Henrietta Swan Leavitt And Cepheid Variables

In the period 1880-1910 a group of assistants at the Harvard Observatory, under the direction of Edward Pickering (1846-1919), carried out an extensive program of photometry, involving the measurement of the brightness of stars from the images they produced on photographic plates. Included in this survey were stars in the Magellanic Clouds. The latter are visible as two patches of light in the southern sky and are resolved by telescope into two systems of stars. Pickering's assistants, most of whom were women, measured the brightness of Magellanic stars recorded on photographic plates exposed at regular intervals using the 24-inch refractor at the Harvard field station in Peru. A distinct class of these stars consisted of what were known as Cepheid variable stars, so named after the prototype 5 Cephei in the northern constellation of Cepheus. Such stars vary in brightness by about one-half to two magnitudes, with a period of anywhere from 1 to 15 days. The part of the cycle where the star is declining in brightness is longer than the part where it is increasing in brightness. Henrietta Swann Leavitt (1868-1921), a supervisor of the Harvard assistants, noticed that there was a relationship between the brightness of a Cepheid-type star in the Small Magellanic Cloud and its period of variability—the brighter the star, the longer its period of variation. The publication by Pickering of Leavitt's result in the Harvard Observatory circular in 1912 ranks as one of the signal discoveries in the history of observational astronomy.

The stars in the Small Magellanic Cloud belong to a localized group that is quite distant from Earth. Viewed from Earth, the relative differences in their distances are very minor so that the apparent brightness of a given star in the cloud compared to other stars in the cloud is also an indication of its real brightness compared to the real brightness of the other stars. It follows that the relationship between period and observed brightness detected by Leavitt is also a relationship between period and absolute or intrinsic luminosity. This fact was explicitly stated in the 1912 paper of Pickering and Leavitt.

The work of American astronomers was closely followed by the Danish astrophysicist Enjar Hertzsprung (1873-1967). Hertzsprung was very interested in how the different characteristics of a star were related to its luminosity. He found that spectral type, a measure of the surface temperature of a star, was related to luminosity, the total energy emitted by a star. As the spectral type of a star moves from red to blue, there is an increase in its luminosity; there is also a special exceptional group of very bright, red stars. When these facts are depicted graphically, one obtains what is known as the Hertszprung-Russell diagram, named for Hertszprung and the American astronomer Henry Russell (1877-1957), who independently arrived at the relation a few years after his Danish colleague. The H-R diagram would become a key to the study of the evolution of stars and is a basic theoretical element in modern stellar physics.

Hertzsprung knew that information about the intrinsic luminosity of a star could be used to determine its distance and immediately recognized the value for this purpose of Leavitt's result. (Leavitt and Pickering also recognized this fact, but as observationalists, refrained from following up on its theoretical implications.) In principle, the period-luminosity relation could be used to determine the distance to Cepheid variables located anywhere in the universe. From observations over time of a given Cepheid variable star one could measure its period, and by means of the period-luminosity relationship one then knew the star's absolute luminosity. Knowing this quantity, and knowing the observed brightness of the star, one could compute its distance. Cepheid variables therefore provided a yardstick to measure stellar distances. It was necessary to calibrate this yardstick, which meant knowing the distance to at least one Cepheid variable. In addition, it was later revealed that there are different types of Cepheid variables, and it was necessary to ensure that the variable in question belonged to the class for which the yardstick was calibrated. Despite the sizeable uncertainties introduced by these considerations, the discovery of the Cepheid distance method was a major leap forward in the scientific project of mapping the universe.

0 0

Post a comment