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Photometric Standard Stars
Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.[1] A current set of photometric-standard stars for UBVRI photometry is that published by Arlo U. Landolt in 1992 in the Astronomical Journal, vol. 104, no. 1, p. 340-371. References[edit]^ Landolt, Arlo U. (1 July 1992). "UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator". The Astronomical Journal. pp. 340–371. Bibcode:1992AJ....104..340L. doi:10.1086/116242
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Star
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
Earth
is the Sun. Many other stars are visible to the naked eye from Earth
Earth
during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, 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
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Stellar Population
In 1944, Walter Baade
Walter Baade
categorized groups of stars within the Milky Way from their spectra. Two main divisions were defined as Population I and II, with another division known as Population III added in 1978[1]. Often now simply abbreviated as either Pop I, II or III, these differences were later shown to be highly significant, dividing stars into classes by their chemical composition or metallicity, whose small proportion of stellar matter consists of the "heavier chemical elements" beyond the more common elements of hydrogen and helium.[2][3] By coincidence, each population group definition has decreasing metal content and increasing age
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Horizontal Branch
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core
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Red Clump
The red clump is a clustering of red giants in the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g
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Asymptotic Giant Branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives. Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity thousands of times greater than the Sun
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Protoplanetary Nebula
A protoplanetary nebula or preplanetary nebula (Sahai, Sánchez Contreras & Morris 2005) (PPN) is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB)[a] phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars (1–8 M☉)
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Planetary Nebula
A planetary nebula, abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.[2] The word "nebula" is Latin for mist or cloud, and the term "planetary nebula" is a misnomer that originated in the 1780s with astronomer William Herschel[dubious – discuss] because, when viewed through his telescope, these objects resemble the rounded shapes of planets. Herschel's name for these objects was popularly adopted and has not been changed.[3][4] They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.[5] Most planetary nebulae form at the end of the star's life, during the red giant phase, when the outer layers of the star are expelled by strong stellar winds
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PG 1159 Star
A PG 1159 star, often also called a pre-degenerate,[1] is a star with a hydrogen-deficient atmosphere that is in transition between being the central star of a planetary nebula and being a hot white dwarf. These stars are hot, with surface temperatures between 75,000 K and 200,000 K,[2] and are characterized by atmospheres with little hydrogen and absorption lines for helium, carbon and oxygen. Their surface gravity is typically between 104 and 106 meters per second squared. Some PG 1159 stars are still fusing helium.[3], § 2.1.1, 2.1.2, Table 2. The PG 1159 stars are named after their prototype, PG 1159-035. This star, found in the Palomar-Green survey of ultraviolet-excess stellar objects,[4] was the first PG 1159 star discovered. It is thought that the atmospheric composition of PG 1159 stars is odd because, after they have left the asymptotic giant branch, they have reignited helium fusion
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Dredge-up
A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star. The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12C/13C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced. The second dredge-up occurs in stars with 4–8 solar masses
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Instability Strip
The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
largely occupied by several related classes of pulsating variable stars:[1] Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap stars (roAps) near the main sequence; RR Lyrae variables
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Luminous Blue Variable
Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in both their spectra and brightness
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Blue Straggler
A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main-sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3.[2][3] Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass
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Supernova
A supernova (/ˌsuːpərnoʊvə/ plural: supernovae /ˌsuːpərnoʊviː/ or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of a massive star's life, whose destruction is marked by one final titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months. SN 1994D
SN 1994D
(bright spot on the lower left), a Type Ia supernova outshining its home galaxy, NGC 4526Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous
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Main Sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung
Ejnar Hertzsprung
and Henry Norris Russell. Stars on this band are known as main-sequence stars or "dwarf" stars.[1][2] These are the most numerous true stars in the universe, and include the Earth's Sun. After condensation of mass and ignition of a star, it generates thermal energy in the dense core region through nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors
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Supernova Impostor
Supernova
Supernova
impostors are stellar explosions that appear at first to be a type of supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae
Eta Carinae
analogs, and giant eruptions of luminous blue variables (LBV).[2] Appearance, origin and mass loss[edit] Supernova
Supernova
impostors appear as remarkably faint supernovae of spectral type IIn—which have hydrogen in their spectrum and narrow spectral lines that indicate relatively low gas speeds. These impostors exceed their pre-outburst states by several magnitudes, with typical peak absolute visual magnitudes of −11 to −14, making these outbursts as bright as the most luminous stars
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.