The Nearby Supernova Revealed

Remember last week when I was talking about the evidence for some ancient-but-astronomically-recent supernovae? It turns out there is other evidence! Evidence that has helped scientists narrow down the potential source locations.

Credit: Gemini South Telescope in Chile. Composite by Travis Rector of the University of Alaska Anchorage

Data from the Cosmic Ray Isotope Spectrometer (CRIS), an instrument aboard NASA’s Advanced Composition Explorer (ACE) spacecraft, has helped us figure out where the recent supernovae might have come from.  CRIS measures what we call cosmic rays, atomic nuclei that have been accelerated across the galaxy at close to the speed of light.  CRIS has been around for a while, and through 17 years of cosmic ray observations, it has seen over 300,000 of them.  But only 15 have been nuclei of Iron-60, which is a key tracer element for recent supernovae.

Iron-60 has a half-life of 2.6 million years, which means any amount of it that was left over from the birth of our solar system has long since decayed, and any new Iron-60 we find is alien in nature.  It’s also a heavy element, and is only formed in extremely energetic processes, such as in a supernova explosion.  This makes it an excellent tracer of recent supernovae.

Finding Iron-60 in any amount is proof that supernovae have recently exploded in our galactic neighbourhood.  We can narrow it down by looking at nearby concentrations of massive stars called OB associations.  These groupings possess stars massive enough to explode as supernovae, and have enough stars so that explosions can occur regularly enough to lead to the Iron-60 we’ve seen.

A few of the potential sources include small regions within the Scorpius and Centaurus constellations, such as Upper Scorpius (83 stars), Upper Centaurus Lupus (134 stars) and Lower Centaurus Crux (97 stars).  These are the largest OB associations nearby and have the highest likelihood of being the source.  All that from a bit of radioactive Iron.

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