![Star-Spectroscope](https://upload.wikimedia.org/wikipedia/commons/8/89/Star-Spectroscope.jpg)
Astronomical spectroscopy is the study of
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
using the techniques of
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
to measure the
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
, including
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
,
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
,
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
,
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
and
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
waves that
radiate from
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
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, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s,
nebula
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ 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 regio ...
e,
galaxies, and
active galactic nuclei
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
.
Background
![Atmospheric electromagnetic opacity](https://upload.wikimedia.org/wikipedia/commons/3/34/Atmospheric_electromagnetic_opacity.svg)
Astronomical spectroscopy is used to measure three major bands of radiation in the electromagnetic spectrum:
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
,
radio waves, and
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
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 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 , breaking down in the lo ...
(O
3) and molecular oxygen (O
2) absorb light with wavelengths under 300 nm, meaning that X-ray and
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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 those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
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 (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
first used a simple prism to observe the refractive properties of light.
In the early 1800s
Joseph von Fraunhofer
Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffract ...
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 sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
,
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, and various stars such as
Betelgeuse
Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion ...
; 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
The Dominion Observatory was an astronomical observatory in Ottawa, Ontario that operated from 1902 to 1970. The Observatory was also an institution within the Canadian Federal Government. The observatory grew out of the Department of the Int ...
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
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
. This wave pattern sets up a reflection pattern similar to the blazed gratings but utilizing
Bragg diffraction
In physics and chemistry , Bragg's law, Georg Wulff, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposit ...
, 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
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...
can be recorded by a detector. Historically,
photographic plate
Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinn ...
s 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
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
by observing the spectrum of
emission line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s of known wavelength from a
gas-discharge lamp. The
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
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
Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
.
Radio spectroscopy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
was founded with the work of
Karl Jansky
Karl Guthe Jansky (October 22, 1905 – February 14, 1950) was an American physicist and radio engineer who in April 1933 first announced his discovery of radio waves emanating from the Milky Way in the constellation Sagittarius. He is considere ...
in the early 1930s, while working for
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
. 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 is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
, 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
Joseph Lade Pawsey (14 May 1908 – 30 November 1962) was an Australian scientist, radiophysicist and radio astronomer.
Education
Pawsey was born in Ararat, Victoria to a family of farmers. At the age of 14 he was awarded a government schol ...
,
Ruby Payne-Scott
Ruby Violet Payne-Scott, BSc (Phys) MSc DipEd (Syd) (28 May 1912 – 25 May 1981) was an Australian pioneer in radiophysics and radio astronomy, and was one of two Antipodean women pioneers in radio astronomy and radio physics at the end of the ...
and
Lindsay McCready
Lindsay may refer to:
People
*Clan Lindsay, a Scottish family clan
*Lindsay (name), an English surname and given name, derived from the Scottish clan name; variants include Lindsey, Lyndsay, Linsay, Linsey, Lyndsey, Lyndsy, Lynsay, Lynsey
Places ...
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
Sir Martin Ryle (27 September 1918 – 14 October 1984) was an English radio astronomer who developed revolutionary radio telescope systems (see e.g. aperture synthesis) and used them for accurate location and imaging of weak radio sourc ...
and Vonberg.
In 1960, Ryle and
Antony Hewish published the technique of
aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation an ...
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 samples of a function into a same-length sequence of equally-spaced samples of the discrete-time Fourier transform (DTFT), which is a comple ...
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
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
.
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 physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects.
He ...
and
Robert Bunsen
Robert Wilhelm Eberhard Bunsen (;
30 March 1811
– 16 August 1899) was a German chemist. He investigated emission spectra of heated elements, and discovered caesium (in 1860) and rubidium (in 1861) with the physicist Gustav Kirchhoff. The Bu ...
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
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identi ...
of the Sun with
emission spectra of known gases, the chemical composition of stars can be determined.
The major
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectru ...
, and the elements with which they are associated, appear in the following table. Designations from the early
Balmer Series
The Balmer series, or Balmer lines in atomic physics, is one of a set of six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered b ...
are shown in parentheses.
