Bang! goes a star.  Watch how fast its contents move, and you know the date, right?  Watch its light curve, and you know the type, right?

Kepler’s Supernova:  We can date Kepler’s Supernova, because Johannes Kepler watched it in 1604 and said it was visible in the daytime for three weeks.  It was a Type 1a supernova, the kind cosmologists use to measure the size and expansion rate  of the universe.  Ah, that things were so simple.  PhysOrg’s article on Kepler’s Supernova revealed some nasty complications.  “Ironically, the precise distance to the remnant of Kepler’s supernova is not very well known,” the article mentioned.  Estimates range from 10,000 to 25,000 light-years. Too bad the nearest Type 1a can’t help calibrate the “linchpin in calibrating standard distance candles” with better precision.  Another complication is that there are “clear signs that the explosive blast wave encountered a dense circumstellar shell” – a phenomenon that could affect the dating of other supernova remnants by slowing their expansion rates.  The Chandra X-Ray Observatory recently re-estimated to “probably greater than about twenty-one thousand light-years, although additional research is needed to strengthen this conclusions.”  What wasn’t mentioned is the undermining effect Chandra’s estimate has on the other techniques used to measure the distance.

Chinese 1006 Supernova:  Type 1a, take five: a supernova witnessed by the Chinese in 1006 – so bright they could read at midnight by its light – was another Type 1a supernova, PhysOrg reported, but “what kind of Ia supernova was it?”  The article went on to describe at least three kinds of events that fit this classification: (a) the blast when a red giant’s gas leaks onto a white dwarf, (b) the kind where two white dwarf stars merge and explode (the rapid kind), and (c), the kind where a white dwarf leaks slowly onto the other white dwarf (the slow kind).  The Chinese Supernova is believed to be the slow kind.  But then the article pulled the rug out from reliable cosmic dating with this statement:

The new finding would mean that there are now five documented type Ia super novae, with four being the rapid kind and just one the slow, leading the research team to suggest that perhaps only twenty percent of all such explosions are of the slow moving variety, which matters because astrophysicists use such explosions to calculate how fast the universe is expanding, which in turn impacts theories on dark energy, which appears to cause the expansion to speed up.

The original paper in Nature said, “It is also the only one whose type has never been disputed.”  Then the paper went on to dispute it.  Of the two kinds of white-dwarf mergers, the team from Spain said,“the relative proportions of their contributions remain a fundamental puzzle in astronomy.”  They came up with a number of 20% for the type the 1006 Supernova represents, for now, “or [it] preferentially involves main-sequence companions with masses more probably below that of the Sun.”  The PhysOrg article (quote above) indicated this is just a suggestion based on statistics.  (Gonzalez Hernandez et al., “No surviving evolved companions of the progenitor of SN 1006,” Nature 489, 27 Sept. 2012, pp. 533–536, doi:10.1038/nature11447)….

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