Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects. Like all Cepheid variables, Type IIs exhibit a

relationship Relationship most often refers to: * Family relations and relatives: consanguinity * Interpersonal relationship, a strong, deep, or close association or acquaintance between two or more people * Correlation and dependence, relationships in mathem ...

between the star's

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

and pulsation period, making them useful as

standard candles The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...

for establishing distances where little other data is available Longer period Type II Cepheids, which are more luminous, have been detected beyond the Local Group in the galaxies

NGC 5128 Centaurus A (also known as NGC 5128 or Caldwell 77) is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable d ...

and

NGC 4258 Messier 106 (also known as NGC 4258) is an intermediate spiral galaxy in the constellation Canes Venatici. It was discovered by Pierre Méchain in 1781. M106 is at a distance of about 22 to 25 million light-years away from Earth. M106 contains ...

Classification

Historically Type II Cepheids were called

W Virginis variable W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days,Wallerstein, G."The Cepheids of Population II and Related Stars" ''Publications of the Astronomical Society of the Pacific'', 114 p.689– ...

s, but are now divided into three subclasses based on the length of their period. Stars with periods between 1 and 4 days are of the BL Herculis subclass and 10–20 days belong to the W Virginis subclass. Stars with periods greater than 20 days, and usually alternating deep and shallow minima, belong to the RV Tauri subclass. RV Tauri variables are usually classified by a formal period from deep minimum to deep minimum, hence 40 days or more. The divisions between the types are not always clearcut or agreed. For example, the dividing line between BL Her and W Vir types is quoted at anything between 4 and 10 days, with no obvious division between the two. RV Tau variables may not have obvious alternating minima, while some W Vir stars do. Nevertheless, each type is thought to represent a distinct different evolutionary stage, with BL Her stars being helium core burning objects moving from the

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) ...

towards the

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(AGB), W Vir stars undergoing hydrogen or helium shell burning on a blue loop, and RV Tau stars being post-AGB objects at or near the end of nuclear fusion. RV Tau stars in particular show irregularities in their light curves, with slow variations in the brightness of both maxima and minima, variations in the period, intervals with little variation, and sometimes a temporary breakdown into chaotic behaviour.

R Scuti R Scuti (''R Sct'') is a star in the constellation of Scutum (constellation), Scutum. It is a yellow supergiant and is a pulsating variable known as an RV Tauri variable. It was discovered in 1795 by Edward Pigott at a time when only a f ...

has one of the most irregular light curves.

Properties

The physical properties of all the type II Cepheid variables are very poorly known. For example, it is expected that they have masses near or below that of the Sun, but there are few examples of reliable known masses.

Period-luminosity relationship

Type II Cepheids are fainter than their classical Cepheid counterparts for a given period by about 1.6 magnitudes. Cepheid variables are used to establish the distance to the Galactic Center,

globular clusters A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member ...

, and

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

Examples

Type II Cepheids are not as well known as their type I counterparts, with only a couple of naked eye examples. In this list, the period quoted for RV Tauri variables is the interval between successive deep minima, hence twice the comparable period for the other sub-types.

References

External links

OGLE Atlas of Variable Star Light Curves - Type II Cepheids
{{Variable star topics *Type II Cepheid Astrometry Standard candles Population II stars