Subject: Can someone please explain?
Posted by: Mixamatosis
Date: Jan 21 17
I've read that it's dangerous to mix ammonia and bleach. Variously I've read that it can produce deadly cyanide gas, chlorine gas (which is said to be bad for you) and even explosions.
However swimming pools are kept fit for use with chlorine, and our urine contains ammonia but then we may clean toilets with bleach. Also many cleaning products contain either ammonia or bleach and it would be easy to use them unthinkingly in combination.
How is it that people aren't generally harmed by these dangers when swimming in swimming pools or doing daily cleaning, or are we being harmed at low level and is the harm cumulative?
Posted by: Mixamatosis
Date: Jan 21 17
I've read that it's dangerous to mix ammonia and bleach. Variously I've read that it can produce deadly cyanide gas, chlorine gas (which is said to be bad for you) and even explosions.
However swimming pools are kept fit for use with chlorine, and our urine contains ammonia but then we may clean toilets with bleach. Also many cleaning products contain either ammonia or bleach and it would be easy to use them unthinkingly in combination.
How is it that people aren't generally harmed by these dangers when swimming in swimming pools or doing daily cleaning, or are we being harmed at low level and is the harm cumulative?
526 replies. On page 6 of 27 pages. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
brm50diboll 
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We don't know what it is we need to know. The difference between applied research and basic research in science is central to that very point. In applied research, scientists are investigating a specific problem with a specific goal in mind, such as, say, curing Stage IV non-small cell lung cancer. But history teaches us that many problems cannot be solved until after there has been a breakthrough in scientific paradigms and methods in general, and that comes through basic research. Basic research is conducted not to solve specific problems, but simply out of a "desire to know", regardless of whether such knowledge has any practical applications or not. It often turns out that such breakthroughs lead to practical applications that were totally unanticipated at the time the original basic research was conducted, but, without which, the applications could never have been achieved. For this reason (among others), a certain amount of basic research is necessary for scientific advancement even though no obvious applications are on the horizon. Now I deliberately leave what I mean by "a certain amount" vague because there is no hard-and-fast rule for answering that, but it is true nevertheless. Example: modern GPS satellites could not function without taking Einstein's theory of relativity into account. If they relied solely on classical Newtonian physics, they would be so inaccurate as to be useless. But obviously, Einstein did not develop his theory of relativity because he envisioned GPS satellites some day. He did it simply because he "wanted to know". There are many basic research discoveries which, at the present time, may not have any economically practical applications. But we cannot say those discoveries are not important because ten years, twenty years fifty years, or hundreds of years from now, they may become the very foundation for some technology not even imagined now. So, to finally get to my point: What we don't know we don't know may actually be quite important. Reply #101. May 31 17, 4:31 PM |
| brm50diboll
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My answer above is in reply to Mix's point on the previous page. Reply #102. May 31 17, 4:33 PM |
| brm50diboll
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There are two factors underlying the luminosity of a star: temperature and surface area. The higher the temperature, the more luminous the star. Simple enough, and, in fact, around 90% of the time, this one factor is enough to explain the luminosity of the star. But what about the exceptions? Consider, once again, the famous star Betelgeuse. Betelgeuse is spectral class M, a cool red star. Since it is cool, its luminosity should be low. But it is extremely luminous, despite being cool. That is because of the second factor: surface area. Betelgeuse has an enormous surface area, because it is a supergiant star. Why? I'll have to get to that later. On the opposite end, consider Sirius B, the companion star to the better-known Sirius A. Sirius B is extremely hot, bluish-white in color. But it also extremely dim for such a hot star. It can only be seen with some of the largest telescopes in observatories (some of the problem is due to the glare from Sirius A.) So Sirius B has an extremely small surface area for a star. In fact, despite being a star more massive than our Sun (which we can calculate through observing the revolutions of the two Sirius stars about their common center of mass and applying Newton's Laws of Motion), it is only a little larger than Earth in terms of its volume and surface area, so it must be extremely dense, denser than any material on Earth (such as osmium, the densest element), by far. Sirius B is called a white dwarf. Very hot, but very small and dim. Luminosity classes have been developed just as spectral classes have. Roman numerals are used for these: I: supergiant, II: bright giant, III: giant, IV: subgiant, and V: main sequence. 90% of stars are predictable luminosity class V main sequence stars, including our Sun. Next time, I will put the spectral classification (x-axis), and luminosity classification (y-axis) together to generate the full HR Diagram and explain what it tells us and why. Reply #103. Jun 04 17, 4:10 PM |
