Most of us, like the early Greek thinkers, believe that stars do not change their properties significantly within the span of human observations.

Any changes which occur in a star are usually expected to take place only over billions of years. Given this backdrop, you can imagine the sensation caused by David Fabricius when he pointed out – as early as 1596 – that the star Mira (in the constellation Cetus) varies in its brightness periodically. It was once again clear nothing remains constant in the cosmos.  Later on, the Italian astronomer Geminiano Montanari noted that the star Algol (in Perseus) also shows similar behaviour.

The study of variability in stellar brightness became a scientific tool in the hands of astronomers only about a century later, mostly due to the work of young astronomer-mathematician, John Goodricke (1764-1786). A son of a British diplomat and Dutch mother, he lost his hearing at the age of five and had to learn lip-reading so as to join the school for normal children. Overcoming these difficulties, he made significant contributions to astronomy. He accurately measured the period over which the brightness changes in Algol in November 1782 and also discovered the variability in the star delta Cephei.  In recognition of these achievements, he was admitted as a Fellow to the Royal Society on 16 April 1786 – when he was only 21 – but unfortunately, died of pneumonia just four days later.

The variability of a class of stars now called Cepheids (of which delta Cephei was the first example) has laid the foundation for measuring extragalactic distances in the universe. These stars exhibit a tight correlation between their mean luminosity and the period of variability.

By measuring the latter, one can infer the former. Since we can also determine how bright it looks when seen from Earth, one can determine the distance to this star fairly accurately.

Goodricke also came up with an explanation for the variability seen in Algol’s brightness. If a distant star is occulted by a less bright object, we will perceive a dimming of its brightness. Algol, which is located about 96 light-years away from us, happens to be a member of a multi-star system. The central star is a massive bluish bright star with a yellow-red giant companion star orbiting around it. When the dimmer of the two comes in front, the light from Algol seems to fade as viewed from Earth.

Most of the change in the luminosity occurs in a span of about 10 hours and hence can be detected within one night.

Currently Algol is visible in the north-west evening sky and during the week 18-24 April, there are three minimas (that is, states when the star is at its dimmest) in Algol’s brightness which are favourable for detection within a single night. They occur at 23:32 hrs, 20:21 hrs and 17:10 hrs on 18th, 21st, 24th of April respectively. You can see for yourself how easy or difficult it was for Goodricke to make the observation!