In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines; each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary due to the temperature of the photosphere, although in some cases there are true abundance differences; the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, M, a sequence from the hottest to the coolest; each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, class D for white dwarfs; the full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K. The conventional color description takes into account only the peak of the stellar spectrum. In actuality, stars radiate in all parts of the spectrum; because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading.
Excluding color-contrast illusions in dim light, there are indigo, or violet stars. Red dwarfs are a deep shade of orange, brown dwarfs do not appear brown, but hypothetically would appear dim grey to a nearby observer; the modern classification system is known as the Morgan–Keenan classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes; those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions.
Main-sequence stars vary in surface temperature from 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are listed from hottest to coldest; the spectral classes O through M, as well as other more specialized classes discussed are subdivided by Arabic numerals, where 0 denotes the hottest stars of a given class. For example, A0 denotes A9 denotes the coolest ones. Fractional numbers are allowed; the Sun is classified as G2. Conventional color descriptions are traditional in astronomy, represent colors relative to the mean color of an A class star, considered to be white; the apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, stars with particular spectral features such as carbon stars may be far redder than any black body.
The fact that the Harvard classification of a star indicated its surface or photospheric temperature was not understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, this was suspected to be true. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere to stellar spectra. Harvard astronomer Cecilia Payne demonstrated that the O-B-A-F-G-K-M spectral sequence is a sequence in temperature; because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals; the Yerkes spectral classification called the MKK system from the authors' initial
Herbig–Haro objects are turbulent looking patches of nebulosity associated with newborn stars. They are formed when narrow jets of ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometres per second. Herbig–Haro objects are ubiquitous in star-forming regions, several are seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec of the source, although some have been observed several parsecs away. HH objects are transient phenomena, they can change visibly over quite short timescales of a few years as they move away from their parent star into the gas clouds of interstellar space. Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium. First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were not recognised as being a distinct type of emission nebula until the 1940s.
The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of star formation when they first analysed the objects, recognised that they were a by-product of the star formation process. Although HH objects are a visible wavelength phenomena, many remain invisible at these wavelengths due to dust and gas envelope and are only visible at infrared wavelengths; such objects, when observed in near infrared, are called MHOs. The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, when he observed the star T Tauri with the 36-inch refracting telescope at Lick Observatory and noted a small patch of nebulosity nearby. However, it was catalogued as an emission nebula becoming known as Burnham's Nebula, was not recognised as a distinct class of object. T Tauri was found to be a young and variable star, is the prototype of the class of similar objects known as T Tauri stars which have yet to reach a state of hydrostatic equilibrium between gravitational collapse and energy generation through nuclear fusion at their centres.
Fifty years after Burnham's discovery, several similar nebulae were discovered which were so small as to be star-like in appearance. Both Haro and Herbig made independent observations of several of these objects in the Orion Nebula during the 1940s. Herbig looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of hydrogen and oxygen. Haro found. Following their independent discoveries and Haro met at an astronomy conference in Tucson, Arizona in December 1949. Herbig had paid little attention to the objects he had discovered, being concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them; the Soviet astronomer Viktor Ambartsumian gave the objects their name, based on their occurrence near young stars, suggested they might represent an early stage in the formation of T Tauri stars. Studies of the HH objects showed they were ionized, early theorists speculated that they were reflection nebulae containing low-luminosity hot stars deep inside.
However, the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. In 1975 American astronomer R. D. Schwartz theorized that winds from T Tauri stars produce shocks in ambient medium on encounter, resulting in generation of visible light. With discovery of collimated jet in HH 46/47, it became clear that HH objects are indeed shock induced phenomenon with shocks being driven by collimated jet from protostars. Stars form by gravitational collapse of interstellar gas clouds; as the collapse increases the density, radiative energy loss decreases due to increased opacity. This raises the temperature of the cloud which prevents further collapse, a hydrostatic equilibrium is established. Gas continues to fall towards the core in a rotating disk; this is called a protostar. Some of the accreting material is ejected out along the star's axis of rotation in two jets of partially-ionized gas; the mechanism for producing these collimated bipolar jets is not understood, but it is believed that interaction between the accretion disk and the stellar magnetic field accelerates some of the accreting material from within a few astronomical units of the star away from the disk plane.
