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.

If one of the former distances were correct, Kothes says, "the minimum energy you need for the synchrotron nebula to be produced is higher than the energy the pulsar has released since its birth." In contrast, the new distance means the nebula is emitting no more energy than the pulsar can provide. As he reports in work to appear in Astronomy & Astrophysics, he's convinced 3C 58 marks the site of the 1181 blast.

If 3C 58 is as close as Kothes thinks, then the 1181 blast was even more of a wimp than astronomers believed. At his proposed distance, the 1181 explosion was roughly a fifth as luminous as the 1987 supernova in the Large Magellanic Cloud, a nearby galaxy, that also emitted less light than the norm. Kothes suggests the two blasts were similar. He estimates the progenitor of 3C 58 began life as a blue star of spectral type O that was 20 to 30 times more massive than the sun.

Of course, if the nebula doesn't mark the blast site, all bets about the progenitor are off. Is the mystery solved?

Not so fast, says Dartmouth College astronomer Robert Fesen. He thinks 3C 58 may be thousands of years old—which means it didn't arise in a blast just 832 years ago. The problem, Fesen says, is that the nebula is large and expanding slowly, which suggests it has been expanding for a long time. But a link to the 1181 supernova means 3C 58 is younger than the Crab Nebula—born in 1054—even though it is larger than the Crab Nebula and is expanding at half the Crab’s speed. Kothes counters that 3C 58's expansion has likely slowed greatly in recent centuries, but Fesen is dubious.

Is 3C 58 really the remnant of the 1181 supernova? "I don't know," Fesen says. "It's in the right spot. There's nothing else in that area. We've looked. If it wasn't 3C 58, then what the heck did the people see in 1181?" Yet the nebula seems too old, he says. "I've puzzled over this for many, many years," Fesen says. "It's a mystery."