Restoration historians in England may be giving astronomers reason to rethink the age of one of the most well known objects in our galaxy, supernova remnant Cassiopeia A (also called “Cas A”).
The Restoration refers to the return of Charles II to England, and thus the restoration of the monarchy. His father, King Charles I, was executed in 1649 at the height of the English Civil War, after which Oliver Cromwell became Lord Protector of England, Scotland, and Ireland. Cromwell ruled for nine years until his death in 1658. Two years later Charles II, who had retreated to mainland Europe, returned to England and assumed his father’s throne.
So far, all of this is established historical fact. What has been largely considered legend up until now is that a bright “star” appeared midday on May 19, 1630, the birth date of Charles II. Long thought to be Restoration propaganda, new historical evidence has emerged that lends credibility to this account. If true, it could be that the bright midday star was the light from supernova Cas A arriving at Earth. This means astronomers may have to rethink their estimate for the age of supernova remnant Cas A, since its appearance has typically been dated to 1667 or 1680.
Although recorded as a bright star in 1680 by John Flamsteed, England’s first Astronomer Royal, the supernova remnant wasn’t rediscovered until 1947 when — having long faded as a visible object — it was detected as a powerful radio source outside of our solar system.
Many descriptions of Cas A state that it exploded in the late 17th century, but this is a bit misleading. Cas A is estimated to be 11,000 light-years away from Earth, which means the supernova actually occurred 11,000 years before Charles II, or anyone else in the 17th century, was born — so what these descriptions mean is that the explosion would be visible to observers in the 17th century, when the light finally reached Earth.
Modern astronomers use the size of the remnant together with a measurement for how fast the remnant is expanding to work backward and calculate when the progenitor star must have exploded. The calculation predicted Cas A would be apparent to Earth observers in 1667. If instead the explosion was actually visible in 1630, it could be that the rate of expansion of the remnant has not been uniform as was assumed. In order to bolster this new hypothesis, however, historians will need to find accounts of the 1630 midday star in other historical records from that time.
Below is a time-lapse vid showing how Cas A has expanded noticeably in just a few years. Keep in mind the remnant is already 10 light-years across after ~340 years, so that material has been expanding very quickly — on average, about 20 million miles per hour.