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87-545: FK Comae Berenices is a variable star that varies in apparent magnitude between 8.14 and 8.33 over a period of 2.4 days. It is the prototype for the FK Comae Berenices (FK Com) class of variable stars. The variability of FK Com stars may be caused by large, cool spots on the rotating surfaces of the stars. This star is thought to be the result of a recent binary merger, resulting in a high rate of both spin and magnetic activity . The spectral class of FK Comae Berenices

174-497: 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

261-445: 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,

348-423: A 6 fold to 30,000 fold change in luminosity. Mira itself, also known as Omicron Ceti (ο Cet), varies in brightness from almost 2nd magnitude to as faint as 10th magnitude with a period of roughly 332 days. The very large visual amplitudes are mainly due to the shifting of energy output between visual and infra-red as the temperature of the star changes. In a few cases, Mira variables show dramatic period changes over

435-531: 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 . 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

522-463: A day. They are thought to have evolved beyond a red supergiant phase, but the mechanism for the pulsations is unknown. The class was named in 2020 through analysis of TESS observations. Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name, these are not explosive events. Protostars are young objects that have not yet completed

609-618: A given constellation, the first variable stars discovered were designated with letters R through Z, e.g. R Andromedae . This system of nomenclature was developed by Friedrich W. Argelander , who gave the first previously unnamed variable in a constellation the letter R, the first letter not used by Bayer . Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries, e.g. RR Lyrae . Later discoveries used letters AA through AZ, BB through BZ, and up to QQ through QZ (with J omitted). Once those 334 combinations are exhausted, variables are numbered in order of discovery, starting with

696-472: 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 , the Moon , Mars , 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

783-458: A laboratory because they are forbidden lines ; the low density of a nebula (one atom per cubic centimetre) allows for metastable 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 :

870-405: 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 2.897 771 955 ... × 10  m⋅K . This equation is called Wien's Law . By measuring the peak wavelength of

957-419: A particular star is variable. Variable stars are generally analysed using photometry , spectrophotometry and spectroscopy . Measurements of their changes in brightness can be plotted to produce light curves . For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to

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1044-419: A period of 0.01–0.2 days. Their spectral type is usually between A0 and F5. These stars of spectral type A2 to F5, similar to δ Scuti variables, are found mainly in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so every 1 to 2 hours. These stars of spectral type A or occasionally F0, a sub-class of δ Scuti variables found on

1131-684: A period of 0.1–1 day and an amplitude of 0.1 magnitude on average. Their spectra are peculiar by having weak hydrogen while on the other hand carbon and helium lines are extra strong, a type of extreme helium star . These are yellow supergiant stars (actually low mass post-AGB stars at the most luminous stage of their lives) which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30–100 days and amplitudes of 3–4 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. Their spectra are of type F or G at maximum light and type K or M at minimum brightness. They lie near

1218-477: A period of decades, thought to be related to the thermal pulsing cycle of the most advanced AGB stars. These are red giants or supergiants . Semiregular variables may show a definite period on occasion, but more often show less well-defined variations that can sometimes be resolved into multiple periods. A well-known example of a semiregular variable is Betelgeuse , which varies from about magnitudes +0.2 to +1.2 (a factor 2.5 change in luminosity). At least some of

1305-521: 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

1392-453: A pulsating star is found in its shifting spectrum because its surface periodically moves toward and away from us, with the same frequency as its changing brightness. About two-thirds of all variable stars appear to be pulsating. In the 1930s astronomer Arthur Stanley Eddington showed that the mathematical equations that describe the interior of a star may lead to instabilities that cause a star to pulsate. The most common type of instability

1479-419: A pulsation is caused by the blocking of the internal energy flow by material with a high opacity, but this must occur at a particular depth of the star to create visible pulsations. If the expansion occurs below a convective zone then no variation will be visible at the surface. If the expansion occurs too close to the surface the restoring force will be too weak to create a pulsation. The restoring force to create

1566-406: A single well-defined period, but often they pulsate simultaneously with multiple frequencies and complex analysis is required to determine the separate interfering periods. In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as stochastic . The study of stellar interiors using their pulsations is known as asteroseismology . The expansion phase of

1653-461: A slightly offset period versus luminosity relationship, so it is always important to know which type of star is being observed. These stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods. They are older than type I Cepheids, belonging to Population II , but of lower mass than type II Cepheids. Due to their common occurrence in globular clusters , they are occasionally referred to as cluster Cepheids . They also have

1740-414: A spectrum can be calibrated by observing the spectrum of emission lines of known wavelength from a gas-discharge lamp . 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 astronomy was founded with the work of Karl Jansky in

1827-483: 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 kelvins . The luminosity of a star is a measure of the electromagnetic energy 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