13LuckyLady
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This thread is like reading a really good book. More please? Reply #104. Jun 06 17, 2:26 PM |
| brm50diboll
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Thank you. Honestly, I would contribute to it every day, but "real life" gets in the way. I'm not quite ready for my next installment, but, as a person who taught science for years, I feel that teaching science, yes, does require some memorization of facts, but the emphasis should be on answering questions like how and why. Why do we know Betelgeuse is a red supergiant? Why should I believe scientists? Those are good questions and deserving of good answers. I'm not writing this because I want to "lord it over" anyone. I'm writing this because I want to explain the difficult, but still explainable, because some people want to know, not because they're trying to become astrophysicists. I'm not, myself, by the way. One thing that I believe is true about all threads, including this one: there are far more readers of threads than contributors. Nothing wrong with being a silent reader of threads. Much of what I write is directed towards those. I am a silent reader of many FT board threads myself I would never contribute to, but find them fascinating nevertheless. Reply #105. Jun 06 17, 9:09 PM |
| 13LuckyLady
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Yep, real life can be a stinker! If you posted every day, I suspect you would tire of sharing fascinating facts. I, however, will never tire of reading them! Back tomorrow for more fun and adventure! Reply #106. Jun 06 17, 9:28 PM |
| 13LuckyLady
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Brian, please explain lake effect snow. (have I mentioned this thread is what I read during breakfast...get those grey/gray cells charged early, I say!) :) Reply #107. Jun 08 17, 8:30 AM |
| brm50diboll
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Water temperatures are generally higher than air temperatures during the winter because of water's high heat capacity. Warmer air holds more water vapor than cooler air. Wind blowing over a large lake in the winter (such as the Great Lakes) warms as it passes over the water and picks up water vapor. Then, when that warmer, moister air reaches the other side of the lake (typically the eastern side, since prevailing winds generally blow from west to east), the air cools again over the land and can no longer hold onto the water vapor it picked up over the lake. If the temperature is below freezing, then the excess water vapor precipitates as snow. So places on the east side of large lakes (such as Buffalo, NY), tend to get a lot of this lake-effect snow in the winter. Reply #108. Jun 08 17, 9:46 AM |
| brm50diboll
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Before I formally introduce the HR Diagram and explain what it tells us, I need to discuss the process that powers the stars - nuclear fusion. Fusion provides an enormous amount of energy by taking advantage of Einstein's famous equation E=mc^2. This is because when small nuclei fuse together to form larger ones, the larger nuclei have slightly less mass than the sum of the masses of the original smaller nuclei, and that "missing mass" was converted into energy. The big problem with fusion and why it isn't used as a practical energy source on Earth is the temperatures required for it to work - in the millions of degrees. Because of these enormously high temperatures required, nuclear fusion is sometimes called thermonuclear fusion. Why? Atomic nuclei are composed of positively charged protons and neutral neutrons and therefore are positively charged. So nuclei repel each other as positive charges repel other positive charges. Then how can fusion take place at all? In fact, how do any nuclei with more than one proton even exist at all? At extremely close distances, the electric repulsion of protons against other protons can and in fact is overcome by a different fundamental force called the *strong nuclear force*, which holds the nuclei together despite proton-proton electric repulsion. But the strong nuclear force only acts over extremely short distances. Two nuclei (both positively charged) will resist being brought together by the electric force, and that force can only be overcome if they get extremely close together so that the strong nuclear force can kick in. But for two nuclei that repel each other to be brought together so close, they must be moving extremely fast. Now, all particles move, and the speed of their movement reflects their temperature. In fact, in physics, the average velocities of particles can be determined using a quantity called kinetic temperature. In order for nuclei to be brought close enough together for the strong nuclear force to kick in and fuse them together, they must be travelling extremely fast, which requires kinetic temperatures in the millions of degrees. At those very high temperatures, all materials are *plasmas*. Plasmas are similar to gases, but the electrons have been stripped from them, so they are charged. Depending on what nuclei a physicist is trying to fuse, the temperatures required varies some and have been determined here on Earth in cyclotron experiments. The lowest temperatures required