Astronomical Spectroscopy

Astronomical spectroscopy Most of the Long-wavelength Visible light infrared spectrum Radio waves observable radio waves Gamma rays, X-rays and ultraviolet observable absorbed by from Earth. blocked. light blocked by the upper atmosphere from Earth, atmospheric (best observed from space). with some gasses (best atmospheric observed distortion. from space). 100 % 50 % opacity Atmospheric 0 % 0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm 1 cm 10 cm 1 m 10 m 100 m 1 km Wavelength Electromagnetic transmittance, or opacity, of the Earth’s atmosphere the signal depending on the frequency. Ozone (O3) and Refereed version molecular oxygen (O2) absorb light with wavelengths un- der 300 nm, meaning that X-ray and ultraviolet spectroscopy require the use of a satellite telescope or rocket mounted detectors.[1]:27 Radio signals have much longer wavelengths than optical signals, and require the use of antennas or radio dishes. Infrared light is absorbed by atmospheric water and carbon dioxide, so while the equipment is similar to that used in optical spectroscopy, satellites are required to record much of the infrared spectrum.[2] 1.1 Optical spectroscopy The Star-Spectroscope of the Lick Observatory in 1898. De- signed by James Keeler and constructed by John Brashear. Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other hot celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical Incident light reflects at the same angle (black lines), but a small composition, temperature, density, mass, distance, lumi- portion of the light is refracted as coloured light (red and blue nosity, and relative motion using Doppler shift measure- lines). ments. Physicists have been looking at the solar spectrum since Isaac Newton first used a simple prism to observe the re- fractive properties of light.[3] In the early 1800s Joseph 1 Background von Fraunhofer used his skills as a glass maker to create very pure prisms, which allowed him to observe 574 dark Astronomical spectroscopy is used to measure three ma- lines in a seemingly continuous spectrum.[4] Soon after jor bands of radiation: visible spectrum, radio, and X- he combined telescope and prism to observe the spec- ray. While all spectroscopy looks at specific areas of trum of Venus, the Moon, Mars, and various stars such the spectrum, different methods are required to acquire as Betelgeuse; his company continued to manufacture and 1 2 2 STARS AND THEIR PROPERTIES sell high-quality refracting telescopes based on his origi- discrete Fourier transforming the incoming signal, recov- nal designs until its closure in 1884.[5]:28–29 ers both the spatial and frequency variation in flux.[14] The resolution of a prism is limited by its size; a larger The result is a 3D image whose third axis is frequency. For this work, Ryle and Hewish were jointly awarded the prism will provide a more detailed spectrum, but the in- [15] crease in mass makes it unsuitable for highly detailed 1974 Nobel Prize in Physics. work.[6] This issue was resolved in the early 1900s with the development of high-quality reflection gratings by 1.3 X-ray spectroscopy J.S. Plaskett at the Dominion Observatory in Ottawa, [5]:11 Canada. Light striking a mirror will reflect at the Main article: X-ray astronomy same angle, however a small portion of the light will be refracted at a different angle; this is dependent upon the indices of refraction of the materials and the wavelength of the light.[7] By creating a “blazed” grating which uti- lizes a large number of parallel mirrors, the small portion 2 Stars and their properties of light can be focused and visualized. These new spec- troscopes were more detailed than a prism, required less 2.1 Chemical properties light, and could be focused on a specific region of the spectrum by tilting the grating.[6] The limitation to a blazed grating is the width of the mirrors, which can only be ground a finite amount before fo- cus is lost; the maximum is around 1000 lines/mm. In or- Continuous spectrum der to overcome this limitation holographic gratings were developed. Volume phase holographic gratings use a thin film of dichromated gelatin on a glass surface, which is subsequently exposed to a wave pattern created by an interferometer. This wave pattern sets up a reflection pattern similar to the blazed gratings but utilizing Bragg Emission lines diffraction, a process where the angle of reflection is dependent on the arrangement of the atoms in the gelatin. The holographic gratings can have up to 6000 lines/mm and can be up to twice as efficient in collecting light as blazed gratings. Because they are sealed between two Absorption lines sheets of glass, the holographic gratings are very versatile, potentially lasting decades before needing replacement.