Amateur astronomers Koichi Nishiyama and Fujio Kabashima in Japan discovered a bright nova in the constellation Cygnus the Swan April 10. Astronomers initially catalog such events as variable stars. This one received the label V2491 Cygni. Recent estimates place the object’s brightness at magnitude 7.6.
The magnitude scale provides a way to compare the brightnesses of celestial objects. The brightest stars have magnitudes of 0 and 1, and the faintest stars visible to the unaided eye from a dark site typically have a magnitude around 6.5. The nova, therefore, lies just below the naked-eye visibility limit. This means you can spot V2491 Cygni easily through binoculars.
To spot the nova, use the finder chart on this page. Cygnus rises in the northeast and is fully visible just after 11 P.M. local time. It continues to climb higher in the sky until dawn. The Moon, a few days after First Quarter, lies across the sky in the constellation Leo the Lion. Moonset occurs around 3:30 A.M. local time. The nova shines brightly enough that moonlight will not interfere with the view.
A nova is an explosion resulting when hydrogen from one star of a binary system falls onto the surface of the second star, which is a white dwarf. White dwarfs represent the last stage in the lives of Sun-like stars. In such cases, the star shines like the Sun from a few billion to about 20 billion years. Energy production exhausts the nuclear fuel in its core, and the core shrinks. This heats up the core, causing the star’s outer layers to expand. As the core cools, it shrinks to form a white dwarf star.
Nishiyama, 70, is from Kurume, Fukuoka-Ken, and Kabashima, 68, from Miyaki-cho, Saga-ken. Both are well-known supernova hunters. Nishiyama takes images with the duo’s 16-inch (0.4 meter) reflector using a charge-coupled device (CCD) camera in their Miyaki Argenteus Observatory. Kabashima then analyzes the images with a personal computer.