for any kind of fusion is still about 10 million kelvin, and is for deuterium-deuterium and deuterium-deuterium fusion. A brief digression is required. There are three isotopes of hydrogen: H-1 or protium, which makes up well over 99% of all the hydrogen in the universe, H-2 or deuterium, which is stable and exists naturally, but is very difficult and expensive to separate from the much more common protium, and H-3 or tritium, which is radioactive and does not exist naturally, but can be manufactured in cyclotrons. The lowest temperature fusion is used in hydrogen bombs here on Earth developed by the physicist Edward Teller in the 1950s. In order to create the high temperatures needed to detonate an H-bomb, a fission explosion with uranium or plutonium must be set off first. Although nuclear fission can be controlled to produce nuclear power in plants across the world (leaving aside all the contentious political and environmental issues they bring with them), at present, nuclear fusion cannot be controlled to produce power because the extremely high temperatures involved tend to vaporize whatever containers they are in, although for many years there has been research into controlling fusion involving experimental reactors called tokomaks. These are not at present an economically feasible source of energy, but research continues. Now, the temperatures required to fuse protium with protium, which is the main type of fusion which occurs in most stars, is even higher than in deuterium fusion (because some of the protons must be converted to neutrons and positrons in the reaction) and cannot be done on a mass scale (more than a few nuclei at a time) here on Earth, but the exact kinetic temperatures required are well understood from laboratory experiments here on Earth. And there are higher types of fusion, involving elements heavier than hydrogen, which can be carried out in cyclotrons here on Earth a few nuclei at a time. In general, the more protons present in the nuclei to be fused, the higher the kinetic temperatures needed to fuse them. So helium fusion into carbon requires temperatures almost ten times as high as protium-protium fusion, and fusing carbon into magnesium even higher temperatures, and so on up to iron, which is the end of the fusion process, because fusion which produces nuclei heavier than iron *consumes* rather than releases energy. All the energies and temperatures required for the various fusion reactions have been tested extensively in cyclotrons on Earth and are well documented and have been for over 50 years. How all this business about fusion ties into the HR Diagram will be discussed in my next installment. Reply #109. Jun 10 17, 8:25 PM |
| brm50diboll
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Correction: the second type of fusion in H-bomb is called deuterium-tritium fusion. I accidentally left out the tritium and repeated the deuterium. Reply #110. Jun 10 17, 8:28 PM |
| brm50diboll
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A little experiment to pass some time by: https://www.youtube.com/watch?v=aBQalkleE7s Reply #111. Jun 11 17, 4:01 PM |
| brm50diboll
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Apparently doesn't work. Too bad. Reply #112. Jun 11 17, 4:02 PM |
| brm50diboll
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We have accumulated data on star temperatures and absolute magnitudes (which correlates with luminosity) on many thousands of stars. If we plot the star temperatures on the x-axis (somewhat backwards, actually, listing the highest temperature blue O stars on the left, with temperatures decreasing as we go to the right, with the low temperature red M stars on the right), and plot absolute magnitude on the y-axis (also somewhat backwards, with the most negative absolute magnitudes (most luminous) at the top with increasing (less luminous) values further down), then we have the HR Diagram. We see very distinct patterns in the full HR Diagram, not just random dots everywhere. About 90% of all stars plotted on the HR Diagram fall onto a curve that slinks diagonally across the diagram from the upper left (hot, bright O stars) down to the lower right (cooler, dim M stars). This diagonal curve is called the Main Sequence and it represents stars which in their cores are fusing hydrogen (H-1, or protium), into helium. We know this because this is the most common type of fusion and (except for the extremely rare deuterium fusion seen in certain "brown dwarfs") is the lowest-temperature type of fusion. But some stars are not on the Main Sequence. There are three other very distinct "bunches" that appear on the HR Diagram. One is in the lower left, stars which are hot but dim, implying very small size. These stars are the white dwarfs, stars that do not undergo fusion at all but are essentially the very slowly cooling cores of stars that have run out of fuel. It takes billions of years for white dwarfs to cool, because they are so small and dense. Sirius B is a well-known white dwarf. In the right a little above the Main Sequence are the Giant stars (usually red, though yellow and orange ones exist.) Giant stars are larger than Main Sequence because hydrogen fusion is sputtering towards an end and helium fusion has (or is nearly) begun, with much higher core temperatures which have caused the outer layers of the star to swell. Paradoxically, although the core temperatures of a Giant star are much higher than the Main Sequence, the surface temperatures are lower because