Presents from the past
ONE of the important rules in science is to keep records of all of the observations, so that it is possible to refer to them later when trying to understand new or more detailed findings.
In astronomy, it has often been the case that examining an old photographic plate of the sky taken many decades ago can help to track down previously unnoticed changes in the light from a star, or the exact position of an asteroid at that earlier time, helping to pin down the size and shape of its orbit around the sun.
Now astronomers have made use of ancient Chinese observations to find that an object that they are observing now is likely to be the remains of a star that was seen to explode — a supernova — more than 1600 years ago, long before it was understood that stars can explode.
In the year 386, the Chinese recorded the appearance of a “guest star’’, which appeared among the stars of the constellation that we call Sagittarius, the Archer. To them, the pattern of stars was called Nan-Dou, and they wrote that: “During the third month of the eleventh year of the Tai-Yuan reign-period of the Jin dynasty, there was a guest star at Nan-Dou until the sixth month, when it was extinguished.”
The Jin Dynasty of the time lasted between the years 265 and 420, and encompassed a large part of the modern area of mainland China.
Observations today at X-ray wavelengths show what astronomers call a supernova remnant, called G7.7-3.7, whose position corresponds quite well with the description recorded by the Chinese.
Such a remnant seen using X-ray telescopes is superheated material resulting from the rapid outflow of the debris from the explosion as it spreads out through space. The astronomers have used observations made several years apart to estimate now long ago the star exploded, and the year 386 is indeed within this possible range.
The information that this supernova was seen for only three months is also very useful, because it suggests that it was a supernova that astronomers call “Type II P’’, which is a special, and uncommon, class of low-brightness supernovae.
Earlier, another supernova remnant, called G11.2-0.3, was suspected of being the one, but this has now been ruled out, leaving G7.73.7 as the likely candidate.
It is very useful to make these definite identifications, because finding examples of the exact years in which the supernovae were seen helps us to date the remnants accurately. The main point is to match the remnant with the ancient observation — and the better the original description, the less difficult is the job.
The Chinese recorded many things in the sky. Another exploding star was the supernova that was seen in the year 1054, the remnant of which is today called the Crab Nebula. It is one of the most striking examples of the result of such an explosion, and it was discovered in 1731 by British astronomer John Bevis.
Chinese astronomers also recorded “broom stars’’, which were comets. The tail of a comet reminded the observers of the appearance of a broom. Those records, too, are useful in identifying ancient movements of comets, and they include a record of Comet Halley that was made in 240BC.
Old descriptions of eclipses are also very helpful to astronomers. Knowing locations on Earth from which the sun was completely blocked out by the moon during a total solar eclipse helps them to work out the exact rotational position of the Earth at the time, and indicates by how much the Earth’s spin has been slowing down.
Even a comment that candles were necessary in the middle of the day can offer important clues!