Compact Binaries

Pairs of Stars Locked in a Mad, Whirling Dance

Spinning Neutron (Neutron: One of the particles in an atomic nucleus. These particles have no electric charge, but they hold together the protons (positive particles in a nucleus), and account for roughly half of the particles in the nucleus. Neutrons are fermions, and are believed to form the majority of the matter in a neutron star.) Stars (Neutron Star: A type of star which is very old, having cooled off and stopped nuclear fusion reactions. When gravity pulls the star down on itself, the electrons and protons are squeezed together, leaving just neutrons. The star is then supported against gravity by "neutron degeneracy pressure" (no two neutrons can be in the same place at the same time). These are produced when a star is too heavy to be a white dwarf (White Dwarf: A type of star which is very old, having cooled off and stopped nuclear fusion reactions. A white dwarf is supported by "electron degeneracy pressure" (no two electrons can be in the same place at the same time). These are produced when a star is not heavy enough to turn into a Neutron Star or a Black Hole. )})|white dwarves}), but not heavy enough to turn into a Black Hole. ) and rippling Black Holes (Black Hole: A region of spacetime (Spacetime: A concept in physics which merges our usual notion of space with our usual notion of time.) where the warpage of both space and time (gravity) is so intense that nothing — even light — can ever escape. Objects may fall in to the Black Hole, but once they pass the Event Horizon (Event Horizon: A surface — like the one surrounding a Black Hole — enclosing a region of space from which nothing (even light) can ever escape.), they can never escape again. Most Black Holes believed to exist are thought to be formed in the collapse of very large stars, or the collision of stars or other Black Holes. )})|blackholes}) will stir up spacetime and give off gravitational waves (Gravitational Wave: A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose. ) very effectively. The more stirring, the better, though. The bigger the lump is, and the faster it moves, the stronger the waves given off will be. There is another, much simpler and more effective way of producing a big "lump" moving quickly.

Everywhere in space, astrophysicists expect to find all types of compact binary (Compact Binary: A specific type of Binary system in which both members are compact (meaning they are White Dwarfs, Neutron Stars, or Black Holes) and have roughly equal mass. )}) — binaries being pairs of stars; compact meaning that the stars are White Dwarfs, Neutron Stars, or Black Holes. Pairs of compact stars orbiting each other closely in this way will be like two enormous lumps moving very quickly. They can be in any combination: a Neutron Star (Neutron Star: A type of star which is very old, having cooled off and stopped nuclear fusion reactions. When gravity pulls the star down on itself, the electrons and protons are squeezed together, leaving just neutrons. The star is then supported against gravity by "neutron degeneracy pressure" (no two neutrons can be in the same place at the same time). These are produced when a star is too heavy to be a white dwarf, but not heavy enough to turn into a Black Hole. ) with a Black Hole (Black Hole: A region of spacetime where the warpage of both space and time (gravity) is so intense that nothing — even light — can ever escape. Objects may fall in to the Black Hole, but once they pass the Event Horizon, they can never escape again. Most Black Holes believed to exist are thought to be formed in the collapse of very large stars, or the collision of stars or other Black Holes. ); a Black Hole with another Black Hole; a White Dwarf with a Neutron Star; etc. Because these stars are compact, and have very intense gravity, they can stir up spacetime very well.

If the Earth were to come too close to a compact star, the same forces that give us tides on the ocean would become so strong that the Earth would be torn to pieces. The same is true—even more so—for other stars like our Sun. They are simply too soft to get very close to a compact star and stay together. Instead, all of their matter would be smeared out around the compact star. Gravitational waves would still be given off, but they would also be "smeared out," and hard to observe.

Thus, the most effective generators of gravitational waves are pairs of compact stars, which can't be ripped apart very easily. Instead, they will be able to come very close to each other and stay intact. To resist the intense pull of gravity, they will need to orbit very quickly so that centrifugal force can hold them apart. In fact, a pair of neutron stars — heavier than our Sun — would be expected to orbit each other dozens or even hundreds of times per second at their closest point! This sort of extreme stirring of spacetime would certainly produce very intense gravitational waves.