the outer layers of the star have moved so far away from the core. Finally, in the upper right corner of the HR Diagram are a few stars that are very cool but very luminous. These are the Supergiants, like Betelgeuse. Supergiant stars have extremely hot, unstable cores that are carrying out high orders of fusion, like fusing carbon into magnesium, which only occurs at temperatures of hundreds of millions of kelvins. This kind of fusion cannot last for very long in stars (a few hundred thousand years, which is extremely short in terms of star lives) because the temperatures are so high and the supply of heavy elements in stars is very limited compared to hydrogen and helium. Supergiant stars are near the end of their lives (and only a small fraction of stars can even become Supergiants - the Sun, for example, is too small to ever become a Supergiant and will "max out" as a Giant star.) Supergiant stars eventually explode spectacularly as supernovas, but that is a story for a different day. Reply #113. Jun 13 17, 2:50 PM |
| brm50diboll
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One of the difficulties I think people have with astronomy is understanding where all the calculated times come from and what the justification for these times are. For example, it is frequently stated Earth is 4.5 billion years old. How did astronomers get that number? There obviously weren't any astronomers around 4.5 billion years ago to witness or document that. And when astronomers predict the Sun will become a red giant in 5 billion years, where does that number come from? It is difficult for many people to conceive of such long timescales, given that a human lifespan is negligible by comparison. But one does not have to live the full length of a long cycle to accurately determine its length. Imagine an alien visitor to a large city who only stayed on Earth one day. In that one day, they would not come anywhere close to seeing an entire human lifespan, yet they could reasonably infer its existence, even without questioning humans. In their one day, they would see a variety of humans of different ages and would quickly conclude these were all of the same species but of different ages, despite not being around long enough to actually witness the aging process. At a hospital, they might see a few births and a few deaths, and careful listening to human's own conversations and they would very quickly accurately determine the nature of the aging process in humans. A similar sort of logical inference process works in allowing us short-lived humans to assess the much longer timescales in astronomy. We cannot see the stellar life cycles in a single human lifespan, or even in all of recorded human history, but we see millions of stars at different points in their life cycles and, with a little use of physics and logical inference, can accurately describe the life cycles of stars despite the immense timescales involved. Radioactivity has very precise mathematical laws governing its workings, laws which are unaffected by such things as temperature. The half-life of uranium-238, for example, is 4.55 billion years, despite its temperature, exposure to light, and many other things. How do we know? It's not like anyone was around 4.55 billion years to see half the U-238 decay. But you don't need that. One second will do, actually. We know what fraction one second is of 4.55 billion years, and that number is very small, but not zero and can easily be calculated. But the number of U-238 atoms in a gram is also easily calculated, and is very large. Through a simple equation from kinetics, we can calculate exactly how many atoms of U-238 should decay (releasing alpha rays) each second by combining the very small fraction above with the very large number of atoms above in the equation. The result is several thousand decays per second, which is easily confirmable by laboratory experiment. We don't have to wait millions of years to calculate the half-lives of long-lived isotopes. All we have to do is accurately measure out a fixed amount of the isotope we're interested in and carefully determine how much decays each second, do a few calculations, and the half-lives are accurately determined, since radioactive decay follows strict physical laws which do not vary over time. Since we know the stable isotopes that radioactive isotopes eventually decay into, by looking at the ratios of isotopes present in a given sample of material, we can quite accurately determine when that material formed from whatever process (such as volcanism) formed it. Now it is true we don't have physical samples of stars as we do with rocks on Earth, but spectroscopy allows us to know the elements and their ratios present in the stars, and the physical laws that work on Earth work the same way in distant stars, so we know how rapidly fusion can take place in laboratory measurements on Earth, and we can extrapolate that to determine how rapidly it takes place in stars. We can calculate, for example, how long it would take to fuse 2.0 × 10^30 kg of hydrogen into helium at the core temperatures which would exist at the pressures in the center of a star of that mass, and, such calculations show it would take billions of years. We know exactly how much fusion is occurring each second in the Sun, for example, by measuring neutrinos created by that fusion here on Earth, although neutrino measurements and neutrino oscillations are extremely complicated and I would have to digress enormously here to even begin to justify where those numbers come from. Reply #114. Jun 19 17, 1:01 AM |
