Astronomical spectroscopy
Encyclopedia
Astronomical spectroscopy is the technique of spectroscopy
used in astronomy
. The object of study is the spectrum
of electromagnetic radiation
, including visible light, which radiates
from star
s and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, but also their motion by Doppler shift measurements.
's initial observations of the light of the Sun
, dispersed by a prism. He saw a rainbow
of colour, and may even have seen absorption lines. These dark bands which appear throughout the solar spectrum were first described in detail by Joseph von Fraunhofer
. Most stellar spectra share these two dominant features of the Sun's spectrum: emission at all wavelengths across the optical spectrum (the continuum) with many discrete absorption lines, resulting from gaps of radiation.
Fraunhofer's original (1817) designations of absorption lines in the solar spectrum
! Letter
! Wavelength
(nm)
! Chemical origin
! Colour range
|-
|A
|759.37
|atmospheric O2
|dark red
|-
|B
|686.72
|atmospheric O2
|red
|-
|C
|656.28
|hydrogen
alpha
|red
|-
|D1
|589.59
|neutral sodium
|red orange
|-
|D2
|589.00
|neutral sodium
|yellow
|-
|E
|526.96
|neutral iron
|green
|-
|F
|486.13
|hydrogen
beta
|cyan
|-
|G
|431.42
|CH molecule
|blue
|-
|H
|396.85
|ionised calcium
|dark violet
|-
|K
|393.37
|ionised calcium
|dark violet
|}>
Fraunhofer and Angelo Secchi
were among the pioneers of spectroscopy of the Sun and other stars. Secchi is particularly noted for classifying stars into spectral types, based on the number and strength of the absorption lines in their spectra. Later the origin of the spectral types was found to be related to the temperature of the surface of the star: particular absorption lines can be observed only for a certain range of temperatures; because only in that range are the involved atomic energy level
s populated.
The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Each element is responsible for a different set of absorption lines in the spectrum, at wavelengths which can be measured extremely accurately by laboratory experiments. Then, an absorption line at the given wavelength in a stellar spectrum shows that the element must be present. Of particular importance are the absorption lines of hydrogen
(which is found in the atmosphere
of nearly every star); the hydrogen lines within the visual spectrum are known as Balmer lines.
In 1868, Sir Norman Lockyer observed strong yellow lines in the solar spectrum which had never been seen in laboratory experiments. He deduced that they must be due to an unknown element, which he called helium
, from the Greek helios (sun). Helium wasn't conclusively detected on earth until 25 years later.
Also in the 1860s, emission lines (particularly a green line) were observed in the corona
l spectrum during solar eclipses that did not correspond to any known spectral lines. Again it was proposed that these were due to an unknown element, provisionally named coronium
. It was not until the 1930s that it was discovered that these lines were due to highly ionised iron
and nickel
, the high ionisation being due to the extreme temperature of the solar corona.
In conjunction with atomic physics and models of stellar evolution
, stellar spectroscopy is today used to determine a multitude of properties of stars: their distance, age, luminosity and rate of mass loss can all be estimated from spectral studies, and Doppler shift studies can uncover the presence of hidden companions such as black hole
s and exoplanets.
was used to describe any fuzzy patch of light that didn't look like a star. Many of these, such as the Andromeda Nebula
, had spectra that looked in many ways a lot like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula
, had very different spectra. When William Huggins
looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong emission lines. These lines did not correspond to any known elements on earth, and so just as helium
had been identified in the Sun, astronomers suggested that the lines were due to a new element, nebulium (occasionally nebulum or nephelium). The hypothetical nebulium that was invoked to account for certain bright lines in gaseous nebulae were shown by Ira Sprague Bowen
in 1927 as due to doubly ionized
oxygen
at extremely low density. As Henry Norris Russell
put it, "Nebulium has vanished into thin air." But nebulae are typically extremely rarefied
, much less dense than the hardest vacuum ever produced on earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as forbidden lines
, and are the strongest lines in most nebular spectra.
look somewhat similar to stellar spectra, as they consist of the light from millions of stars combined. Galactic spectroscopy has led to many fundamental discoveries. Edwin Hubble
discovered in the 1920s that, apart from the nearest ones (those in what is known as the Local Group
), all galaxies are receding from the Earth. The further away a galaxy, the faster it is receding (see Hubble's Law
). This was the first indication that the universe originated from a single point, in a Big Bang
.