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1914-486: 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 can be also detected due to their radial velocity ; as they orbit around each other one star may be moving towards

2001-405: A value of 5.670 374 419 ... × 10  W⋅m ⋅K . Thus, when both luminosity and temperature are known (via direct measurement and calculation) the radius of a star can be determined. 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

2088-593: A well established period-luminosity relationship, and so are also useful as distance indicators. These A-type stars vary by about 0.2–2 magnitudes (20% to over 500% change in luminosity) over a period of several hours to a day or more. Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as Dwarf Cepheids . They often show many superimposed periods, which combine to form an extremely complex light curve. The typical δ Scuti star has an amplitude of 0.003–0.9 magnitudes (0.3% to about 130% change in luminosity) and

2175-448: Is G4 III, although it is considered unusual in having very broad absorption lines as well as some emission lines . The broadened spectral lines are due to rapid rotation. The rotation rate of FK Comae Berenices is unusually fast for a cool giant star . It is speculated that this is due to the merger of a contact binary pair of stars into a single star. The rotation produces extremely strong magnetic fields which are expected to brake

2262-400: 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. Physicists have been looking at the solar spectrum since Isaac Newton first used a simple prism to observe the refractive properties of light. In the early 1800s Joseph von Fraunhofer used his skills as

2349-458: Is frequency. For this work, Ryle and Hewish were jointly awarded the 1974 Nobel Prize in Physics . 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 and Robert Bunsen described the phenomena behind these dark lines. Hot solid objects produce light with

2436-407: Is longer, appearing redder than the source. Conversely, the wavelength of blueshifted light is shorter, appearing bluer than the source light: where λ 0 {\displaystyle \lambda _{0}} is the emitted wavelength, v 0 {\displaystyle v_{0}} is the velocity of the object, and λ {\displaystyle \lambda }

2523-421: Is named after Beta Cephei . Classical Cepheids (or Delta Cephei variables) are population I (young, massive, and luminous) yellow supergiants which undergo pulsations with very regular periods on the order of days to months. On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae , the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids

2610-659: Is now known as the Tholen classification , the C-types are made of carbonaceous material, S-types consist mainly of silicates , 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

2697-718: Is often much smaller, with the more rapid primary variations are superimposed. The reasons for this type of variation are not clearly understood, being variously ascribed to pulsations, binarity, and stellar rotation. Beta Cephei (β Cep) variables (sometimes called Beta Canis Majoris variables, especially in Europe) undergo short period pulsations in the order of 0.1–0.6 days with an amplitude of 0.01–0.3 magnitudes (1% to 30% change in luminosity). They are at their brightest during minimum contraction. Many stars of this kind exhibits multiple pulsation periods. Slowly pulsating B (SPB) stars are hot main-sequence stars slightly less luminous than

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2784-412: Is related to oscillations in the degree of ionization in outer, convective layers of the star. When the star is in the swelling phase, its outer layers expand, causing them to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the gas more transparent, and thus makes it easier for the star to radiate its energy. This in turn makes the star start to contract. As

2871-471: Is the Hubble Constant , and d {\displaystyle d} is the distance from Earth. Redshift (z) can be expressed by the following equations: In these equations, frequency is denoted by f {\displaystyle f} and wavelength by λ {\displaystyle \lambda } . The larger the value of z, the more redshifted the light and the farther away

2958-547: 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

3045-455: Is the speed of light. 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

3132-495: Is the star Delta Cephei , discovered to be variable by John Goodricke a few months later. Type II Cepheids (historically termed W Virginis stars) have extremely regular light pulsations and a luminosity relation much like the δ Cephei variables, so initially they were confused with the latter category. Type II Cepheids stars belong to older Population II stars, than do the type I Cepheids. The Type II have somewhat lower metallicity , much lower mass, somewhat lower luminosity, and

3219-454: Is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of asteroseismology . A Blue Large-Amplitude Pulsator (BLAP) is a pulsating star characterized by changes of 0.2 to 0.4 magnitudes with typical periods of 20 to 40 minutes. A fast yellow pulsating supergiant (FYPS) is a luminous yellow supergiant with pulsations shorter than

3306-405: The electron has either the same spin or the opposite spin of the proton . 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: Using this information, the shape of the Milky Way has been determined to be a spiral galaxy , though the exact number and position of

3393-485: The instability strip , that swell and shrink very regularly caused by the star's own mass resonance , generally by the fundamental frequency . Generally the Eddington valve mechanism for pulsating variables is believed to account for cepheid-like pulsations. Each of the subgroups on the instability strip has a fixed relationship between period and absolute magnitude, as well as a relation between period and mean density of