| brm50diboll
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How stars die (and when) is a function of original stellar mass. Why? All stellar evolution is a consequence of a war between two relatively simple opposing forces: gravity, which tends to cause the gas to fall inwards towards the center, and energy production, which exerts an outward pressure on the gas. For stars on the Main Sequence, (anywhere, from large hot blue O stars down to small cool red M dwarfs), these opposing forces are in balance for long periods of time, but eventually, the balance is lost and the star moves off the Main Sequence to a fate determined by the temperature and pressure in its core, which is determined by its mass. The core temperature isn't actually determined by the type of nuclear fusion present; in fact, it works the other way around. That is, the type of nuclear fusion present is determined by the core temperature, which is generated by the pressure of the gas first, even before the fusion begins. In a star like the Sun, gravity first has the upper hand during the early protostar stage before fusion begins, so the gas contracts and pressure and temperature at the center build up until the threshold temperature in the core for hydrogen fusion is reached, at which point a balance is achieved and the star enters the Main Sequence with a core temperature in the hydrogen fusion range, and the core neither contracts further nor expands. Celestial bodies a little too small to achieve core temperatures necessary to initiate hydrogen fusion are "failed stars" known as "brown dwarfs", typically about the radius of our planet Jupiter but with masses between 13 and 80 times Jupiter's mass. The largest group of brown dwarfs are able to briefly initiate deuterium fusion, but deuterium is so rare that it doesn't last long, so, in any case, brown dwarfs slowly cool over billions of years without ever having entered the Main Sequence. It is presumed that the red dwarfs will eventually leave the Main Sequence and fail to achieve helium fusion and will end up as "helium white dwarfs", but because red dwarfs burn through their hydrogen supply so slowly, they have lifespans of trillions of years, so none of them have actually left the Main Sequence yet. The smallest stars that have left the Main Sequence are a little smaller than the Sun is, and, when they were on the Main Sequence, they were K class orange dwarfs. But these stars, as well as the sunlike G yellow dwarfs and F yellow-white dwarfs, are all large enough that, when they left the Main Sequence, core temperatures became hot enough to initiate helium fusion. Why? Because when the supply of hydrogen at the center of their cores began to exhaust, heavier helium accumulated there and gravity once again got the upper hand, causing the core to contract and core temperatures to rise. But the rise in core temperatures paradoxically caused the outer layers of the star to move outward and cool, turning it into a red giant. Eventually, the core temperature got high enough for helium fusion into carbon to begin. The initiation of helium fusion is called the helium flash, and, curiously enough, does not result in an immediate change in the outward appearance of the red giant star, although over time, the giant star first shrinks as the new heat from helium fusion causes the core to separate into two separate layers, or shells: an inner helium-burning shell at the very center of the core, surrounded by a hydrogen-burning outer shell. But this arrangement is not completely stable, so the core and the whole star pulsates in temperature, size, and brightness, becoming a variable star. Eventually the helium at the center of the core is exhausted, and carbon accumulates, and gravity once again gets the upper hand, causing the core to start contracting again and the temperatures to rise, but this time, the process stops because the temperature never gets high enough at the center for carbon fusion to begin. Instead, a phenomenon known as electron degeneracy pressure stops the core from contracting any further. The balance is now completely lost. The outer layers of the star drift away from the exposed carbon core, briefly fluorescing under UV light from the core for a few thousand years as a planetary nebula, such as the famed Ring Nebula in Lyra. But eventually those outer layers move so far away they stop fluorescing, and the exposed carbon core is now a white dwarf, like Sirius B. White dwarfs take billions of years to cool, as fusion has now stopped. It is presumed they will eventually become "black dwarfs", but none have cooled enough for that to happen yet. This pattern: Main Sequence to red giant to planetary nebula to white dwarf is the eventual future of our Sun. We know that because many stars with the same mass as the Sun but which happened to form earlier than the Sun did have already progressed through these various stages. We can calculate stars' masses by using Newton's Gravitational Laws in binary star systems which are quite common. For the stars which were originally on the upper end of the Main Sequence, the O, B, and A stars, a somewhat different fate awaits them. These larger stars burn through their hydrogen very quickly, so most of them have long since left the Main Sequence and we have seen what happens to them: they become Supergiants and eventually go supernova. I'll discuss why their fates are different from the sunlike stars another time. Reply #115. Jun 25 17, 12:11 AM |
