Star clusters are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars contain fewer than a few hundred members, are very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space though they are no longer gravitationally bound. Star clusters visible to the naked eye include the Pleiades and the Beehive Cluster. Globular clusters are spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30 light years across, they consist of old Population II stars—just a few hundred million years younger than the universe itself—which are yellow and red, with masses less than two solar masses. Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases to end as white dwarfs.
Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions. In our galaxy, globular clusters are distributed spherically in the galactic halo, around the galactic centre, orbiting the centre in elliptical orbits. In 1917, the astronomer Harlow Shapley made the first reliable estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However improved distance measurements to globular clusters using the Hipparcos satellite and accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less. Our galaxy has about 150 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79.
Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand. A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age; the brightest globular cluster in the northern hemisphere is Messier 13 in the constellation of Hercules. Super star clusters are large regions of recent star formation, are thought to be the precursors of globular clusters. Examples include Westerlund 1 in the Milky Way. Open clusters are different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, are always found within spiral arms, they are young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 for example. They form from H II regions such as the Orion Nebula. Open clusters contain up to a few hundred members, within a region up to about 30 light-years across.
Being much less densely populated than globular clusters, they are much less gravitationally bound, over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can result in the ejection of stars, a process known as'evaporation'; the most prominent open clusters are the Hyades in Taurus. The Double Cluster of h+Chi Persei can be prominent under dark skies. Open clusters are dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die. Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are used as standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe. Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts.
Embedded clusters are groups of young stars that are or encased in an Interstellar dust or gas, impervious to optical observations. Embedded clusters form in molecular clouds, when the clouds begin to form stars. There is ongoing star formation in these clusters, so embedded clusters may be home to various types of young stellar objects including protostars and pre-main-sequence stars. An example of an embedded cluster is the Trapezium cluster in the Orion Nebula. In ρ Ophiuchi cloud core region there is an embedded cluster; the embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered leading to the disruption of a star cluster.
Most young embedded clusters disperse shortly after the end of star formation. The open clusters fou
Stellar nucleosynthesis is the theory explaining the creation of chemical elements by nuclear fusion reactions between atoms within stars. Stellar nucleosynthesis has occurred continuously since the original creation of hydrogen and lithium during the Big Bang, it is a predictive theory that today yields excellent agreement between calculations based upon it and the observed abundances of the elements. It explains why the observed abundances of elements in the universe grow over time and why some elements and their isotopes are much more abundant than others; the theory was proposed by Fred Hoyle in 1946, who refined it in 1954. Further advances were made to nucleosynthesis by neutron capture of the elements heavier than iron, by Margaret Burbidge, Geoffrey Burbidge, William Alfred Fowler and Hoyle in their famous 1957 B2FH paper, which became one of the most cited papers in astrophysics history. Stars evolve because of changes in their composition over their lifespans, first by burning hydrogen helium, progressively burning higher elements.
However, this does not by itself alter the abundances of elements in the universe as the elements are contained within the star. In its life, a low-mass star will eject its atmosphere via stellar wind, forming a planetary nebula, while a higher–mass star will eject mass via a sudden catastrophic event called a supernova; the term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a pre-supernova massive star. Those massive stars are the most prolific source of new isotopes from carbon to nickel; the advanced sequence of burning fuels is driven by gravitational collapse and its associated heating, resulting in the subsequent burning of carbon and silicon. However, most of the nucleosynthesis in the mass range A = 28–56 is caused by the upper layers of the star collapsing onto the core, creating a compressional shock wave rebounding outward; the shock front raises temperatures by 50%, thereby causing furious burning for about a second. This final burning in massive stars, called explosive nucleosynthesis or supernova nucleosynthesis, is the final epoch of stellar nucleosynthesis.
A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. The need for a physical description was inspired by the relative abundances of isotopes of the chemical elements in the solar system; those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions. This suggested a natural process, not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light. In 1920, Arthur Eddington, on the basis of the precise measurements of atomic masses by F. W. Aston and a preliminary suggestion by Jean Perrin, proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium and raised the possibility that the heavier elements are produced in stars.
This was a preliminary step toward the idea of nucleosynthesis. In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier; the Gamow factor was used in the decade that followed by Atkinson and Houtermans and by Gamow himself and Edward Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors. In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium, he defined two processes. The first one, the proton–proton chain reaction, is the dominant energy source in stars with masses up to about the mass of the Sun; the second process, the carbon–nitrogen–oxygen cycle, considered by Carl Friedrich von Weizsäcker in 1938, is more important in more massive main-sequence stars.
These works concerned the energy generation capable of keeping stars hot. A clear physical description of the proton–proton chain and of the CNO cycle appears in a 1968 textbook. Bethe's two papers did not address the creation of heavier nuclei, however; that theory was begun by Fred Hoyle in 1946 with his argument that a collection of hot nuclei would assemble thermodynamically into iron Hoyle followed that in 1954 with a large paper describing how advanced fusion stages within massive stars would synthesize the elements from carbon to iron in mass. This is the first work of stellar nucleosynthesis, it and Hoyle's 1954 paper provided the roadmap to how the most abundant elements on Earth had been synthesized within stars from their initial hydrogen and helium, making clear how those abundant elements increased their galactic abundances as the galaxy aged. Hoyle's theory was expanded to other processes, beginning with the publication of a review paper in 1957 by Burbidge, Burbidge and Hoyle.