At these distances the outflow is divergent, with fanning out at an angle in the range of 10−30°, but it becomes collimated at distances of tens to hundreds of astronomical units from the source, as its expansion is constrained. The jets carry away the excess angular momentum resulting from accretion of material onto the star, which would otherwise cause the star to rotate and disintegrate; when these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects. Electromagnetic emission from HH objects is caused when shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces". Spectroscopic observations of their doppler shifts indicate velocities of several hundred kilometres per second, but the emission lines in those spectra are weaker than what would be expected from such high speed collisions; this suggests that some of the material they are co
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has been classified as a "rare-earth element". Yttrium is always found in combination with lanthanide elements in rare-earth minerals, is never found in nature as a free element. 89Y is the only stable isotope, the only isotope found in the Earth's crust. In 1787, Carl Axel Arrhenius found a new mineral near Ytterby in Sweden and named it ytterbite, after the village. Johan Gadolin discovered yttrium's oxide in Arrhenius' sample in 1789, Anders Gustaf Ekeberg named the new oxide yttria. Elemental yttrium was first isolated in 1828 by Friedrich Wöhler; the most important uses of yttrium are LEDs and phosphors the red phosphors in television set cathode ray tube displays. Yttrium is used in the production of electrodes, electronic filters, superconductors, various medical applications, tracing various materials to enhance their properties. Yttrium has no known biological role.
Exposure to yttrium compounds can cause lung disease in humans. Yttrium is a soft, silver-metallic and crystalline transition metal in group 3; as expected by periodic trends, it is less electronegative than its predecessor in the group and less electronegative than the next member of period 5, zirconium. Yttrium is the first d-block element in the fifth period; the pure element is stable in air in bulk form, due to passivation of a protective oxide film that forms on the surface. This film can reach a thickness of 10 µm; when finely divided, yttrium is unstable in air. Yttrium nitride is formed; the similarities of yttrium to the lanthanides are so strong that the element has been grouped with them as a rare-earth element, is always found in nature together with them in rare-earth minerals. Chemically, yttrium resembles those elements more than its neighbor in the periodic table, if physical properties were plotted against atomic number, it would have an apparent number of 64.5 to 67.5, placing it between the lanthanides gadolinium and erbium.
It also falls in the same range for reaction order, resembling terbium and dysprosium in its chemical reactivity. Yttrium is so close in size to the so-called'yttrium group' of heavy lanthanide ions that in solution, it behaves as if it were one of them. Though the lanthanides are one row farther down the periodic table than yttrium, the similarity in atomic radius may be attributed to the lanthanide contraction. One of the few notable differences between the chemistry of yttrium and that of the lanthanides is that yttrium is exclusively trivalent, whereas about half the lanthanides can have valences other than three; as a trivalent transition metal, yttrium forms various inorganic compounds in the oxidation state of +3, by giving up all three of its valence electrons. A good example is yttrium oxide known as yttria, a six-coordinate white solid. Yttrium forms a water-insoluble fluoride and oxalate, but its bromide, iodide and sulfate are all soluble in water; the Y3+ ion is colorless in solution because of the absence of electrons in the d and f electron shells.
Water reacts with yttrium and its compounds to form Y2O3. Concentrated nitric and hydrofluoric acids do not attack yttrium, but other strong acids do. With halogens, yttrium forms trihalides such as yttrium fluoride, yttrium chloride, yttrium bromide at temperatures above 200 °C. Carbon, selenium and sulfur all form binary compounds with yttrium at elevated temperatures. Organoyttrium chemistry is the study of compounds containing carbon–yttrium bonds. A few of these are known to have yttrium in the oxidation state 0; some trimerization reactions were generated with organoyttrium compounds as catalysts. These syntheses use YCl3 as a starting material, obtained from Y2O3 and concentrated hydrochloric acid and ammonium chloride. Hapticity is a term to describe the coordination of a group of contiguous atoms of a ligand bound to the central atom. Yttrium complexes were the first examples of complexes where carboranyl ligands were bound to a d0-metal center through a η7-hapticity. Vaporization of the graphite intercalation compounds graphite–Y or graphite–Y2O3 leads to the formation of endohedral fullerenes such as Y@C82.
Electron spin resonance studies indicated the formation of 3 − ion pairs. The carbides Y3C, Y2C, YC2 can be hydrolyzed to form hydrocarbons. Yttrium in the Solar System was created through stellar nucleosynthesis by the s-process, but by the r-process; the r-process consists of rapid neutron capture of lighter elements during supernova explosions. The s-process is a slow neutron capture of lighter elements inside pulsating red giant stars. Yttrium isotopes are among the most common products of the nuclear fission of uranium in nuclear explosions and nuclear reactors. In the context of nuclear waste management, the most important isotopes of yttrium
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
Strontium is the chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element, chemically reactive; the metal forms a dark oxide layer. Strontium has physical and chemical properties similar to those of its two vertical neighbors in the periodic table and barium, it occurs mainly in the minerals celestine and strontianite, is mined from these. While natural strontium is stable, the synthetic 90Sr isotope is radioactive and is one of the most dangerous components of nuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium, on the other hand, is not hazardous to health. Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank. Strontium was first isolated as a metal in 1808 by Humphry Davy using the then-newly discovered process of electrolysis. During the 19th century, strontium was used in the production of sugar from sugar beet.
