Wednesday, February 15, 2006
Recurrent Nova Brightens in Ophiuchus.
Now that the skies have cleared somewhat after this weekend's snowstorm, there is something interesting happening in the early morning skies. A recurrent nova has brightened to naked-eye visibility in the constellation Ophiuchus. Sky and Telescope excerpt:
For first time in 21 years, the famous recurrent nova RS Ophiuchi has erupted into naked-eye visibility.
As late as February 10, 2006, the star was still magnitude 11.0, where it spends most of its time with only minor fluctuations. But on the morning of February 13th it was reported shining at magnitude 4.8. The outburst was discovered at February 12.8 Universal Time by Kiyotaka Kanai and Hiroaki Narumi in Japan, who estimated it to be magnitude 4.5 at the time.
RS Oph is located in eastern Ophiuchus near Serpens Cauda. This part of the sky is moderately well up in the southeast just before the first light of dawn. The star lies at right ascension 17h 50.2m, declination –6° 43' (equinox 2000.0).
RS Ophiuchi, like other cataclysmic variable stars, is a close binary. A red giant of spectral type M2 is spilling gas onto a small, dense, blue companion star. The hot blue star erupted in 1898, 1933, 1958, 1967, and 1985, and it may also have had an outburst in 1945.
Since I'm up at about 5:00 AM every morning to prepare for work, I have the opportunity to see this. Ophiuchus is a large constellation that lies just north of Scorpius, and, as it lies along the Milky Way, it contains dozens of deep-sky objects like globular clusters and planetary nebulae (as well as six stars that lie within 20 light-years of our Solar System--four of which are Sun-like in size and temperature). Now, this recurrent nova will bear watching to see if it continues to brighten.
This sort of thing happened a few years ago when Delta Scorpii brightened from magnitude 2.3 to 1.7, nearly to the point of being a first-magnitude star. It does not appear that RS Ophiuchi will get quite that bright, but it is still a significant event to monitor.