| brm50diboll
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In my last missive, I mentioned "electron degeneracy pressure". I should try to explain that and a few other things before moving onto how big stars die. Besides energy production (like fusion), what else opposes the inward contracting force of gravity. After all, Earth is stable and not contracting, so *something* must be opposing the inward force of gravity in Earth, and it certainly isn't fusion. Leaving aside energy production, there are four "categories" I want to describe that oppose the inward contracting force of gravity: 1) Ordinary atomic pressure. This is by far the most common opposing force, and also the weakest. Atoms can be squeezed together under pressure, but only up to a point. That is because atoms consist of a tiny positive (but massive) nucleus surrounded by "shells" of electrons. When two atoms are squeezed close to each other, the outer electrons of one from will get close to the outer electrons of the second atom and repel. This is ordinary atomic pressure, and it is what keeps most ordinary matter, including meteoroids, comets, asteroids, dwarf planets, planets, and even brown dwarfs from contracting any further. But if the pressure is high enough, ordinary atomic pressure fails, leading to: 2) Electron degeneracy pressure. This is what is active in white dwarfs. There are several kinds, actually, but carbon white dwarfs are the most common. In them, atoms are broken down by the intense pressure and what you have is a "plasma sea" of electrons and nuclei moving around at very high speeds. In white dwarfs, they compress much further and denser than atoms can, but eventually, the electrons come so close together they repel each other very strongly. But white dwarfs can only get as massive as 1.4 solar masses. Above that, electron degeneracy pressure also fails, leading us to: 3) Neutron degeneracy pressure. Now the electrons are shoved into the nuclei, joining with the protons to create neutrons and compressing further. But eventually even tightly packed neutrons (despite being neutral) will repel each other because neutrons are composed of charged quarks and at extremely close distances, quark-quark interactions occur that prevent further compression in remnant bodies between 1.4 and 3.0 solar masses. Such bodies are called neutron stars. But if the body has a mass over 3.0 solar masses (the so-called Chandrasekhar limit), then: 4) No force known to physics can stop the contraction of gravity. In this case, all the mass (HUGE), is compressed to a single point called *the singularity*. The result is a black hole. More about cases 3 and 4 next time. Reply #116. Jun 27 17, 10:16 AM |
| brm50diboll
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Got my mass limits confused. The Chandrasekhar limit is 1.4 solar masses. The Tolman-Oppenheimer-Volkoff limit is 3.0 solar masses. Not the first time I've been wrong about something and won't be the last, either. Comes from writing posts that are too long about complicated subjects off the top of my head without checking details until afterward. Reply #117. Jun 27 17, 3:00 PM |
| brm50diboll
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Large stars, in addition to having much shorter lives than small stars, lose proportionately more mass during the late stages of their lives than small stars do. There is a significant difference between the mass a large star has at when it enters the Main Sequence and the mass it has left when it finally dies. The question of how large a star must be in order for it to end its life as a supernova explosion can therefore be answered two ways: What must the minimum mass of the core be to trigger the actual supernova explosion at the end of a large star's life? And, What must the minimum starting mass of the progenitor star be so that it will have enough mass left to trigger the supernova explosion at the end of its life? Roughly speaking, the answer to the first question is about three solar masses, and the answer to the second question is about ten solar masses, since about seven solar masses gets "shed" during the late stages of the life of a ten solar mass star. Spectral class A (white stars) are "on the borderline" as to whether they can go supernova or not. The smaller end cannot, but the larger end probably can. Astrophysicists are still working out where the exact "break point" is when supernova explosions become possible. The famous star Sirius A (the brightest star in the night sky) is a Class A Main Sequence star, but, at only about 2.1 solar masses, is probably a little too small to go supernova when the time comes. Unlike the Sun, which will end its fusion at helium-to-carbon, Sirius A is big enough to get to the next step, carbon-to-magnesium, but not any further. So eventually, Sirius A will end up as a red bright giant that dies as a magnesium-silicon-sulfur white dwarf without going supernova. By the time we get to the upper end of Class A, and certainly the even larger Class B and Class O stars, now supernova explosions become virtually certain. These huge stars spend only a few million years on the Main Sequence as very hot blue stars, and even shorter amounts of time as supergiants, a few hundred thousand years at most. Despite shedding enormous amounts of mass in their outer layers as supergiants, their onion-shaped cores (with multiple different types of fusion going on from the outermost to innermost