
Astronomical spectroscopy is the study of
astronomy
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
using the techniques of
spectroscopy
Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
Spectro ...
to measure the
spectrum
A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
, including
visible light,
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
,
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
,
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
and
radio
Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
waves that
radiate from
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s and other celestial objects. A stellar spectrum can reveal many properties of stars, such as their chemical composition, temperature, density, mass, distance and luminosity. Spectroscopy can show the velocity of motion towards or away from the observer by measuring the
Doppler shift. Spectroscopy is also used to study the physical properties of many other types of celestial objects such as
planet
A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s,
nebula
A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...
e,
galaxies, and
active galactic nuclei.
Background

Astronomical spectroscopy is used to measure three major bands of radiation in the electromagnetic spectrum:
visible light,
radio wave
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s, and
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
s. While all spectroscopy looks at specific bands of the spectrum, different methods are required to acquire the signal depending on the frequency.
Ozone
Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
(O
3) and molecular oxygen (O
2) absorb light with wavelengths under 300 nm, meaning that X-ray and
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
spectroscopy require the use of a satellite telescope or
rocket mounted detectors.
Radio signals have much longer wavelengths than optical signals, and require the use of
antennas or radio dishes.
Infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
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.
Optical spectroscopy

Physicists have been looking at the solar spectrum since
Isaac Newton
Sir Isaac Newton () was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian, and author. Newton was a key figure in the Scientific Revolution and the Age of Enlightenment, Enlightenment that followed ...
first used a simple prism to observe the refractive properties of light.
In the early 1800s
Joseph von Fraunhofer used his skills as a glassmaker to create very pure prisms, which allowed him to observe 574 dark lines in a seemingly continuous spectrum.
Soon after this, he combined telescope and prism to observe the spectrum of
Venus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
, the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
,
Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
, and various stars such as
Betelgeuse; his company continued to manufacture and sell high-quality refracting telescopes based on his original designs until its closure in 1884.
The resolution of a prism is limited by its size; a larger prism will provide a more detailed spectrum, but the increase in mass makes it unsuitable for highly detailed work.
This issue was resolved in the early 1900s with the development of high-quality reflection gratings by
J.S. Plaskett at the
Dominion Observatory in Ottawa, Canada.
Light striking a mirror will reflect at the 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.
By creating a
"blazed" grating which utilizes a large number of parallel mirrors, the small portion of light can be focused and visualized. These new spectroscopes were more detailed than a prism, required less light, and could be focused on a specific region of the spectrum by tilting the grating.
The limitation to a blazed grating is the width of the mirrors, which can only be ground a finite amount before focus is lost; the maximum is around 1000 lines/mm. In order 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 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 sheets of glass, the holographic gratings are very versatile, potentially lasting decades before needing replacement.
Light
dispersed by the grating or prism in a
spectrograph can be recorded by a detector. Historically,
photographic plates were widely used to record spectra until electronic detectors were developed, and today optical spectrographs most often employ
charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
s (CCDs). The wavelength scale of a spectrum can be
calibrated by observing the spectrum of
emission lines of known wavelength from a
gas-discharge lamp
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionization, ionized gas, a plasma (physics), plasma.
Typically, such lamps use a
noble gas (argon, neon, krypton, and x ...
. The
flux scale of a spectrum can be calibrated as a function of wavelength by comparison with an observation of a standard star with corrections for atmospheric absorption of light; this is known as
spectrophotometry.
Radio spectroscopy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies Astronomical object, celestial objects using radio waves. It started in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observat ...
was founded with the work of
Karl Jansky in the early 1930s, while working for
Bell Labs
Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
. He built a radio antenna to look at potential sources of interference for transatlantic radio transmissions. One of the sources of noise discovered came not from Earth, but from the center of the
Milky Way
The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
, in the constellation
Sagittarius. In 1942,
JS Hey captured the Sun's radio frequency using military radar receivers.
Radio spectroscopy started with the discovery of the
21-centimeter H I line in 1951.
Radio interferometry
Radio interferometry was pioneered in 1946, when
Joseph Lade Pawsey,
Ruby Payne-Scott and Lindsay McCready used a
single antenna atop a sea cliff to observe 200 MHz solar radiation. Two incident beams, one directly from the sun and the other reflected from the sea surface, generated the necessary interference. The first multi-receiver interferometer was built in the same year by
Martin Ryle and Vonberg.
