Evidence of two massive near-Earth supernovas was announced this week in a pair of papers published in the journal Nature. Brian Fields, a professor and the department chair of astronomy at the University of Illinois, is an expert on near-Earth supernovas and their detection. Fields talked with News Bureau physical sciences editor Liz Ahlberg about the new findings and how nearby supernovas could affect life on Earth.
What is a near-Earth supernova? How does it affect Earth?
A supernova is the explosion that marks the spectacular death of a massive star. During their brief lives, massive stars are extremely luminous, but burn out quickly. Then they collapse under their own weight until rebounding and launching a powerful blast wave that violently flings most of the star’s material into space.
The main danger posed from a nearby supernova comes from the high-energy radiation it produces. First, an intense burst of X-rays and gamma rays from the explosion would be absorbed by Earth’s atmosphere, resulting in the complete destruction of stratospheric ozone. In the absence of ozone, ultraviolet rays from the sun would be unshielded and wreak havoc with the biosphere. Later, as the supernova blast arrives at the solar system, it would bring high-energy particles (cosmic rays) that would create a second wave of ozone destruction.
If the supernova is close enough, it could even lead to a mass extinction of life. This possibility is the Holy Grail – or rather the Unholy Grail – of nearby supernova studies. The two explosions that were described in this week’s findings are outside of this “kill zone” but still close enough to have showered material on Earth.
How do we know that these supernovas exploded where and when they did?
If the supernova is close enough, the blast will engulf the solar system and deliver supernova material to Earth. The material will literally rain down upon Earth, and find its way to natural archives like sea sediments. But how can we be sure that we have found supernova material in a deep-ocean sample?
In the blizzard of nuclear reactions in a supernova, a wide array of nuclei are born. Most are stable, but some supernova products are unstable. These radioactive nuclei are destined to decay into a more stable nucleus, with different species of radioactive nuclei taking different amounts of time to decay. We are particularly interested in radioactive iron-60 because it lives for a few million years. This is perfect: The iron-60 survives long enough to escape the supernova, but not so long that any could be left over from Earth’s formation.
Thus, if we find a geological sample of iron-60, it had to be made recently and somehow brought to Earth. Supernovas are the only plausible sites for creating iron-60 and delivering significant amounts to Earth. Iron-60 is a “smoking gun” of a supernova.
What did the two Nature papers find?
One reported important new iron-60 measurements, and the other was a theoretical study that interprets the data. The new measurements, by Anton Wallner and collaborators, shed new light in several ways. Evidence had been discovered previously of a nearby supernova about 2 to 3 million years ago, but the new data show indication of a second iron-60 signal about 8 million years ago – pointing to at least one more nearby supernova. Also, they measured iron-60 in sea sediments that allowed them to trace this history of how the supernova debris was delivered over time. The deposition lasted about 1 million years, much longer than the simplest expectations for an isolated supernova; this poses an interesting problem for this new field of “supernova archaeology.”
The second paper, by Dieter Breitschwerdt and collaborators, presents nice astrophysics detective work on the origin of the supernovas that led to the iron-60. The authors showed that it is possible to build a coherent story that weaves together disparate pieces of evidence. They point out that the nearby Scorpius-Centaurus association of massive stars likely represents the cousins of more massive stars that have already exploded. They calculate the effects of these explosions and show that they not only could lead to multiple iron-60 pulses on Earth, but also that they could be responsible for creating the Local Bubble – an enormous, hot, rarefied cavity of gas that surrounds the sun.
These were estimated to have exploded millions of years ago. What would happen if such a supernova exploded now? Are there any nearby stars that could reach that point?
The massive stars that explode as supernovas are extremely luminous. One so close would be the brightest star in the sky by far. This is doubly good news. First, we can be confident that there are no near-Earth supernovas threatening us today. We can sleep well. This also means that if, someday in the distant future, our descendants are threatened by a nearby massive star, it will not sneak up on them, and they will have time to make disaster plans.
To contact Brian Fields, call 217-333-5529; email bdfields@illinois.edu.
