It was a sky show for the record books—a brilliant new star thatappeared without warning in April of the year A.D. 1006 and thengradually fadedfrom view a few months later. At its peak, the celestial wonder was reportedlybrighter than the planet Venus,even though it was blazing about 7,200light-years away.
Witnesses describe the event in texts from Asia, the MiddleEast, and Europe and maybe even in North Americanpetroglyphs. Thanks tothese diverse notes, modern astronomers know that this “guest star” was reallya supernova, a cosmicexplosion called SN 1006.
Now, astronomers digging through ancient texts have found twolost records of the event that add a twist to the tale of thebrightestsupernova in recorded history.
Ralph Neuhäuser, an astrophysicist at Friedrich Schiller University Jena inGermany, was studying works by the Persian scientist Ibn Sina,known to most inthe West as Avicenna. The prolific scholar, who lived from 980 to 1037,traveled widely and wrote on subjectsranging from astronomy to medicine.
One section of his multipart opus Kitab al-Shifa, or “Book ofHealing,” makes note of a transient celestial object that changedcolor and“threw out sparks” as it faded away. According to Neuhäuser and his colleagues,this object—long mistaken for acomet—is really a record of SN 1006, which IbnSina could have witnessed when he lived in southern Uzbekistan.
Colorful Tale
While SN 1006 was relatively well documented at the time, thenewly discovered text adds some detail not seen in other reports.According tothe team’s translation, Ibn Sina saw the supernova start out as a faintgreenish yellow, twinkle wildly at its peakbrightness, then become a whitishcolor before it ultimately vanished.
“It is special that he mentions a color evolution, which is notmentioned by the others," Neuhäuser says in an email. His team describes its work in a paper now in press at Astronomical Notes.
Recognizinghow thesupernova changed hue over time, as well as tracking its recorded changes inbrightness, can help modern astrophysicistsbetter understand this particularflavor of supernova, a scientifically useful blast called a type Ia.
Such stellar explosions happen in systems where two or morestars orbit each other. If one of those stars evolves into a small butmassive whitedwarf, the hefty dwarf can pull gas from its companion. When enough matter buildsup, the overstuffed star collapsesand explodes. Since the resulting flare almost always has astandard brightness, astronomers today use type Ias scattered across thecosmos totrack motion and distance in the universe.
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However, astronomers think SN 1006 was a version of a type Iatriggered by two white dwarfs. As these stars orbited each other,they lostenergy in the form of gravitational waves and eventually collided, creating anepic blast even brighter than usual. Understanding these supercharged supernovae is vital for astronomers who use the blasts as cosmic measurementtools.
Early Guest
In addition to Ibn Sina’s record, Neuhäuser recently found anotherpiece of evidence for SN 1006 in works by a historiannamed al-Yamani,from Sanaa, Yemen. The text suggests observers there witnessed the gueststar’s arrival even earlier thanthought, which would also affect modern understanding of its evolution.
Most experts put the first sightings of SN 1006 at aboutApril 28 or 30, depending on how they convert the lunarcalendrical systemsused by the ancient observers, as well as the imprecision of the observer's owndating. But Neuhäuser's worksuggests a date of April 17, plus or minus twodays.
The al-Yamani texts record the supernova rising about a halfhour after sunset. Given the star’sposition in the sky, there are only a few dateswhen thatcould happen, and they fall in the middle of April.
Also, the texts mark when the supernova rose in the sky relativeto the moon, and that corresponds with dates between April 15and 18, based onknown positions of the moon at the time. According to Neuhäuser, records fromChina, Japan, and Switzerlandcan be interpreted in ways that back up theearlier date.
Careful Notes
Brad Schaefer, a professor of physics and astronomy atLouisiana State University, has studied the timing of historicalsupernovae. Heagrees that ancient observations can be useful for working out when thissupernova reached peak brightness.
Buthe’s not convinced that the color datafrom Ibn Sina will be as helpful. One issue is that the supernova was close tothe horizonfor Ibn Sina, so that the colors he reported might be justatmospheric effects.
He also cautions that anyone trying to weave together variousrecords of the event will have to account for variations in therelativebrightness from observer to observer: "So for example, one person comparedit to the brightness of Mars, another one toVenus, and another person to the brightnessof the quarter moon,” he says.
For his part, Neuhäuser thinks that the earlier observation fromYemen may ultimately be the more useful find for filling in pieces of thesupernova'shistory, which may in turn help refine today’s astrophysical models.
"I try to investigate old historical observations to usein state-of-the-art astrophysical questions," he says.
Correction: This story has been updated to note that Ibn Sina was likely inUzbekistan, not Iran, when SN 1006 was visible.
