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GK Persei

GK Persei

This column’s companion picture looks like something that might be seen during an Independence Day celebration next week. The spherical display of sparkling colored lights, evident in the online version of the column, closely resembles the so-called “peony” style of fireworks commonly exhibited at pyrotechnic shows. The dazzling feature in the picture was, in fact, created by an explosive event, but on a scale much grander than any July 4th fireworks show.

This feature is informally known as the Firework Nebula. Astronomers today refer to it as GK Persei and have debated its true nature since Thomas Anderson discovered it in 1901. Thomas David Anderson was a Scottish clergyman and amateur astronomer. His interest in space was kindled at the age of five, when his father pointed out Donati’s Comet from outside their Edinburgh home; half way around the world, from a porch in Boston, 3-year-old Percival Lowell saw the same comet, a memory that later fueled his passion for astronomy and led him to found his astronomical observatory in Flagstaff.

As an adult, Anderson spent long hours gazing at the heavens, mastering the night sky. At about 2:40 am on February 22, 1901, he was walking home and, while glancing up toward the constellation Perseus, saw a fairly bright star that he had never noticed before — evidence that he truly had become a night sky expert. The next day he reported his discovery to the Royal Observatory at Edinburgh and this apparent new star soon became known as Nova Persei 1901.

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Astronomers at Harvard College Observatory later examined the observatory’s comprehensive collection of celestial photographs and realized that Nova Persei 1901 existed before Anderson’s observation. One of these photographs, captured just two days before Anderson’s discovery, showed the object, but about 10,000 times fainter than when Anderson saw it. Anderson had not discovered a newly formed star, but rather a star that happened to flare up and briefly became one of the brightest features in the night sky. This was an object that astronomers today call a cataclysmic variable star.

Cataclysmic variable stars are binary stars consisting of a white dwarf and a secondary star. The white dwarf started out as a sun-like star, but 100,000 years ago expanded and ultimately shed its outer layers, leaving behind the very dense remnant core that we know as a white dwarf (in about five billion more years our sun will do this too!). If this white dwarf is close enough to the secondary star, it will cannibalize its partner, drawing material in and forming a flattened structure of matter around itself known as an accretion disc.

Eventually the increasing pressure within the accretion disc triggers thermonuclear fusion ― like a nuclear bomb ― and a resulting massive explosion. This blows the accumulated material off into space and can create a fireworks-like display, as in the case of the picture of GK Persei.

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