Appendix: Electromagnetic Spectrum
Appendix: Electromagnetic Spectrum
The entirety of light, or radiation, in the Universe spans more than just the light we see with our naked eyes. This continuous range of light is called the electromagnetic (EM) spectrum and includes radiation at every frequency or wavelength. The EM spectrum has been broken into regions (see the table below) ranging from the highest-energy gamma rays to the lowest-energy radio waves.
| Region | Wavelengths (m) | Frequencies (Hz) | Temperature (K) |
| Radio | > 1 x 10-1 | < 3 x 109 | < 0.029 |
| Microwave | 1 x 10-3 to 1 x 10-1 | 3 x 109 to 3 x 1011 | 2.9-0.029 |
| Infrared | 7 x 10-7 to 1 x 10-3 | 3 x 1011 to 4 x 1014 | 4140-2.9 |
| Visible | 4 x 10-7 to 7 x 10-7 | 4 x 1014 to 7 x 1014 | 7245-4140 |
| Ultraviolet | 1 x 10-8 to 4 x 10-7 | 7 x 1014 to 3 x 1016 | 290,000-7245 |
| X-ray | 1 x 10-11 to 1 x 10-8 | 3 x 1016 to 3 x 1019 | 290,000,000-290,000 |
| Gamma-ray | < 1 x 10-11 | > 3 x 1019 | >290,000,000 |
In addition to its usefulness in astronomy, each region of the EM spectrum is important for everyday uses as well. On the high-energy side of the EM spectrum, we use gamma rays to power nuclear reactors and for medical testing involving radioactive isotopes. In space we observe gamma-ray bursts. These result from a collision between a black hole and a neutron star and produce enough energy that they are easily seen across the Universe.
On Earth, we use X-rays to check for broken bones and decay in our teeth. In astronomy, X-rays are used to study neutron stars and black holes in our Galaxy and to find quasars throughout the Universe. Astronomers also observe X-rays in the winds of hot stars in the Milky Way.
Most of the ultraviolet (UV) radiation is absorbed in the atmosphere before it reaches the ground. The little that does make it through can be harmful to humans, causing skin cells to mutate and become cancerous. In astronomy, UV light is used to study active star formation and for probing the atmospheres of cooler stars.
The visible part of the spectrum is that region of the EM spectrum where the Sun emits its peak intensity. This is the main reason that humans have evolved to see in this region of the spectrum. Our eyes can simply be thought of as filters that see only in the visible part of the EM spectrum. With a prism, this region can be split into its component colors from red to violet. Visible light reveals the main stellar populations in galaxies.
Infrared (IR) light is used to track cloud cover in meteorology and for seeing objects in the dark with night-vision goggles. Humans radiate in the IR (about 9,300 nm) with a body temperature of about 36o C (98o F). In the Galaxy, infrared light is emitted by dust, which can absorb a huge fraction of incoming starlight before re-radiating it. IR telescopes can penetrate this dust to see stars and planets embedded in clouds of star-forming regions, like the Orion Nebula.
Microwave radiation is used to heat food and for cellular communications. Astronomers observe microwave light in galaxies and quasars, but for the most part, the microwave bands are dominated by the cosmic microwave background, the light left from the Big Bang.
Finally, the least energetic form of light is radio. On Earth, we receive radio light for AM and FM broadcasts (wavelengths of AM are around 1 kilometer, while FM lies near 10 meters), television (UHF and VHF signals are around 0.1 to 1 meter), and radar. Astronomers use radio telescopes to detect the locations of large hydrogen clouds where stars form, revealing the size and structure of the Galaxy.
Astronomers use the EM spectrum to probe objects at varying depths. Some regions, like the visible and parts of the radio spectrum, are observed on Earth. Most regions are absorbed by our atmosphere, requiring us to build space telescopes to see these wavelengths.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott