(ORDO NEWS) — If Stephen Smartt is lucky, he may one day receive a message that will give an astrophysicist early warning that one of the most unusual phenomena known to science is about to light up the night sky.
Signals from automated telescopes and underground detectors will indicate that a star in our galactic neighborhood has just gone supernova.
A supernova occurs when a star destroys itself so much that it can outshine the light of an entire galaxy. In the last thousand years, only five of them have been visible to the naked eye. Ironically, they all happened before the invention of the telescope.
“We know about supernovae from their appearance in other galaxies and from the remnants left in our own galaxy,” says Smartt, an astrophysicist at Queen’s University Belfast. “But we would like to see one of them appear close enough to us so that we can study it with modern telescopes and detectors.”
When a supernova erupts, it showers space with heavy elements – so observing such a star nearby would provide valuable information about the creation of matter in our galaxy.
“Most of the elements heavier than oxygen were created in a supernova and then scattered through space,” says Prof Mark Sullivan of the University of Southampton.
“These atoms provide the galaxy with the material needed for life. The calcium in your bones and the iron in your blood, and the gold in the ring on your finger, were all created by supernova explosions.”
This image continues to excite writers and artists. According to Jeanette Winterson, astronomers have proven that our first true parent was a star and that we are made up of elements that are “long-lived radioactive nuclear waste from a supernova explosion.” Or, to put it more simply: “We are stardust.”
The most common type of supernova occurs when a very large star runs out of fuel, halting the fusion process that keeps it shining.
The outer layers of the star fall inwards, protons and electrons are crushed together, forming neutrons, which are collected in a superdense ball. Matter continues to rain down on this neutron ball and then bounces back, causing a shock wave that destroys the star.
All that remains after the star is a neutron sphere, which is so dense that its matchbox would weigh about 3 billion tons. And if the progenitor star that led to the supernova was especially large, then this neutron star will become so heavy that it forms a black hole from which nothing can escape, not even light.
This is a core-collapse supernova, and it can release more energy than our Sun in its entire 10 billion years of its life. If a star in our galaxy, too far away to be seen with the naked eye on Earth, goes supernova, it will suddenly shine so brightly that it can be seen in daylight.
Scientists estimate that, on average, about 20 supernovae occur in a galaxy like ours every thousand years. However, only five have been observed in the last millennium. East Asian and Arabic records indicate that supernovae occurred in 1006, 1054 and 1181, while European documents say that they occurred in 1572 and 1604.
The first of this last pair flared up in November 1572 and was seen by the Danish astronomer Tycho Brahe. “A strange star suddenly appeared overhead, sparkling with a radiant brilliance,” he recalled.
“I stood still and watched… When I became convinced that no star of this kind had ever shone before, I was so perplexed by the improbability of what was happening that I began to doubt the faith of my own eyes.”
But if supernovae are so bright, why have we only discovered five in the last 1,000 years? Why haven’t we seen the number close to 20 that other galaxies are talking about?
The answer is simple, Sullivan says. “Our galaxy is like a flat plate, and our solar system is about two-thirds of the way to the edge. A supernova that occurs on the other side of the plate will simply be obscured by all the dust and stars that are in the center of the galaxy.”
Since then, astronomers have observed supernovae in other galaxies and studied the remnants of those that occurred inside our own galaxy. These include the glowing filaments of the Crab Nebula, the remnants of a supernova that lit up the night sky in 1054 AD and has been spreading through space ever since.
Such galactic debris testifies to the enormous destruction that supernovae bring. However, according to scientists, these stellar convulsions are also an important engine of creation.
In addition to showering the cosmos with heavy elements on which life depends, they also play a key role in the formation of planets and stars, says astrophysicist Cosimo Inserra of Cardiff University.
“The supernova sends shockwaves through the galaxy that hit the clouds of gas and dust in space, compressing them so that protostars begin to form at their centers. Eventually, nuclear fusion begins, which ignites the hydrogen reserves in the star, and it begins to glow. Planets form that orbit around a star, which is probably how our sun and solar system came into being.”
