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The Cepheids are a class of pulsating variable stars ; Delta Cephei has a minimum size of 40 solar diameters and a maximum size of 46 solar diameters.
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Cepheids and are
Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period-luminosity relation are more metal rich than those found in the LMC.
Classical Cepheids ( or Delta Cephei variables ) are population I yellow supergiants which undergo pulsations with very regular periods on the order of days to months.
Cepheids are important because they are a type of standard candle.
These stars are somewhat similar to Cepheids, but are not as luminous.
Due to their common occurrence in globular clusters, they are occasionally referred to as cluster Cepheids.
Delta Scuti ( δ Sct ) variables are similar to Cepheids but rather fainter, and with shorter periods.
Cepheid variables are divided into several subclasses which exhibit markedly different masses, ages, and evolutionary histories: Classical Cepheids, Type II Cepheids, Anomalous Cepheids, and Dwarf Cepheids.
Classical Cepheids are population I variable stars which are 4 – 20 times more massive than the Sun, and up to 100, 000 times more luminous.
Cepheids are yellow supergiants of spectral class F6 – K2 and their radii change by (~ 25 % for the longer-period I Carinae ) millions of kilometers during a pulsation cycle.
Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established.
Type II Cepheids ( also termed Population II Cepheids ) are population II variable stars which pulsate with periods typically between 1 and 50 days.
Type II Cepheids are typically metal-poor, old (~ 10 Gyr ), low mass objects (~ half the mass of the Sun ).
Type II Cepheids are divided into several subgroups by period.
Type II Cepheids are used to establish the distance to the Galactic center, globular clusters, and galaxies.
Classical Cepheids are younger and more massive population I stars, whereas Type II Cepheids are older fainter population II stars.
Cepheids and class
Comparing the light curve, the amplitude and the radial velocity variations as compared to the light curve, Type II Cepheids constitute a different class of star with a luminosity relation offset from that of the δ Cepheids.
W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10 – 20 days, and are of spectral class F6 – K2.
From 1915 to the 1930s, the RR Lyraes became increasingly accepted as a class of star distinct from the Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences from classical Cepheids, being mostly metal-poor, Population II stars.
Cepheids and variable
However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.
A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars.
The mechanism is believed to be Eddington pulsations, like for the yellow Cepheids ( see above ), but with molecular hydrogen as the variable opacity layer of the star instead of helium.
However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.
A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds.
Initial studies of Cepheid variable distances were complicated by the inadvertent admixture of classical Cepheids and Type II Cepheids.
This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables.
The stellar population includes four Cepheids and RV Tauri variables, plus at least one RR Lyrae variable for which a period is known.
As a member of the category of variable stars known as classical Cepheids, it has a regular pulsation frequency that is determined by its mass.
6822, A Remote Stellar System wherein he identified 15 variable stars ( 11 of which were Cepheids ).
Cepheids and stars
In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster — the most distant stars resolved have up to hundred million light years away ( see Cepheids ).
More recently, this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars and the tip of the red giant branch.
* Cepheids and cepheid-like stars They have short periods ( days to months ) and their luminosity cycle is very regular ;
Type II Cepheids ( historically termed W Virginis stars ) have clock 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 also belong to Population II, compared to Population I of δ Cepheids, and so have a lower metallicity.
The luminosity of Type II Cepheids is, on average, less than classical Cepheids by about 1. 5 magnitudes ( but still brighter than RR Lyrae stars ).
They were first recognized as being distinct from classical Cepheids by Walter Baade in 1942, in a study of Cepheids in the Andromeda Galaxy that proposed that stars in that galaxy were of two populations.
In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor " Population II " stars.
Cepheids and ;
Several times as many RR Lyraes are known as all Cepheids combined ; in the 1980s, about 1900 were known in globular clusters.
Along with Delta Cephei, Zeta Geminorum and Beta Doradus, it is one of the most prominent naked eye Cepheids ; that is, both the star itself and the variation in its brightness can be distinguished with the naked eye.
Cepheids and Delta
Classical Cepheids, Type II Cepheids, RR Lyrae variables and Delta Scutis belong to the instability strip which is believed to be driven by Eddington pulsations in helium, while for the Beta Cepheids the pulsation mechanism is unknown.
This period-luminosity relationship was first established for Delta Cepheids by Henrietta Swan Leavitt.
Classical Cepheids ( also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables ) undergo pulsations with very regular periods on the order of days to months.
* Classical Cepheids include: Eta Aquilae, Zeta Geminorum, Beta Doradus, RT Aurigae, Polaris, as well as the namesake Delta Cephei.
* Dwarf Cepheids include: Delta Scuti, SX Phoenicis.
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