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 atmo- sphere

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 spec- troscopy 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 ob- jects. Spectroscopy can be used to derive many proper- ties 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 mir- rors, 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 de- pendent 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. Zwicky hypothesized that there 14 must be a great deal of non-luminous matter in the galaxy 5000 K clusters, which became known as dark matter.[26] Since 12 Classical theory (5000 K) his discovery, astronomers have determined that a large 10 portion of galaxies (and most of the universe) is made up of dark matter. In 2003, however, four galaxies (NGC 8 821, NGC 3379, NGC 4494, and NGC 4697) were found 6 to have little to no dark matter influencing the motion of

4000 K the stars contained within them; the reason behind the 4 lack of dark matter is unknown.[27]

Spectral radiance (kW · sr ⁻ ¹ m ² nm ¹) 2 3000 K In the 1950s, strong radio sources were found to be as-

0 sociated with very dim, very red objects. When the first 0 0.5 1 1.5 2 2.5 3 spectrum of one of these objects was taken there were ab- Wavelength (μm) sorption lines at wavelengths where none were expected. It was soon realised that what was observed was a normal Black body curves for various temperatures. galactic spectrum, but highly red shifted.[28][29] These were named quasi-stellar radio sources, or quasars, by In 1860 Gustav Kirchhoff proposed the idea of a black Hong-Yee Chiu in 1964.[30] Quasars are now thought to body, a material that emits electromagnetic radiation at [20][21] be galaxies formed in the early years of our universe, with all wavelengths. In 1894 Wilhelm Wien derived an their extreme energy output powered by super-massive expression relating the temperature (T) of a black body black holes.[29] to its peak emission wavelength (λₐₓ).[22] The properties of a galaxy can also be determined by an- alyzing the stars found within them. NGC 4550, a galaxy in the Virgo Cluster, has a large portion of its stars ro- λmaxT = b tating in the opposite direction as the other portion. It is b is a constant of proportionality called Wien’s displace- believed that the galaxy is the combination of two smaller ment constant, equal to 2.8977729(17)×10−3 m K.[23] galaxies that were rotating in opposite directions to each This equation is called Wien’s Law. By measuring the other.[31] Bright stars in galaxies can also help determine 4 5 MOTION IN THE UNIVERSE

the distance to a galaxy, which may be a more accurate • The intensity of the 21 cm line gives the density and method than parallax or standard candles.[32] number of atoms in the cloud

• The temperature of the cloud can be calculated 4 Interstellar medium Using this information the shape of the Milky Way has The interstellar medium is matter that occupies the space been determined to be a spiral galaxy, though the exact between star systems in a galaxy. 99% of this matter number and position of the spiral arms is the subject of is gaseous - hydrogen, helium, and smaller quantities of ongoing research.[38] other ionized elements such as oxygen. The other 1% is dust particles, thought to be mainly graphite, silicates, [33] and ices. Clouds of the dust and gas are referred to as 4.2 Complex molecules nebulae. There are three main types of nebula: absorption, Main article: List of interstellar and circumstellar reflection, and emission nebulae. Absorption (or dark) molecules nebulae are made of dust and gas in such quanti- ties that they obscure the starlight behind them, mak- Dust and molecules in the interstellar medium not only ing photometry difficult. Reflection nebulae, as their obscures photometry, but also causes absorption lines name suggest, reflect the light of nearby stars. Their in spectroscopy. Their spectral features are generated spectra are the same as the stars surrounding them, by transitions of component electrons between differ- though the light is bluer; shorter wavelengths scatter ent energy levels, or by rotational or vibrational spec- better than longer wavelengths. Emission nebulae emit tra. Detection usually occurs in radio, microwave, or in- light at specific wavelengths depending on their chemical frared portions of the spectrum.[39] The chemical reac- composition.[33] tions that form these molecules can happen in cold, dif- fuse clouds[40] or in the hot ejecta around a white dwarf [41] 4.1 Gaseous emission nebulae star from a nova or supernova. Polycyclic aromatic hy- drocarbons such as acetylene (C2H2) generally group to- [42] In the early years of astronomical spectroscopy, scien- gether to form graphites or other sooty material, but [43] tists were puzzled by the spectrum of gaseous nebulae. In other organic molecules such as acetone ((CH3)2CO) 1864 William Huggins noticed that many nebulae showed and buckminsterfullerenes (C60 and C70) have been [41] only emission lines rather than a full spectrum like stars. discovered. From the work of Kirchhoff, he concluded that nebu- lae must contain “enormous masses of luminous gas or vapour.”[34] However, there were several emission lines 5 Motion in the universe that could not be linked to any terrestrial element, bright- est among them lines at 495.9 nm and 500.7 nm.[35] These lines were attributed to a new element, nebulium, until Ira Bowen determined in 1927 that the emission lines were from highly ionised oxygen (O+2).[36][37] These emission lines could not be replicated in a laboratory because they are forbidden lines; the low density of a nebula (one atom per cubic centimetre)[33] allows for metastable ions to de- cay via forbidden line emission rather than collisions with other atoms.[35] Not all emission nebulae are found around or near stars where solar heating causes ionisation. The majority of gaseous emission nebulae are formed of neutral hydro- gen. In the ground state neutral hydrogen has two pos- sible spin states: the electron has either the same spin or Redshift and blueshift the opposite spin of the proton. When the atom transi- tions between these two states, it releases an emission or Stars and interstellar gas are bound by gravity to form [33] absorption line of 21 cm. This line is within the radio galaxies, and groups of galaxies can be bound by gravity [35] range and allows for very precise measurements: in galaxy clusters.[44] With the exception of stars in the Milky Way and the galaxies in the Local Group, almost • Velocity of the cloud can be measured via Doppler all galaxies are moving away from us due to the expansion shift of the universe.[17] 5.3 Binary stars 5