In 1960, Ryle and
Antony Hewish published the technique of
aperture synthesis to analyze interferometer data. The aperture synthesis process, which involves
autocorrelating and
discrete Fourier transform
In mathematics, the discrete Fourier transform (DFT) converts a finite sequence of equally-spaced Sampling (signal processing), samples of a function (mathematics), function into a same-length sequence of equally-spaced samples of the discre ...
ing the incoming signal, recovers both the spatial and frequency variation in flux. The result is a
3D image whose third axis is frequency. For this work, Ryle and Hewish were jointly awarded the 1974
Nobel Prize in Physics
The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...
.
X-ray spectroscopy
Stars and their properties
Chemical properties
Newton used a prism to split white light into a spectrum of color, and Fraunhofer's high-quality prisms allowed scientists to see dark lines of an unknown origin. In the 1850s,
Gustav Kirchhoff
Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German chemist, mathematician, physicist, and spectroscopist who contributed to the fundamental understanding of electrical circuits, spectroscopy and the emission of black-body ...
and
Robert Bunsen described the phenomena behind these dark lines. Hot solid objects produce light with a
continuous spectrum, hot gases emit light at specific wavelengths, and hot solid objects surrounded by cooler gases show a near-continuous spectrum with dark lines corresponding to the emission lines of the gases.
By comparing the
absorption lines of the Sun with
emission spectra of known gases, the chemical composition of stars can be determined.
The major
Fraunhofer lines, and the elements with which they are associated, appear in the following table. Designations from the early
Balmer Series are shown in parentheses.
Not all of the elements in the Sun were immediately identified. Two examples are listed below:
*In 1868
Norman Lockyer and
Pierre Janssen independently observed a line next to the sodium doublet (D
1 and D
2) which Lockyer determined to be a new element. He named it
Helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, but it wasn't until 1895 the element was found on Earth.
*In 1869 the astronomers
Charles Augustus Young and
William Harkness independently observed a novel green emission line in the
Sun's corona during an eclipse. This "new" element was incorrectly named
coronium, as it was only found in the corona. It was not until the 1930s that
Walter Grotrian and
Bengt Edlén discovered that the spectral line at 530.3 nm was due to
highly ionized iron (Fe
13+).
Other unusual lines in the coronal spectrum are also caused by highly charged ions, such as
nickel
Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
and
calcium
Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
, the high ionization being due to the extreme temperature of the
solar corona
In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...
.
To date more than 20 000 absorption lines have been listed for the
Sun between 293.5 and 877.0 nm, yet only approximately 75% of these lines have been linked to elemental absorption.
By analyzing the
equivalent width of each spectral line in an emission spectrum, both the elements present in a star and their relative abundances can be determined.
Using this information stars can be categorized into
stellar populations; Population I stars are the youngest stars and have the highest metal content (the Sun is a Pop I star), while Population III stars are the oldest stars with a very low metal content.
Temperature and size

In 1860
Gustav Kirchhoff
Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German chemist, mathematician, physicist, and spectroscopist who contributed to the fundamental understanding of electrical circuits, spectroscopy and the emission of black-body ...
proposed the idea of a
black body, a material that emits electromagnetic radiation at all wavelengths.
In 1894
Wilhelm Wien derived an expression relating the temperature (T) of a black body to its peak emission wavelength (λ
max):
:
''b'' is a
constant of proportionality called ''Wien's displacement constant'', equal to This equation is called
Wien's Law. By measuring the peak wavelength of a star, the surface temperature can be determined.
For example, if the peak wavelength of a star is 502 nm the corresponding temperature will be 5772
kelvin
The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...
s.
The
luminosity
Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...
of a star is a measure of the
electromagnetic energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radia ...
output in a given amount of time.
Luminosity (L) can be related to the temperature (T) of a star by:
:
,
where R is the radius of the star and σ is the
Stefan–Boltzmann constant, with a value of Thus, when both luminosity and temperature are known (via direct measurement and calculation) the radius of a star can be determined.
Galaxies
The spectra of
galaxies look similar to stellar spectra, as they consist of the combined light of billions of stars.
Doppler shift studies of
galaxy clusters by
Fritz Zwicky in 1937 found that the galaxies in a cluster were moving much faster than seemed to be possible from the mass of the cluster inferred from the visible light. Zwicky hypothesized that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as
dark matter
In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
.
Since his discovery, astronomers have determined that a large portion of galaxies (and most of the universe) is made up of dark matter. In 2003, however, four galaxies (NGC 821,
NGC 3379, NGC 4494, and
NGC 4697) were found to have little to no dark matter influencing the motion of the stars contained within them; the reason behind the lack of dark matter is unknown.
In the 1950s, strong radio sources were found to be associated with very dim, very red objects. When the first spectrum of one of these objects was taken there were absorption lines at wavelengths where none were expected. It was soon realised that what was observed was a normal galactic spectrum, but highly red shifted.
