used for astronomical purposes. The boundaries between some of the spectra can vary and are not universally agreed upon. Data listed here represents what is generally agreed upon by the sources used. (Sources listed in Bibliography below. ) "Cosmic-rays" are high energy sub-atomic particles that exhibit properties within the X-ray and Gamma-ray bands.

 

Electromagnetic Spectrum Bands (Detail)

Radio

In the United States, the U.S. Department of Commerce Office of Spectrum Management manages the use of the electromagnetic spectrum between 9 kHz and 300 GHz where there are many small allocations for radio astronomy, space operation and space research. The electromagnetic spectrum is not allocated below 9 kHz. The full list is available at U.S. Frequency Allocations Chart (PDF). Information regarding radio spectrum allocation and management around the world is available from Wikipedia.

Astronomical objects such as quasars, radio galaxies, the center of the Milky Way Galaxy, pulsars, masers and other emission by interstellar molecules including the 21-centimeter line of neutral hydrogen, solar flares, sunspots, and the cosmic microwave background can be imaged in the radio bands. Energetic objects including pulsars and quasars can also be seen in radio frequencies. This spectra is also useful in observing hot gases from the interstellar medium; spectral line radiation from atomic and molecular transitions that occur in the interstellar medium or in the gaseous envelopes around stars; and pulsed radiation from fast-spinning neutron stars with powerful magnetic fields. [1]

The first radio antenna used to identify an astronomical radio source was one built by Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, in 1931. His was an inadvertant discovery which eventually lead to continued exploration in the radio wavelengths. Radio wavelengths are very long and require antennas on the order of several hundred meters. The world's largest filled-aperture telescope (i.e., a full dish) is the Arecibo radio telescope located in Arecibo, Puerto Rico, with a 305-meter dish that is fixed in the ground. The largest individual radio telescope of any kind is the RATAN-600 located near Nizhny Arkhyz, Russia, which consists of a 576-meter circle of rectangular radio reflectors, each of which can be pointed towards a central conical receiver. China officially started construction of the world's largest single-aperture radio telescope in 2009, the FAST. The FAST, or Five hundred meter Aperture Spherical Telescope, will have 4,600 panels and be similar in design to the Arecibo Observatory, utilizing a natural hollow (karst) to provide support for the telescope dish. As the name suggests, it will have a diameter of 500m. It will use an adaptive surface that adjusts to create parabolas in different directions, with an effective dish size of 300m. This means that, unlike Arecibo, it will not be confined to pointing directly upwards, but capable of covering the sky within 40 degrees from the zenith. Cost and physical constraints make these very large telescopes impractical. Radio interferometry is an inovative means of connecting multiple smaller antennas to "infer" an antenna the size of the waves to be received, such as the Very Large Array telescope in Socorro, New Mexico. [2]

Microwave

Cosmic microwave background radiation is a constant low-level background noise permeating the universe that remains from the earliest days of the universe, the "afterglow" of the big bang. The precise temperature fluctuations in the microwave background from region to region speak specifically about how this modern structure formed. An example of how microwave radiation is used in astronomy, NASA's Wilkinson Microwave Anisotropy Probe (WMAP) has observed the cosmic microwave background. WMAP data will help refine theories about the evolution of the universe and reveal new insights into the theory of inflation and the nature of the dark energy. [3]

The segment of microwave between 1 and 10 GHz, plus "spot" bands up to 25 GHz, is favored for the search for signals that might originate from extraterrestrial civilizations due to minimal natural celestial background noise or static interferance. The Search for ExtraTerrestrial Intelligence (SETI) is focused in this range. [4]

Sub-millimeter

Sub-millimeter is also known as Terahertz radiation, which is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. Planned telescopes operating in the submillimeter include the Atacama Large Millimeter Array and the Herschel Space Observatory. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space. [5] In astronomy, objects such as stars are frequently regarded as black bodies, though this is often a poor approximation. An almost perfect black-body spectrum is exhibited by the cosmic microwave background radiation. Hawking radiation is the hypothetical black-body radiation emitted by black holes. [6]

The submillimeter bandwidth is ideally suited for studies of the structure and motions of the matter that forms stars; of the spiral structure of galaxies, as outlined by their giant molecular clouds; and of quasars and active galactic nuclei. [7]

Infrared

  • Far Infrared: Cold dust emmisions, galaxy central regions, very cold molecular clouds. Incl. thermal IR.
  • Mid Infrared: Planets, comets, asteroids, starlight warmed dust, protoplanetary disks.
  • Near Infrared: Cooler red stars, red giants, dust is transparent.

