3.3: The Electromagnetic Spectrum

Types of Light

Objects in the universe send out an enormous range of electromagnetic radiation. Scientists call this range the electromagnetic spectrum, which they have divided into a number of categories. The spectrum is shown in Figure \(\PageIndex{1}\), with some information about the waves in each part or band. The Electromagnetic Spectrum, or EMS, is the range of frequencies of electromagnetic radiation. From shortest wavelengths to longest, the groups in the electromagnetic spectrum are gamma-rays, X-rays, ultraviolet, optical, infrared, microwaves, and radio waves. This is also the order from highest to lowest energy.

Figure \(\PageIndex{1}\) : The Electromagnetic Spectrum. The electromagnetic spectrum spans from gamma rays, the shortest wavelength and highest energy, to radio waves, the longest wavelength and lowest energy. (CC BY-SA 3.0; Jonathan S Urie via Wikimedia Commons) Accessible description of Figure \(\PageIndex{1}\).

Grouping by Wavelength

Astronomers use all parts of the electromagnetic spectrum because each type has its advantages and disadvantages. Some wavelength ranges can only be observed from space, because they are absorbed or scattered by Earth's atmosphere. The main region that is easily observed on the surface of Earth is visible light, and even that light is disturbed by turbulence in the atmosphere. Figure \(\PageIndex{2}\) includes examples of the extra information found at different wavelengths of electromagnetic radiation. Image (a) is a visible light image, with straight lines connecting the bright stars to form the outline of the constellation. Image (b) is the same area as Image (a) in X-rays, revealing many bright objects that are absent in Image (a). Image (c) is an infrared image, that includes only a few stars, and reveals delicate wisps of clouds which get quite bright and dense in the vicinity of the Orion nebula, near Orion's belt and sword. Images like (b) and (c) are sometimes called false color-images because they use visible colors to represent wavelengths that are not detectable by the human eye.

Figure \(\PageIndex{2}\) : Orion in Different Wavelengths. The same region of sky appears different depending on wavelength: (a) visible light traces the familiar stars of the constellation, (b) X-rays reveal point-like sources not seen in visible light, and (c) infrared light highlights glowing dust clouds near the Orion Nebula. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{2}\).

Electromagnetic radiation with the shortest wavelengths, no longer than 0.01 nanometer, is categorized as gamma rays (1 nanometer = 10^–9 meters). The name gamma comes from the third letter of the Greek alphabet: gamma rays were the third kind of radiation discovered coming from radioactive atoms when physicists first investigated their behavior. Because gamma rays carry a lot of energy, they can be dangerous for living tissues. Gamma radiation is generated deep in the interior of stars, as well as by some of the most violent phenomena in the universe, such as the deaths of stars and the merging of stellar corpses. Gamma rays coming to Earth are absorbed by our atmosphere before they reach the ground (which is a good thing for our health); thus, they can only be studied using instruments in space.

Electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers is referred to as X-rays. Being more energetic than visible light, X-rays are able to penetrate soft tissues but not bones, and so allow us to make images of the shadows of the bones inside us. While X-rays can penetrate a short length of human flesh, they are stopped by the large numbers of atoms in Earth's atmosphere with which they interact. Thus, X-ray astronomy (like gamma-ray astronomy) could not develop until we invented ways of sending instruments above our atmosphere. Figure \(\PageIndex{3}\) is an image of the entire sky seen in x-rays, with different colors (red, yellow, and blue) representing different x-ray energies. Red outlines the glow from a hot local bubble of gas all around us, blown by one or more exploding stars in our cosmic vicinity. Yellow and blue show more distant sources of X-rays, such as remnants of other exploded stars or the active center of our galaxy.

Figure \(\PageIndex{3}\) : X-Ray Sky. This false-color, all-sky X-ray map is tilted so the Milky Way's disk runs across the center; red, yellow, and blue mark X-rays of increasing energy, from a nearby hot gas bubble to more distant sources near the galaxy's center. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{3}\).

Radiation intermediate between X-rays and visible light is ultraviolet (meaning higher energy than violet). Outside the world of science, ultraviolet light is sometimes called “black light” because our eyes cannot see it. Ultraviolet radiation is mostly blocked by the ozone layer of Earth's atmosphere, but a small fraction of ultraviolet rays from our Sun do penetrate to cause sunburn or, in extreme cases of overexposure, skin cancer in human beings. Ultraviolet astronomy is also best done from space.

