New Nova in Cygnus
BY MARK ARMSTRONG
ASTRONOMY NOW
Posted: April 15, 2008

Another nova has been discovered in the constellation of Cygnus following hot on the heels of last month’s magnitude +8 outburst. International Astronomical Union Circular (IAUC) no. 8934, issued by the Central Bureau for Astronomical Telegrams (CBAT) reports that the nova, now given the variable star designation of V2491 Cygni, was discovered at magnitude +7.7 on 10 April around 17.30h UT by Koichi Nishiyama, Kurume, Fukuoka-ken, Japan, and Fujio Kabashima, Miyaki-cho, Saga-ken, Japan.

A couple of hours later an apparent independent discovery was made by Z. –w. Jin and X. Gao at Xingming Observatory, Mt. Nanshan, China. A precise position has been obtained with coordinates R.A. = 19h 43m 01.96s, Decl. = +32° 19’ 13”.8 (equinox 2000.0), which places the nova in the south-west quadrant of Cygnus, roughly midway between Albireo (beta) and eta. The closest bright star to the nova is mag. +5.9 SAO 68730, 7 arcminutes to north-west.

The prolific Italian amateur astronomers E. Guido and G. Sostero obtained confirming images on April 11 using a 0.25-m reflector based in New Mexico, USA, operated remotely on the Internet. They report a slight fade over a few hours from mag. +7.54 to +7.77. Interestingly they scanned the Palomar Oschin Schmidt telescope plate from 3 August 1995 and an approx. mag. +18 red star is present at the reported position of the nova.

An object’s true nature cannot really be established until a spectrum has been obtained and CBAT announced that low-resolution spectra of V2491 Cyg obtained on April 11 by K. Ayani, Bisei Astronomical Observatory and K. Matsumoto, Osaka Kyoiku University, Japan, indicate that the object is a nova in its early phase of outburst. Fe II emission may be present. Classical novae are a class of cataclysmic variables with the explosive mechanism thought to be the outburst of material on the surface of a very dense star called a white dwarf in a close binary system with a main sequence (Sun-like) star. The main sequence star is losing material to the white dwarf, resulting in the material forming a disc around the dwarf. Over periods of between 10-100,000 years enough material has spiraled down on to its surface to trigger a thermonuclear explosion. This massive event causes a rise in brightness of some 7 to 16 magnitudes, but does not result in the complete destruction of the star, unlike a supernova. Novae are unpredictable events with perhaps as many as 40 outbursts per year in a galaxy the size of our Milky Way.

As with last month’s nova this one is a morning object but Cygnus rises a little earlier now so observations should be possible around 02h BST when the nova is about 30 degrees above the eastern horizon. The spectra revealed the nova was in its early stages but it looks to have faded by a magnitude from bang up to date observations (pun intended). So you will need to be quick to catch a glimpse of it in large binoculars and small telescopes. Whilst gazing at it in the eyepiece reflect upon the unimaginably powerful forces that were unleashed in this colossal outburst.