It was a sky show for the record books—a brilliant new star that appeared without warning in April of the year A.D. 1006 and then gradually faded from view a few months later. At its peak, the celestial wonder was reportedly brighter than the planet Venus, even though it was blazing about 7,200 light-years away.
Witnesses describe the event in texts from Asia, the Middle East, and Europe and maybe even in North American petroglyphs. Thanks to these diverse notes, modern astronomers know that this “guest star” was really a supernova, a cosmic explosion called SN 1006.
Now, astronomers digging through ancient texts have found two lost records of the event that add a twist to the tale of the brightest supernova in recorded history.
Ralph Neuhäuser, an astrophysicist at Friedrich Schiller University Jena in Germany, was studying works by the Persian scientist Ibn Sina, known to most in the West as Avicenna. The prolific scholar, who lived from 980 to 1037, traveled widely and wrote on subjects ranging from astronomy to medicine.
One section of his multipart opus Kitab al-Shifa, or “Book of Healing,” makes note of a transient celestial object that changed color and “threw out sparks” as it faded away. According to Neuhäuser and his colleagues, this object—long mistaken for a comet—is really a record of SN 1006, which Ibn Sina could have witnessed when he lived in northern Iran.
While SN 1006 was relatively well documented at the time, the newly discovered text adds some detail not seen in other reports. According to the team’s translation, Ibn Sina saw the supernova start out as a faint greenish yellow, twinkle wildly at its peak brightness, then become a whitish color before it ultimately vanished.
“It is special that he mentions a color evolution, which is not mentioned by the others," Neuhäuser says in an email. His team describes its work in a paper now in press at Astronomical Notes.
Recognizing how the supernova changed hue over time, as well as tracking its recorded changes in brightness, can help modern astrophysicists better understand this particular flavor of supernova, a scientifically useful blast called a type Ia.
Such stellar explosions happen in systems where two or more stars orbit each other. If one of those stars evolves into a small but massive white dwarf, the hefty dwarf can pull gas from its companion. When enough matter builds up, the overstuffed star collapses and explodes. Since the resulting flare almost always has a standard brightness, astronomers today use type Ias scattered across the cosmos to track motion and distance in the universe.
However, astronomers think SN 1006 was a version of a type Ia triggered by two white dwarfs. As these stars orbited each other, they lost energy in the form of gravitational waves and eventually collided, creating an epic blast even brighter than usual. Understanding these supercharged supernovae is vital for astronomers who use the blasts as cosmic measurement tools.
In addition to Ibn Sina’s record, Neuhäuser recently found another piece of evidence for SN 1006 in works by a historian named al-Yamani, from Sanaa, Yemen. The text suggests observers there witnessed the guest star’s arrival even earlier than thought, which would also affect modern understanding of its evolution.
Most experts put the first sightings of SN 1006 at about April 28 or 30, depending on how they convert the lunar calendrical systems used by the ancient observers, as well as the imprecision of the observer's own dating. But Neuhäuser's work suggests a date of April 17, plus or minus two days.
The al-Yamani texts record the supernova rising about a half hour after sunset. Given the star’s position in the sky, there are only a few dates when that could happen, and they fall in the middle of April.
Also, the texts mark when the supernova rose in the sky relative to the moon, and that corresponds with dates between April 15 and 18, based on known positions of the moon at the time. According to Neuhäuser, records from China, Japan, and Switzerland can be interpreted in ways that back up the earlier date.
Brad Schaefer, a professor of physics and astronomy at Louisiana State University, has studied the timing of historical supernovae. He agrees that ancient observations can be useful for working out when this supernova reached peak brightness.
But he’s not convinced that the color data from Ibn Sina will be as helpful. One issue is that the supernova was close to the horizon for Ibn Sina, so that the colors he reported might be just atmospheric effects.
He also cautions that anyone trying to weave together various records of the event will have to account for variations in the relative brightness from observer to observer: "So for example, one person compared it to the brightness of Mars, another one to Venus, and another person to the brightness of the quarter moon,” he says.
For his part, Neuhäuser thinks that the earlier observation from Yemen may ultimately be the more useful find for filling in pieces of the supernova's history, which may in turn help refine today’s astrophysical models.
"I try to investigate old historical observations to use in state-of-the-art astrophysical questions," he says.