by Alok Jha
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The end of our solar system would come in stages. For decades, no one would be able to work out why the number of asteroids hitting the Earth was suddenly shooting up. Astronomers would eventually pick up some wobbles in the orbits of the outer planets, as if a giant invisible hand was knocking them around. Any clouds of dust and gas around the solar system would begin to glow as they became sucked into the black hole and released intense electromagnetic radiation.
As it moved closer to the solar system, the black hole’s immense well of gravity would rip some planets apart and swallow others whole. When it reached Earth, it would draw us into the mysterious void at its center, a place with a vast appetite for anything the universe can throw at it, a place with infinite destructive potential.
What is a black hole?
Anything can fall into a black hole, but nothing can get out. Stars and planets can disappear and anything that gets too close will be torn apart into its constituent atoms. These cosmic objects are a one-way ticket to mystery, a place where known physics seems to break down and the space we are all familiar with becomes supremely strange.
A black hole is a dead star. After billions of years of shining and fusing hydrogen at its center, a star will run out of fuel and start to collapse. The collapse increases the temperature and pressure at the center of the dying star, and the energy levels rise high enough to start fusing the helium there into carbon and oxygen. Later, the helium runs out and the collapse starts again, until the pressure at the core is high enough to begin fusing the carbon.
A star will go through several stages of burning successively heavier fuels before the end of its life: a supermassive star will go through phases fueled by neon, oxygen and silicon. By the time any star is producing iron at its core, it is near to the end. Fusing iron is no good for a star, since it would consume, rather than release, energy. At this point the star is no longer able to hold itself up against further collapse—from being millions of miles wide, it collapses into a dot (called a singularity) that is smaller than the full stop at the end of this sentence.
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Once something enters the event horizon, it loses all hope of exiting.
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Albert Einstein’s general theory of relativity predicts that if matter is compressed into a small enough space, the resulting gravity becomes so strong that nothing nearby can escape the pull. The boundary of the region where the gravity of a collapsed star beats every other force around is called the event horizon. Pass this point and there is no coming back, not even for the massless particles of light.
“Once something enters the event horizon, it loses all hope of exiting. Whatever light the falling body gives off is trapped, too, so an outside observer never sees it again,” says Pankaj Joshi, a physicist at the Tata Institute of Fundamental Research in Mumbai. “It ultimately crashes into the singularity.”
Only the biggest stars collapse into black holes. The Sun, for example, could not naturally become a black hole, because it does not contain enough matter to create the intense gravity needed to overcome the repulsive forces that exist between subatomic particles (the strong and weak nuclear forces and electromagnetism, all of which are usually many orders of magnitude stronger than gravity). Only stars with a mass six times greater than that of the Sun can typically become black holes.
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THE MILKY WAY CONTAINS
10 million dead stars (potential black holes)
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How many are there?
Given that they do not emit light, it is impossible to directly detect or image a black hole. Having said that, scientists infer the presence of these objects based on the effects they have on their local environments.
If anything passes near its event horizon, the black hole’s intense gravity will begin to suck in material. A star could end up in orbit, for example, and slowly lose its mass to the hole. As the outer layers are drawn in, the black hole acts like a power plant, releasing gravitational potential energy that energizes intense beams of X-rays and jets of gas. These fly out from the region around the black hole, and are detectable by satellites and instruments on Earth.
Evidence from the past few decades suggests that the biggest black holes are responsible for keeping galaxies together. In 2007, astrophysicists finally confirmed that the Milky Way contained a supermassive black hole at its center, something that had been suspected for many years beforehand.
There are probably more than 10 million dead stars in the Milky Way that could be candidates for black holes. They are likely to have compressed themselves to point-sized singularities with event horizons around 24 km (15 miles) wide. They are probably cannibalizing everything that wanders into their vicinity right now.
What would happen to Earth near a black hole?
The disaster scenario for our solar system would unfold if any object, such as a planet, got stuck within the event horizon of a black hole. In that region, the gravity would totally dominate, and because of the sharp rate of increase of the forces, different ends of an object would feel different amounts of force. If your head was nearer the hole than your feet, for example, the atoms in your hair would feel a stronger force than those in your toes. This difference would quickly tear you apart, turning you into a spaghetti-like line of atoms moving toward the singularity.
No one knows what actually happens at the singularity, given that nothing, not even information, can escape.
The black hole would not have to sit on top of the Earth, however, for its effects to change the course of human civilization forever. If it came within a billion miles, its gravitational forces would knock the Earth out of its current orbit and into a dangerous elliptical path around the Sun. In this new orbit, winters would regularly drop to -50°C and summers would reach hundreds of degrees Celsius. It is hard to imagine much life surviving at these extremes.
