Hubble view of a Cepheid variable star
Many stars do not consistently shine with the same magnitude or brightness. Such stars are called variables, their brightness normally changing in cycles which can last from hours to years. Some exhibit irregular non-cyclical variation. Stellar variability can have many causes. These include changes caused by flares arising from the outer layers, wave-like motions on the surface resulting from fluctuations in the flow of energy from their interiors, having a non-spherical shape, having a surface which has some brighter areas than others, and stars in binary systems which are eclipsed by their companion.
Cepheid variables are an important group of very luminous yellow giant or supergiant pulsating variable stars with masses three times that of the Sun. They are named after the prototype Delta Cephei in the far-northern constellation of Cepheus. Their variability is a result of expansion and contraction caused by fluctuations in the energy emitted from their active cores. These stars exhibit radial pulsations, a form of pulsation in which they expand and contract symmetrically over their whole surface. The changes in radius are accompanied by variations in brightness, surface temperature, and spectrum (colour). Their size at maximum is typically 7-15% larger than at their minimum, differences which are in the millions of kilometres in distance. Cepheid variables pulsate with a simple fundamental mode, their pulsation reflecting the simple pattern of period expansion and contraction of the stars outer layers. This contrasts with the more complex vibrations found in other variable stars where the basic frequency is overlaid with others, like the harmonics in musical tones.
Cepheids pulsate in regular periods of a few days, a pattern which astronomers plot on a light curve which shows a sharp rise in brightness towards maximum followed by a slower dimming towards minimum. The first variable star was identified in 1781 by the amateur English astronomer John Goodricke, but it was in 1784 that he identified Delta Cephei, the variable which was later realised to be the first known example of what became the Cepheid variables, variables with a unique set of characteristics. Delta Cephi is not only important for its initial discovery, but because its own distance is amongst the most precisely established, enabling it to function as a particularly important calibrator for distance measurements.
The importance of these stars for astronomy was identified in 1908 by the American astronomer Henrietta Swan Leavitt when she discovered that their period was directly related to their absolute magnitude (brightness) in an almost linear way. The longer the period, the greater the luminosity. The resulting consistent period-luminosity relationship, which she published in 1912, has given Cepheids the status of being important ‘standard candles’ for establishing galactic and extra-galactic distance scales.
Leavitt’s proof enabled others to use Cepheid variables in a number of important ways. In 1915, Harlow Shapely used those he found in globular clusters* to identify the size and shape of the Milky Way, and the position of the Sun within it. In 1924, Edwin Hubble established the distance of Cepheid variables in the Andromeda galaxy, demonstrating that these variables were not members of the Milky Way. This put to rest the Island Universe debate, which was concerned with whether the Milky Way and the Universe were synonymous. Later that decade, Hubble also used Cepheids in the work which eventually identified Hubble’s law and evidence of the expansion of the Universe.
Two distinct types have been identified. More numerous are classical Cepheids aka Type 1 Cepheids, Delta Cephei Cepheids. These are Population I stars (stars which, like the Sun, lie in the disk of the galaxy and follow roughly circular orbits) with absolute magnitudes 0.7 – 2 magnitudes brighter than Type II Cepheids. The pulsation periods for classical Cepheids range from days to months. They are 4-20 times more massive than the Sun, and up to 100,000 times more luminous. Stars of this type are used to determine distances to galaxies within the Local Group (of which the Milky Way is a member) and are also the means by which the Hubble constant (the speed of an objects recession in the expanding universe) can be established. They have also been used to clarify many characteristics of the Milky Way including the Sun’s height above the galactic plane, and the galaxy’s local spiral structure.
Type II Cepheids are older Population II stars (stars found in the halo and central bulge of the galaxy, those in the halo following very elliptical orbits). They are dimmer than classical Cepheids, have smaller masses (half the mass of the Sun) and brightness, and also contain lower concentrations of heavy elements. Their pulsation periods are much shorter than those of the classical Cepheids, typically between 1 and 50 days. This type is divided into three subgroups by period length. The BL Herculis sub-class has pulsation periods of 1-4 days, W Virginis Cepheids 10-12 days, and the RV Tauri sub-class periods of greater than 20 days. Type II Cepheids are used to establish distances to the galactic centre, globular clusters and other galaxies.
Sources: Ridpath, I (Ed) 2007 Oxford dictionary of astronomy 2nd ed, Astronomy (Dorling Kindersley – Eyewitness companions,www.en.wikipedia.org
*Objects written in bold are covered in more detail in other parts of this series.