In the late 18th century a young man named John Goodricke discovered that the brightness of the second brightest star named Algol in the constellation of Perseus varied. Led to his discovery that eventually led to a new branch in astronomy – astronomy of variable stars. Goodricke had proposed that Algol was not a single star but two stars which were rotating around each other. When the fainter star came between the earth and the star, the total brightness reduced, such stars are called extrinsic variable stars. The graph of time versus the brightness of the variable star is called a light curve. (See NC newsletter Nov and Dec. 2020).
As time passed, John went discovered more stars whose brightness changed. In 1784 he discovered that the brightness of the fourth brightest star in the constellation of Cepheus, changed over 5 days 8hours and 47.4 minutes. The light curve of this star was differed from that of Algol. The brightness of the star rose quickly and then faded slowly. (See figure 1.)
Later more variable stars were discovered. Though the pattern of variation in their brightness seemed to be identical, they appeared at different periods. The stars brightened quickly and faded slowly but some stars would do so in just a few days time and some others would take a longer time, even up to 100 days. The stars whose brightness varied with a pattern similar to that of Delta Cepheii are collectively called Delta Cepheid variables or simply Cepheid variables.
In 1894, Russian astronomer A. A. Belopolasky used the spectroscopic method to study the expansion and contraction of the stars and discovered that these stars are actually expanding and contracting. In the first decade of 1900 American astronomer Henrietta Swan Levitt, working at the Harvard College Observatory, made a very significant discovery. She was studying stars in Large and Small Magellanic Clouds (LMC/SMC).
Large and Small Magellanic clouds are nebulae or patches of light seen from the southern hemisphere of the earth. These patches were known to Persian astronomers. However, it was through writings of Ferdinand Magellan, the Portuguese explorer, in 1519, that the European astronomers learnt about these celestial clouds which now bear his name.
Levitt, during her studies, discovered many Cepheid variables in LMC and SMC. Most interestingly, she found that, the duration of brighter stars were uniformly longer than those of fainter stars. She reported her discovery in 1912. In her report, she published a graph of magnitude (the term used for indicating the brightness of stars) versus their duration. (See fig 2.)
In her own words, “A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods.”
It did not take time for astronomers to understand the significance of this important discovery, which was the direct correlation between the brightness of the stars and their distance. As all the stars in LMC or SMC can be considered to be at nearly the same distance from the earth this relation can be used to estimate the distance between the stars. This correlation is now called the period-luminosity relation. Luminosity is the total energy or intrinsic brightness of the stars.
Simultaniously, some other astronomers were measuring distances to stars by the method of parallax. The method is similar to the one used by geographical surveyors.
Leavitt died at an early age of 53 due to cancer. But her work was continued by another young astronomer Harlow Shapley. He studied Cepheids in a cluster of stars called globular clusters and found that thus were not centred around the Sun, as was believed then, but at a point somewhere in the constellation of Sagittarius. It may be noted that until this time humanity had no knowledge of the actual extent of our universe.
In 1924, Edwin Hubble, working at the Mount Wilson Observatory was observing Cepheid variables in Andromeda nebula. He estimated the distance to the nebula to be 1.5 million lightyears from the earth. This distance is far too larger than that of any celestial object ever measured from the earth. Eventually, it was realized that the Andromeda nebula was actually a galaxy and was at a distance of 2.5 million lightyears from the earth.
In 1941 British Astronomer, Arthur Eddington found the true cause for the variation of brightness of a Cepheid variable. He theoretically showed that variability of Cepheids is due to the evolutionary process of the star. At a stage in its life cycle, a star rapidly expands due to excess energy pressure generated inside it. It cools as the energy is radiated out and slowly contracts.
As the cause for the variation in brightness of Cepheid variable stars is due to the changes taking place inside the stars, these stars are called intrinsic variable stars.
There are other types of intrinsic variable stars but Cepheids have played a major role in scaling our universe.
Celestial events for viewing pleasure 