The image is a four minute exposure taken on the night of September 26th 1994 (UT of observation 27/09/94:07:21) using a red-sensitive (R-band) filter. The brightness of the image has been converted to color (a technique called pseudo-color) using a simple orange/red-shading-to-white transformation, purely to make a more attractive picture. The photograph shows a region 120 arc seconds square. The approximate "seeing" is 1.2 arc seconds.
The unusual nova shell GK Per is the result of Nova Persei 1901, a nova which exploded in 1901 about 1500 light-years away in the direction of the constellation Perseus. Classical novae, nowadays categorized as cataclysmic variables, normally comprise a hot white dwarf with accretion disk, and a cool mass-transferring companion. At some point, hydrogen burning triggers thermonuclear runaway in the accreted matter, leading to an explosive shock wave and an expanding shell.
The first bright nova of the 20th century and the first to be subjected to detailed spectroscopy and photometry, Nova Persei 1901 reached a maximum brightness of 0.2m and declined very rapidly thereafter to its current minimum of 13.1. GK Per is unique among classical novae, having the longest known period (almost two days) and showing dwarf nova-like outbursts of about 3 mag. It also contains an evolved secondary (type K2IV), while all others have main sequence companions.
Expanding nebulosity was detected as early as 1902, and apparent superlight speeds were explained as reflection from dust grains, which has only been seen in one other nova. The ejecta show an asymmetry unique among novae, contain as much as one ten-thousandth of a solar mass, and have speeds reaching 1200 km/s. The unusual features of this `firework burst' nebula are best explained by expansion into an interstellar medium that is considerably denser than the average, although it is not yet clear why this should be so.
Location: 03 27 47.4 +43 44 05 (1950.0), distance 1500 light-years, size about 0.7 light-years (this piece: structures associated with this nova stretch some twenty times farther out).
More: nebulae page, WIYN nebulae page.