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Every year astronomers see hundreds of supernovae erupt in other galaxies, but from such great distances these stellar explosions look only like bright dots. Researchers therefore prize the few supernovae that past observers witnessed in the Milky Way, where telescopes can scrutinize the wreckage. Since the year A.D. 1000, skywatchers have seen five of our galaxy's stars die in brilliant explosions. Now a new distance determination to the most mysterious of these is yielding new insight into its nature.
All five stars blew up thousands of light-years away, so their light took many millennia to reach us. But observers can recognize celestial events only when their light strikes Earth, and astronomers therefore usually say they occur that same year. Four of the five post-1000 supernovae are famous: A 1006 explosion in the southern sky was the brightest in recorded history; a 1054 supernova in the constellation Taurus spawned the well-known Crab Nebula; and supernovae in 1572 and 1604 bear the names of two Renaissance astronomers, Tycho (Brahe) and (Johannes) Kepler.
That leaves a puzzling explosion that Chinese and Japanese observers recorded in 1181. "The event was there, we know it happened—we know it from several independent sources—and the descriptions are very similar," says Roland Kothes, an astronomer at the Dominion Radio Astrophysical Observatory in British Columbia. "The length [of the explosion] is too long for a nova."
Starting in August 1181 a "guest star" appeared out of nowhere in Cassiopeia, a W-shaped constellation in the northern sky. Even at its peak brightness the newcomer was much fainter than the four other bright supernovae of the second millennium, which outshone every nighttime star. The guest star merely matched Vega, the fifth-brightest star in the night. Six months later, it vanished.
Until astronomers located the remnant of the blast, no one could say what type of star had blown up. Was it a massive star, like the Crab's progenitor, or a tiny white dwarf, like the stars of 1006, 1572 and 1604? Four decades ago astronomy historian F. Richard Stephenson provided a crucial clue when he linked the 1181 blast to 3C 58, a nebula in Cassiopeia that emits radio waves. X-ray observations later revealed that the nebula harbors a pulsar, the fast-spinning collapsed core of an exploded massive star.
An early estimate placed the nebula 27,000 light-years from Earth; later work reduced that distance to 10,000 light-years. But even that figure is so great it prompted astronomers to question the connection between the 1181 supernova and 3C 58. Now Kothes has remeasured 3C 58’s distance, finding the nebula to be just 6,500 light-years from Earth, which he says reaffirms the link to the supernova.
To gauge 3C 58’s distance, astronomers exploit the Milky Way's rotation, measuring velocities of hydrogen clouds in front of the nebula to deduce how fast it revolves around the galaxy's center. From knowing how the Milky Way rotates, astronomers derive the distance from Earth to the nebula. But Kothes says the nebula inhabits the Milky Way's Perseus arm, the next spiral arm out from ours, which pushes hydrogen clouds toward the galactic center and thereby alters their velocities. By correcting for this disturbance, he finds a much closer distance for the nebula.
The new distance changes the properties astronomers deduce for the nebula. Because 3C 58 is closer, it follows that it must produce less synchrotron radiation—which electrons emit as they whirl around magnetic field lines—than previously thought. The ultimate energy source for this radiation is the nebula's pulsar. Astronomers use the pulsar's age and spin to calculate how much energy it has injected into the nebula.