5.1 Doppler effect and redshift This motion can cause confusion when looking at a solar or galactic spectrum, because the expected redshift based The motion of stellar objects can be determined by look- on the simple Hubble law will be obscured by the peculiar ing at their spectrum. Because of the Doppler effect, ob- motion. For example, the shape and size of the Virgo jects moving towards us are blueshifted, and objects mov- Cluster has been a matter of great scientific scrutiny due ing away are redshifted. The wavelength of redshifted to the very large peculiar velocities of the galaxies in the light is longer, appearing redder than the source. Con- cluster.[51] versely, the wavelength of blueshifted light is shorter, ap- pearing bluer than the source light: 5.3 Binary stars λ − λ v 0 = 0 λ0 c

where λ0 is the emitted wavelength, v0 is the velocity of the object, and λ is the observed wavelength. Note that v<0 corresponds to λ<λ0, a blueshifted wavelength. A redshifted absorption or emission line will appear more towards the red end of the spectrum than a stationary line. In 1913 Vesto Slipher determined the Andromeda Galaxy was blueshifted, meaning it was moving towards the Milky Way. He recorded the spectra of 20 other galaxies — all but 4 of which were redshifted — and was able to calculate their velocities relative to the Earth. Edwin Hubble would later use this information, as well as his own observations, to define Hubble’s law: The further a galaxy is from the Earth, the faster it is moving away Two stars of different size orbiting the center of mass. The spec- from us.[17][45] Hubble’s law can be generalised to trum can be seen to split depending on the position and velocity of the stars.

Just as planets can be gravitationally bound to stars, pairs v = H d 0 of stars can orbit each other. Some binary stars are vi- sual binaries, meaning they can be observed orbiting each where v is the velocity (or Hubble Flow), H0 is the Hubble Constant, and d is the distance from Earth. other through a telescope. Some binary stars, however, are too close together to be resolved.[52] These two stars, Redshift (z) can be expressed by the following [46] when viewed through a spectrometer, will show a com- equations: posite spectrum: the spectrum of each star will be added In these equations, frequency is denoted by f and wave- together. This composite spectrum becomes easier to de- length by λ . The larger the value of z, the more red- tect when the stars are of similar luminosity and of dif- shifted the light and the farther away the object is from ferent spectral class.[53] the Earth. As of January 2013, the largest galaxy redshift Spectroscopic binaries can be also detected due to their of z~12 was found using the Hubble Ultra-Deep Field, radial velocity; as they orbit around each other one star corresponding to an age of over 13 billion years (the uni- [47][48][49] may be moving towards the Earth whilst the other moves verse is approximately 13.82 billion years old). away, causing a Doppler shift in the composite spectrum. The Doppler effect and Hubble’s law can be combined to The orbital plane of the system determines the magnitude vHubble form the equation z = c , where c is the speed of of the observed shift: if the observer is looking perpen- light. dicular to the orbital plane there will be no observed ra- dial velocity.[52][53] For example, if you look at a carousel from the side, you will see the animals moving toward and 5.2 Peculiar motion away from you, whereas if you look from directly above they will only be moving in the horizontal plane. Objects that are gravitationally bound will rotate around a common center of mass. For stellar bodies, this motion is known as peculiar velocity, and can alter the Hubble Flow. Thus, an extra term for the peculiar motion needs 6 Planets, asteroids, and comets to be added to Hubble’s law:[50] Planets and asteroids shine only by the reflected light of their parent star, while comets both absorb and emit light vtotal = H0d + vpec at various wavelengths. 6 8 REFERENCES

6.1 Planets 7 See also

The reflected light of a planet contains absorption bands • Atomic and molecular astrophysics due to minerals in the rocks present for rocky bod- ies, or due to the elements and molecules present in • Emission spectrum the atmospheres of gas giants. To date almost 1000 • exoplanets have been discovered. These include so-called Gunn-Peterson trough Hot Jupiters, as well as Earth-like planets. Using spec- • Lyman-alpha forest troscopy, compounds such as alkali metals, water vapor, carbon monoxide, carbon dioxide, and methane have all • Photometry (astronomy) been discovered.[54] • Prism • 6.2 Asteroids Spectrometer

Asteroids can be classified into three major types accord- ing to their spectra. The original categories were created 8 References by Clark R. Chapman, David Morrison, and Ben Zell- ner in 1975, and further expanded by David J. Tholen [1] Foukal, Peter V. (2004). Solar Astrophysics. Weinheim: in 1984. In what is now known as the Tholen classifica- Wiley VCH. p. 69. ISBN 3-527-40374-4. tion, the C-types are made of carbonaceous material, S- [2] “Cool Cosmos - Infrared Astronomy”. California Institute types consist mainly of silicates, and X-types are 'metal- of Technology. Retrieved 23 October 2013. lic'. There are other classifications for unusual asteroids. C- and S-type asteroids are the most common asteroids. [3] Newton, Isaac (1705). Oticks: Or, A Treatise of the Reflec- In 2002 the Tholen classification was further “evolved” tions, Refractions, Inflections and Colours of Light. Lon- into the SMASS classification, expanding the number of don: Royal Society. pp. 13–19. categories from 14 to 26 to account for more precise spec- [55][56] [4] Fraunhofer, Joseph (1817). “Bestimmung des Brechungs- troscopic analysis of the asteroids. und des Farben-Zerstreuungs - Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre”. Annalen der Physik 6.3 Comets 56 (7): 282–287. Bibcode:1817AnP....56..264F. doi:10.1002/andp.18170560706.

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