July 2015
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It's a fact!

Centenary celebrations to mark discovery of Proxima Centauri

100 years ago South Africa made a significant discovery, which still today marks major international discoveries in Astronomy. We discovered the closest star to the Sun, Proxima Centauri.

Proxima Centauri lies in the constellation of Centaurus (the Centaur), just over four light-years from Earth. It is invisible to the naked eye when viewed from the surface of Earth. It possesses a very low average luminosity, and has a mass only about an eighth of that of the Sun. Occasionally the brightness of Proxima increases because of what astronomers call a "flare star". This happens when convection processes within the star's body give rise to random, powerful changes in brightness.

At a mere four light-years away, Proxima Centauri is our closest stellar neighbour (shown to scale in this image, between the Sun and Jupiter)
In this Hubble telescope photo, Proxima Centauri shines as a bright point-like object, but that appearance is deceiving


Proxima Centauri was discovered in 1915 by the Scottish-born astronomer Robert Innes, then Director of the Union Observatory in Johannesburg currently known as the Johannesburg Observatory¹. Innes suggested that the star is part of the three-star Alpha Centauri system, the other two stars being Alpha Centauri A and B. In fact, it is still not certain whether Proxima is part of the Alpha system.

Alpha (α) and Beta (β) Centauri are the pointer stars of the Southern Cross; α Centauri is the brighter of the two and lies furthest from the cross. For many years astronomers believed that α Centauri was the closest star to our solar system.

Although Alpha had been thoroughly observed by Innes, with his vast experience and passion for observing double stars he suspected that α-Centauri might have a companion. While comparing photographic plates that were taken five years apart using the Union Observatory's² Franklin-Adams camera³, Innes observed that a certain faint star had moved. He found that this movement was about the same as that of α-Centauri. He announced his discovery in a paper dated 12 October 1915, entitled "A faint star of large proper motion".

After further investigation, he concluded that it was closer to the sun than Alpha. In 1917 he proposed the new star should be called Proxima Centauri, proxima being the Latin word for "nearest".

We do not know on what day exactly Innes discovered Proxima as a large proper motion star, but the evidence suggests it was just before 5 October 1915, the day he sent the paper containing the announcement to be printed. We know that on 20 October he wrote of his discovery to a friend, stating he had discovered the star a few days previously.

This discovery was of great significance for the Union Observatory and for South African astronomy as a whole.

It was only in 1928 that H. Alden of the Yale Observatory on the Wits campus in Johannesburg proved with reasonable certainty that Proxima is really closer to our solar system than Alpha.

Proxima Centauri was discovered in 1915 by the Scottish-born astronomer Robert Innes, then Director of the Union Observatory in Johannesburg (Image: Wikipedia)
The Sun's closest neighbours (Image: NASA)

Centenary celebrations

Plans are in the pipeline to celebrate the 100-year anniversary of the discovery of Proxima Centauri by Robert Innes. Several activities are to take place across the country leading up to the main event, which will be held at the Johannesburg Observatory.

Further information will be published in the next edition of GetSETgo.

¹ The Johannesburg Observatory is now owned by the National Research Foundation and operated by SAASTA.

² The Union Observatory (1912) started as the Transvaal Meteorological Observatory (1903) and was wholly a South African institution. The observatory building still exists in modified form.

³ The Franklin-Adams telescope, whose camera was used to discover the star, was donated to the Transvaal Observatory in 1909. It still exists at the Broederstroom site now owned by Tshwane University of Technology.

By Shadrack Mkansi, SAASTA