Many objects in the universe that are too cool and/or faint to be detected in visible light, can be seen in the infrared. These include cool stars, infrared galaxies, clouds of particles around stars, nebulae, interstellar molecules, brown dwarfs, and extrasolar planets. In the case of dust, infrared astronomers can either see through it or focus on it, depending on the wavelengths they choose. At near infrared wavelengths, dust is essentially transparent, enabling far-flung views along the dusty plane of the Milky Way that are impossible in visible light. The galactic bulge, for example, shows up particularly well in near infrared. On the other hand, at mid infrared wavelengths astronomers pick up radiation generated by dust itself, which is valuable in studying such phenomena as protoplanetary disks and possible extrasolar planets. Work in cosmology also depends crucially on infrared observations. The expansion of the universe means that all of the ultraviolet and much of the visible light from distant sources is shifted into the infrared part of the spectrum. [8]

Optical

Astronomical observation at infrared, visible and ultraviolet wavelengths.

Visible-light astronomy encompasses a wide variety of observations via light sensitive telescopes. It includes imaging, where a picture of some sort is made of the object; photometry, where the amount of light coming from an object is measured, spectroscopy, where the distribution of that light with respect to its wavelength is measured, and polarimetry where the polarisation state of that light is measured. Visible astronomy also includes looking up at night (skygazing). [9]

Ultraviolet

The hottest and the most active objects in the cosmos give off large amounts of ultraviolet energy, sheding light on galactic evolution and structure. However, ultraviolet light is absorbed by Earth's atmosphere, so observations at these wavelengths must be performed in the upper atmosphere or from space-based observatories. Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of the hottest regions in space including the interstellar medium and hot young stars. Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. UV observations thus provide essential information about galactic evolution. [10]

X-ray

As with all observations in all non-visible spectra, X-ray observations enable us to "see" beyond the limits of human vision. X-ray observatories are used to explore various astrophysical objects from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super-soft x-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies also emit X-rays. Although the more energetic X-rays, photons with an energy greater than 30 keV, can penetrate the air at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. Therefore, X-ray observations are generally space-based although some observations can be made in the upper reaches of Earth's atmosphere. X-rays are classified as "soft" from 10 to 0.1 nm (about 0.12 to 12 keV), and as "hard" from 0.1 nm to 0.01 nm (about 12 to 120 keV). X-rays in the 2.48 to 0.248 nm (0.5 to 5 keV) range, is the range where most celestial sources give off the bulk of their energy. [11]

Gamma-ray

Observing the Universe in gamma-rays allows us to examine how matter and radiation interact with each other under extreme conditions, such as where temperatures are hundreds of millions of degrees, matter is very dense, or magnetic fields are very strong. Some specific targets include: Gamma-ray Bursts, Black Holes and Neutron Stars, Supernovae, Pulsars, Diffuse Emission, Active Galaxies including Seyferts and Quasars, and other Unidentified Sources. Unfortunately, gamma-ray detectors have to contend with a large contamination from cosmic rays. Cosmic rays - elementary particles which are come from all parts of the sky - often affect gamma-ray detectors in a similar manner to the source photons. This background must be suppressed in order to obtain a pure photonic signal. [12]

Gamma-ray detectors can be placed in two broad classes. The first are what would typically be called spectrometers or photometers in optical astronomy. These are instruments which are "light buckets" and focus on a region of the sky containing the object of interest collecting as many photons as possible. These types of detectors typically use scintillators or solid-state detectors to transform the gamma-ray into optical or electronic signals which are then recorded. The second class are detectors which perform the difficult task of gamma-ray imaging. Detectors of this type either rely on the nature of the gamma-ray interaction process such as pair production or Compton scattering to calculate the arrival direction of the incoming photon, or use a device such as a coded-mask to allow an image to be reconstructed. [13]