At the peak of production of television cathode ray tubes, as much as 75 percent of strontium consumption in the United States was used for the faceplate glass. With the replacement of cathode ray tubes with other display methods, consumption of strontium has declined. Strontium is a divalent silvery metal with a pale yellow tint whose properties are intermediate between and similar to those of its group neighbors calcium and barium, it is harder than barium. Its melting and boiling points are lower than those of calcium; the density of strontium is intermediate between those of calcium and barium. Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C; the standard electrode potential for the Sr2+/Sr couple is −2.89 V midway between those of the Ca2+/Ca and Ba2+/Ba couples, close to those of the neighboring alkali metals. Strontium is intermediate between calcium and barium in its reactivity toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas.
Strontium metal burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature, it forms only the oxide spontaneously. Besides the simple oxide SrO, the peroxide SrO2 can be made by direct oxidation of strontium metal under a high pressure of oxygen, there is some evidence for a yellow superoxide Sr2. Strontium hydroxide, Sr2, is a strong base, though it is not as strong as the hydroxides of barium or the alkali metals. All four dihalides of strontium are known. Due to the large size of the heavy s-block elements, including strontium, a vast range of coordination numbers is known, from 2, 3, or 4 all the way to 22 or 24 in SrCd11 and SrZn13; the Sr2 + ion is quite large. The large size of strontium and barium plays a significant part in stabilising strontium complexes with polydentate macrocyclic ligands such as crown ethers: for example, while 18-crown-6 forms weak complexes with calcium and the alkali metals, its strontium and barium complexes are much stronger.
Organostrontium compounds contain one or more strontium–carbon bonds. They have been reported as intermediates in Barbier-type reactions. Although strontium is in the same group as magnesium, organomagnesium compounds are commonly used throughout chemistry, organostrontium compounds are not widespread because they are more difficult to make and more reactive. Organostrontium compounds tend to be more similar to organoeuropium or organosamarium compounds due to the similar ionic radii of these elements. Most of these compounds can only be prepared at low temperatures. For example, strontium dicyclopentadienyl, Sr2, must be made by directly reacting strontium metal with mercurocene or cyclopentadiene itself; because of its extreme reactivity with oxygen and water, strontium occurs only in compounds with other elements, such as in the minerals strontianite and celestine. It is kept under a liquid hydrocarbon such as mineral kerosene to prevent oxidation. Finely powdered strontium metal is pyrophoric, meaning that it will ignite spontaneously in air at room temperature.
Volatile strontium salts impart a bright red color to flames, these salts are used in pyrotechnics and in the production of flares. Like calcium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, strontium metal dissolves directly in liquid ammonia to give a dark blue solution. Natural strontium is a mixture of four stable isotopes: 84Sr, 86Sr, 87Sr, 88Sr, their abundance increases with increasing mass number and the heaviest, 88Sr, makes up about 82.6% of all natural strontium, though the abundance varies due to the production of radiogenic 87Sr as the daughter of long-lived beta-decaying 87Rb. Of the unstable isotopes, the primary decay mode of the isotopes lighter than 85Sr is electron capture or positron emission to isotopes of rubidium, that of the iso
A molecular cloud, sometimes called a stellar nursery, is a type of interstellar cloud, the density and size of which permit the formation of molecules, most molecular hydrogen. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most used to determine the presence of H2 is carbon monoxide; the ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps; these clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is the densest part of the medium, comprising half of the total gas mass interior to the Sun's galactic orbit.
The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs from the center of the Milky Way. Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy; that molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of 50 to 75 parsecs, much thinner than the warm atomic and warm ionized gaseous components of the ISM; the exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances.