Not all of the elements in the Sun were immediately identified. Two examples are listed below.
*In 1868
Norman Lockyer
Sir Joseph Norman Lockyer (17 May 1836 – 16 August 1920) was an English scientist and astronomer. Along with the French scientist Pierre Janssen, he is credited with discovering the gas helium. Lockyer also is remembered for being the f ...
and
Pierre Janssen
Pierre Jules César Janssen (22 February 1824 – 23 December 1907), usually known as Jules Janssen, was a French astronomer who, along with English scientist Joseph Norman Lockyer, is credited with discovering the gaseous nature of the solar ...
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 el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, but it wasn't until 1895 the element was found on Earth.
*In 1869 the astronomers
Charles Augustus Young
Charles Augustus Young (15 December 1834 – 4 January 1908) one of the foremost solar spectroscopist astronomers in the United States. He observed solar eclipses and worked on spectroscopy of the Sun. He observed a solar flare with a spec ...
and
William Harkness
William Harkness (December 17, 1837 – February 28, 1903) was an astronomer. He was born at Ecclefechan, Scotland, a son of James (1803–78) and Jane (née Wield) Harkness. His father was a pastor and moved the family to the United States. H ...
independently observed a novel green emission line in the
Sun's corona
A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma.
The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total so ...
during an eclipse. This "new" element was incorrectly named
coronium
Coronium, also called newtonium, was the name of a suggested chemical element, hypothesised in the 19th century. The name, inspired by the solar corona, was given by Gruenwald in 1887. A new atomic thin green line in the solar corona was then con ...
, as it was only found in the corona. It was not until the 1930s that
Walter Grotrian
Walter Robert Wilhelm Grotrian (21 April 1890 in Aachen; † 3 March 1954 in Potsdam) was a German astronomer and astrophysicist.
Grotrian studied the emission line from the solar corona in the green region of the spectrum; this emission line co ...
and
Bengt Edlén
Bengt Edlén (2 November 1906, Gusum – 10 February 1993, Lund) was a Swedish professor of physics and astronomer who specialized in spectroscopy. He was the first who identified the unknown coronal spectral lines in the Corona, which was u ...
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 with 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 slow to ...
and
calcium
Calcium is a chemical element with the 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 ...
, the high ionization being due to the extreme temperature of the
solar corona
A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma.
The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total solar ...
.
To date more than 20 000 absorption lines have been listed for the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
between 293.5 and 877.0 nm, yet only approximately 75% of these lines have been linked to elemental absorption.
By analyzing the 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
During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.
In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926:
Baade noticed th ...
; Population I stars are the youngest stars and have 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.
Temperature and size
![Black body](https://upload.wikimedia.org/wikipedia/commons/1/19/Black_body.svg)
In 1860
Gustav Kirchhoff
Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects.
He ...
proposed the idea of a
black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
, a material that emits electromagnetic radiation at all wavelengths.
In 1894
Wilhelm Wien
Wilhelm Carl Werner Otto Fritz Franz Wien (; 13 January 1864 – 30 August 1928) was a German physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the emission of a blackbody ...
derived an expression relating the temperature (T) of a black body to its peak emission wavelength (λ
max).
:
''b'' is a
constant of proportionality
In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called the coefficient of proportionality or proportionality constan ...
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 5778
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
s.
The
luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
of a star is a measure of the
electromagnetic energy
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
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 cluster
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...
s by
Fritz Zwicky
Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) 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 an ...
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
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
.
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
NGC 4697 (also known as Caldwell 52) is an elliptical galaxy some 40 to 50 million light-years away in the constellation Virgo (constellation), Virgo. It is a member of the NGC 4697 Group, a group of galaxies also containing NGC 4731 and several ...
) 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 luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass rangi ...
, by
Hong-Yee Chiu
Hong-Yee Chiu (; born October 1932) is a Taiwanise-American astrophysicist, at NASA for 35 years, and successful publisher of EHGBooks micro-publishing company. He was born in Shanghai, China.