However, supernovae pose a threat. “If one were to occur within 20 parsecs – roughly 60 light-years – of Earth, its intense cosmic rays could destroy our protective ozone layer, allowing increased levels of ultraviolet radiation from the sun to reach us,” says Sullivan. However, only one star very close to Earth can have such an impact, and there are currently no candidate stars near us that are ready to destroy themselves in this way, he adds.
On the other hand, it is obvious that supernovae have exploded near the Earth in the past as well. As evidence, scientists point to the discovery of a radioactive isotope of iron known as iron-60, which was found in seafloor sediments dating back 2.5 million years ago and other sediments dating back about 7 million years.
Iron-60 is produced by supernovae, and these deposits indicate that at least two supernovae have exploded near Earth in the past 10 million years, probably at a distance of about 100 parsecs, or 320 light-years.
What impact this had on the planet is unknown. “It’s possible that cosmic ray activity has increased, and this could have affected the formation of clouds on Earth or reduced the amount of solar radiation reaching the earth,” says Professor John Ellis of King’s College London. “This could have caused climate change, which in turn could have affected the course of human evolution.”
Apart from the rather startling prospect that the appearance of Homo sapiens could have been shaped by local supernovae, these discoveries also suggest that there could have been enough of them to have a real impact on life in the early stages of our planet’s history.
“If you find two that happened fairly close to the Earth within the last 10 million years, that means there must have been hundreds in the last billion years,” says Ellis.
“Some of them were quite distant … but a few were close, say, at a distance of 10 parsecs.” And we must be clear: if a supernova exploded 10 parsecs from our planet, it would most likely cause a mass extinction. ”
The Earth has experienced at least five mass extinctions, each of which destroyed thousands of species of animals, plants and marine life, and at least one of them was caused by an extraterrestrial agent: an asteroid that crashed into Earth at the end of the Cretaceous period 66 million years ago, destroying the dinosaurs.
Other mass extinctions are blamed on terrestrial catastrophes, such as large-scale volcanism. However, scientists now suspect that an otherworldly event is to blame in one other case. They point to rocks formed at the end of the Devonian period 360 million years ago, when another mass extinction occurred that destroyed ammonites, trilobites and other early life forms.
According to astronomer Brian Fields of the University of Illinois Urbana-Champaign, these rocks contain hundreds of thousands of generations of plant spores that appear to have been incinerated by ultraviolet light – evidence of a long-term ozone depletion event.
“We hypothesize that one or more supernova explosions at a distance of about 65 light-years from Earth could have caused a long-term loss of ozone,” he says.
This explosion would first wash the Earth with powerful X-ray and gamma radiation, and then its fragments would crash into the planet, depriving it of the protective ozone layer. This astronomical double whammy would expose the planet’s surface to lethal radiation for up to 100,000 years and lead to a mass extinction.
Currently, scientists are looking for further evidence for this idea. They abandoned the search for iron-60 atoms because they decay too quickly to survive 360 million years after the Late Devonian mass extinction.
Instead, they plan to look for atoms of the plutonium-244 isotope, which is also produced in supernova explosions and can persist for several hundred million years. These studies are already underway.
Meanwhile, scientists are gearing up to respond as quickly as possible to the first signs of a nearby supernova. It is very important that these first signals will not come from flashes of light, but from underground detectors designed to detect the smallest entity in the universe – neutrinos.
“Neutrinos are the first thing to appear after a supernova,” says Smartt. They are so small that they are very difficult to detect, and devices should be installed in such places that they do not pick up false signals from other sources.
“However, if enough of them are detected, an automatic warning will be sent out, and the arrays of telescopes that we use to study the night sky will be turned towards the sources of these neutrinos. Then we will be ready to study the first bursts of radiation and light coming from a supernova, and watch it unfold.”
Although scientists are confident that a supernova will occur in 2022, the question of whether it will occur in our galaxy is quite another. In any given year, this is an unlikely prospect. On the other hand, one day this may happen in our galactic neighborhood. If that happens, astronomers say they will be ready.
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