Observations

Centenary of discovery of Proxima Centauri’s…proximity

Robert Thorburn Ayton Innes
Robert Thorburn Ayton Innes

What is the closest star, in the night-time sky, to Earth?

Almost everyone answers Alpha Centauri, the brighter of the pair of Pointer stars. But Alpha Centauri is in fact a system of three stars. Two orbit close together and their combined light is what our naked-eye sees as the brighter of the Pointer stars, and what most of us would call Alpha Centauri.

The third star of the system is too faint to see without a large telescope but it is the closest of the three. We call this star Proxima Centauri.

How do we know Proxima Centauri is the closest? We could measure its distance via the parallax method. Or we could measure how fast it is moving across the sky – its proper motion – and infer that it is the closest because it has the greatest proper motion.

On October 12, 1915 astronomer Robert T. A. Innes, working in South Africa, placed two glass plate negatives of the sky into a blink comparator. This device allowed him to rapidly flip between viewing one plate then the other. He noticed one faint star jumped back and forth as he blinked the plates – it had moved in the five years separating the exposure of the plates. His measurements showed its proper motion and the direction it was heading were very similar to those of the star-pair we call Alpha Centauri. It was almost certainly part of that star system and therefore nearby. Later parallax measurements, by Innes and others, proved it was the closest known star at a distance of 4.22 light years.

If you want to know what 4.22 light years feels like try walking the Solar System model on the Melbourne shoreline.

A blink comparator (H10185) used at Sydney Observatory and made by H. F. Pinnock c1960.
A blink comparator (H10185) used at Sydney Observatory and made by H. F. Pinnock c1960.

Before moving to South Africa Innes lived for several years in Australia. He arrived in Sydney in 1890, as a 28 year old, to establish a wine & spirit business. He was already an accomplished mathematician and had been appointed a Fellow, no less, of the Royal Astronomical Society at age 17. In Sydney he got acquainted with local astronomers: Henry Chamberlain Russell at Sydney Observatory, John Tebbutt at Windsor and Walter Gale (after whom Gale crater on Mars, where the Curiosity rover is presently roaming, is named). While in Sydney he observed double stars and investigated the orbital motions of planets. He also helped establish the NSW branch of the British Astronomical Association, a group that continues to meet at Sydney Observatory to this day as the Sydney City Skywatchers.

Proxima Centauri is a faint red dwarf flare star. David Malin, UKS, AAO, APOD
Proxima Centauri is a faint red dwarf flare star. David Malin, UKS, AAO, APOD

Viewing Proxima Centauri The star is too faint to view with the naked eye and it lies four Moon-widths from the star we call Alpha Centauri (the bright Pointer star). Theoretically it should be visible through a good full-size pair of binoculars from a dark site. If you want to take up that challenge this article will help you find it! However, it is easier to find with a well setup computerised telescope. From Sydney Observatory last week I used our new 16-inch diameter DFM telescope. After allowing my eyes to adjust to the dark and some careful comparison with the star map on the PC, there it was – a pale pink-red spot!

 

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