When stars collide: Remarkable event goes virtually unnoticed


One of the most significant observations in the history of science occurred recently, and it got almost as much notice in the press as the color of Melania Trump’s gardening clogs.

On Monday, scientists all over the world held press conferences to report that, for the first time ever, we were able to observe two neutron stars colliding in an event called a kilonova. “So what” might be the first thought that pops into your mind, which might account for the relatively light coverage of this event. The people who publish news don’t spend much time or effort on events that nobody cares about. So let me tell you what’s up with this, and why this is a perfect example of why we, as a human society, need to be more aware of the sciences.

I wrote a column some time back about stars and the processes that go on inside them, turning hydrogen into helium and other reactions that generate such incredible amounts of energy that we can feel the heat from 92 million miles away. That story, of hydrogen atoms (one proton and one electron) fusing together to produce helium (two protons and two electrons) and other “light” elements, is the story about how the universe, which in the beginning, during the Big Bang, was almost entirely composed of hydrogen, came to give rise to the rest of the 92 elements that make up all matter in the universe. This, however, is not just about science — it is also about history. It is the history of how the universe in which we live came to be. This event that merited little more than a brief mention in most media, showed us how much of what our world and our universe is made of came to be.

Physicists, chemists and astronomers have theorized for generations on how the elements came to exist. The nuclear fusion of lighter elements, like hydrogen, into heavier elements, like helium and lithium is pretty straightforward. All stars can produce these light elements. In certain stars, with masses larger than our Sun, these smaller atoms can continue to fuse with each other, forming elements like carbon and oxygen. In the largest stars, particularly when they reach the ends of their life cycles, elements as heavy as iron can be produced. Iron is element number 26 on the Periodic Chart. There are 66 elements heavier than iron on the Periodic Chart, and none of them can be produced through this process of fusion inside of stars. The reason for this is that, for the elements up to and including iron, the fusion reactions that are necessary to produce them can occur inside stars because there is enough energy produced in the reactions under the conditions found inside these stars that the reactions will proceed spontaneously. That’s why stars smaller than our Sun can only produce helium and lithium from hydrogen — they are too small. Their masses aren’t large enough to produce enough pressure in their cores or to produce enough heat to sustain the fusion of hydrogen, helium and lithium into any of the heavier elements. Bigger stars can, which is why they can produce the other elements, up to iron.

So where did all the other elements come from? These are elements like nickel, copper, zinc, lead, silver, gold, radon and uranium. These elements are produced in reactions that require the input of energy — more energy than any star can produce during its normal life cycle. Stars do have life cycles. They all start out as mostly hydrogen, but as they convert their hydrogen into helium and (depending on their mass) other elements, they change. Let’s use our Sun as an example. As our Sun converts hydrogen into helium, the helium will accumulate at the core, forcing the hydrogen reactions to move out closer to the surface. As the hydrogen fusion moves outward, the Sun (or similar stars) will expand in size, becoming

red giants. When our Sun reaches this stage in a few billion years, it will expand to the point where it engulfs the Earth. Eventually, these stars will convert all their hydrogen into helium (or as discussed above, into whatever elements their masses will allow) at which point the stars will collapse on themselves, forming what are called white dwarf stars. As stars collapse into white dwarfs, they eject matter into space that becomes the building blocks of new planets. These are called planetary nebulae (“nebulae” is the plural of “nebula”).

Stars that are much larger than the Sun evolve over a different course. Their mass is so large that they do not go through the expansion phase as heavier elements accumulate in their cores. Their cores just get denser and denser, as they produce more and more iron, which is the final and heaviest product of fusion in a massive star. At some point, the core of these stars will become so massive that the cores will collapse in a gravitational implosion, releasing unfathomable amounts of energy in an explosion called a supernova. Depending on how fast this collapse happens and what the star is made of, one of three outcomes will occur. In one case, the star will pretty much blow up, scattering its mass out into space. In another case, the core will be so massive that it continues to collapse until it forms a supermassive body that we refer to as a black hole. A black hole isn’t a hole at all. It is a mass so dense that it produces such intense gravity that it does not allow anything, even light, to escape. The third possible result of a supernova is the formation of a neutron star, like those that were just observed colliding. Neutron stars are super-dense masses composed mostly of neutrons, which are the uncharged particles in the nuclei of atoms.

These two invisible, incredibly dense remnants of supernovas that occurred about 10 billion years ago were once massive stars that used up their nuclear fuel at an incredible rate. The burned very bright and very hot, but for a relatively short time. Once their hydrogen had been converted by fusion into heavier and heavier elements until enough iron built up in their cores, these massive stars exploded in supernovas. They were just the right size that some of their matter condensed into the super-dense neutron star. If they had been bigger, the remnant would have been so massive that it would have continued to collapse into an even more dense black hole. These two neutron stars were speeding through a galaxy (a galaxy is a dense collection of billions of stars, all moving together thorough space, linked to each other by gravity) about 130 million light-years away. Our galaxy is the Milky Way. The galaxy where this collision occurred is known by the decidedly unromantic name of NGC 4993.

The two neutron stars had being rotating about each other for billions of years, spiraling closer and closer together until they collided, sending vast amounts of energy, in the form of gamma rays, X-rays and gravitational waves, out into space. On Aug. 17, astronomers, first in the U.S., then in Europe and South America, heard the first signals to reach Earth from that collision. Mind you, the actual event happened 130 million years ago — it took that long for the gravity waves to travel from NGC 4993 to the detectors on Earth. At the same time that the gravity waves were being detected, gamma-ray telescopes picked up the radiation signal from the event, proving it was the collision of two neutron stars. Fortunately, there were three gravity wave detectors online when the event was recorded. The last of the three just joined the network only two weeks before. Combining the information from these different sources, the location of the source of the signal was able to be pinpointed. Without those three sources, we would not have known where the collision occurred and we would not have been able to study it in detail.

But we were able to locate it, and we were able to photograph it and measure it. What we were able to see was how the heavy elements of the universe came to be. In the blast following the collision, so much energy was released that the neutrons that made up the neutron stars were hurled out into space, combining with other atoms at incredible velocities. Neutrons slammed into other atoms, and new atomic nuclei were formed. One estimate is that there was gold (a heavy metallic element) equal to 40 to 100 times the mass of the Earth produced in the collision. Add to this the silver, uranium and other heavy elements. This type of event is where all the elements found in the universe comes from. All the gold, silver and other elements on Earth were originally formed in a collision like this (along with a few other related processes). Those elements, formed in the hearts of stars in their last cataclysmic moments, formed into chunks of rock, metal and ice, traveling through space until eventually crashing into a bigger rock that was starting to form around a small yellow star in a distant part of the galaxy that would one day be called the Milky Way. One day, a member of a very bright, curious species of life that would develop on that planet would dig up a piece of that gold and fashion it into a ring to use as an ornament. The ring on your finger takes on a bit of a different value when you consider how the gold from which it was made came to end up on your hand.

On Monday, the scientific community announced that we had witnessed, for the first time, how that gold, and a great many other elements that are essential to and part of our world, were made. And most people never heard the news.

Michael J. Howard, Ph.D., is vice president for education and research at Baptist Health Madisonville. He can be reached by email at madisonvillescience@gmail.com or via Twitter at @madville_sci.



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