Electromagnetic radiation with wavelengths between roughly 400 and 700 nm is called visible light because these are the waves that human vision can perceive. This is also the band of the electromagnetic spectrum that most readily reaches Earth's surface. These two observations are not coincidental: human eyes evolved to see the kinds of waves that arrive from the Sun most effectively. Visible light penetrates Earth's atmosphere effectively, except when it is temporarily blocked by clouds.

Between visible light and radio waves are the wavelengths of infrared or heat radiation. Astronomer William Herschel first discovered infrared in 1800 while trying to measure the temperatures of different colors of sunlight spread out into a spectrum. He noticed that when he accidentally positioned his thermometer beyond the reddest color, it still registered heating due to some invisible energy coming from the Sun. This was the first hint about the existence of the other (invisible) bands of the electromagnetic spectrum, although it would take many decades for our full understanding to develop.

A heat lamp radiates mostly infrared radiation, and the nerve endings in our skin are sensitive to this band of the electromagnetic spectrum. Infrared waves are absorbed by water and carbon dioxide molecules, which are more concentrated low in Earth's atmosphere. For this reason, infrared astronomy is best done from high mountaintops, high-flying airplanes, and spacecraft.

After infrared comes the familiar microwave, used in short-wave communication and microwave ovens. Wavelengths vary from 1 millimeter to 1 meter and are absorbed by water vapor, which makes them effective in heating foods. The “micro-” prefix refers to the fact that microwaves are small in comparison to radio waves, the next on the spectrum. You may remember that tea—which is full of water—heats up quickly in your microwave oven, while a ceramic cup—from which water has been removed by baking—stays cool in comparison.

All electromagnetic waves longer than microwaves are called radio waves, but this is so broad a category that we generally divide it into several subsections. Among the most familiar of these are radar waves, which are used in radar guns by traffic officers to determine vehicle speeds, and AM radio waves, which were the first to be developed for broadcasting. The wavelengths of these different categories range from over a meter to hundreds of meters, and other radio radiation can have wavelengths as long as several kilometers.

With such a wide range of wavelengths, not all radio waves interact with Earth's atmosphere in the same way. FM and TV waves are not absorbed and can travel easily through our atmosphere. AM radio waves are absorbed or reflected by a layer in Earth's atmosphere called the ionosphere (the ionosphere is a layer of charged particles at the top of our atmosphere, produced by interactions with sunlight and charged particles that are ejected from the Sun).

Table \(\PageIndex{1}\) summarizes the bands of the electromagnetic spectrum and indicates the temperatures and typical astronomical objects that emit each kind of electromagnetic radiation. While at first, some of the types of radiation listed in the table may seem unfamiliar, you will get to know them better as your astronomy course continues. You can return to this table as you learn more about the types of objects astronomers study. Note that the shortest wavelength radiation detects the highest temperature objects. For example, the solar corona is detected in X-rays and has a temperature of 10⁶–10⁸ K. Meanwhile, long wavelength radio waves detect objects cooler than 10,000 K such as cold gas in space.

Table \(\PageIndex{1}\): Types of Electromagnetic Radiation

Type of Radiation	Wavelength Range (nm)	Radiated by Objects at This Temperature	Typical Sources
Gamma rays	Less than 0.01	More than 10^8 K	Produced in nuclear reactions; require very high-energy processes
X-rays	0.01–20	10^6–10^8 K	Gas in clusters of galaxies, supernova remnants, solar corona
Ultraviolet	20–400	10^4–10^6 K	Supernova remnants, very hot stars
Visible	400–700	10^3–10^4 K	Stars
Infrared	10^3–10^6	10–10^3 K	Cool clouds of dust and gas, planets, moons
Microwave	10^6–10^9	Less than 10 K	Active galaxies, pulsars, cosmic background radiation
Radio	More than 10^9	Less than 10 K	Supernova remnants, pulsars, cold gas

Further Exploration

• Watch Neil deGrasse Tyson and Chuck Nice explain The Electromagnetic Spectrum on Star Talk.