Doppler shift studies of clusters of galaxies by Fritz Zwicky
found that most galaxies were moving much faster than seemed to be possible, from what was known about the mass of the cluster. Zwicky hypothesised that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as dark matter
.
sources were found to be associated with very dim objects that seemed to be very blue. These were named Quasi-stellar radio sources, or quasars. When the first spectrum of one of these objects was taken, it was something of a mystery, with absorption lines at wavelengths where none were expected. It was soon realised that what was being seen was a normal galactic spectrum, but highly redshift
ed. According to Hubble's Law
, this implied that the quasar must be extremely distant, and therefore highly luminous. Quasars are now thought to be galaxies forming, with their extreme energy output being powered by super-massive black hole
s.
s and asteroid
s shine only by reflecting the light of their parent star. The reflected light contains absorption bands due to mineral
s in the rocks present for rocky bodies, or due to the elements and molecules present in the atmospheres of the Gas giants
. Asteroids can be classified into three main types, according to their spectra: the C-types
are made of carbonaceous material, S-types
consist mainly of silicates, and M-types
are 'metallic'. C- and S-type asteroids are the most common.
and/or chemical reactions. Nearby comets can even be seen in X-ray as solar wind ions flying to the coma
are neutralized, and cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet. Many organic chemicals
are known to exist in comets, and it has been suggested that cometary impacts provided the Earth with much of the water for its ocean
s and the chemicals necessary for the formation of life
. It has even been suggested that life may have been brought to earth from interstellar space by comets (the Panspermia theory).
. At the frequencies that molecules, atoms, or ions emit and absorb strongly, most light from deeper parts of the photosphere is absorbed before escaping. The light we see at those absorbed frequencies comes from still shallower depths. The shallow regions are cooler, so the light coming from them is weaker. Thus the spectrum contains relatively dark bands at the characteristic frequencies of constituents of the outer layers.
can be used to view solar spectra. Note that looking at the Sun
directly or by reflection can result in temporary or permanent loss of vision. Proper equipment must be used, and children should be supervised.
Recently, there has been a resurgence in amateur astronomical spectroscopy. It is possible to use a digital camera
or telescope to easily view spectra of astronomical objects.
Inexpensive diffraction grating
s like the Paton Hawksley Star Analyser or the Rainbow Optics Star Spectroscope can be used to split the starlight. Software like RSpec can be used to plot the curves.
An excellent introduction to amateur spectroscopy is Astronomical Spectroscopy for Amateurs, published in 2011.
Also, Sky & Telescope Magazine has produced an excellent video interview that explains how to get started.
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
used in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
. The object of study is the spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, including visible light, which radiates
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
from star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, but also their motion by Doppler shift measurements.
Stars
Astronomical spectroscopy began with Isaac NewtonIsaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
's initial observations of the light of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, dispersed by a prism. He saw a rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
of colour, and may even have seen absorption lines. These dark bands which appear throughout the solar spectrum were first described in detail by Joseph von Fraunhofer
Joseph von Fraunhofer
Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...
. Most stellar spectra share these two dominant features of the Sun's spectrum: emission at all wavelengths across the optical spectrum (the continuum) with many discrete absorption lines, resulting from gaps of radiation.
Fraunhofer's original (1817) designations of absorption lines in the solar spectrum
Letter (alphabet)
A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....
! Wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
(nm)
! Chemical origin
! Colour range
|-
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Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
alpha
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Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
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Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
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Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
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Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
beta
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Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
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Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
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Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
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Fraunhofer and Angelo Secchi
Angelo Secchi
-External links:...
were among the pioneers of spectroscopy of the Sun and other stars. Secchi is particularly noted for classifying stars into spectral types, based on the number and strength of the absorption lines in their spectra. Later the origin of the spectral types was found to be related to the temperature of the surface of the star: particular absorption lines can be observed only for a certain range of temperatures; because only in that range are the involved atomic energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s populated.
The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Each element is responsible for a different set of absorption lines in the spectrum, at wavelengths which can be measured extremely accurately by laboratory experiments. Then, an absorption line at the given wavelength in a stellar spectrum shows that the element must be present. Of particular importance are the absorption lines of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(which is found in the atmosphere
Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. It is divided into several regions of distinct character:...
of nearly every star); the hydrogen lines within the visual spectrum are known as Balmer lines.
In 1868, Sir Norman Lockyer observed strong yellow lines in the solar spectrum which had never been seen in laboratory experiments. He deduced that they must be due to an unknown element, which he called helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, from the Greek helios (sun). Helium wasn't conclusively detected on earth until 25 years later.
Also in the 1860s, emission lines (particularly a green line) were observed in the corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
l spectrum during solar eclipses that did not correspond to any known spectral lines. Again it was proposed that these were due to an unknown element, provisionally named coronium
Coronium
Coronium was the name of a suggested chemical element, hypothesised in the 19th century. It was named after the solar corona.During the total solar eclipse of 7 August 1869, a green emission line of wavelength 530.3 nm was observed in the coronal spectrum...
. It was not until the 1930s that it was discovered that these lines were due to highly ionised iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, the high ionisation being due to the extreme temperature of the solar corona.
In conjunction with atomic physics and models of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
, stellar spectroscopy is today used to determine a multitude of properties of stars: their distance, age, luminosity and rate of mass loss can all be estimated from spectral studies, and Doppler shift studies can uncover the presence of hidden companions such as black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s and exoplanets.
Nebulae
In the early days of telescopic astronomy, the word nebulaNebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
was used to describe any fuzzy patch of light that didn't look like a star. Many of these, such as the Andromeda Nebula
Andromeda nebula
Andromeda nebula may refer to:* Andromeda Galaxy, a spiral galaxy in the Andromeda constellation* Andromeda , a 1957 science fiction novel by Ivan Efremov* The Andromeda Nebula, a 1967 Russian film based on the above novel...
, had spectra that looked in many ways a lot like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula
Cat's Eye Nebula
The Cat's Eye Nebula is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets, bubbles and sinewy arc-like features...
, had very different spectra. When William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...
looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong emission lines. These lines did not correspond to any known elements on earth, and so just as helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
had been identified in the Sun, astronomers suggested that the lines were due to a new element, nebulium (occasionally nebulum or nephelium). The hypothetical nebulium that was invoked to account for certain bright lines in gaseous nebulae were shown by Ira Sprague Bowen
Ira Sprague Bowen
Ira Sprague Bowen was an American astronomer. In 1927 he discovered that nebulium was not really a chemical element but instead doubly ionized oxygen.-Life and work:...
in 1927 as due to doubly ionized
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
at extremely low density. As Henry Norris Russell
Henry Norris Russell
Henry Norris Russell was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung–Russell diagram . In 1923, working with Frederick Saunders, he developed Russell–Saunders coupling which is also known as LS coupling.-Biography:Russell was born in 1877 in Oyster Bay, New...
put it, "Nebulium has vanished into thin air." But nebulae are typically extremely rarefied
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, much less dense than the hardest vacuum ever produced on earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as forbidden lines
Forbidden mechanism
In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient route...
, and are the strongest lines in most nebular spectra.
Galaxies
The spectra of galaxiesGalaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
look somewhat similar to stellar spectra, as they consist of the light from millions of stars combined. Galactic spectroscopy has led to many fundamental discoveries. Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
discovered in the 1920s that, apart from the nearest ones (those in what is known as the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
), all galaxies are receding from the Earth. The further away a galaxy, the faster it is receding (see Hubble's Law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
). This was the first indication that the universe originated from a single point, in a Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
.