Cosmic-ray

So-called "cosmic-rays" were discovered in 1912 by Victor Hess. The term "cosmic-ray" is a misnomer. Initially, these sub-atomic particles were thought to be electromagnetic in nature. In the 1930's Cosmic-rays were determined to be electrically charged sub-atomic particles. The energy of cosmic-rays is usually measured in units of MeV, for mega-electron volts, or GeV, for giga-electron volts. Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light). The number of cosmic rays with energies beyond 1 GeV decreases by about a factor of 50 for every factor of 10 increase in energy. It is believed that most galactic cosmic-rays derive their energy from supernova explosions and other high energy events. Cosmic-rays include essentially all of the elements in the periodic table. Cosmic rays are electrically charged and are deflected by magnetic fields. Cosmic rays are also a hazard to electronic instrumentation in space; impacts of heavily-ionizing cosmic ray nuclei can cause computer memory bits to "flip" or small microcircuits to fail. [14] Cosmic-rays are labelled depending upon their origin: Solar Energetic Particles – associated with solar flares and other energetic solar events.; Galactic Cosmic Rays – coming from outside the solar system; Anomalous Cosmic Rays – coming from the interstellar space at the edge of the heliopause. [15] Point of origin is difficult to determine as cosmic-ray particle trajectory is affected by magnetic fields.

Cosmic-rays offer one of the few ways in which scientists can actually sample real matter (from protons up through the heaviest elements -- the actinides) outside of our solar system. By identifying the various nuclei that are dispersed throughout our Galaxy, scientists hope to unravel the mechanisms that actually produce these nuclei – from stellar nucleosynthesis to nucleosynthesis within supernovae to nuclear fragmentation. [15]

It is important to remember that cosmic-rays are particles – the nuclei of elements in the periodic table. They have nothing to do with the cosmic microwave background. The cosmic background radiation is the thermal radiation left over from the Big Bang – it consists of photons only – not particles. The only thing they have in common is the word "cosmic." [15]

 

NASA Missions - Spectra Studies

The image above indicates the range of electromagnetic spectra studies by various NASA space missions.
(Source: NASA.)

 

Units of Measure

Standard Prefixes

Standard prefixes for the SI (International Standard) units of measure
Multiples
(Large)		 Name   deca- hecto- kilo- mega- giga- tera- peta- exa- zetta- yotta-
Symbol   da h k M G T P E Z Y
Factor 100 101 102 103 106 109 1012 1015 1018 1021 1024
 
Subdivisions
(Small)		 Name   deci- centi- milli- micro- nano- pico- femto- atto- zepto- yocto-
Symbol   d c m µ n p f a z y
Factor 100 10−1 10−2 10−3 10−6 10−9 10−12 10−15 10−18 10−21 10-24

Standard prefixes for the SI units of measure.
Source: Wikipedia. Table links lead to content in Wikipedia.

Energy – Electron Volt

An electron volt (eV), is a unit of energy used to describe the total energy carried by a particle. It is the energy gained by an electron (or proton, same size of electric charge) moving through a voltage difference of one volt.

  • 1 keV = 1 kilo-electron volt = 1,000 eV – typical of dental X-rays.
  • 1 MeV = 1 mega-electron volt = 1 million eV – typical of radioactive decay particles.
  • 1 GeV = 1 giga-electron volt = 1 billion eV – the equivalent energy of a proton (hydrogen nucleus) at rest.