These were named ''quasi-stellar radio sources'', or
quasars
A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...
, by
Hong-Yee Chiu in 1964.
Quasars are now thought to be galaxies formed in the early years of our universe, with their extreme energy output powered by super-massive
black hole
A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
s.
The properties of a galaxy can also be determined by analyzing the stars found within them.
NGC 4550, a galaxy in the Virgo Cluster, has a large portion of its stars rotating in the opposite direction as the other portion. It is believed that the galaxy is the combination of two smaller galaxies that were rotating in opposite directions to each other.
Bright stars in galaxies can also help determine the distance to a galaxy, which may be a more accurate method than
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
or
standard candles.
Interstellar medium
The interstellar medium is matter that occupies the space between
star systems in a galaxy. 99% of this matter is gaseous –
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
,
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, and smaller quantities of other ionized elements such as
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. The other 1% is dust particles, thought to be mainly
graphite
Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
,
silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
s, and ices.
Clouds of the dust and gas are referred to as
nebula
A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...
e.
There are three main types of nebula:
absorption,
reflection, and
emission nebulae. Absorption (or dark) nebulae are made of dust and gas in such quantities that they obscure the starlight behind them, making
photometry
Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electr ...
difficult. Reflection nebulae, as their name suggest, reflect the light of nearby stars. Their spectra are the same as the stars surrounding them, though the light is bluer; shorter wavelengths scatter better than longer wavelengths. Emission nebulae emit light at specific wavelengths depending on their chemical composition.
Gaseous emission nebulae
In the early years of astronomical spectroscopy, scientists were puzzled by the spectrum of gaseous nebulae. In 1864
William Huggins noticed that many nebulae showed only emission lines rather than a full spectrum like stars. From the work of Kirchhoff, he concluded that nebulae must contain "enormous masses of luminous gas or vapour."
However, there were several emission lines that could not be linked to any terrestrial element, brightest among them lines at 495.9 nm and 500.7 nm.
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).
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)
allows for
metastable
In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...
ions to decay via forbidden line emission rather than collisions with other atoms.
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 hydrogen. In the
ground state neutral hydrogen has two possible
spin states: the
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
has either the same spin or the opposite spin of the
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
. When the atom transitions between these two states, it releases an emission or absorption line of 21 cm.
This line is within the radio range and allows for very precise measurements:
*Velocity of the cloud can be measured via
Doppler shift
*The intensity of the 21 cm line gives the density and number of atoms in the cloud
*The temperature of the cloud can be calculated
Using this information, the shape of the Milky Way has been determined to be a
spiral galaxy
Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' , though the exact number and position of the spiral arms is the subject of ongoing research.
Complex molecules
Dust and molecules in the interstellar medium not only obscures photometry, but also causes absorption lines in spectroscopy. Their spectral features are generated by transitions of component electrons between different energy levels, or by rotational or vibrational spectra. Detection usually occurs in radio, microwave, or infrared portions of the spectrum.
The chemical reactions that form these molecules can happen in cold, diffuse clouds
or in dense regions illuminated with
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
light.
Most known compounds in space are
organic, ranging from small molecules e.g.
acetylene
Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
C
2H
2 and
acetone
Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
(CH
3)
2CO;
to entire classes of large molecule e.g.
fullerenes
A fullerene is an allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may have hollow sphere- ...
and
polycyclic aromatic hydrocarbons; to
solid
Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
s, such as
graphite
Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
or other
sooty material.
Motion in the universe

Stars and interstellar gas are bound by gravity to form galaxies, and groups of galaxies can be bound by gravity in
galaxy clusters.
With the exception of stars in the
Milky Way
The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
and the galaxies in the
Local Group, almost all galaxies are moving away from Earth due to the
expansion of the universe
The expansion of the universe is the increase in proper length, distance between Gravitational binding energy, gravitationally unbound parts of the observable universe with time. It is an intrinsic and extrinsic properties (philosophy), intrins ...
.
Doppler effect and redshift
The motion of stellar objects can be determined by looking at their spectrum. Because of the
Doppler effect, objects moving towards someone are
blueshifted, and objects moving away are
redshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...
ed. The wavelength of redshifted light is longer, appearing redder than the source. Conversely, the wavelength of blueshifted light is shorter, appearing bluer than the source light:
:
where
is the emitted wavelength,
is the velocity of the object, and
is the observed wavelength. Note that v<0 corresponds to λ<λ
0, 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 four of which were redshifted — and was able to calculate their velocities relative to the Earth.
Edwin Hubble
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble proved that many objects previously ...
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.