[8] Newton used a prism to split white light into a spectrum of color, and Fraunhofer’s high-quality prisms allowed sci- entists to see dark lines of an unknown origin. It was not 1.2 Radio spectroscopy until the 1850s that Gustav Kirchhoff and Robert Bunsen would describe the phenomena behind these dark lines— hot solid objects produce light with a continuous spec- Radio astronomy was founded with the work of Karl Jan- trum, hot gasses emit light at specific wavelengths, and sky in the early 1930s, while working for Bell Labs. He hot solid objects surrounded by cooler gasses will show a built a radio antenna to look at potential sources of in- near-continuous spectrum with dark lines corresponding terference for transatlantic radio transmissions. One of to the emission lines of the gasses.[5]:42–44[16] By compar- the sources of noise discovered came not from Earth, but ing the absorption lines of the sun with emission spectra from the center of the Milky Way, in the constellation of known gasses, the chemical composition of stars can Sagittarius.[9] In 1942, JS Hey captured the sun’s radio be determined. frequency using military radar receivers.[1]:26 The major Fraunhofer lines, and the elements they are Radio interferometry was pioneered in 1946, when associated with, are shown in the following table. Desig- Joseph Lade Pawsey, Ruby Payne-Scott and Lindsay Mc- nations from the early Balmer Series are in parentheses. Cready used a single antenna atop a sea cliff to observe 200 MHz solar radiation. Two incident beams, one Not all of the elements in the sun were immediately iden- directly from the sun and the other reflected from the tified. Two examples are listed below. sea surface, generated the necessary interference.[10] The first multi-receiver interferometer was built in the same • In 1868 Norman Lockyer and Pierre Janssen inde- year by Martin Ryle and Vonberg.[11][12] In 1960, Ryle pendently observed a line next to the sodium doublet and Antony Hewish published the technique of aperture (D1 and D2) which Lockyer determined to be a new synthesis to analyze interferometer data.[13] The aper- element. He named it Helium, but it wasn't until ture synthesis process, which involves autocorrelating and 1895 the element was found on Earth.[5]:84–85 3 • In 1869 the astronomers Charles Augustus Young peak wavelength of a star, the surface temperature can and William Harkness independently observed a be determined.[16] For example, if the peak wavelength novel green emission line in the Sun’s corona dur- of a star is 502 nm the corresponding temperature will ing an eclipse. This “new” element was incorrectly be 5778 Kelvin. named coronium, as it was only found in the corona. The luminosity of a star is a measure of the It was not until the 1930s that Walter Grotrian and electromagnetic energy output in a given amount of Bengt Edlén discovered that the spectral line at [24] 13+ [17] time. Luminosity (L) can be related to the tempera- 530.3 nm was due to highly ionized iron (Fe ). ture (T) of a star by Other unusual lines in the coronal spectrum are also caused by highly charged ions, such as nickel and calcium, the high ionization being due to the ex- 2 4 treme temperature of the solar corona.[1]:87,297 L = 4πR σT where R is the radius of the star and σ is the Stefan– To date more than 20 000 absorption lines have been Boltzmann constant, with a value of 5.670367(13)×10−8 listed for the Sun between 293.5 and 877.0 nm, yet only W m−2 K−4.[25] Thus, when both luminosity and temper- approximately 75% of these lines have been linked to el- ature are known (via direct measurement and calculation) [1]:69 emental absorption. the radius of a star can be determined. By analyzing the width of each spectral line in an emis- See also: Luminosity and Magnitude (astronomy) sion spectrum, both the elements present in a star and their relative abundances can be determined.[7] Using this information stars can be categorized into stellar popula- tions; Population I stars are the youngest stars and have 3 Galaxies the highest metal content (our Sun is a Pop I star), while Population III stars are the oldest stars with a very low metal content.[18][19] The spectra of galaxies look similar to stellar spectra, as they consist of the combined light of millions of stars. Doppler shift studies of galaxy clusters by Fritz Zwicky in 2.2 Temperature and size 1937 found that most galaxies were moving much faster than seemed to be possible from what was known about UV VISIBLE INFRARED the mass of the cluster.

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