A compact binary would form as a result of the chance encounter of two compact stars. They might be moving in different directions, and happen to pass very close, causing them to give off gravitational waves and — along with the waves — energy. After losing this energy, each star would be unable to escape the other's gravity, and they would start to orbit each other.

The three phases of a binary black hole's merger, as seen in gravitational waves. The long inspiral begins very far in the past, growing more pronounced as it approaches merger. During merger, there is a rich variety of effects which will give us greater insight into General Relativity, and the objects we observe using gravitational waves. The ringdown is a very quick reduction in the size of the waves as the system settles down — it sounds much like a drum being hit once.As the stars circle each other, they continue to give off gravitational waves, gradually coming closer and closer, moving faster and faster. This part of the encounter is called the inspiral (Inspiral: The gradually-shrinking orbit of a binary system. As the pair of stars in the binary orbit each other, they give off energy in the form of gravitational waves. This lost energy draws them closer in their orbit — eventually resulting in a Merger (Merger: The portion of the Inspiral of a binary system in which the individual objects are highly distorted, and their orbit is changing rapidly. This portion is not well-understood, and must be simulated using Numerical Relativity (Numerical Relativity: The branch of Relativity research which deals with simulating the development of Spacetime, using computers. This is believed to be the only possible way to understand things like the merger of two Black Holes.).).). The amplitude (Amplitude: The height of the peak of a wave, measured relative to its center. Equivalently, the depth of the trough of a wave.) and frequency (Frequency: The number of occurrences of something in a given period (Period: The length of time between two events. For a wave, this is usually the length of time it takes two successive peaks to pass a given point. This number is simply 1 divided by the Frequency of the wave.) of time. For a wave, this is might be the number of times the wave peaks in one second. )}) of the gravitational waves will increase (the waves will become larger, and more frequent), until the stars are nearly touching each other. At this point, a messy merger of the two objects will take place. Finally, the combined object will go on rotating, giving off quickly-dying gravitational waves in the ringdown (Ringdown: The portion of an Inspiral, following the Merger, when the two objects have combined into one. During this brief period, the combined object will settle down by giving off gravitational waves.) portion.

Physicists know that these pairs of stars exist. In fact, one such binary has even been used to demonstrate the existence of gravitational waves. In 1993, Russell Hulse and Joseph Taylor won the Nobel Prize in Physics for this demonstration. They used observations of a binary consisting of a Pulsar (Pulsar: A neutron star with a very high rate of spin (Spin: An intrinsic property of particles. (That is, a property which does not change. Mass and electric charge are examples of intrinsic properties.) Spin is related to the usual notion of spin, though it is a little more difficult to understand. Spin comes in units of 1/2, so that a particle may have a spin of 0, 1/2, 1, 3/2, and so on. A particle's spin determines whether it is a Fermion or a Boson.), and very intense magnetic fields. The pulsar gives off beams of radiation along its magnetic poles. If these poles are not aligned with the spin poles, the beam will sweep around like the beam of a lighthouse. )}), with some dense companion star. The pulsar's precisely-timed flashes came slightly earlier or later than they would otherwise, depending on which way the pulsar was moving in its dance with the other star. The timing of this variation gives a direct measurement of the timing of the orbit. Hulse and Taylor showed that the orbit took less and less time with each pass — in exactly the way predicted by General Relativity (General Theory of Relativity: Einstein's version of the laws of physics, when there is gravity. Building on the Special Theory of Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity becomes very nearly the same as Newton's theory, which is much easier to use. ) becomes very nearly the same as Newton's theory, which is much easier to use. ), this theory generalizes Einstein's work so that the laws of physics must be the same for all observers (Observer: A person or piece of equipment that measures something in physics. Frequently, we speak of an observer measuring time or a distance in a particular place. ), even in gravity. Einstein showed that gravity is best understood as a warping of the geometry of spacetime, rather than as a pulling of objects on each other. The crucial idea is that objects move along geodesics — which are determined by the warping of spacetime — while spacetime is warped by massive objects according to the formula \(G = 8 π T\). ) — as the binary gave off energy in the form of gravitational waves in the slow Inspiral portion of the stars' encounter.