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An Extra-Starry Starry Night

Alan Gilmore

It was Saturday evening and mostly cloudy. Nigel Frost and I had been running an Open Night at the Mt John Observatory, a fund-raiser for the Tekapo school. We closed down before 10pm and adjourned to the house for supper.

The sky was clearing when Nigel left, so I went back to the OC dome to try some photometry. While checking the sky at about 10:40pm I noticed an unfamiliar triangle of stars northwest of the Southern Cross.

I didn't get excited -- I've been checking out "new" stars for 40 years, all of them turning out to be old residents of Norton's Star Atlas. However, Norton's showed only two stars in the area, not three.

Still not really believing it, I took the atlas over to the B&C dome housing the Boller & Chivens 61-centimetre telescope where Pam Kilmartin and the rest of the MOA (Microlensing Observation in Astrophysics) team were working. They all emerged to compare the sky with the atlas while I went back to the OC to get a photometric measure.

The MOA committee confirmed that the star was not in Norton's. Pam checked the emails, expecting to find a blizzard of reports of a naked-eye nova. There were none. In the meantime I had put an attenuating mask on the OC (cardboard technology developed for observing bright variables) and compared the new star with bright comparison stars.

I phoned Karen Pollard and Jennifer McSaveney at the one-metre telescope and suggested that they get a spectrum of the new star ASAP. Quick reduction of the photometry showed an ultraviolet excess, a fairly sure indicator of a nova. It was V magnitude 2.9, similar to the fourth star of the Southern Cross.

Off to the office to draft an email to send to the International Astronomical Union's Central Bureau and to nova observers. When I logged in, there was a lone report of the nova from Australian amateur Peter Williams. He'd seen it about the same time I did. Nothing from South America where it might have been noticed many hours earlier.

Karen phoned to say the star's spectrum showed H-alpha with a P-Cygni profile (a blue-shifted absorption on the edge of an emission line), another nova signature. I emailed that observation to the Central Bureau too.

Meanwhile the MOA team had taken CCD exposures of the star. It was a large puff-ball of light among stars of 10th magnitude and fainter. From the image's diffraction spikes Pam was able to estimate its central pixel coordinates. With those, and similar coordinates for surrounding catalogued stars, she calculated the nova's position and this was sent off in a further email to the IAU Bureau.

Karen and Jennifer downloaded a digital survey image of the region. Using the MOA coordinates, they were able to identify a plausible progenitor star of around 16th magnitude -- that made the rise around 13 magnitudes, or an increase in brightness of around 160,000 times. It was 6,500 light years away.

Karen reported that the spectrum of the nova was altering visibly during the night. Spectra by several observers later showed that the gas shell was expanding at between 2,300 and 3,600 kilometres per second, depending on which hydrogen lines were measured.

Photometry was less exciting. Instead of dramatically rising or falling in brightness as I expected, the nova edged up by just 0.05 magnitude (5%) in the next three hours. After 3:30am, it was too low in the sky for accurate measures. Much redrafting of reference files and reduction of data followed till dawn, along with emails.

There was still nothing back from the IAU Bureau when we went to bed at 7:30am. We were up again about 12:30 to find an IAU Circular detailing the various Mt John observations and noting Peter Williams and me as the only discoverers. The nova was given the variable star designation V382 Velorum, and it looks like it was the tenth brightest nova ever observed.

On Sunday evening the nova was 0.2 mag brighter and redder. The UV excess had all but disappeared. On Monday it was 0.6 magnitude fainter with a large UV excess.

Weather prevented further photometry but quick eyeballing between clouds showed the nova fading about half a magnitude a day. By the weekend it was hard to see with the naked eye, particularly in the increasing moonlight.

So, was the discovery pure luck? Well, having a clear sky that night was certainly lucky. But having years of star-gazing experience was similarly essential. And, dare I say it myself, an element of dedication also helped -- it was Saturday night, after all, and I had already done 60 hours work that week.

An Extra-Starry Starry Night Figure A (21KB)
From left: Alan Gilmore, Pam Kilmartin, Yutaka Matsubara and Nick Rattenbury, star-hunters at large.

Alan Gilmore works at the Mt John Observatory in Tekapo