If the black hole’s gravity managed to eject us from the solar system altogether, our planet would end up wandering through deep space without a source of energy to keep us warm or make plants grow. Life on Earth would freeze to death in a matter of months.
Any object in the region around a black hole would be drawn in by the immense gravitational force. Anything within the event horizon would be on a course for eventual destruction—there is no escape from this region, not even for light. The accretion disc, mass drawn toward the central body, emits a powerful jet of charged particles.
Is a black hole headed our way?
Black holes, like any other cosmic object governed by gravity, would orbit the center of the galaxy and also any other massive objects nearby. Their immense gravitational effects on the objects around them mean that we should notice if they turned up at the solarsystem’s edge. But whereas the approach of a star would be obvious, given that it would shine, and astronomers would be able to measure the compression of the oncoming light waves to work out when it would reach the Earth, “seeing” black holes would be more difficult.
Black holes, as we have said before, do not emit any light. If they happen to eat something on the way to us, we might see a flash of X-rays or be able to detect some superhot gas jets. Beyond that, there would be little warning apart from the effects of the black hole’s gravity as it began to reach the outer edges of the solar system. Perhaps a decade or two before the close encounter, the rocks and comets at the furthest reaches of the solar system would start to be thrown around. The black hole might even dislodge an asteroid big enough to cause catastrophic damage if it hit the Earth—though that is a doomsday scenario all by itself (see p.137).
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Life on Earth would freeze to death in a matter of months.
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As the black hole got closer, the shower of asteroids into the solar system would increase and the outer planets might get knocked out of orbit. By then we would know something untoward was up.
But what could we really do against the colossal cosmic forces and energies that would be
unleashed upon us if a black hole came near? Unless we were safely ensconced on another world far away, our planet and people would have to face some horrifying consequences.
Mind you, chance is on our side. Even though there are probably around 10 million black holes in the Milky Way, our galaxy is a vast place that stretches for around 100,000 light years and contains hundreds of billions of stars. We have no idea how many planets (and possibly civilizations) have been destroyed in the vicious maw of a black hole, but it is safe to say that they were the (extremely) unlucky ones.
Gamma Rays from Space
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It started with a blinding flash in the sky, a sign that the Earth’s atmosphere had been hit by the most intense radiation imaginable. For billions of years, that gamma radiation had zoomed unimpeded through interstellar space at the speed of light. When it crashed into our planet’s atmosphere, its immense energy was dumped directly into the air molecules it found, tearing them apart.
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The upper atmosphere started to cook. The protective ozone layer disintegrated and all organic matter on the Earth’s surface was left exposed to the deadly ultraviolet rays streaming in every day from our Sun. The combination of cosmic gamma rays and local UV rays meant that over the next few months, our planet’s surface became largely sterilized of life.
This is what happens when a planet finds itself in the way of one of the biggest explosive events in the universe: the death throes of a supermassive star in the moments before it collapses to become a black hole. As it died, billions of years ago, that star shot two concentrated beams of gamma rays into space, streams of such immense energy that they would easily destroy anything in their path. It was just unlucky that so much time later, our blue-green planet filled with life happened to wander into that firing line.
What is a gamma-ray burst?
Describing anything as “the biggest in the universe” perhaps seems excessive. After all, the universe is a vast place and we have not yet searched every corner—how do we know something is the biggest? Normally, scientists agree with this sentiment and shy away from such superlatives. But there is one thing that astrophysicists do not hesitate to throw into the category of “biggest”—gamma-ray bursts. These jets of radiation are the result of the largest explosions known to exist, the final moments of stars at least 15 times more massive than our Sun.
A star forms when a cloud of hydrogen is drawn together by the mutual gravity of all the atoms. When it becomes sufficiently dense, the gas in the center will start to fuse and release energy, making the cloud shine. As more gas fuses, the star continues to shine.
Eventually, after several stages of fusion and billions of years of shining, the star will run out of fuel. At this point, it will collapse into a ball of waste, composed of heavy elements that cannot be fused any further. If it is a particularly big star, the subsequent collapse of its insides causes its outer layers to explode into a supernova, an event so bright that it can briefly outshine all the other stars in an entire galaxy.
An artist’s image of the radiation (purple shell) from a gamma-ray burst (GRB) event (upper left) hitting the Earth (lower right). The radiation, X-rays and gamma rays would not be visible.
