Read our COVID-19 research and news.

New Explanation for Ancient 'Po Star'

"During the third month of the third year of Jian-Ping in the period of the king of Han Ai, a po star was seen near He-Gu," recorded the Chinese scribe Ban Gu 2 millennia ago. Two present-day Chinese astronomers suggest that the ancients saw a superbright exploding star called a hypernova. Its remnants persist, they say, in the form of a flashing gamma ray beacon called a soft gamma repeater (SGR). If they are right, pinning an age on this type of exotic star--only four of which are known--would help astronomers understand how they form and evolve.

From ancient times on, the Chinese were diligent recorders of new features in the night sky. They noted 103 "po stars"--faint point sources--between 2320 B.C. and A.D. 1911. Zhenru Wang and her colleague Zhiyuan Li, together with classicist Yi Zhao, all from Nanjing University, realized that the location of the po star recorded by Ban Gu made it a candidate for the precursor to the gamma repeater SGR1900+14. Wang and Li then analyzed data on the timing of its gamma ray pulses to fix the age of SGR1900+14. The star was born about 2000 years ago, right about the time Ban Gu made his sighting, the team will report in the 10 April issue of The Astrophysical Journal.

Astronomers believe that SGRs are special spinning neutron stars, but they are not sure where they come from or how they really work. The finding could provide some clues, says Wang. Because SGR1900+14 is very distant, its precursor must have been very bright to have been visible from Earth, she says. This, and the fact that the po star was visible for no more than a month, suggest that the ancients may have seen an especially bright star explosion called a hypernova. If the link holds, it suggests not only that SGRs arise from the ashes of a hypernova, but also that they are surprisingly new. Most neutron stars are millions of years old, she says, so anything under 10,000 years old qualifies as a baby.

"It would certainly be interesting if the SGR were linked with this po star," says Peter Woods, an astronomer at NASA's National Space Science and Technology Center in Huntsville, Alabama. But he and Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics both think the connection is weak. "This is a very crowded and complicated area of the sky," Gaensler says, so position is hard to justify as evidence.

Related sites
Wang's home page
Nanjing University Department of Astrononmy home page
Further information on soft gamma repeaters
Further information on sgr1900+14