Sue French’s interesting article, "Overlooked in Ophiuchus" ( Sky & Telescope: July 2002, page 88), might have gone on to mention one of the most unusual stars, not just in Ophiuchus but in our entire galaxy. That star is RS Ophiuchi, a repeating nova that has had at least four outbursts since 1898. It rose in a few hours from 12th magnitude to 5th or brighter in 1933, 1958, 1967, and 1985.
Since it's now many years since the last outburst, another one may occur at any time. The star lies at right ascension 17h 50.2m, declination –6° 43' (equinox 2000.0), and is well placed for viewing by Northern and Southern Hemisphere observers during the middle of each year.
RS Oph has been of great interest to me, because I happened to be the observer on the Mount Wilson 60-inch telescope on the night of its outburst in 1958. On July 14th I was photographing the spectra of stars in the open cluster NGC 6940 in Vulpecula with the telescope’s Cassegrain spectrograph and "short camera." We received a phone call from Leland Cunningham at Berkeley (who had a reputation for being a late-night worker), and he relayed a telegram announcing the outburst of RS Oph. I immediately threw away the partially exposed plate and turned the telescope to the new position. And there, in the center of the field, was a bright purple star.
Spectral Results of RS Ophiuchi's 1958 Eruption
Now, purple stars are no more common than purple cows, so I knew that I had an unusual object. I quickly switched to the spectrograph’s long camera, which gave moderate resolution at 20 angstroms per millimeter. Parts of the spectra recorded on July 14th, 17th, and 25th are shown below. In addition to the very broad spectral lines formed in the outward-moving gas at a velocity of about 3,000 kilometers per second, there are a variety of sharp lines not usually seen in novae. They reveal the presence of quiescent circumstellar gas that surrounded the whole system prior to the outburst.
A detailed study of these sharp lines showed that those requiring high density were present only on the first night. Lines that formed at lower density, such as doubly ionized neon (Ne III), were seen later. The whole phenomenon could be explained by an outward-moving shock wave that marked the leading edge of the nova’s ejecta and was rapidly sweeping up the circumstellar gas. As the shock reached the outer layers of the gas, it raised the temperature to about 2 million degrees Kelvin and excited the same emission lines that are seen in the Sun’s corona.
Spectra taken that first night showed features not seen even one night later, nor in any other nova. So I urge readers to keep an eye or, rather, binoculars on the RS Oph region. Be prepared to notify the International Astronomical Union’s Central Bureau for Astronomical Telegrams immediately upon recognizing that a new outburst is beginning.
For a little fun at the same time, it would be easy enough to record a visual light curve for RS Oph’s neighbor, the Cepheid variable Y Oph (marked "Y" on the chart on page 1). It varies from visual magnitude 5.9 to 6.5 in a 17-day period.