Mystery Behind Supernova SN 1006 Solved?

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Historical accounts from all over the world describe a spectacularly bright “guest star” in the night sky during the spring of 1006 — what we now know as a supernova (SN 1006). Now astronomers think they have pinpointed the probable cause of that massive explosion, one thousand years later: a merging of two white dwarf stars.

SN 1006 made quite a splash on its debut around May 1, 1006, in the constellation Lupus (the Wolf) just south of Scorpio. The critics raved! Monks in a Benedictine abbey in Switzerland marveled at the star’s brightness, and commented on the variability of its light, “sometimes contracted, sometimes diffused, and moreover sometimes extinguished” — likely due to atmospheric conditions, since the star was visible fairly low in the southern horizon.

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Chinese astronomers from that era described how the supernova’s light was sufficient to illuminate objects on the ground. And in 2006, two astronomers speculated that a Native American petroglyph (rock carving) found in Arizona could depict the 1006 supernova event.

The carving showed an object resembling a star hovering over a scorpion symbol. (As cool as it sounds, the drawing is quite crude (see left) and several leading archaeoastronomers are highly skeptical of the claim.)

The Egyptian/Arabic physician and astronomer Ali ibn Ridwan noted that “the sky was shining” from the light of SN 1006, adding, “the intensity of its light was a little more than a quarter than of moon light.” He also compared its brightness as being three times greater than Venus.

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Back in 2003, Frank Winkler, an astronomer at Middlebury College, was able to ascertain just what ibn Ridwan described by using basic mathematics to precisely estimate the brightness of SN 1006. Since Winkler knew it was a Type 1a, he also knew its luminosity, since all such supernovae produce the same amount of light. That’s why they’re so useful as “standard candles” to measure astronomical distances — or to pinpoint just how fast the expansion of our universe is accelerating.

Winkler combined that knowledge with precise digital observations of a shell of glowing hydrogen gas around the remnant, and concluded that at its peak, “In the spring of 1006, people could probably have read manuscripts at midnight by its light.”

As bright as SN 1006 may have shone one thousand years ago, today its remnant is barely visible. In fact, astronomers didn’t even identify it until 1965. And there is still much to be learned from studying its properties — like what kind of star may have produced such an explosion.

Most supernovae occur when a single star dies, specifically those of sufficiently large mass, namely, 1.4 times the mass of the sun. This is known as the Chandrasekhar limit. Stars of lesser mass usually end their lives as white dwarf stars.

But SN 1006 is the kind of supernova that tends to occur in binary star

systems, usually one white dwarf and one normal star. The latter sloughs off matter onto its white dwarf companion, which explodes when its mass hits the Chandrasekhar limit, leaving behind the normal stellar companion, which must live out the rest of its life alone, pining for its lost partner.

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However, according to co-author Pilar Ruiz-Lapuente of the University of Barcelona, in this case, there does not seem to be a surviving companion star. “The existing stars in the area have been studied, regarding distance and possible contamination by elements of the supernova, and the results show that there is no star that could be considered the progenitor of this explosion,” he explained via press release.

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Instead, he and his colleague, Jonay Gonzales Hernandez concluded that the event must have resulted from a collision or merger of two white dwarf stars. Such an event would also produce a supernova explosion — only one that leaves no trace, other than the glowing remnant we see today.

It’s kind of cosmically romantic when you think about it: two white dwarf stars, joined by gravity, dying together in a spectacular burst of energy that is seen all over the globe. Compare that to the fate of the companion star in the system that produced Tycho Brahe’s supernova of 1572, which the astronomers did find, languishing in stellar widowhood.

Ruiz-Lapuente and Gonzalez Hernandez have studied five supernovae to date, and only once (with SN 1572) have they found a likely companion star. This could mean that merging white dwarfs might be a more common pathway to these types of stellar explosions than previously thought.

Next up in their continued analysis: Kepler’s supernova of 1604!

Images: (top) Composite image of SN 1006 remnant, combining x-ray, optical, and radio data. Sources: NASA/Chandra (x-ray); NRAO/AUI/NSF/GBT/VLA (radio); NOAO/AURA/NSF/CTIO (optica). (bottom) Pteroglyph that may (or may not) depict SN 1006. Credit: John Barentine, Apache Point Observatory.