This review paper collected and refined earlier research into a cited picture that gave promise of accounting for the observed relative abundances of the elements.
A variable star is a star whose brightness as seen from Earth fluctuates. This variation may be caused by a change in emitted light or by something blocking the light, so variable stars are classified as either: Intrinsic variables, whose luminosity changes. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth. Many most, stars have at least some variation in luminosity: the energy output of our 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. Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti pulsated in a cycle taking 11 months; 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, 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. Chi Cygni was identified in 1686 by G. Kirch R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known. John Goodricke himself discovered Beta Lyrae. Since 1850 the number of known variable stars has increased especially after 1890 when it became possible to identify variable stars by means of photography; the latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, over 10,000'suspected' variables. The most common kinds of variability involve changes in brightness, but other types of variability occur, in particular changes in the spectrum. By combining light curve data with observed spectral changes, astronomers are able to explain why a particular star is variable.
Variable stars are 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 well established. Peak brightnesses in the light curve are known as maxima. 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 the world and shares the data with the scientific community. From the light curve the following data are derived: are the brightness variations periodical, irregular, or unique? What is the period of the brightness fluctuations? What is the shape of the light curve? From the spectrum the following data are derived: what kind of star is it: what is its temperature, its luminosity class? is it a single star, or a binary? does the spectrum change with time?
Changes in brightness may depend on the part of the spectrum, observed if the wavelengths of spectral lines are shifted this points to movements strong magnetic fields on the star betray themselves in the spectrum abnormal emission or absorption lines may be indication of a hot stellar atmosphere, or gas clouds surrounding the star. In 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 gives a clue as to the changes that occur in a variable star. For example, evidence for 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 is related to oscillations in the degree of ionization in outer, convective layers of the star.
Suppose the star is in the swelling phase. Its outer layers expand; because of the decreasing temperature the degree of ionization decreases. This makes the gas more transparent, thus makes it easier for the star to radiate its energy; this in turn will make the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. Thi
A giant star is a star with larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III; the terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905. Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as hypergiants. A hot, luminous main-sequence star may be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is. A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence; the behaviour of a post-main-sequence star depends on its mass. For a star with a mass above about 0.25 solar masses, once the core is depleted of hydrogen it contracts and heats up so that hydrogen starts to fuse in a shell around the core.
The portion of the star outside the shell expands and cools, but with only a small increase in luminosity, the star becomes a subgiant. The inert helium core continues to grow and increase temperature as it accretes helium from the shell, but in stars up to about 10-12 M☉ it does not become hot enough to start helium burning. Instead, after just a few million years the core reaches the Schönberg–Chandrasekhar limit collapses, may become degenerate; this causes the outer layers to expand further and generates a strong convective zone that brings heavy elements to the surface in a process called the first dredge-up. This strong convection increases the transport of energy to the surface, the luminosity increases and the star moves onto the red-giant branch where it will stably burn hydrogen in a shell for a substantial fraction of its entire life; the core continues to gain mass and increase in temperature, whereas there is some mass loss in the outer layers. § 5.9. If the star's mass, when on the main sequence, was below 0.4 M☉, it will never reach the central temperatures necessary to fuse helium.
P. 169. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf. § 4.1, 6.1. According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe. In stars above about 0.4 M☉ the core temperature reaches 108 K and helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process.§ 5.9, chapter 6. When the core is degenerate helium fusion begins explosively, but most of the energy goes into lifting the degeneracy and the core becomes convective; the energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, the star moves from the red-giant branch to the horizontal branch. Chapter 6; when the core helium is exhausted, a star with up to about 8 M☉ has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell.
As with the earlier collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up, causes a dramatic increase in size and luminosity. This is the asymptotic giant branch analogous to the red-giant branch but more luminous, with a hydrogen-burning shell contributing most of the energy. Stars only remain on the AGB for around a million years, becoming unstable until they exhaust their fuel, go through a planetary nebula phase, become a carbon–oxygen white dwarf. § 7.1–7.4. Main-sequence stars with masses above about 12 M☉ are very luminous and they move horizontally across the HR diagram when they leave the main sequence becoming blue giants before they expand further into blue supergiants, they start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without a strong increase in luminosity. At this stage they have comparable luminosities to bright AGB stars although they have much higher masses, but will further increase in luminosity as they burn heavier elements and become a supernova.
