Project VS-COMPAS | Variable Stars Common Observation Mission in Particular Areas of the Sky

Project VS-COMPAS

Variable Stars Common Observation
Mission in Particular Areas of the Sky

1347 stars discovered as of Nov 15, 2015

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Pulsating Variable Stars

In October 1595 the Dutchman David Fabricus observed the star omicron Ceti to disappear. The same star was noted to vary in brightness during 1638-39 by another Dutch observer and became known as Mira (the "Wonderful") due to its behaviour. It was eventually found to have a period of about 334 days and was the first pulsating variable discovered. Its light curve was different to that of Algol which was correctly inferred to be an eclipsing binary by the brilliant young English astronomer John Goodricke in 1782.

Pulsating variable stars are intrinsic variables as their variation in brightness is due to a physical change within the star. Pulsating variables periodically expand and contract their surface layers. In the process they change their size, effective temperature and spectral properties. The different types of pulsating variable are distinguished by their periods of pulsation and the shapes of their light curves. The pulsations may be radial or nonradial. A radially pulsating star remains spherical in shape, while in the case of nonradial pulsations the star's shape periodically deviates from a sphere, and even neighboring zones of its surface may have opposite pulsation phases.

There are several type groups of pulsating stars and some of the key types are described briefly below. The two most important types are:

Cepheids and cepheid-like stars
They have short periods (days to months) and their luminosity cycle is very regular.

Long period variables
Their period is longer, on the order of a year, and much less regular.

Depending on the period value, on the mass and evolutionary status of the star, and on the scale of pulsational phenomena, the following types of pulsating variables may be distinguished.

Cepheid Variable Stars

DCEP : Classical Cepheids – Radially pulsating, Cepheid variables pulsate with periods from 1 to 135 days, with light variations from 0.1 to 2 magnitudes in V. These massive stars have high luminosity and are of F spectral class at maximum, and G to K at minimum. The later the spectral class of a Cepheid, the longer it takes to complete a cycle. 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. On September 10, 1784 Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.

With Cepheid variables, the brighter the star, the longer it takes to complete a cycle. The exact relation depends on the metallicity of the star which categorize them into Population I and Population II. Population I stars are generally found closer to the galactic plane and relatively high concentrations of metals compared to Population II stars which are found around the galactic halo. Note that Population II Cepheids are also known as W Virginis Cepheids as opposed the Population I designation as classical Cepheids. Categorizing the stars may be done by analyzing absorption lines through spectroscopy. In general Population I Cepheids are brighter than Population II Cepheids with the same period.

Cepheids are important because they are a type of standard candle. Their luminosity is directly related to their period of variation, with a slight dependence on metallicity as well. The longer the pulsation period, the more luminous the star. A relationship between the period and luminosity for Delta Cepheids was first established in 1908 by Henrietta Swan Leavitt. Edwin Hubble used this method to prove that the so-called spiral nebulae are in fact distant galaxies.

CW : W Virginis variables – These are pulsating variables of the galactic spherical component (old disk) population with periods of approximately 0.8 to 35 days and amplitudes from 0.3 to 1.2 mag in V. Type II Cepheids are population II stars and are thus old, typically metal-poor, low mass objects. They were historically called W Virginis variables, but are now divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Viriginis subclass, and stars with periods greater than 20 days belong to the RV Tauri subclass. Type II Cepheids constitute a different class of star with a luminosity relation offset from that of the Delta Cepheids, also belong to Population II, so have a lower metallicity. For an equal period value, the W Vir variables are fainter than the Delta Cep stars by 0.7 - 2 mag.

W Vir variables are present in globular clusters and at high galactic latitudes. They may be separated into the following subtypes:

CWA - W Vir variables with periods longer than 8 days (W Vir);

CWB - W Vir variables with periods shorter than 8 days (BL Her).

Some DCEP and CW stars are quite often called Cepheids because it is often impossible to discriminate between them on the basis of the light curves for periods in the range 3 - 10 days. However, these are distinct groups of entirely different objects in different evolutionary stages. One of the significant spectral differences between W Virginis stars and Cepheids is the presence, during a certain phase interval, of hydrogen-line emission in the former and of Ca II H and K emission in the latter.

RR : RR Lyrae variables –

DSCT : δ Scuti variables –

SXPHE : SX Phoenicis variables –

RV : RV Tauri variables –

BCEP : β Cephei variables –

ACYG : α Cygni variables –

ZZ : ZZ Ceti variables –

Long-period Variables

M : Mira-like variables –

SR : Semiregular variables –

L : Slow irregular variables –

PVTEL : PV Telescopii variables –

PV Telescopii variable is a type of variable star that is established in the General Catalogue of Variable Stars with the acronym PVTEL. This class of variables are defined as "helium supergiant Bp stars with weak hydrogen lines and enhanced lines of He and C". That is, the hydrogen spectral lines of these stars are weaker than normal for a star of stellar class B, while the lines of helium and carbon are stronger. The prototype for this category is PV Telescopii (PV Tel), which undergoes small but complex luminosity variations and radial velocity fluctuations. The PV Tel stars are extremely hydrogen-deficient compared to other B-class stars and vary in luminosity on time scales ranging from a few hours to several years. As of 2008, there are twelve confirmed PV Tel variables in the General Catalogue of Variable Stars.