Ancient Stellar Explosion comes to Light
By Amanda Jermyn

It’s not often that historical records can be linked to astronomical events, because long ago, before telescopes, and even with the help of early telescopes, most events occurring in the night sky would have had to be quite close to be bright enough to be visible. However, astronomers recently observed echoes of an ancient exploding star that can be linked to 1,500-year-old Chinese records of an unusually bright object appearing in the night sky.

The story begins in 2010 with the discovery by amateur astronomers of a strange looking nebula, a cloud of gas and dust in outer space. The nebula, called Te 11, in the Orion constellation, could have been the ejected envelope of an old star like our sun or the result of the violent explosion of a star. An international team has recently solved this mystery, publishing their findings in the Monthly Notices of the Royal Astronomical Society. Observations by astronomers primarily from the University of Cape Town using the Southern African Large Telescope (SALT) and other telescopes at the South African Astronomical Observatory (SAAO) in Sutherland, South Africa, revealed that the nebula was likely formed in a stellar explosion just over 1,500 years ago.

The object at the center of the nebula turned out to be a double star system, with the stars orbiting each other about every three hours. By noting variations in brightness over several years the astronomers were able to record changes in the binary star system’s light. These variations led them to conclude that the center of the nebula contains a dwarf nova, a white dwarf star that is accreting, or sucking in, material from its binary companion star. While the exact mechanism is not fully understood, it is thought that the bright and dim periods are the result of instability in the rate of accretion. It turns out that the combination of a dwarf nova with a nebula is extremely rare.

Key to solving the mystery of the Te 11 nebula was using the huge light collecting power of the 11 meter Southern African Large Telescope (SALT) to determine the temperature of one of the stars in the nebula, and hence calculate its distance. This distance, around 1,000 light years, placed the nebula on the edge of the Orion-Eridanus super bubble, an area in space hundreds of light years across, filled with gas blown there by multiple supernovae explosions and interstellar wind.

Studying Chinese historical records, team member Brian Warner of the University of Cape Town found mention of a bright guest star in the constellation Orion in the year 483 CE close to the position of the nebula Te 11. This suggests that what we are seeing now when we view Te 11 are the remains of that same stellar explosion that occurred over 1,500 years ago. At that time the star would have outshone all the stars in the Orion constellation, appearing as bright as Jupiter in the night sky. Chinese viewers would have had no trouble viewing such a bright object with the unaided eye.

The two stars in the nebula Te 11 orbit each other closely, separated only by about twice the distance from the earth to the moon. It is hoped that some of the new generation telescopes coming online soon will be able to discover more of these unusual compact binaries, and in the process find out more about their formation and evolution.

Join the Springfield Stars Club on Tuesday, October 25th at 7pm at the Springfield Science Museum for a talk by Richard Sanderson entitled “A Review of Summer Astronomy Events.” Sanderson is curator of physical science at the Springfield Science Museum and manages the museum’s Seymour Planetarium and Observatory. He is an astronomy writer and co-author of the 2006 book, “Illustrated Timeline of the Universe.” Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

On Friday, November 4th at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Astronomy educator Ed Faits will speak on “Cruising the Universe: Through Our Solar System and Beyond.” A fee of $3 for adults and $2 for children under 18 will be charged.