Stars in the 8-12 M☉ range have somewhat intermediate properties and have been called super-AGB stars. They follow the tracks of lighter stars through RGB, HB, AGB phases, but are massive enough to initiate core carbon burning and some neon burning, they form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova, or they may leave behind an oxygen–neon white dwarf. O class main sequence stars are highly luminous; the giant phase for such stars is a brief phase of increased size and luminosity before developing a supergiant spectral luminosity class. Type O giants may be more than a hundred thousand times as luminous as the sun, brighter than many supergiants. Classification is complex and difficult with small differences between luminosity classes and a continuous range of intermediate forms; the most massive stars develop giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to the surface and high luminosity which produces a powerful stellar wind and causes the star's atmosphere to expand.
A star whose initial mass is less than 0.25 M☉ will not become a giant star at all. For most of th
A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite. Globular clusters are tightly bound by gravity, which gives them their spherical shapes, high stellar densities toward their centers; the name of this category of star cluster is derived from globulus -- a small sphere. A globular cluster is sometimes known, more as a globular. Globular clusters are found in the halo of a galaxy and contain more stars, are much older, than the less dense, open clusters which are found in the disk of a galaxy. Globular clusters are common. Larger galaxies can have more: The Andromeda Galaxy, for instance, may have as many as 500; some giant elliptical galaxies, such as M87, have as many as 13,000 globular clusters. Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, every large galaxy surveyed, has been found to possess a system of globular clusters; the Sagittarius Dwarf galaxy, the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way.
This demonstrates. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear, it does appear clear that globular clusters are different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy, rather than as a separate galaxy. The first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4 in 1764; the first eight globular clusters discovered are shown in the table. Subsequently, Abbé Lacaille would list NGC 104, NGC 4833, M55, M69, NGC 6397 in his 1751–52 catalogue; the M before a number refers to Charles Messier's catalogue, while NGC is from the New General Catalogue by John Dreyer. When William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters.
Herschel discovered another 36 himself and was the first to resolve all of them into stars. He coined the term "globular cluster" in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789; the number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 152 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20; these additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way. Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers, he examined the RR Lyrae variables in the clusters and used their period–luminosity relationship for distance estimates. It was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, the large majority are located in the celestial sky centered on the core.
In 1918, this asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of the galaxy. By assuming a spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center. While his distance estimate was in significant error, it did demonstrate that the dimensions of the galaxy were much greater than had been thought, his error was due to interstellar dust in the Milky Way, which absorbs and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are. Shapley's measurements indicated that the Sun is far from the center of the galaxy contrary to what had been inferred from the nearly distribution of ordinary stars. In reality, most ordinary stars lie within the galaxy's disk and those stars that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer. In 1927–29, Shapley and Sawyer categorized clusters according to the degree of concentration each system has toward its core; the most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class In 2015, a new type of globular cluster was proposed on the basis of observational data, the dark globular clusters; the formation of globular clusters remains a poorly understood phenomenon and it remains uncertain whether the stars in a globular cluster form in a single generation or are spawned across multiple generations over a period of several hundred million years. In many globular clusters, most of the stars are at approxima
In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russell diagram which forms a loop towards the blue side of the diagram. Blue loops can occur for red supergiants asymptotic giant branch stars; some stars may undergo more than one blue loop. Many pulsating variable stars such as Cepheids are blue loop stars. Stars on the horizontal branch are not referred to as on a blue loop though they are temporarily hotter than on the red giant or asymptotic giant branches. Loops occur far too to be observed for individual stars, but are inferred from theory and from the properties and distribution of stars in the H-R diagram. Most stars on the red giant branch have an inert helium core and remain on the RGB until a helium flash moves them to the horizontal branch. However, stars more massive than about 2.3 M☉ do not have an inert core.
They smoothly ignite helium before reaching the tip of the red-giant branch and become hotter while they burn helium in their cores. More massive stars become hotter during this phase and stars from about 5 M☉ upwards are treated as experiencing a blue loop, which lasts on the order of a million years; this type of blue loop occurs only once in the lifetime of a star. Stars on the asymptotic giant branch have inert cores of carbon and oxygen, alternately fuse hydrogen and helium in concentric shells around the core; the onset of helium shell burning causes a thermal pulse and in some cases this will cause the star to temporarily increase its temperature and execute a blue loop. Many thermal pulses may occur as the shells alternately switch on and off, multiple blue loops can occur in the same star. Red supergiants are massive stars that have left the main sequence and expanded and cooled, their high luminosity and low surface gravity means they are losing mass. The most luminous red supergiants can lose mass enough that they become hotter and smaller.
In the most massive stars, this can result in the star evolving permanently away from the red supergiant stage to become a blue supergiant, but in some cases the star will execute a blue loop and return to being a red supergiant. Stars which are executing blue loops cross the yellow portion of the H-R diagram above the main sequence, so that many of them cross a region called the instability strip because the outer layers of stars in that region are unstable and pulsate. Stars from the asymptotic giant branch that cross the instability strip during a blue loop are thought to become W Virginis variables. More massive stars, crossing the instability strip during a blue loop from the red giant branch, are thought to make up the δ Cephei variables. Both types of star have luminous and unstable photospheres at this stage of their lives and have the spectra of supergiants, although most are not massive enough to fuse carbon or reach a supernova