Career
Chiu graduated from National Taiwan Univers ...
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 region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
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 lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
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 the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
,
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, and smaller quantities of other ionized elements such as
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
. The other 1% is dust particles, thought to be mainly
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
,
silicate
In chemistry, 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 al ...
s, and ices.
Clouds of the dust and gas are referred to as
nebula
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ 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 regio ...
e.
There are three main types of nebula:
absorption,
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in ...
, 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 electro ...
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
Sir William Huggins (7 February 1824 – 12 May 1910) was an English astronomer best known for his pioneering work in astronomical spectroscopy together with his wife, Margaret.
Biography
William Huggins was born at Cornhill, Middlesex, in ...
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
Nebulium was a proposed element found in astronomical observation of a nebula by William Huggins in 1864. The strong green emission lines of the Cat's Eye Nebula, discovered using spectroscopy, led to the postulation that an as yet unknown e ...
, 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
In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particul ...
; the low density of a nebula (one atom per cubic centimetre)
allows for
metastable
In chemistry and physics, metastability denotes an intermediate Energy level, energetic state within a dynamical system other than the system's ground state, state of least energy.
A ball resting in a hollow on a slope is a simple example of me ...
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 ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
has either the same spin or the opposite spin of the
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
. 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 class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''[ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...]
light.
Most known compounds in space are organic compound, organic, ranging from small molecules e.g. acetylene C
2H
2 and acetone (CH
3)
2CO;
to entire classes of large molecule e.g. fullerenes
and polycyclic aromatic hydrocarbons; to solids, such as
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
or other sooty material.
Motion in the universe
![Redshift blueshift](https://upload.wikimedia.org/wikipedia/commons/e/e4/Redshift_blueshift.svg)
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 is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
and the galaxies in the Local Group, almost all galaxies are moving away from us due to the Metric expansion of space, expansion of the universe.
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 us are blueshifted, and objects moving away are redshifted. 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 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 from us.
Hubble's law can be generalised to
:
where
is the velocity (or Hubble Flow),
is the hubble's law#Measured values of the Hubble constant, Hubble Constant, 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, 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
![Wiki Spect Binaries v2](https://upload.wikimedia.org/wikipedia/commons/d/d2/Wiki_Spect_Binaries_v2.gif)
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 angular resolution, 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 Stellar classification, spectral class.
Binary star#Spectroscopic binaries, Spectroscopic binaries 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 (astronomy), 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, if you look at a carousel from the side, you will see the animals moving toward and away from you, whereas if you look from directly above they will only be moving in the horizontal plane.
Planets, asteroids, and comets
Planets, asteroids, and comets all reflect light from their parent stars and emit their own light. For cooler objects, including Solar System planets and asteroids, most of the emission is at infrared wavelengths we cannot see, but that are routinely measured with spectrometers. 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 cover (such as the gas giants,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, and Saturn's satellite Titan (moon), Titan), the spectrum is mostly or completely due to the atmosphere alone.
Planets
The reflected light of a planet contains absorption bands due to minerals in the rocks present for rocky bodies, or due to the elements and molecules present in the atmosphere. To date over 3,500 exoplanets have been discovered. These include so-called Hot Jupiters, 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 Asteroid spectral types#Tholen classification, Tholen classification, the C-type asteroid, C-types are made of carbonaceous material, S-type asteroid, S-types consist mainly of silicates, and X-type asteroid, 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 Asteroid spectral types#SMASS classification, SMASS classification, expanding the number of categories from 14 to 26 to account for more precise spectroscopic analysis of the asteroids.
Comets
![Spectrum of Comet Hyakutake](https://upload.wikimedia.org/wikipedia/commons/a/a4/Spectrum_of_Comet_Hyakutake.gif)
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 by sunlight and/or chemical reactions. For example, the chemical composition of C/2012 S1, Comet ISON
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 (cometary), coma are neutralized. The cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet.
See also
References
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Astronomical spectroscopy,
Spectroscopy
Observational astronomy