Ice Core Records: From Volcanoes to Supernovas (Part II)
Narrator (April Hobart, CXC)
Cassiopeia A is one of the most famous objects in the night sky. Modern-day telescopes like Chandra reveal a spectacular debris field that glows brightly across the electromagnetic spectrum from radio waves to infrared light and, of course, X-rays.
While astronomers know that Cassiopeia A, or Cas A for short, is the aftermath of a massive star that exploded, it is unclear exactly when the explosion took place.
Supernova explosions in the Milky Way are relatively rare – with one going off roughly every 50 years or so in our Galaxy. For those that exploded centuries ago, it is often difficult to identify their exact birthdate since, of course, there were no telescopes to record them. Getting a precise date is important because the age helps determine many other properties of the explosion.
Astronomers today can get a loose estimate of age by looking at how fast the supernova remnant is expanding and working backward. To really nail the date, however, it is helpful to have some sort of historical evidence. For example, did anyone mention the sighting of a new star in the sky?
In the case of Cas A, the expansion that astronomers observe now indicates that the star would have exploded – and been seen on Earth – around 1671 A.D, give or take a few years. Unfortunately, there are no accounts from anywhere in the world around that time that mention the appearance of what would have been a very bright new star as with other historic supernovas like the Crab, Kepler, and Tycho.
The closest official account of Cas A comes in 1680 when the English Royal Astronomer John Flamsteed catalogued a star roughly in the area where Cas A is found. The problem is no one else ever recorded seeing Flamsteed's star so it's not considered an open and shut case.
In recent years, there have been arguments that Cas A exploded about 50 or 60 years earlier. This line of reasoning, made by astronomer Martin Lunn, suggests that a "noon-day" star noted at the birth of the future King Charles the Second is, in fact, Cas A.
With this astronomical and historical controversy brewing, is there some other way to settle this question? Dr. Gisela Dreschoff, a geophysicist, has been studying ice cores in the Antarctic and Greenland for years. Initially trying to date solar proton events, Dr. Dreschoff realized these cores may also contain evidence of events from much farther away. Certain signatures of nitrates in the ice may, in fact, be signals of supernovas that exploded long ago.
So we compared 1200 years (because South Pole is only 1200 years) so 1200 years at the South Pole with 1200 years at Vostok Station, and there we found a peak near 1006, 1054 (Crab Nebula), 1320, and 1572, and 1604 (which would be Kepler, before that Tycho and so forth) so that looked very encouraging.
If these other, well-documented supernova explosions left clues in the ice cores, could Cas A have done the same?
A report came out. I think I saw it first in Science News by Ron Cowen. 340 years ago. Cas A could have occurred 340 years ago. Now if I calculate back, well 1667 looks pretty good to me, and at that time, this is now 1667, in the middle of the modern minimum. And at that time, there was a volcanic eruption in the Azores, not far from Greenland, and saw a beautiful volcanic signal. So I know the year. That is 1667 and there is a beautiful, not as big as 1700, but a beautiful nitrate enhancement. And so we suggested this could be Cas A.
But what about 1630? Could there be evidence in the ice core data that Cas A really went off at the time of King Charles the Second's birth?
I got an email from Martin Lunn in England, asking me, well, do we have any of those data going further back in time, like in the early 1600s, 1630. I said, yeah, well, I can send it to you, I can send you all my data, which I did. And so I learned from him that he was interested in the year 1630 because King Charles was born at the time, and some, you know, real optical observation was made. And I told him, I'm sorry, 1630, in my data at least, there is no signal.
What could finally settle this debate? Often in science, the answer is more data. In the case of Cas A, that data could come from astronomical observations from space, historical accounts on Earth, or maybe from ice cores extracted from deep under ground. Wherever the next clue comes from, the story of Cassiopeia A and its past is not yet over and will continue to fascinate us well into the future.