3480-525: The Beta Cephei stars, with longer periods and larger amplitudes. The prototype of this rare class is V361 Hydrae , a 15th magnitude subdwarf B star . They pulsate with periods of a few minutes and may simultaneous pulsate with multiple periods. They have amplitudes of a few hundredths of a magnitude and are given the GCVS acronym RPHS. They are p-mode pulsators. Stars in this class are type Bp supergiants with

3567-510: 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

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3654-565: The asteroids. 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 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

3741-575: The basis for all subsequent work on the subject. The latest edition of the General Catalogue of Variable Stars (2008) lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 'suspected' variables. The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the spectrum . By combining light curve data with observed spectral changes, astronomers are often able to explain why

3828-436: The contraction phase of a pulsation can be pressure if the pulsation occurs in a non-degenerate layer deep inside a star, and this is called an acoustic or pressure mode of pulsation, abbreviated to p-mode . In other cases, the restoring force is gravity and this is called a g-mode . Pulsating variable stars typically pulsate in only one of these modes. This group consists of several kinds of pulsating stars, all found on

3915-427: The discovery of the 21-centimeter H I line in 1951. 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

4002-522: The discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669; John Goodricke gave the correct explanation of its variability in 1784. Chi Cygni was identified in 1686 by G. Kirch , then R Hydrae in 1704 by G. D. Maraldi . By 1786, ten variable stars were known. John Goodricke himself discovered Delta Cephei and Beta Lyrae . Since 1850,

4089-417: The distance to a galaxy, which may be a more accurate method than parallax or standard candles . The interstellar medium is matter that occupies the space between star systems in a galaxy. 99% of this matter is gaseous – hydrogen , helium , and smaller quantities of other ionized elements such as oxygen . The other 1% is dust particles, thought to be mainly graphite , silicates , and ices. Clouds of

4176-430: The dust and gas are referred to as nebulae . 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 difficult. Reflection nebulae, as their name suggest, reflect the light of nearby stars. Their spectra are the same as the stars surrounding them, though

4263-463: The early 1930s, while working for Bell Labs . 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 , in the constellation Sagittarius . In 1942, JS Hey captured the Sun's radio frequency using military radar receivers. Radio spectroscopy started with

4350-570: The early years of our universe, with their extreme energy output powered by super-massive black holes . 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

4437-555: 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 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

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4524-407: 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. In 1860 Gustav Kirchhoff proposed the idea of a black body ,

4611-559: The energy output of the Sun , for example, varies by about 0.1% over an 11-year solar cycle . An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol . Aboriginal Australians are also known to have observed the variability of Betelgeuse and Antares , incorporating these brightness changes into narratives that are passed down through oral tradition. Of

4698-443: The entire star expands and shrinks as a whole; and non-radial , where one part of the star expands while another part shrinks. Depending on the type of pulsation and its location within the star, there is a natural or fundamental frequency which determines the period of the star. Stars may also pulsate in a harmonic or overtone which is a higher frequency, corresponding to a shorter period. Pulsating variable stars sometimes have

4785-510: The exception of stars in the Milky Way and the galaxies in the Local Group , almost all galaxies are moving away from Earth due to the expansion of the universe . 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 redshifted . The wavelength of redshifted light

4872-471: 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: 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

4959-459: The gas is thereby compressed, it is heated and the degree of ionization again increases. This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression (swelling and shrinking) is maintained. The pulsation of cepheids is known to be driven by oscillations in the ionization of helium (from He to He and back to He ). In

5046-422: 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 , and Saturn 's satellite Titan ), the spectrum is mostly or completely due to the atmosphere alone. The reflected light of a planet contains absorption bands due to minerals in the rocks present for rocky bodies, or due to

5133-417: 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 devices (CCDs). The wavelength scale of

5220-456: The horizontal plane. 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

5307-447: The instability strip, cooler than type I Cepheids more luminous than type II Cepheids. Their pulsations are caused by the same basic mechanisms related to helium opacity, but they are at a very different stage of their lives. Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of spectral classes B ep to A ep Ia. Their periods range from several days to several weeks, and their amplitudes of variation are typically of

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5394-408: The light is bluer; shorter wavelengths scatter better than longer wavelengths. Emission nebulae emit light at specific wavelengths depending on their chemical composition. 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

5481-414: 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

5568-406: The main sequence. They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude. The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years. Mira variables are Asymptotic giant branch (AGB) red giants. Over periods of many months they fade and brighten by between 2.5 and 11 magnitudes ,

5655-456: The modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught. In this way,

5742-568: 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 , by Hong-Yee Chiu in 1964. Quasars are now thought to be galaxies formed in

5829-534: The next. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima. Amateur astronomers can do useful scientific study of variable stars by visually comparing the star with other stars within the same telescopic field of view of which the magnitudes are known and constant. By estimating the variable's magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around