As if supernovas themselves were not impressive enough, if the star is at the biggest end of the spectrum, the explosion will be so huge that it is called a hypernova. This type of explosion can emit as much energy in just a few seconds as a typical star (our Sun, say) might release in its entire 10-billion-year lifetime.
As part of its explosion, a hypernova sends two concentrated jets of gamma-ray photons shooting off in opposite directions from its poles. This burst of gamma rays, the most energetic electromagnetic radiation there is, can last for anything from a few milliseconds to several minutes. In that time, it will shine about a million trillion times as brightly as the Sun, making it temporarily the brightest source of gamma rays in the observable universe.
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These jets of radiation are the result of the largest explosions known to exist, the final moments of stars at least 15 times more massive than our Sun.
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According to NASA, the gamma-ray bursts (GRBs) of longest duration originate at the furthest edges of the observable universe, and the stars linked to the explosions are typically in the order of several billion light years away. That means that any gamma-ray photons coming from them would take billions of years to reach us at the speed of light (300,000 km/s). Given that the Earth is just over 4 billion years old, it is entirely feasible that some of the GRBs scientists see in the sky today actually happened when our planet was still in its earliest stages of formation, well before life even started to evolve.
Using space telescopes, scientists can detect around one GRB a day from various directions in the universe. Fortunately, all these known events have taken place well outside our galaxy. If a GRB did occur in our galaxy and one of the gamma-ray jets happened to be pointed at Earth, we would be in a lot of trouble.
What would it do to Earth and life here?
If a GRB did have the Earth in its sights, our planet’s atmosphere would get pummeled. The incoming gamma rays would likely cause a blinding lightshow as they hit the Earth and knocked electrons off the atoms they encountered. At that point, however, we would not feel any effects on the surface. In the upper atmosphere, the gamma rays would begin splitting nitrogen and oxygen molecules and forcing them to react with each other, causing the creation of toxic brown nitrogen oxide. This is a greenhouse gas that can blot out the Sun and which also also destroys ozone. As soon as the GRB hit the Earth, the ozone layer would be under threat, and in very short order, our planet would be vulnerable to rapid mass extinction.
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OZONE LAYER AFTER A GAMMA-RAY BURST
5 weeks later 50% destroyed
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Charles Jackman, of NASA’s Goddard Space Flight Center, has worked out that even a short GRB near the Earth would destroy half of the planet’s ozone layer within just a few weeks. Five years later, at least ten percent of the ozone would still be missing.
“Nearly all the energy goes into atmospheric chemistry,” wrote physicist Larissa M. Ejzak in a research paper published in the Astrophysical Journal in 2006 that modeled the effects of a nearby GRB on the Earth. “The primary chemical effect of the incident radiation is to break the strong chemical bonds of O2 and N2, making possible the formation of molecules that are normally present in very low abundances in the atmosphere. NO and NO2 are in this class; they also catalyze the destruction of ozone.” She calculated that it would take nearly a decade for the atmosphere to recover from such a burst.
Without the protective ozone shield, harmful ultraviolet rays from the Sun would penetrate to the surface of our planet and start tearing through DNA in living things. For humans, the effects would be gradual: first of all we would notice our skin getting sunburnt more quickly, but underneath, our cells would be quietly ravaged by the UV rays. The rates of skin cancer would skyrocket.
Other animals and plants would suffer as their own cells were unable to reproduce or were killed outright because of the widespread DNA damage. The UV rays would only penetrate the top of the oceans, but this would be enough to kill all the tiny photosynthetic plankton that sit at the root of the oceanic food chain. Remove these and there is much less oxygen being put into the atmosphere and far less food for the animals further underwater.
Perhaps it is a sign of the Earth’s resilience that the ozone layer would heal itself after a decade. The bad news is that a decade without phytoplankton would not leave much alive in the oceans.
Is it likely?
Some scientists believe that a gamma-ray burst was behind an infamous mass extinction in which 60 percent of marine invertebrates were destroyed. “At least five times in the history of life, the Earth has experienced mass extinctions that eliminated a large percentage of the biota,” says Adrian Melott, a physicist at the University of Kansas who has exam
ined the effects that GRBs might have on the Earth. “Many possible causes have been documented, and GRBs may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB.”
He added: “A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life. We don’t know exactly when one came, but we’re rather sure it did come—and left its mark.”
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A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life. We don’t know exactly when one came, but we’re rather sure it did come—and left its mark.
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What is less certain is how often these huge events occur in our vicinity. In a 2004 paper published in the International Journal of Astrobiology, Melott made an educated guess that a dangerously close GRB should occur on average two or more times per billion years. So going by statistics alone, we are not due one for at least another 500 million years.