Presents from the past

Mercury (Hobart) - - NEWS FEATURE - MAR­TIN GE­ORGE Mar­tin Ge­orge is man­ager of the Launceston Plan­e­tar­ium at QVMAG.

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!

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