O'Connell effect

Lore & Background

The O'Connell effect was named after the astronomer Daniel Joseph Kelly O'Connell, SJ of Riverview College in New South Wales, who in 1951 studied this phenomenon and distinguished it from the so-called periastron effect described by earlier authors, as it does not necessarily appear near the periastron, when tidal effects and an increase in mutual radiation may cause an increase in luminosity. The effect manifests as unequal out-of-eclipse brightness maxima, with the maximum following the primary minimum nearly always brighter (positive O'Connell effect), while the reverse case is referred to as the negative O'Connell effect. The difference increases with the ellipticity of the stars, and the differences in their sizes and densities, and spectral differences have been observed between subsequent maxima.

Reader's Guide

The O'Connell effect has been observed in a variety of binary configurations, including over-contact, semi-detached, and near contact systems, making a single explanation difficult. In some systems such as CG Cygni, RT Lacertae, XY Ursae Majoris, or YY Eridani, the luminosity difference between subsequent maxima has been found to be variable, while in others it is relatively stable. Several mechanisms have been proposed to account for the effect: an asymmetric distribution of starspots, impacts of one-way gas streams between the components, or the flow of circumstellar matter asymmetrically deflected due to Coriolis forces. The effect's presence across diverse system types and its variable nature suggest that various mechanisms may be responsible for its manifestation, and no single cause has been established.

Did You Know?

• The effect was distinguished from the periastron effect in 1951.
• The positive O'Connell effect occurs when the maximum following the primary minimum is brighter than the preceding one.

The O'Connell Effect: A Named Anomaly

In 1951, Daniel Joseph Kelly O'Connell published a paper titled "The so-called periastron effect in close eclipsing binaries," in which he documented a puzzling phenomenon observed in the light curves of eclipsing binary star systems. Specifically, he noted that the two maxima in these light curves differed from one another in ways that the previously recognized periastron effect could not account for. This observation became so closely associated with his name that the anomaly is now commonly referred to as the O'Connell effect. Remarkably, despite nearly three-quarters of a century passing since its formal description, no definitive physical explanation has yet been established for the phenomenon. The effect emerged from O'Connell's broader body of observational work on binary star systems, a field in which he was particularly well known. His discovery stands as a testament to the power of meticulous photometric observation to reveal gaps in theoretical understanding, leaving astronomers with a named anomaly that continues to resist full explanation.

Early Life and Formative Years

Daniel Joseph Kelly O'Connell entered the world on 25 July 1896 in Rugby, England, the son of an Irish father, also named Daniel O'Connell, and an English mother, Rosa Susannah Helena O'Connell. His childhood was cut short by tragedy: by the age of eleven, both parents had died, and the boy was sent to Ireland to attend Clongowes Wood College, a boarding school operated by the Society of Jesus. In 1913 he formally joined the Jesuit order, a commitment he would maintain for the remainder of his life. Two years later, at Rathfarnham Castle—another Jesuit-run institution—he crossed paths with Father William O'Leary, and together they constructed a seismograph. Because World War I had led to a ban on radios, O'Connell acquired a transit telescope that had once belonged to Lord Rosse and used it to keep accurate time for their seismological instruments. This early, hands-on encounter with both seismology and astronomical instrumentation set the trajectory for a career that would span multiple disciplines.

Academic Path and Observational Work

O'Connell earned his BSc in mathematics and physics from University College Dublin in 1919, followed by an MSc in pure mathematics the next year. A scholarship had taken him toward research at Cambridge, but a persistent lung condition forced a change of plans; medical advice pointed him toward a warmer climate. After a period studying philosophy and observing variable stars at St Ignatius' College in Valkenburg, Netherlands, he relocated to Australia in 1922, where he completed his Jesuit regency by teaching mathematics and physics at St Ignatius' College, Riverview. In 1923 he became assistant director of the Riverview Observatory under Edward Francis Pigot, a role he held until 1926. After theological studies and ordination in 1928, he spent formative time at Harvard Observatory in 1931, absorbing lectures on photographic photometry from Fred Wipple and learning about Cecilia Payne's variable-star research. Returning to Riverview in 1933, he applied photographic photometry to variable stars alongside seismology. He was elected a fellow of the Royal Astronomical Society and the Royal Society of New South Wales in 1935, became director of Riverview in 1938, and received his DSc from the National University of Ireland in 1949.

Vatican Leadership and Final Years

In 1952 O'Connell left Australia to assume the directorship of the Vatican Observatory in Rome, where he installed the institution's largest instrument, a 60/90-cm Schmidt telescope for stellar observation. When asked about his dual role as priest and scientist, he noted that the two were distinct and that there was no opposition between them. His research earned him a nomination to the Pontifical Academy of Sciences on 24 September 1964, and he became its third president in 1968. He organized two major study weeks for the Academy—one on Stellar Populations in 1957 and another on Nuclei of Galaxies in 1970—and edited the proceedings of each into published volumes. When Pope Pius XII faced criticism for remarks during the second study week, O'Connell defended the pontiff, noting that the Pope had carefully studied the technical material and shaped it to highlight the religious and ethical dimensions he wished to convey. O'Connell retired from the Vatican Observatory in 1970 and from the Academy in 1972. He died in Rome in 1982 at the Jesuit headquarters, remembered by colleagues as a man who treasured his friendships and was always nurturing new ones.