A&B's Astronomy Lab, Fall 2001, No. 4

Different Wavelengths of Light

Have you heard that we only can see a part of the whole spectrum of the light?
Why it that? What if your eyes are sensitive to other wavelengths, not an "optical" range?
How and why astronomers see the sky in diffrent wavelengths?

Electromagnetic Spectrum

The electromagnetic (EM) spectrum is just a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Radiation is energy that travels and spreads out as it goes-- visible light that comes from a lamp in your house or radio waves that come from a radio station are two types of electromagnetic radiation. Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events create higher energy radiation than cool objects. Only extremely hot objects and particles moving at very high velocities can create high-energy radiation like x-rays and gamma-rays.

Radio... yes, this is the same kind of energy that radio stations emit into the air for your boom box to capture and turn into your favorite Mozart, Madonna, or Coolio tunes. But radio waves are also emitted by other things... such as stars and gases in space. You may not be able to dance to what these objects emit, but you can use it to learn about what they are made of and how much of the stuff they have.

Microwaves... they will cook your popcorn in just a few minutes! In space, microwaves are used by astronomers to learn about the structure of our own Galaxy.

Infrared... we often think of this as being the same thing as 'heat', because it makes our skin feel warm. In space, IR emission maps the dust between stars.
Visible... yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars... also by fast-moving particles hitting other particles.
Ultraviolet... we know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn!

X-rays... your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X-rays.

Gamma-rays... radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.

Absorption and Emission Spectra

A continuum spectrum results when the gas pressures are higher, so that lines are broadened by collisions between the atoms until they are smeared into a continuum. We may view a continuum spectrum as an emission spectrum in which the lines overlap with each other and can no longer be distinguished as individual emission lines.
Emission spectra are produced by thin gases in which the atoms do not experience many collisions (low density). The emission lines correspond to photons of discrete energies that are emitted when excited atomic states in the gas make transitions back to lower levels.
An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines in the spectrum.

Astronomy in Different Wavelengths
Since human eyes have been tuned to our Sun, people used to see the Universe only in optical wavelength. However, in modern astronomy, as instruments improved, we can see the universe in different wavelengths. The diagrams below (our Milky Way in multi-wavelengths) show what kinds of activities we can wee with "different wavelength eyes".
Radio Continuum (408 MHz). Intensity of radio continuum emission from surveys with ground-based radio telescopes (Jodrell Bank MkI and MkIA, Bonn 100 meter, and Parkes 64 meter). At this frequency, most of the emission is from the scattering of free electrons in interstellar plasmas (hot, ionized interstellar gas). Some emission also comes from electrons accelerated in strong magnetic fields. The large arc apparent near the center of the image is the remnant plasma from a supernova explosion that occurred thousands of years ago relatively close to the Sun (on the scale of the Milky Way).

Atomic Hydrogen. Column density of neutral atomic hydrogen from radio surveys of the 21-cm transition of hydrogen. The 21-cm emission traces the "warm" interstellar medium, which on a large scale is organized into diffuse clouds of gas and dust that have sizes of up to hundreds of light years. The data shown here are a composite of several surveys with ground-based telescopes in the northern and southern hemispheres.

Molecular Hydrogen. Column density of molecular hydrogen inferred from the intensity of the J = 1-0 line of carbon monoxide (CO). Molecular hydrogen is difficult to observe directly. The ratio of CO to Molecular hydrogen is fairly constant, so it is used instead to trace the cold, dense molecular hydrogen gas. Such gas is concentrated in the spiral arms in discrete "molecular clouds," which are often sites of star formation. The data shown here are a composite of surveys taken with twin 1.2-m millimeter-wave telescopes, one in New York City and the other on Cerro Tololo in Chile.

Infrared. Composite of mid and far-infrared intensity images taken by the Infrared Astronomical Satellite (IRAS) in 12, 60, and 100 micron wavelength bands. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds.

Near Infrared. Composite near-infrared intensity observed by the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) in the 1.25, 2.2, and 3.5 micron wavelength bands. Most of the emission is from cool, low-mass K stars in the disk and bulge of the Milky Way. Interstellar dust does not strongly obscure emission at these wavelengths; the maps trace emission all the way through the Galaxy, although absorption in the 1.25 band is evident toward the Galactic center region.

Optical. Intensity of red-band (0.6 micron) visible light from a photomosaic taken with a very wide-field camera at northern and southern observatories. Owing to the strong obscuration by interstellar dust the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way, which has a diameter on the order of 100,000 light years. Nebulosity from hot, low-density gas is widespread in the image. Dark patches are due to absorbing dust clouds, which are evident in the Molecular Hydrogen and Infrared maps as emission regions. The photographs do not cover the entire sky, and coordinate distortions in this image have not been corrected.

X-Ray. Composite X-ray intensity observations by an instrument on the Roentgen Satellite (ROSAT) in three broad, X-ray bands centered at 0.25 keV, 0.75 keV, and 1.5 keV. Shown is extended soft X-ray emission from tenuous hot gas. At the lower energies the cold interstellar gas strongly absorbs X-rays, and clouds of gas are seen as shadows against background X-ray emission. Color variations indicate variations of the absorption or of the temperatures of the emitting regions. The Galactic plane appears blue because only the highest-energy X-rays can pass through the large column densities of gas. The large loop near the center of the image is the North Polar Spur, an old supernova remnant. Many of the white sources are younger, more compact and more distant supernova remnants.

