Neutron Stars: The Most Extreme Objects

TITLE: Neutron Stars: the most extreme objects A teaspoon of it would weigh about 10 million tons. Its gravitational field is intense. It is a collapsed star so dense that electrons and protons do not exist separately, but are fuzed to form neutrons. It acts like an enormous magnet. Can you guess which are we talking about? Of course, a neutron star! Today we are going to explore the nature of these objects, their birth, and their most prominent and relevant characteristics. Follow me! The term neutron star as generally used today refers to a star with a mass M on the order of 1.5 solar masses and a radius of about 12 km, with a density of about 5 to 10 times the nuclear equilibrium density of neutrons and protons found in laboratory nuclei, which makes it one of the densest forms of matter in the observable universe. But how did we discover them? It was 1967. Jocelyn Bell, the name of a famous scientist that passed to history, was at that time a research student at Cambridge University. She was studying distant radio sources with a special detector that had been designed and built by her adviser Antony Hewish to find rapid variations in radio signals. Her job was simple, yet fascinating: she had to go through all the reams of paper showing where the telescope had surveyed the sky, in order to search for interesting phenomena. In September 1967, Bell discovered what she called “a bit of scruff”—a strange radio signal unlike anything seen before. In which sense it was classified as strange? Because the source, in the constellation of Vulpecula, which was rapid, sharp and intense, had an extremely regular pulse of radio radiation. It seems more precise than a clock, and it pulsed every 1.33728 seconds. Now, this is an uncommon thing to find in our universe. The only pulsating radio sources with such a regular pulse could perhaps be built by humans (or at least another extraterrestrial form of life), but we had no clue about the existence of such precise signals, such precise clocks within the chaos of the universe. Spoiler: it was not an alien civilization! However, radio astronomers, jokingly, dubbed the source “LGM”, which stands for Little green men. Soon, three similar sources were discovered in separated directions in the sky. This was clearly the start of something new, this was surely an important discovery that had to be further explored. By today, more than 2500 such sources have been discovered; they are now called pulsars, short for “pulsating radio sources.” Astronomers wanted to know the formation process behind the birth of a pulsar. When they found that at the center of the Crab Nebula there was a pulsar emitting sharp bursts that occurred 30 times each second, they started to think that maybe they could’ve been connected to the death of stars. In fact, the pulsar was just in the region of the supernova remnant. They could’ve been elusive corpses of massive stars! Another thing to explain was: how made up a pulsar? With the help of observation, astronomers eventually created a model of the pulsar and concluded that they must’ve been spinning neutron stars. One has to imagine a pulsar as a lighthouse on the sea. In order to warn ships in all directions, the light sweeps its beam across the dark sea. If you are a mariner, you see a pulse of light every time the beam points at you, in your direction. This was exactly what pulsars did. Radiation from a very small region of the neutron star gives us a pulse of radiation each time the beam points toward Earth. And this happens very regularly, and in a very short period of time. The only way a neutron star can do that job is by being a very small object, so small that it can turn very rapidly. In fact, we know pretty well from the conservation of angular momentum, that if an object gets smaller, it can spin more rapidly. Now, considering the short period of a neutron star’s pulse, astronomers concluded that such an object should’ve had a diameter of only 10 to 20 km. If you think that pulsars are star remnants, one thing becomes clear: all the huge volume the star owned (the radius of the sun is, for example, about 696.340 km), had to be squeezed in such a small region of space, making it an extraordinarily dense object. Also, any magnetic field that existed in the original star will be highly compressed when the core collapses to a neutron star. Protons and electrons are trapped in this spinning field and eventually they can escape from it, but this turns out to happen only if they enter a cone of light. There is just a specific path for them to escape the strong magnetic field of the pulsar, and it is the one through this light cone, that in the end reaches us and does the “lighthouse” thing. The cone axis however is defined by the star’s rotation. SO, IN SHORT TERMS: The radiation itself is confined to a narrow beam, which explains why the pulsar acts like a lighthouse. As the rotation carries the first one and then the other magnetic pole of the star into our view, we see a pulse of radiation each time. This explanation of pulsars in terms of beams of radiation from highly magnetic and rapidly spinning neutron stars is a very clever idea. But what evidence do we have that it is the correct model? This is a question that astronomers asked themselves. One way to test the model is, for example, to try to measure the mass of a pulsar, which was done and gave us the following result: a pulsar is about 1.4 to 1.8 solar