Black Holes and Neutron Stars
Star Birth, Death, Rebirth
Stars are part of a complex, balanced cycle of birth, death, and rebirth. A young generation of stars condenses from the gas thrown off by its parent generation. These burn for millions, sometimes billions of years, going through various life stages depending on their composition and mass. In their final stage they disperse most of their matter into space through supernovae (Supernova: Violently exploding stars which shine very brightly for days or weeks. They occur when the fuel for nuclear reactions is used up, and a star cools. Gravity pulls all the matter down toward the star's center. If this happens quickly, nuclear reactions may suddenly begin again, detonating the star in a nuclear explosion. )}), strong winds, or violent pulsations. The next generation of stars forms from this recycled gas.
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. ) Lock Up Matter Forever
But in each successive generation, some stars end as neutron stars, each one as heavy as our solar system (that includes you, Pluto!), but compressed into a sphere the size of a large city. The matter locked in 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. ) does not contribute to the next generation. A neutron star never collapses, it never explodes. It doesn't burn anything, it just gets colder and colder. It is one of the few terminal graveyards for stellar matter.
Well, Not Neutron Stars in Binaries
But in fact, some neutron stars, after sitting dormant for a billion years, go through a spectacular, though brief, resurrection. Neutron stars in close binaries with 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}) eventually merge with their companions, spewing exotic matter back into space and flashing brilliantly for less than a second, before falling into their companion 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. ). To understand this process, the 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.).) of a neutron star and a black hole, let's follow the life of one imaginary representative binary.
Birth, Evolution, Two SNe
Our star system is born when a collapsing cloud of gas, recycled from the previous generation, fragments into two distinct young stars. This is a very common formation scenario. Well over half of the the stars in the night sky compose multiple-star systems. This imaginary binary comprises two very heavy stars: one is 30 times, and the other is 15 times as massive as the Sun. Because they're so heavy, their nuclear-burning cores are compressed more tightly than the Sun's, making them burn hotter, and expending their fuel much faster.
Their orbital separation is similar to the distance between Mercury and the Sun. They orbit this way for 10 million years, until they run out of fuel. The heaviest goes first, its core collapses to a black hole as its outer shell explodes, driving off more than half the star. What's left is a black hole of 10 solar masses, but small enough to fit inside of Luxembourg. Soon (maybe a million years later) its lighter companion explodes in a much more energetic supernova (Supernova: Violently exploding stars which shine very brightly for days or weeks. They occur when the fuel for nuclear reactions is used up, and a star cools. Gravity pulls all the matter down toward the star's center. If this happens quickly, nuclear reactions may suddenly begin again, detonating the star in a nuclear explosion. ), flinging off all but about one solar mass of matter. What remains from this second explosion forms into a neutron star, about the same diameter as the black hole. This second explosion serendipitously pushes the pair of stars closer together, so close that they now orbit several times every day, with a separation about 10 times that of the Earth and Moon. They orbit much faster than the Earth and Moon because their masses are so much greater.
Orbit Evolution, Begin Merger
Next comes a long, uneventful 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.). With masses this large, moving this fast, the curvature of spacetime actually ripples in sympathy. The binary begins to lose orbital energy this way, and the NS gets closer to the BH, at a rate of 5 mm per orbit. It's a long wait until they make contact, much longer than the lifetimes of their progenitor stars. The orbits get faster and faster, though, and more and more energetic. After a billion years they are so close, they could fit onto Africa. They orbit 5 times every second. In another 10 minutes they could easily fit inside Lake Victoria, and they are orbiting more than 100 times per second. They are just about to merge.
Tidal Disruption
At this distance, the neutron star begins to distort, as seen in the image. Because it is now so close to the black hole, its left and right sides get tugged in different directions, and its sides closest and furthest from the hole get tugged with different strengths. These “tidal” forces simultaneously squeeze the star left to right like a tube of toothpaste and stretch it front to back like taffy. The star disrupts, violently, and a portion of its trailing edge shoots out and away from the black hole, as seen in the next image.
Outflow: Kilonova and R-Process
The erupting action is so powerful that some of the matter, maybe 1/10 of the mass of the Sun, flies off into space, as an expanding tail, never to return. It is liberated from what had seemed like permanent captivity in the NS, and becomes part of the next generation of stars. And this matter is unique. Having just come from the neutron star, it is extremely neutron-rich. As it flies off it decompresses and cools, and like a glass of water in the freezer, little crystals begin to condense out of it. These are rare heavy nuclei (Nucleus: The central part of an atom, which contains Neutrons and Protons. Electrons are usually found around the Nucleus. Strictly speaking, this is the only part of an atom involved in Nuclear Reactions (Fission or Fusion). ), with far too many neutrons to be stable. In the same way that Uranium-235 decays through a chain of 10 or so unstable elements to Lead-207, these rare, heavy nuclei decay to more stable nuclei through a cascade of reactions. This neutron-rich cascade produces a unique spectrum of elements. When this matter is “recycled”, it will contribute rare elements that are hard to make in normal stellar evolution. For example, most of the Holmium and Europium, which we find in relative abundance on Earth, were likely made in this manner.
