Type II Cepheids are

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

s 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 variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

s, Type IIs exhibit a relationship between the star's

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

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 ...

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 The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...

in the galaxies NGC 5128 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 an ...

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, and are of spectral class F6 – K2. They were first recognized as being distinct from classical Cepheids by Walter Baade in 1942, in ...

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 proc ...

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 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–Russel ...

, 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 has one of the most irregular light curves.

Properties

The physical properties of all the type II Cepheid variables are 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 The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

globular clusters A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

, and

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and 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 Sys ...

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