SkyEye

In light we find such energy,
but it's one part upon the chart
which shows us more we would explore
if we could watch infinity.
— Robert J. Tiess, "Electromagnetic Spectrum", 2019

The Different Parts of the Electromagnetic Spectrum

The electromagnetic spectrum is comprised of all the different types of electromagnetic radiation, ranging from radio waves with wavelengths that can be measured in kilometres to visible light to gamma rays which have wavelengths smaller than an atomic nucleus. Electromagnetic radiation is catagorised by its wavelength (λ) or frequency (f). The two quantities are related by the formula λ = c/f where c is the speed of light.

Gamma Ray

This radiation has the shortest wavelength (measured in picometres) and thus, the highest frequency. Discovered in 1900 by French chemist and physicist Paul Ulrich Villard (1860–1934), gamma radiation is classified as ionising radiation, meaning it possesses sufficient energy to detach electrons from atoms or molecules. Gamma rays occur naturally from radioactive decay, interactions between cosmic rays and the atmosphere, and from a variety of astronomical sources such as solar flares, supernovae, quasars, and gamma ray bursts (GRBs).

X-ray

Still often called Röntgen rays after their discoverer, German physicist Wilhelm Conrad Röntgen (1845–1923), X-rays range in wavelength from picometres to nanometres. X-rays have wide medical applications but they are a form of ionising radiation and can cause harm in high doses or after repeated exposure. High-energy astronomical events produce X-rays which can be detected by orbiting X-ray satellites, but the Earth's atmosphere blocks extraterrestrial X-rays from reaching the ground.

Ultraviolet

Ultraviolet radiation is sometimes split into extreme ultraviolet (shorter wavelengths) and near ultraviolet (longer wavelengths). The Sun is the main source of ultraviolet radiation on Earth with the Earth's atmosphere blocking the shortest wavelengths. Some animals can see in the near ultraviolet and sunscreens are formulated to combat the damaging effects on skin of the longest ultraviolet wavelengths, namely, UV-A, UV-B, and UV-C. Ultraviolet radiation was discovered in 1801 by German physicist Johann Wilhelm Ritter (1776–1810).

Visible

Light in this category is visible to the human eye and typically ranges from 380 nanometres to 780 nanometres. Violet has the shortest wavelength, followed by blue, green, yellow, orange, and on to red which has the longest.

Infrared

Astronomer William Herschel (1738–1822) published his discovery of infrared radiation in 1800. It is often subdivided into near, mid, and far ranges, with wavelengths measured in micrometres. Around half of the Sun's output is in the infrared region but the Earth's atmosphere absorbs most of this energy. Infrared radiation has a number of uses, including night-vision devices, thermography, and cooking. Astronomers use infrared satellites to look through dusty regions of space.

Microwave

Well known for their use in microwave ovens, microwaves have wavelengths ranging from millimetres to centimetres. Astronomical sources of microwaves include the Sun and the cosmic microwave background.

Radio

Radio waves have the longest wavelengths, ranging from a centimetre to thousands of kilometres. Although artificially-produced radio waves are perhaps the best known, radio waves occur naturally, produced by lightning and by a number of astronomical sources. Some astronomical radio waves penetrate the Earth's atmosphere and are detected by special radio telescopes on the ground. However, the longest wavelengths are blocked.

Stellar Spectra

Spectroscopy, the study of electromagnetic spectra, allows astronomers to examine many physical properties of stars and other celestial bodies. These properties include chemical composition, temperature, density, pressure, rotation, and magnetism, as well as motion toward or away from the observer. Sir Isaac Newton (1642–1726/7) was the first scientist to use a simple prism to split white sunlight into separate colours. In 1802, English chemist and physicist William Hyde Wollaston (1766–1828) noticed that the solar spectrum had dark lines running through it. Twelve years later, German physicist Joseph von Fraunhofer (1787–1826) independently discovered the same phenomenon. Further experimentation by Fraunhofer led to the detection of similar dark lines in stellar spectra. It was later found that these lines were atomic and molecular absorption lines, some of them originating in the stars and others caused by absorption by molecules in Earth's atmosphere. Absorption lines caused by atmospheric intereference are termed telluric lines. These dark lines are still call Fraunhofer lines today.

Stars are organised according to the characteristics present in their spectra. Most stars are classified according to their surface temperature using the Harvard system. This uses letters — O, B, A, F, G, K, M — with type O being the hottest and type M the coolest. Each letter type is further subdivided using numbers zero through nine with zero being the hottest and nine the coolest. As an example, a star of type B3 is hotter than a star of type B6, and an O9 star is hotter than a K2 star. There are several other letters used for special kinds of stars which fall outside this sequence, such as D for white dwarfs, C for carbon stars, and L, T, and Y for very cool objects such as brown dwarfs. A luminosity class is usually appended to the spectral type. The Yerkes system uses Roman numerals (plus zero) to indicate whether the star is some kind of supergiant or giant star, a subgiant star, or a dwarf star. Hypergiants are assigned 0, supergiants are types Ⅰa, Ⅰb, or Ⅰab, bright giants are Ⅱ, giants are Ⅲ, subgiants are Ⅳ, dwarfs are Ⅴ, and subdwarfs are Ⅵ. Additional nomenclature to address peculiarities of a spectrum can be added. These are usually lower case Latin alphabet letters and denote such things are emission lines (e), broad or nebulous absorption lines due to rapid rotation (n), or sharp or narrow lines (s), to name but a few. The Sun is of spectral type G2Ⅴ, a cooler main sequence or dwarf star.

The Solar Spectrum

Below is a representation of some of the stronger Fraunhofer lines found in the visible portion of the Sun's spectra. The line widths correspond to the equivalent line width of the actual absorption line. The strongest lines that were identified by Fraunhofer himself are labelled.

In the violet and blue part of the solar spectrum, the lines come thick and fast. Dominating the region between 380 and 480 nanometres (3800 to 4800 Angstroms) are the H and K lines of singly ionised calcium (Ca Ⅱ). The next strongest lines are two from the neutral hydrogen (H Ⅰ) Balmer series, h and f, now called the Hδ and Hγ lines. The g (neutral calcium Ca Ⅰ), and e and d lines (neutral iron FeⅠ) also stand out. The G line is more of a band of closely spaced lines, primarily comprised of methylidyne (CH), neutral calcium, and neutral iron, along with a few other elements in various ionisation states.

The lines begin to thin out as the wavelengths get longer. Between 480 and 580 nanometres is found the strong F line which corresponds to the Balmer Hβ line. Neutral iron is responsible for the fainter c and E lines. The quartet of b lines belong to neutral iron and neutral magnesium (Mg Ⅰ).

Line C is the strong Balmer Hα line, and is located in the region between 480 and 580 nanometres. Also prominent is the doublet of D lines which are generated by neutral sodium (Na Ⅰ). However, the a line does not originate on the Sun but in Earth's atmosphere. This is a telluric line generated by moledular oxygen (O₂). Atmospheric contamination becomes more dominant at longer wavelengths.

Atmospheric contamination due to molecular oxygen and water vapour (H₂O) begins to dominate the spectrum at the extreme red end of the scale, with the B and A Fraunhofer lines both due to O₂. However, some of the fainter lines originate on the Sun and represent a variety of elements and molecules, including aluminium (Al), calcium, carbon (C), neutral and singly-ionised iron, nickel (Ni), silicon (Si), titanium (Ti), zirconium (Zr), and cyanide (CN), to name but a few.