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The variable stars in our sky

The variable stars in our sky

In mid-September last year, a quiet, unassuming New Zealand amateur astronomer by the name of Albert Jones died. He started making observations of stars whose brightness varies over time in the 1940s and by 1963 Albert had logged around 100,000 observations. Like botany and palaeontology, astronomy is a scientific discipline that amateurs can still make significant contributions to, and Albert was one such contributor.

Even today it’s tempting to think of the stars as unchanging and eternal, but we only have to look at images of the violent surface of our own star, the Sun, to see that this can’t be so. Stars are “born”, “live”, and “die”, just on longer time scales than we can imagine. At some point in every star’s life, it can be classified as “variable”. The number of variable stars listed in the International Variable Star Index is around a quarter of a million. The international database hosted by the American Association of Variable Star Observers (AAVSO) contains tens of millions of observations of such objects, mostly submitted by amateurs.

On 14 August 2013 Koichi Itagaki, an amateur astronomer in Yamagata, Japan spotted a “new star”, as people centuries ago would have conceived it, on an image he had taken of the constellation Delphinus, it was in fact a “nova”, eventually earning the catalogue entry V0339 Delphini.

A nova results from a runaway thermonuclear explosion at the surface of a white dwarf star after years of gas exchange from a companion star onto the dwarf. In less than an hour, a shell of material begins to expand at around a thousand kilometres per second. Unlike a supernova, such an event doesn’t destroy the progenitor star system, nor does it release as much energy.

HyperNova1_LG 600px(w)

By 17 August, the nova had peaked in brightness, becoming briefly visible to the unaided eye from a suitably dark location. The rise from pre-nova to peak brightness represents an increase in luminosity of thousands of times that of our Sun.

This relatively rare event went largely unreported in the mainstream media, but to many amateur astronomers, the nova was captivating.

These things are unpredictable. From one night to the next, you don’t know what you’ll see. A nova may steadily dim for a while, only to rise again unexpectedly or level out before continuing downward in brightness. Whether armed with a pair of binoculars or a digital camera attached to a telescope, amateurs had collected and reported more than 50,000 observations of V0339 Delphini between the middle of August and late October, and observations continue even now.

A bright object such as this would quickly saturate a detector connected to a large professional telescope, but amateurs with smaller telescopes or binoculars, sometimes just with the unaided eye, record such objects night after night.

One of the stars that Albert Jones observed is the pulsating variable star, R Carinae, located not too far from the Southern Cross. This star goes from being visible to the unaided eye in dark skies to visible in binoculars, then only in a telescope in around 300 days. The light curve below shows visual observations since 2003.

RCarSince2003 600px(w)

We have records going back to 1891 for this star. This year, R Carinae’s maximum was lower than any on record. Even relatively sedate stars like this one hold surprises from year to year. What it signals for the star’s future evolution remains to be seen, but such change is not uncommon.

In the Southern Hemisphere there are fewer observers than in the north. We have an opportunity here to contribute to basic science, just by making simple, careful observations. On some nights, my observations of the nova in Delphinus were the only ones recorded for a few hours. Given the unpredictable nature of such a beast, a southern observer could be the first to notice a significant change and report it via the Internet.

This is even more obviously the case for another recent bright nova near one of the “pointers” to the Southern Cross. Within a day of discovery by John Seach (an amateur in NSW) on December 3 2013, it quickly rose from obscurity to become brighter than V0339 Delphini. Coverage of this nova is currently considerably less than for the one in Delphinus because of its southerly sky location and visibility in the early morning hours.

Science is not possible without data. Just as the basic stellar position measurements of Tycho Brahe in the 17th century permitted Johannes Kepler to dare to propose that planetary orbits might be something other than circular, the work of people like Albert Jones helps professional astronomers and increasingly, a new breed of amateurs, to refine our collective knowledge of the distance to and nature of the stars.

Amateur astronomers were making contributions to basic science long before the meme “citizen science” became part of popular culture. Albert’s contributions will continue to be of value long after his passing, and like him, everyone can play a part in doing real science.

Links & Notes

By David Benn

Feature image “Keplers Supernova” sourced from Wikimedia Commons and authored by NASA/ESA/JHU/R.Sankrit & W.Blair.
Body image 1 “HyperNova1 LG” sourced from Wikimedia Commons and authored by NASA/GSFC/Dana Berry.
Body image 2 “RCarSince2003” supplied by author.

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