Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona
The Firework Nebula (also known as GK Persei) is a nova remnant, an expanding cloud of gas and dust of about 0.7 light-year across, formed by Nova Persei 1901, a nova which exploded on Feb. 23, 1901 although it was first detected a year later, on 21 February, 1902, by the Scottish clergyman Thomas David Anderson. The nova occurred some 1,500 light-years away in the northern constellation of Perseus and is expanding at about 1,200 kilometers per second.
A nova is an explosion in a white dwarf star in a binary system,what brightens the white dwarf for tens to hundreds of days. If these two stars are close enough, the white dwarf captures matter from its companion. However, this material does not accrete directly onto the white dwarf, but forms an accretion disk around it. Then, material in the disk slowly drifts inward and accretes onto the surface of the white dwarf. An envelope of hydrogen-rich material builds up on its surface. The intense heat and pressure at the base of this envelope leads to a thermonuclear explosion as hydrogen is burned to helium. The explosion blows off the outer layers of the envelope.
Nova Persei reached a maximum magnitude of 0.2, the brightest nova of modern times until Nova Aquilae 1918. It was fading very rapidly thereafter to its current minimum of 13.1. From 1966 to the present, Nova Persei (now more commonly called GK Persei) has shown dwarf-nova-like outbursts of about 3 magnitudes, typically lasting about two months and occurring about every 3 years.
Around 1980, the discovery of X-rays coming from this system led scientists to reclassify the system as an intermediate polar, a magnetic cataclysmic variable in which the white dwarf component generates an extremely powerful magnetic field (on the order of 106 to 107 gauss). Thus, GK Persei has seemed to change from a classical nova, to more recently, a dwarf nova-type cataclysmic variable star. In fact, GK Persei is a giant among the intermediate polars, with a highly developed disk.
The Firework Nebula, which contains as much as one ten-thousandth of a solar mass, shows an asymmetry unique among novae. This can probably be explained by the expansion into an interstellar medium that is considerably denser than the average, although it is not yet clear why this should be so.