Doppler shift studies of clusters of galaxies by Fritz Zwicky
Fritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....
found that most galaxies were moving much faster than seemed to be possible, from what was known about the mass of the cluster. Zwicky hypothesised that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
.
Quasars
In the 1950s, some strong radioRadio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
sources were found to be associated with very dim objects that seemed to be very blue. These were named Quasi-stellar radio sources, or quasars. When the first spectrum of one of these objects was taken, it was something of a mystery, with absorption lines at wavelengths where none were expected. It was soon realised that what was being seen was a normal galactic spectrum, but highly redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
ed. According to Hubble's Law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
, this implied that the quasar must be extremely distant, and therefore highly luminous. Quasars are now thought to be galaxies forming, with their extreme energy output being powered by super-massive black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s.
Planets and asteroids
PlanetPlanet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s shine only by reflecting the light of their parent star. The reflected light contains absorption bands due to mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s in the rocks present for rocky bodies, or due to the elements and molecules present in the atmospheres of the Gas giants
Gas Giants
Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...
. Asteroids can be classified into three main types, according to their spectra: the C-types
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
are made of carbonaceous material, S-types
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
consist mainly of silicates, and M-types
M-type asteroid
M-type asteroids are asteroids of partially known composition; they are moderately bright . Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts,...
are 'metallic'. C- and S-type asteroids are the most common.
Comets
The spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet, as well as emission lines from gaseous atoms and molecules excited by sunlight fluorescenceFluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
and/or chemical reactions. Nearby comets can even be seen in X-ray as solar wind ions flying to the coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
are neutralized, and cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet. Many organic chemicals
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
are known to exist in comets, and it has been suggested that cometary impacts provided the Earth with much of the water for its ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s and the chemicals necessary for the formation of life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
. It has even been suggested that life may have been brought to earth from interstellar space by comets (the Panspermia theory).
Mechanisms
Absorption spectra of nebulae and planetary atmospheres (and gases in terrestrial experiments) arise simply when molecules absorb light of certain frequencies and re-emit it, if at all, in directions away from the observer. Absorption spectra of stars arise from a different mechanism. Starlight at most frequencies is emitted from close enough to the surface that it can escape; this outer layer is called the photospherePhotosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
. At the frequencies that molecules, atoms, or ions emit and absorb strongly, most light from deeper parts of the photosphere is absorbed before escaping. The light we see at those absorbed frequencies comes from still shallower depths. The shallow regions are cooler, so the light coming from them is weaker. Thus the spectrum contains relatively dark bands at the characteristic frequencies of constituents of the outer layers.
Amateur Spectroscopy
It is not hard to view spectra of astronomical objects. A simple, homemade DVD spectrographSpectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
can be used to view solar spectra. Note that looking at the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
directly or by reflection can result in temporary or permanent loss of vision. Proper equipment must be used, and children should be supervised.
Recently, there has been a resurgence in amateur astronomical spectroscopy. It is possible to use a digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
or telescope to easily view spectra of astronomical objects.
Inexpensive diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s like the Paton Hawksley Star Analyser or the Rainbow Optics Star Spectroscope can be used to split the starlight. Software like RSpec can be used to plot the curves.
An excellent introduction to amateur spectroscopy is Astronomical Spectroscopy for Amateurs, published in 2011.
Also, Sky & Telescope Magazine has produced an excellent video interview that explains how to get started.
See also
- Gunn-Peterson troughGunn-Peterson troughIn astronomical spectroscopy, the Gunn-Peterson trough is a feature of the spectra of quasars due to the presence of neutral hydrogen in the Intergalactic Medium . The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the Lyman-alpha...
- Lyman-alpha forestLyman-alpha forestIn astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....
- PhotometryPhotometry (astronomy)Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
- SpectrometerSpectrometerA spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
- Emission spectrumEmission spectrumThe emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....