Gamma Ray. Intensity of high-energy gamma-ray emission observed by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO). The map includes all photons with energies greater than 100 MeV. At these extreme energies, most of the celestial gamma rays originate in collisions of cosmic rays with nuclei in interstellar clouds, and hence the Milky Way is a diffuse source of gamma-ray light. Superimposed on the diffuse light of the Milky Way are several gamma-ray pulsars, e.g., the Crab, Geminga, and Vela pulsars along the Galactic plane on the right-side of the image. Away from the plane, many of the sources are known to be active Galactic nuclei.

Electromagnetic Spectrum
The electromagnetic (EM) spectrum is just a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Radiation is energy that travels and spreads out as it goes-- visible light that comes from a lamp in your house or radio waves that come from a radio station are two types of electromagnetic radiation. Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events create higher energy radiation than cool objects. Only extremely hot objects and particles moving at very high velocities can create high-energy radiation like x-rays and gamma-rays.
	Radio... yes, this is the same kind of energy that radio stations emit into the air for your boom box to capture and turn into your favorite Mozart, Madonna, or Coolio tunes. But radio waves are also emitted by other things... such as stars and gases in space. You may not be able to dance to what these objects emit, but you can use it to learn about what they are made of and how much of the stuff they have.
	Microwaves... they will cook your popcorn in just a few minutes! In space, microwaves are used by astronomers to learn about the structure of our own Galaxy.
	Infrared... we often think of this as being the same thing as 'heat', because it makes our skin feel warm. In space, IR emission maps the dust between stars. Visible... yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars... also by fast-moving particles hitting other particles. Ultraviolet... we know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn!
	X-rays... your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X-rays.
	Gamma-rays... radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.

Astronomy in Different Wavelengths
Since human eyes have been tuned to our Sun, people used to see the Universe only in optical wavelength. However, in modern astronomy, as instruments improved, we can see the universe in different wavelengths. The diagrams below (our Milky Way in multi-wavelengths) show what kinds of activities we can wee with "different wavelength eyes".
	Radio Continuum (408 MHz). Intensity of radio continuum emission from surveys with ground-based radio telescopes (Jodrell Bank MkI and MkIA, Bonn 100 meter, and Parkes 64 meter). At this frequency, most of the emission is from the scattering of free electrons in interstellar plasmas (hot, ionized interstellar gas). Some emission also comes from electrons accelerated in strong magnetic fields. The large arc apparent near the center of the image is the remnant plasma from a supernova explosion that occurred thousands of years ago relatively close to the Sun (on the scale of the Milky Way).
	Atomic Hydrogen. Column density of neutral atomic hydrogen from radio surveys of the 21-cm transition of hydrogen. The 21-cm emission traces the "warm" interstellar medium, which on a large scale is organized into diffuse clouds of gas and dust that have sizes of up to hundreds of light years. The data shown here are a composite of several surveys with ground-based telescopes in the northern and southern hemispheres.
	Molecular Hydrogen. Column density of molecular hydrogen inferred from the intensity of the J = 1-0 line of carbon monoxide (CO). Molecular hydrogen is difficult to observe directly. The ratio of CO to Molecular hydrogen is fairly constant, so it is used instead to trace the cold, dense molecular hydrogen gas. Such gas is concentrated in the spiral arms in discrete "molecular clouds," which are often sites of star formation. The data shown here are a composite of surveys taken with twin 1.2-m millimeter-wave telescopes, one in New York City and the other on Cerro Tololo in Chile.
	Infrared. Composite of mid and far-infrared intensity images taken by the Infrared Astronomical Satellite (IRAS) in 12, 60, and 100 micron wavelength bands. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds.
	Near Infrared. Composite near-infrared intensity observed by the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) in the 1.25, 2.2, and 3.5 micron wavelength bands. Most of the emission is from cool, low-mass K stars in the disk and bulge of the Milky Way. Interstellar dust does not strongly obscure emission at these wavelengths; the maps trace emission all the way through the Galaxy, although absorption in the 1.25 band is evident toward the Galactic center region.
	Optical. Intensity of red-band (0.6 micron) visible light from a photomosaic taken with a very wide-field camera at northern and southern observatories. Owing to the strong obscuration by interstellar dust the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way, which has a diameter on the order of 100,000 light years. Nebulosity from hot, low-density gas is widespread in the image. Dark patches are due to absorbing dust clouds, which are evident in the Molecular Hydrogen and Infrared maps as emission regions. The photographs do not cover the entire sky, and coordinate distortions in this image have not been corrected.
	X-Ray. Composite X-ray intensity observations by an instrument on the Roentgen Satellite (ROSAT) in three broad, X-ray bands centered at 0.25 keV, 0.75 keV, and 1.5 keV. Shown is extended soft X-ray emission from tenuous hot gas. At the lower energies the cold interstellar gas strongly absorbs X-rays, and clouds of gas are seen as shadows against background X-ray emission. Color variations indicate variations of the absorption or of the temperatures of the emitting regions. The Galactic plane appears blue because only the highest-energy X-rays can pass through the large column densities of gas. The large loop near the center of the image is the North Polar Spur, an old supernova remnant. Many of the white sources are younger, more compact and more distant supernova remnants.
	Gamma Ray. Intensity of high-energy gamma-ray emission observed by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO). The map includes all photons with energies greater than 100 MeV. At these extreme energies, most of the celestial gamma rays originate in collisions of cosmic rays with nuclei in interstellar clouds, and hence the Milky Way is a diffuse source of gamma-ray light. Superimposed on the diffuse light of the Milky Way are several gamma-ray pulsars, e.g., the Crab, Geminga, and Vela pulsars along the Galactic plane on the right-side of the image. Away from the plane, many of the sources are known to be active Galactic nuclei.