A vast assemblage of molecular gas with a mass of 103 to 107 times the mass of the Sun is called a giant molecular cloud. GMCs are around 15 to 600 light-years in diameter. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun; the substructure of a GMC is a complex pattern of filaments, sheets and irregular clumps. The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia; the concentration of dust within molecular cores is sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large. These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt; the most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs. The Sagittarius region is chemically rich and is used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules; the densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are included in the same studies. In 1984 IRAS identified a new type of diffuse molecular cloud; these were diffuse filamentary clouds. These clouds have a typical density of 30 particles per cubic centimeter; the formation of stars occurs within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity; the physics of molecular clouds is poorly much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are supersonic but comparable to the speeds of magnetic disturbances; this state is thought to lose energy requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most the effects of massive stars—before a significant fraction of their mass has become stars. Molecular clouds, GMCs, are
An S-type star is a cool giant with equal quantities of carbon and oxygen in its atmosphere. The class was defined in 1922 by Paul Merrill for stars with unusual absorption lines and molecular bands now known to be due to s-process elements; the bands of zirconium monoxide are a defining feature of the S stars. The carbon stars have more carbon than oxygen in their atmospheres. In most stars, such as class M giants, the atmosphere is richer in oxygen than carbon and they are referred to as oxygen-rich stars. S-type stars are intermediate between normal giants, they can be grouped into two classes: intrinsic S stars, which owe their spectra to convection of fusion products and s-process elements to the surface. The intrinsic S-type stars are on the most luminous portion of the asymptotic giant branch, a stage of their lives lasting less than a million years. Many are long period variable stars; the extrinsic S stars are less luminous and longer-lived smaller-amplitude semiregular or irregular variables.
S stars are rare, with intrinsic S stars forming less than 10% of asymptotic giant branch stars of comparable luminosity, while extrinsic S stars form an smaller proportion of all red giants. Cool stars class M, show molecular bands, with titanium oxide strong. A small proportion of these cool stars show correspondingly strong bands of zirconium oxide; the existence of detectable ZrO bands in visual spectra is the definition of an S-type star. The main ZrO series are: α series, in the blue at 464.06 nm, 462.61 nm, 461.98 nm β series, in the yellow at 555.17 nm and 571.81 nm γ series, in the red at 647.4 nm, 634.5 nm, 622.9 nmThe original definition of an S star was that the ZrO bands should be detectable on low dispersion photographic spectral plates, but more modern spectra allow identification of many stars with much weaker ZrO. MS stars, intermediate with normal class M stars, have detectable ZrO but otherwise normal class M spectra. SC stars, intermediate with carbon stars, have weak or undetectable ZrO, but strong sodium D lines and detectable but weak C2 bands.
S star spectra show other differences to those of normal M class giants. The characteristic TiO bands of cool giants are weakened in most S stars, compared to M stars of similar temperature, absent in some. Features related to s-process isotopes such as YO bands, SrI lines, BaII lines, LaO bands, sodium D lines are all much stronger. However, VO bands are absent or weak; the existence of spectral lines from the period 5 element Technetium is expected as a result of the s-process neutron capture, but a substantial fraction of S stars show no sign of Tc. Stars with strong Tc lines are sometimes referred to as Technetium stars, they can be of class M, S, C, or the intermediate MS and SC; some S stars Mira variables, show strong hydrogen emission lines. The Hβ emission is unusually strong compared to other lines of the Balmer series in a normal M star, but this is due to the weakness of the TiO band that would otherwise dilute the Hβ emission; the spectral class S was first defined in 1922 to represent a number of long-period variables and stars with similar peculiar spectra.
Many of the absorption lines in the spectra were recognised as unusual, but their associated elements were not known. The absorption bands now recognised as due to ZrO are listed as major features of the S-type spectra. At that time, class M was not divided into numeric sub-classes, but into Ma, Mb, Mc, Md; the new class S was left as either S or Se depending on the existence of emission lines. It was considered that the Se stars were all LPVs and the S stars were non-variable, but exceptions have since been found. For example, π1 Gruis is now known to be a semiregular variable; the classification of S stars has been revised several times since its first introduction, to reflect advances in the resolution of available spectra, the discovery of greater numbers of S-type stars, better understanding of the relationships between the various cool luminous giant spectral types. The formalisation of S star classification in 1954 introduced a two-dimensional scheme of the form SX,Y. For example, R Andromedae is listed as S6,6e.
X is the temperature class. It is a digit between 1 and 9, intended to represent a temperature scale corresponding to the sequence of M1 to M9; the temperature class is calculated by estimating intensities for the ZrO and TiO bands summing the larger intensity with half the smaller intensity. Y is the abundance class, it is a digit between 1 and 9, assigned by multiplying the ratio of ZrO and TiO bands by the temperature class. This calculation yields a number which can be rounded down to give the abundance class digit, but this is modified for higher values: 6.0 – 7.5 maps to 6 7.6 – 9.9 maps to 7 10.0 – 50 maps to 8 > 50 maps to 9In practice, spectral types for new stars would be assigned by referencing to the standard stars, since the intensity values are subjective and would be impossible to reproduce from spectra taken under different conditions. A number of drawbacks came to light as S stars were studied more and the mechanisms behind the spectra came to be understood; the strengths of the ZrO and TiO are influenced both by actual abundances.
The S stars represent a continuum from having oxygen more abundant than carbon to carbon being more abundant than oxygen. When carbon becomes more abundant than oxygen, the free