Additionally, the decay from unstable to stable nuclei heats up the tail like a nuclear reactor. About a day after the merger, once the tail has expanded to the distance of Pluto from our Sun, it heats up so much that it begins to shine, like a dim star. This is called a kilonova, and it lasts for about a week before the material cools back down and goes dark.
Accretion Disk: Life and Behavior
But let's return back in time and space to the black hole, right after the star disrupts. What happens to the rest of the matter? Much of it falls into the black hole. But some of it whips around the black hole in a tight swirling orbit, and slams back into itself. This material, much denser than lead, much heavier than the earth, slamming into itself at very nearly the speed (Speed: For a wave, the speed of a particular point (such as its crest).) of light (Speed of Light: A constant of Nature. This speed is precisely 299,792,458 meters per second, or roughly 670,616,629 miles per hour. One of the most unusual discoveries of science has been the fact that all Observers measure light as moving at exactly this speed, even if those observers are moving relative to each other. This fact is one of the basic ingredients in Einstein's 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.
).), heats up due to the incredible shock. It swirls around the black hole and forms a bagel-shaped accretion disk, as seen in the figure below. As it swirls and shocks, the fluid goes from a negligible temperature to almost 100 billion degrees. This temperature increase takes only 10 ms! Over the next second this hot accretion disk swirls many hundreds of times, losing matter to the black hole until the disk is gone.
Accretion & Radiation
The process of accretion is very important for the final stage of the black hole–neutron star merger. As the disk swirls around the black hole it drains into it, like a reservoir of water drains through its dam. And like a hydroelectric dam, the faster the fluid flows, the more energy it generates. In the case of the black hole–accretion disk system, the generated energy is thermal, the fluid gets really hot as it falls into the black hole. This 100 billion degree fluid radiates as if it were a star, only it is much more luminous than a star. And there's one other big difference. The accretion disk is extremely dense, still much more dense than lead. This keeps photons (Photon: An Elementary Particle which carries the energy of light. The photon is a Boson, and has no mass. It always moves at the Speed of Light. ) from escaping. So unlike a star, which radiates photons, visible to the human eye, the accretion disk shines predominantly with neutrinos (Neutrino: A type of particle which has no charge and an extremely small mass. It is a Fermion, and is extremely difficult to stop or to detect. Nonetheless, they are produced in large numbers. The Sun, for example, sends 30 million neutrinos through every square inch of the Earth every single second. They are so hard to stop, however, that if a neutrino were sent through a solid light year of lead, it would still have a 50:50 chance of flying right through without stopping. ), exotic particles that can escape, but which we can't see.
Neutrinos
Neutrinos are extremely lightweight particles, so lightweight they're always kicked at high speed (the speed of light, actually) from the particle interactions in which they're generated. If you slam cannon balls and golf balls together, the golf balls always come out fast; it's the same with neutrinos. They're also weakly interacting. “Weak” here means both “they interact through the weak force (one of the four fundamental forces in the Standard Model)” and “they barely interact at all” (which is partly why the weak force got its name). Neutrinos are so weakly interacting that they can fly through a light year (Light Year: The distance traveled by light in one year. This is roughly 1013 kilometers, or 6 × 1012 miles. )}) of lead and never touch an atom of it. They're formed in overwhelming quantities in hot neutron-rich matter, which is exactly what our accretion disk is composed of! Because they can escape, and photons cannot, neutrinos rather than photons are the important radiating particle in black hole–neutron star mergers.
The End
From the moment our NS disrupts to when the last bit of the accretion disk falls down the black hole, less than a second passes. During this second, the extremely bright neutrino (Neutrino: A type of particle which has no charge and an extremely small mass. It is a Fermion, and is extremely difficult to stop or to detect. Nonetheless, they are produced in large numbers. The Sun, for example, sends 30 million neutrinos through every square inch of the Earth every single second. They are so hard to stop, however, that if a neutrino were sent through a solid light year of lead, it would still have a 50:50 chance of flying right through without stopping. ) emission may be powerful enough to create a gamma ray burst.