South Africa can claim a unique astronomical distinction: the first ever measurement of the distance to a star. In 1833 Alpha Centauri was the first star to have had its distance measured and it stayed the nearest star known until 82 years later when a closer one was found. The new champion, another South African discovery, was Proxima Centauri. To this day it remains the record holder and Alpha the runner-up.

This book tells the story of the discoveries and the rather curious characters involved in making them, starting with the first suggestion that the distance to a star might be measurable and continuing right up to the present-day investigations of Alpha and Proxima. The treatment is mainly historical, with little that is mathematically challenging! Proxima will be enjoyed by astronomers and non-astronomers alike.

Chapter 1: The distance of alpha Centauri

In 1820 the Royal Observatory near Cape Town was founded to improve navigation by getting better positions for the southern stars. By 1828 its instruments, among the most precise constructed up to that time, were ready for action. In 1832, the bright double star Alpha Centauri already formed part of the observing programme, but acting on a tip from a soldier-turned astronomer, Manuel Johnson who worked on the island of St Helena, Thomas Henderson and his assistant William Meadows started observing it more intensively and soon had enough readings to get its distance. Their work was afterwards refined by Thomas Maclear.

Royal Observatory ca 1843, (© Museum of the History of Science Oxford)

Chapter 2: The nearest double star

Alpha Centauri has been known to be a double since 1689. William Herschel showed that many doubles are in fact in orbit around one another and in the 19th century it became possible to measure the masses of its two components using this fact. Over 400 observations of their relative positions have been published and it now is one of the best-understood binaries. Late in the 19th century its distance was established at essentially it modern value by Gill and Elkin at the Royal Observatory.

T. Maclear (Courtesy Prof B. Warner)

Chapter 3: Proxima

Eighty-two years after the Cape discovery, Robert Innes at the Union Observatory in Johannesburg found a fainter star near alpha Cen that he suspected was a third but somewhat separated member of the same system. Its distance was measured over the following two years by Joan Voûte at the Royal Observatory and by Innes himself, unknown to each other. Innes prematurely declared it to be closer than alpha and named it 'Proxima Centauri', or 'Proxima' for short. Voûte's better-quality observations suggested it was no closer then alpha but, in fact, he was wrong and Innes was right, largely by chance!

R. Innes © SAAO

J. Voute © SAAO

Chapter 4: Establishing the proximity of Proxima

The certainty that it truly was the closest star had to wait until 1928 when Harold Alden, based at the Yale Southern Observatory in Johannesburg with a superior telescope, made further measurements, though his results were not accepted without controversy. It took many more observations and finally the results from the Hipparcos satellite before the astronomical world was absolutely convinced that Proxima really was the nearest star.

Proxima Cen colour composite

Proxima Centauri, the nearest star. Proxima is stretched out and multi-coloured in this picture because it moved during the long times between the three exposures that were required. Here, infrared light is mapped as red, red light as green and blue light as blue. Besides being the nearest star, Proxima is the 13th fastest moving one. It moves by the diameter of the full Moon in about 500 years. By 2000, about 15 years after these images were taken, it was at the position shown by the faint cross.

Chapter 5: Modern studies of Alpha and Proxima

Because Alpha and Proxima are the nearest stars of their types to us, they are still observed with great interest. Their light can be analysed to find their chemical compositions. The components of Alpha resemble our own Sun. All three stars have been investigated for the presence of planets with ever-increasing sensitivity. The methods used for making planetary searches are described.

No certain planets have been found around either component of Alpha Cen or Proxima as of 2015 although Dumusque et al (Nature Online, 17 October 2012) claimed the discovery of an approximately solar-mass planet in orbit around Alpha 2 Cen with a period of 3.236 days. The planet-star distance is only about 4% of the earth-Sun distance and implies that it will be far too hot to support life. Their finding has, however, been received with scepticism,

The 80-year orbit of the binary star Alpha Cen.

The periastron is the closest they are apart and the apastron is the furthest.

Chapter 6: Bibliography

Suggestions for further reading and references.

Index

Readers' comments:

"Ian Glass's book on Proxima Cen is eminently readable, giving full scope to the human foibles of the scientists who have worked on this nearest of known stars, and showing how the process of science can sharpen our understanding despite individual obstinacy, prejudice or error. It follows efforts to answer seemingly simple questions about Proxima and Alpha Centauri, explaining in clear, plain language how the necessary scientific concepts work, and shows how and why we still cannot say with certainty what the answers to some of these questions are." - C.D. Laney

"Certainly the most complete account of man's epic quest for finding the closest star that started in the 1830s, continuing all the way into the space age. Intriguing is the fierce competition that existed between astronomers to be the first to claim this achievement ..." - W.P. Koorts

"The distance to alpha Centauri was first measured by Henderson, an astronomer at the then Royal Observatory (now SA Astronomical Observatory) here in Cape Town in 1833 ... Ian Glass has written a short book on this and the discovery of the nearest star to our Sun, Proxima Centauri. Simply called Proxima, it is a wonderful story ..." - Case Rijsdijk, Weekend Argus.

"This clear, interesting, and accessibly-written book explains how the orbit of the earth in Copernicus's Sun-centred Solar System provides a natural baseline for finding the distance to the stars by trigonometry, and why the actual measurement proved so difficult." Elisabeth Lickindorf, QUEST, Science for South Africa.

"In Proxima, Ian Glass has managed well to convey the excitement of a scientific discovery in progress. This book is a valuable contribution to our understanding of a small but significant aspect of the history of astronomy and of our attempts to establish our place in the Universe. It now forms part of the record of the history of science in South Africa." M. Soltynski, MNASSA.

"Proxima: The Nearest Star ... is a charming little book and seems designed primarily for the interested layman, yet it will appeal equally to astronomers with a passion for the history of our discipline. I thoroughly recommend it as a valued and eminently affordable addition to your bookshelf." Wayne Orchiston, Journal of Astronomical History and Heritage.