masses. Surprisingly (or not), this was just what was predicted by the theory. But we can have another (maybe even better) confirmation of that, considering the Energy conservation. When the high-energy charged particles from the neutron star pulsar hit the slower-moving material from the supernova, they energize this material and cause it to “glow” at many different wavelengths—just what we observe from the Crab Nebula. The pulsar beams are a power source that “light up” the nebula long after the initial explosion of the star that made it. Who “pays the bills” for all the energy we see coming out of a remnant like the Crab Nebula? After all, when energy emerges from one place, it must be depleted in another. The ultimate energy source in our model is the rotation of the neutron star, which propels charged particles outward and spins its magnetic field at enormous speeds. As its rotational energy is used to excite the Crab Nebula year after year, the pulsar inside the nebula slows down. As it slows, the pulses come a little less often; more time elapses before the slower neutron star brings its beam back around. Several observations around the years have shown that the Crab Nebula pulse is not as perfect and regular as we originally thought. In fact, it is gradually slowing down. And maybe this is even more fascinating, because, despite the fact that pulses are not really constant, it slows down with the regularity of a swiss clock! So, based on our model, in theory, if we can measure the slowing down of a pulsar (and we need sophisticated instruments in order to do that), we can calculate how much rotation energy the neutron star is losing. To the satisfaction of astronomers, the rotational energy lost by the pulsar turns out to be the same as the amount of energy emerging from the nebula surrounding it. In other words, the slowing down of a rotating neutron star can explain precisely why the Crab Nebula is glowing with the amount of energy we observe. It’s all beautiful. But we are not done yet! Why? Because now we need to know how many pulsars are born, for example, in a year! That is to say, we wish to know the pulsar formation rate. From observations of pulsars discovered so far, astronomers have concluded that the star formation rate is 1/25 ore 1/100 years, that is to say, a new pulsar is born somewhere in the galaxy every 25 to 100 years (that is also the same rate at which supernovae are estimated to occur). But we can’t see as many pulsars as we predict. Why is that? Calculation suggests that the typical lifetime of a pulsar is about 10 million years; after that, the neutron star no longer rotates fast enough to produce significant beams of particles and energy and is no longer observable. So, the lighthouse is no more in function, but this doesn’t mean it isn’t there! If one plots the graph of energy of a pulsar vs its age, one soon discovers that there is a region of the plot in which we can’t see any pulsar. This zone is called “pulsar’s graveyard”. We estimate that there are about 100 million neutron stars in our Galaxy, most of them rotating too slowly to come to our notice. So, resuming, a young pulsar has a (pretty) regular period that will slow down, and its period is short, its beam is sharp. An older pulsar, instead, has a longer period and the beam of light loses its initial energy, and eventually, we know it will end in the “pulsar’s graveyard”, where the beam is too faint to be observed. Another reason why the number population prediction of pulsar doesn’t match the observed number lies in the fact that, for their nature, pulsars behave as a lighthouse. In fact, let’s consider our lighthouse model again. On Earth, all ships approach on the same plane—the surface of the ocean—so the lighthouse can be built to sweep its beam over that surface. But in space, objects can be anywhere in three dimensions. As a given pulsar’s beam sweeps over a circle in space, there is absolutely no guarantee that this circle will include the direction of Earth. In fact, if you think about it, many more circles in space will not include Earth than will include it. Thus, we estimate that we are unable to observe a large number of neutron stars because their pulsar beams miss us entirely. We would like to end this video by recalling a beautiful thing that happens when two neutron stars merge. A neutron star merger is a type of stellar collision. It occurs in a fashion similar to the rare brand of type Ia supernovae resulting from merging white dwarfs. When two neutron stars orbit each other closely, they spiral inward as time passes due to gravitational radiation. When the two neutron stars meet, their merger leads to the formation of either a more massive neutron star, or a black hole (depending on the mass of the remnant). The first such observation, which took place in August of 2017, made history for being the first time that both gravitational waves and light were detected from the same cosmic event. All of this was made possible thanks to LIGO and VIRGO, the famous network of interferometers that observe the deep sky in order to detect gravitational waves. Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. They were predicted by Einstein’s general relativity, and now we know they are real. Isn’t science beautiful? “This video ends here! Thanks for watching everyone! What do you think about pulsars? Let us know in the comment below! See you next time on the channel!”

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