Education Talk

Richard Stratford: ‘Messier and Herschel: Opening the Doors of the Sky’

Summary: The talk begins with a brief summary of developments in astronomy during the 17^th century, in particular the establishment of the heliocentric cosmology and the law of universal gravitation, which made it possible to predict the movements of the Moon and the planets with unprecedented accuracy. These movements, combined with accurate positions of the stars, could be used for navigation and to solve the longitude problem. As a by-product, the movements of comets became understandable and predictable; they were now regarded as natural phenomena rather than as signs and portents.

Charles Messier (1730-1817) began his career by searching for Halley’s Comet, whose re-appearance had been predicted in 1758. He was not the first person to see the comet, and did not see it until 21 January 1759. However, he had independently discovered Comet de la Nux in August 1758, and he went on to discover 13 comets of his own, with about eight independent discoveries of comets found by other astronomers.

Between 1771 and 1781, Messier published three catalogues of ‘nebulae’, amounting to 103 objects. These included a supernova remnant (M1), globular clusters, open clusters, emission nebulae (H II regions), planetary nebulae, and external galaxies.

Messier’s work was followed by that of William Herschel (1738-1822) and his sister Caroline (1750-1848). Herschel first tried to detect stellar parallaxes by measuring the relative positions of double stars. Although he failed in this attempt, he discovered evidence of orbital motion in several systems, and proposed that these were true binary stars orbiting around their centre of gravity, rather than being chance alignments of stars at different distances. Between 1782 and 1821 he published catalogues containing more than 500 double stars.

Herschel’s second attempt relied on the assumption that all stars had about the same brightness, and therefore that he could estimate their relative distances from their apparent brightness. By counting stars in the field of his telescope, he obtained a ‘grindstone’ model for the shape of the Galaxy, whose diameter he estimated to be 1000 times the distance of Sirius (about 9000 light-years in modern terms).