Hubble's law can be generalised to:
:
where
is the velocity (or Hubble Flow),
is the
Hubble Constant
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faste ...
, and
is the distance from Earth.
Redshift (z) can be expressed by the following equations:
In these equations, frequency is denoted by
and wavelength by
. The larger the value of z, the more redshifted the light and the farther away the object is from the Earth. As of January 2013, the largest galaxy redshift of z~12 was found using the
Hubble Ultra-Deep Field
The Hubble Ultra-Deep Field (HUDF) is a List of deep fields, deep-field image of a small region of outer space, space in the constellation Fornax, containing an estimated 10,000 galaxies. The original data for the image was collected by the Hub ...
, corresponding to an age of over 13 billion years (the universe is approximately 13.82 billion years old).
The Doppler effect and Hubble's law can be combined to form the equation
,
where c is the speed of light.
Peculiar motion
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 to be added to Hubble's law:
:
This motion can cause confusion when looking at a solar or galactic spectrum, because the expected redshift based on the simple Hubble law will be obscured by the peculiar motion. For example, the shape and size of the
Virgo Cluster has been a matter of great scientific scrutiny due to the very large peculiar velocities of the galaxies in the cluster.
Binary stars

Just as planets can be gravitationally bound to stars, pairs of stars can orbit each other. Some
binary stars are visual binaries, meaning they can be observed orbiting each other through a telescope. Some binary stars, however, are too close together to be
resolved.
These two stars, when viewed through a spectrometer, will show a composite spectrum: the spectrum of each star will be added together. This composite spectrum becomes easier to detect when the stars are of similar luminosity and of different
spectral class.
Spectroscopic binaries
A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...
can be also detected due to their
radial velocity; as they orbit around each other one star may be moving towards the Earth whilst the other moves away, causing a Doppler shift in the composite spectrum. The
orbital plane of the system determines the magnitude of the observed shift: if the observer is looking perpendicular to the orbital plane there will be no observed radial velocity.
For example, a person looking at a
carousel from the side will see the animals moving toward and away from them, whereas if they look from directly above they will only be moving in the horizontal plane.
Planets, asteroids, and comets
Planet
A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s,
asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s, and
comet
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s all reflect light from their parent stars and emit their own light. For cooler objects, including
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
planets and asteroids, most of the emission is at infrared wavelengths we cannot see, but that are routinely measured with
spectrometer
A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomeno ...
s. For objects surrounded by gas, such as comets and planets with atmospheres, further emission and absorption happens at specific wavelengths in the gas, imprinting the spectrum of the gas on that of the solid object. In the case of worlds with thick atmospheres or complete cloud or haze cover (such as the four
giant planets,
Venus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
, and
Saturn
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
's satellite
Titan), the spectrum is mostly or completely due to the atmosphere alone.
Planets
The reflected light of a planet contains absorption bands due to
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
s in the rocks present for rocky bodies, or due to the elements and molecules present in the atmosphere. To date over 3,500
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
s have been discovered. These include so-called
Hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter (i.e. Jupiter analogue, Jupiter analogues) but that have very short orbital periods (). The close proximity to t ...
s, as well as Earth-like planets. Using spectroscopy, compounds such as alkali metals, water vapor, carbon monoxide, carbon dioxide, and methane have all been discovered.
Asteroids
Asteroids can be classified into three major types according to their spectra. The original categories were created by Clark R. Chapman, David Morrison, and Ben Zellner in 1975, and further expanded by
David J. Tholen in 1984. In what is now known as the
Tholen classification, the
C-types are made of carbonaceous material,
S-types consist mainly of
silicates
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used for an ...
, and
X-types are 'metallic'. There are other classifications for unusual asteroids. C- and S-type asteroids are the most common asteroids. In 2002 the Tholen classification was further "evolved" into the
SMASS classification, expanding the number of categories from 14 to 26 to account for more precise spectroscopic analysis of the asteroids.
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 to
fluorescence
Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...
by sunlight and/or chemical reactions. For example, the chemical composition of
Comet ISON
Comet ISON, formally known as C/2012 S1, was a sungrazing comet from the Oort cloud which was discovered on 21 September 2012 by Vitaly Nevsky (Віталь Неўскі, Vitebsk, Belarus) and Artyom Novichonok (Артём Новичонок, Ko ...
was determined by spectroscopy due to the prominent emission lines of cyanogen (CN), as well as two- and three-carbon atoms (C
2 and C
3).
Nearby comets can even be seen in X-ray as solar wind ions flying to the
coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...
are neutralized. The cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet.
See also
References
{{Authority control
Spectroscopy
Observational astronomy