Shapley Constellations

A low magnification cruise around the Cloud’s five Shapley constellations is a journey of both mind and imagination

The Cloud’s five constellations

In 1931 Harlow Shapley noted 15 “small irregular star clouds” and that “nearly all of them appear to be distinct physical organisations.” In 1951 he noted that many of them have 10 times the diameters and luminosities of M11 and the Pleiades and he asked, “Should such widespread assemblies be called subclouds or subclusters or would it be better to designate them constellations? They are doubtless comparable to the Orion, Scorpius and Vela aggregations of bright galactic stars.”

In 1953 Virginia McKibben Nail and Harlow Shapley published their paper, Magellanic Clouds VII. Star colors and luminosities in five constellations, in which they wrote, “We have examined five stellar aggregations in the Large Magellanic Cloud that average something more than one thousand light years in diameter. We call them constellations because of their rough similarity in size, population, and star colors to the Orion constellation and similar stellar associations in our galactic system. The luminosities of the blue member stars range from -8 to -5 in absolute photographic magnitude. A few exceedingly red supergiant stars also appear in these constellations, as well as a large number of blue stars fainter than M = -5, as in the Orion, Perseus, and Scorpius-Centaurus groups.” Thus, the Large Magellanic Cloud acquired its constellations.

Constellation V lies in the bar of the Cloud, and the others are on the north side. Although the areas in each constellation are square – varying from 17′ x 17′ to 33′ x 33′ – the constellations are all elongated or irregular in outline.

The great arc of stars in the supergiant shell LMC 4, the Quadrant, is usually – and quite erroneously – identified with Constellation III. Westerlund & Mathewson (1966) noted on a UV plate ‘the great arc of the bright blue stars’, saying that ‘Shapley called this arc Constellation III’. Olsen et al. (1997) also called the Quadrant Constellation III in their fig. 1, yet in the text they refer to the entire LMC 4 as Constellation III. Sidney van den Bergh (1981) also called the whole LMC 4 region Constellation III. However, McKibben Nail & Shapley designated NGC 1974 as the identifier of Constellation III, including an area of 28′ x 28′ around NGC 1974, which lies in the Sextant (its stars form a perfect sixth of a ring). They also noted that Constellation III is a triple cluster; the Sextant’s arc of bright stars contains the three NGC open clusters: NGC 1974 itself, NGC 1955 and NGC 1968 (as well as SL 456 at its western end, which was discovered in 1963).