Mystery of historical Nova solved

Astronomers identify the author of a stellar explosion in 1437

In a nova, a white dwarf sucks hydrogen from his companion. If it collects too much gas, however, it becomes unstable and repels parts of the gas envelope in an expo. © NASA / CXC / M.Weiss
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Cosmic Archeology: In 1437, Korean court astrologers suddenly observed a bright star burst in the sky. Now astronomers have discovered which star system was responsible for this nova. On historical photo plates, the researchers also discovered that this white dwarf lived through several minor eruptions in the 1930s and 1940s. For the first time a Nova, a Nova remnant and dwarf novae were detected in the same star, the researchers in the journal "Nature".

On March 11, 1437, astrologers at the court of the Korean kingdom Joseon noticed a new celestial phenomenon: in the tail of the constellation Scorpio, a star suddenly shone a hundred thousand times brighter than before and outshined its entire surroundings. For 14 days this glow lasted, then the star faded again. The scholars wrote down their observations - documenting one of the best-described historical star explosions.

From historical records, astronomers today conclude that the event must have been a nova: a white dwarf had sucked hydrogen from its partner star until it became unstable. In the resulting explosion, he hurled a large portion of his gas envelope into space - and lit up brightly. Which star, however, caused this Nova almost 600 years ago, remained unknown until now.

Explosion relic in scorpion

This mystery has now been solved by Michael Shara of the American Museum of Natural History in New York and his colleagues. They scoured for the images of the Anglo-Australian Telescope in New South Wales, because this had the sky region around the constellation Scorpio particularly detailed screened.

This gas cloud is the remnant of Nova Scorpii from 1437 © K. Ilkiewicz and J. Mikolajewska

In fact, the astronomers found what they were looking for: they came across a photograph from 1985 in which a sheath-shaped gas cloud was visible. At the center of this explosion relic, however, no matching star was seen. But about 15 arcseconds from the center lay a star that seemed to be emitting varying amounts of UV and X-rays. Could this be the White Dwarf who caused the "Nova Scorpii 1437" baptized explosion? display

Reconstructed earlier position

A fortunate coincidence helped the researchers to make that clear. Because in order to reconstruct whether this star, including Gash lle, stood in the right place with the records in 1437, they needed more data on the movement of this object. The crucial clue came from a nearly 100-year-old photographic plate from the Harvard Observatory. On the recording of 1923, the former position of the star was visible.

"Thanks to this recording, we were able to determine how the star has moved since then, and to calculate this for 600 years, " explains Shara. "And there he was: He was then right in the center of our blast shell. This convinced us that he must have been the person responsible. "According to this, the star was 1.43 arc seconds to the east in 1437 and 3.1 arc seconds north to its current position.

"This is the first nova of Chinese, Korean or Japanese records of the last 2, 500 years that could be safely attributed to their origin, " says Shara.

These recordings from 1942 show a dwarf nova outbreak of the star system. Harvard DASCH

First Nova, then dwarf novae?

Even more exciting, however: Further historical photographic plates from the years 1934, 1935 and 1942 revealed that this star was living through three so-called dwarf novels. The brightness of the star increased by two to four magnitudes. Unlike the nova, however, the gases do not explode on the surface of the star. Instead, gas is emitted in the outermost accretion disk in the direction of brittleness.

Astronomers have long suspected that these weaker eruptions occur whenever a white dwarf draws gas from his companion very slowly. They have already been observed in some so-called cataclysmic changers. However, it was unclear whether these dwarf novas can also appear as after-effects of a nova until now. Because the historical recordings document now for the first time dwarf nova in a previously exploded as a nova star.

Cyclic metamorphosis

"Like an egg, a caterpillar, a pupa and a butterfly are just different stages of the same insect, we now have strong evidence that these binaries also go through different stages, " says Shara. After a nova, therefore, a phase follows as nova ähnlicher changeable first. Some centuries later, the gas transfer slows down and only dwarf novices are found. Even later, the system enters a rest phase, only to restart the cycle some 100, 000 years later.

"So far, we've never had enough data to follow a complete cycle, " says Shara. "But when we were able to link the almost 600-year-old Korean nova observation with the dwarf nova recordings and the still-visible nova remnant, that was the breakthrough." (Nature, 2017; doi: 10.1038 / nature23644)

(American Museum of Natural History, 01.09.2017 - NPO)