The molecules in our atmosphere have energies around 0.03 eV. The Sun's plasma and Earth's magnetosphere contain particles that are much more energetic. Protons in the magnetosphere typically have energies of 1 keV to 10 keV. And particles having still higher energies are quite common throughout the Universe. [16]

Radio Frequency – Hertz

Frequency of oscialtion between two polar ends of a scale over a period of time. The standard unit of radio frequency is the Hertz, abbreviated as "Hz" and defined as the number of cycles per second. Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894) was a German physicist who clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves. The SI unit hertz (Hz) was established in his honor by the IEC in 1930 for frequency. [17]

Temperature – Kelvin

Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an "absolute" scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the temperature at which the particle constituents of matter have minimal motion and can be no colder. Absolute zero is 0 K or −273.15 °C. [18]

The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero (0 K). The Kelvin scale and the kelvin are named after the Irish physicist and engineer William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale." Unlike the degree Fahrenheit, and degree Celsius, the kelvin is not referred to as a "degree", nor is it typeset with a degree symbol; that is, it is written K and not °K.

"The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water." 13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of 2H per mole of 1H, 0.000 379 9 mole of 17O per mole of 16O, and 0.002 005 2 mole of 18O per mole of 16O." [19]

 

Atmoshperic Opacity

Earth's atmosphere blocks or absorbs a significant portion of the electromagnetic spectrum coming from space.  Most of the spectra that pass through the atmosphere are in the visible light and some radio-wave bands. Spetra that are absorbed tend to warm the absorbing gases. The image below provides a quick view of the spectra that are blocked or absorbed versus those that can pass through, or are "transmitted" by the atmosphere.

Atmospheric electromagnetic transmittance or opacity. Source: WikiCommons.

 

Biblography

  1. Electromagnetic Spectrum - The Internet Encyclopedia of Science.
  2. Electromagnetic Spectrum - Wikipedia.
  3. History of Gamma-ray Astronomy - Imagine the Universe, NASA.
  4. Submillimeter Wave Astronomy Satellite - Goddard Space Flight Center, NASA.
  5. What is Submillimeter Astronomy - Arizona Radio Observatory.
  6. The Electromagnetic Spectrum - HyperPhysics, C. R. Nave, Department of Physics and Astronomy, Georgia State University.
  7. The Electromagnetic Spectrum, Radio Bands - HyperPhysics, C. R. Nave, Department of Physics and Astronomy, Georgia State University.
  8. The Electromagnetic Spectrum, Infrared through Gama-ray Bands - HyperPhysics, C. R. Nave, Department of Physics and Astronomy, Georgia State University.

References

  1. Radio Astronomy - The Internet Encyclopedia of Science, David Darling, Ph.D.
  2. Radio Telescope - Wikipedia.
  3. The Anisotropy of the Microwave Background - Imagine the Universe, NASA.
  4. SETI - The Internet Encyclopedia of Science, David Darling, Ph.D.
  5. Terahertz Radiation - Wikipedia.
  6. Black Body - Wikipedia.
  7. Submillimeter Wave Astronomy - The Internet Encyclopedia of Science, David Darling, Ph.D.
  8. Infrared Astronomy - The Internet Encyclopedia of Science, David Darling, Ph.D.
  9. Visible-light Astronomy - Wikipedia.
  10. Ultraviolet Astronomy - Wikipedia.
  11. X-ray Astronomy - Wikipedia.
  12. History of Gamma-ray Astronomy - Imagine the Universe, NASA.
  13. Gamma-Ray Telescopes & Detectors - Imagine the Universe, NASA.
  14. Cosmic Rays - A general introduction to the field of cosmic rays. R. A. Mewaldt, California Institute of Technology. Pub. Macmillan Encyclopedia of Physics, 1996.
  15. Cosmic Rays, Energetic Particles, and Plasma - Cosmicopia, Astrophysics Science Division, Goddard Space Flight Center, NASA.
  16. Energetic Particles - Cosmicopia, , Astrophysics Science Division, Goddard Space Flight Center, NASA.
  17. Hertz - Wikipedia.
  18. Thermodynamic Temperature - Wikipedia.
  19. History of Gamma-ray Astronomy - Imagine the Universe, NASA.

Related EoC Articles

Related EoC Topics

The following topics are listed under "Observatories & Telescopes" and provide detailed information on facilities within the specific electromagnetic spectra.

External Links

 

Citation

Wallace, Matthew H., Assoc. Editor EoC. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Electromagnetic / Radio Wave Spectrum." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published April 24, 2009].
<http://www.cosmosportal.org/articles/view/138959/>

 

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