5916-462: The number of known variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography. In 1930, astrophysicist Cecilia Payne published the book The Stars of High Luminosity, in which she made numerous observations of variable stars, paying particular attention to Cepheid variables . Her analyses and observations of variable stars, carried out with her husband, Sergei Gaposchkin, laid

6003-508: 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 z = v Hubble c {\displaystyle z={\frac {v_{\text{Hubble}}}{c}}} , where c

6090-448: The order of 0.1 magnitudes. These non-radially pulsating stars have short periods of hundreds to thousands of seconds with tiny fluctuations of 0.001 to 0.2 magnitudes. Known types of pulsating white dwarf (or pre-white dwarf) include the DAV , or ZZ Ceti , stars, with hydrogen-dominated atmospheres and the spectral type DA; DBV , or V777 Her , stars, with helium-dominated atmospheres and

6177-405: The order of 0.1 magnitudes. The light changes, which often seem irregular, are caused by the superposition of many oscillations with close periods. Deneb , in the constellation of Cygnus is the prototype of this class. Gamma Doradus (γ Dor) variables are non-radially pulsating main-sequence stars of spectral classes F to late A. Their periods are around one day and their amplitudes typically of

6264-529: The prefixed V335 onwards. Variable stars may be either intrinsic or extrinsic . These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated U Geminorum stars after the first recognized star in the class, U Geminorum . Examples of types within these divisions are given below. Pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into: radial , where

6351-609: The process of contraction from a gas nebula to a veritable star. Most protostars exhibit irregular brightness variations. Stellar spectrum Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation , including visible light , ultraviolet , X-ray , infrared and radio waves that radiate from stars 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

6438-500: The semi-regular variables are very closely related to Mira variables, possibly the only difference being pulsating in a different harmonic. These are red giants or supergiants with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic. Many variable red giants and supergiants show variations over several hundred to several thousand days. The brightness may change by several magnitudes although it

6525-486: 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. 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

6612-455: The spectral type DB; and GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars may be subdivided into DOV and PNNV stars. The Sun oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as helioseismology . Oscillations in the Sun are driven stochastically by convection in its outer layers. The term solar-like oscillations

6699-403: 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

6786-447: The spectrum, different methods are required to acquire the signal depending on the frequency. Ozone (O 3 ) and molecular oxygen (O 2 ) absorb light with wavelengths under 300 nm, meaning that X-ray and ultraviolet 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 light

6873-593: The spectrum. The chemical reactions that form these molecules can happen in cold, diffuse clouds or in dense regions illuminated with ultraviolet light. Most known compounds in space are organic , ranging from small molecules e.g. acetylene C 2 H 2 and acetone (CH 3 ) 2 CO; to entire classes of large molecule e.g. fullerenes and polycyclic aromatic hydrocarbons ; to solids , such as graphite or other sooty material. Stars and interstellar gas are bound by gravity to form galaxies, and groups of galaxies can be bound by gravity in galaxy clusters . With

6960-400: The spiral arms is the subject of ongoing research. 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

7047-464: The star to a slower rotation rate. Analysis of variability due to star spots on the surface show that the star rotates at different speeds at different latitudes. FK Comae Berenices is listed as a companion to the slightly brighter HD 117567. The two are not thought to be physically associated, with HD 117567 being a much closer F2 main sequence star. Variable star Many, possibly most, stars exhibit at least some oscillation in luminosity:

7134-488: The star. The period-luminosity relationship was first established for Delta Cepheids by Henrietta Leavitt , and makes these high luminosity Cepheids very useful for determining distances to galaxies within the Local Group and beyond. Edwin Hubble used this method to prove that the so-called spiral nebulae are in fact distant galaxies. The Cepheids are named only for Delta Cephei , while a completely separate class of variables

7221-470: 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 planets , nebulae , galaxies , and active galactic nuclei . Astronomical spectroscopy is used to measure three major bands of radiation in the electromagnetic spectrum: visible light , radio waves , and X-rays . While all spectroscopy looks at specific bands of

7308-474: 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 ). These emission lines could not be replicated in

7395-421: The world and shares the data with the scientific community. From the light curve the following data are derived: From the spectrum the following data are derived: In very few cases it is possible to make pictures of a stellar disk. These may show darker spots on its surface. Combining light curves with spectral data often gives a clue as to the changes that occur in a variable star. For example, evidence for

7482-420: 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. Hubble's law can be generalised to: where v {\displaystyle v} is the velocity (or Hubble Flow), H 0 {\displaystyle H_{0}}

7569-446: 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 transforming the incoming signal, recovers both the spatial and frequency variation in flux. The result is a 3D image whose third axis

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