layers of the core) are still large enough to push fusion all the way to its theoretical end: iron. Yes, iron. Elements lighter than iron can be fused to release energy (albeit at phenomenally high temperatures), but fusion of iron and elements heavier than iron does not produce energy, it consumes it. So when iron begins to accumulate in the center of the core of a supergiant star, that is the death knell! From the time iron production begins to the supernova explosion is difficult to determine looking at only the outer appearance of an elderly supergiant, but when it happens, the star has only a few days left, maybe less, according to theoretical physics models. Betelgeuse is certainly near the end of its life, but it probably hasn't begun to produce iron yet, according to most studies. There is iron in the outer layers of Betelgeuse, but Betelgeuse is not a first-generation star, and the iron was present in the original nebula that Betelgeuse first formed in a few million years ago from a previous supernova. When iron begins to form at the center of a supergiant's core, gravity once again takes the upper hand and the core starts to collapse (all the layers). At the center, iron begins to fuse to form even heavier elements, but unlike previously, energy is now absorbed rather than produced. This absorption of energy at the center of the collapsing core has the effect of *accelerating* the collapse. In a few seconds, the core collapses to only about 20 miles in diameter. Electron degeneracy fails and electrons are shoved into protons to form neutrons. In all but the very largest supergiants, neutron degeneracy abruptly stops the collapse, producing a massive shock wave that in just a few seconds, rips through the outer part of the core and into the outer layers of the supergiant, becoming a supernova explosion. Supernova explosions are the only known natural way elements heavier than iron can be created. All the gold and uranium on Earth, for example, were once created by a supernova explosion around five billion years ago, creating the nebula that eventually developed into our Solar System (and probably several others, now long-separated as the Sun has gone around the Milky Way several times since it originally formed 4.5 billion years ago.) The inner core remnant of a supernova explosion is a neutron star, and young neutron stars spin extremely rapidly and generate radio emissions and are known as pulsars. After several thousand years, the pulsars lose their radio emissions, but still remain dead neutron stars only about 20 miles in diameter but with about twice the Sun's mass or so. In the really biggest supergiants, neutron degeneracy cannot stop the core collapse, so a black hole forms. Black holes are sufficiently interesting they deserve a full discussion to themselves. Sometime later, though. I'm tired now. Reply #118. Jun 30 17, 6:10 PM |
| brm50diboll
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A few loose ends to tie up before discussing black holes. I keep using the term supernova, which assumes there is such a thing as a nova (no "super".) Certainly there is, but a supernova is not just a really big nova. They are two completely different celestial phenomena. Old style: plurals are novae and supernovae. But I get sloppy sometimes and write novas and supernovas. What is a nova? The literal meaning of the word is "new", as early astronomers at first believed novae to be new stars. What they saw was the appearance of something they believed to be a new star (definitely not a planet or comet) in a part of the sky where no star had been observed previously. But even the early astronomers knew something wasn't quite right with novae being new stars, because once they appeared, they invariably faded slowly over a few days or weeks and became invisible. With the advent of the telescope and more modern astronomy, faded novae could still be seen, and, in a few cases, stars too dim to be seen with the naked eye but seen with telescopes were found to become naked eye novae. Further analysis of novae eventually showed they were typically white dwarfs in close binary systems with a partner, typically a red giant, that the white dwarf was stealing mass from. Hydrogen gas from the outer layer of the larger star was being pulled to the surface of the extremely hot (usually carbon) white dwarf and accumulating. At a certain point, enough hydrogen will have accumulated on the surface of the white dwarf to have the right density to ignite hydrogen fusion there. But this was hydrogen fusion on the surface, not the core of a star (and normally there is no fusion in white dwarfs anyway, since they are the burnt out cores of dead stars.) So when the hydrogen fusion began, it did so explosively and flared for only a few days or weeks before the hydrogen had been turned into helium and the star returned to being a dim white dwarf. So novae are not exploding stars, supernovae are. To complicate matters further, there are several subtypes of both novae and supernovae, but I don't really want to get into that now, except to say that "recurrent novae" are a rare but interesting phenomenon, and in some cases leads to an unusual variant of supernova with no remnant afterward. I should also point out that binary stars, or more properly, multiple star systems with two, three, four or even more stars all close together and orbiting a common center of mass are actually quite common. Close to half of all stars belong to multiple star systems. Reply #119. Jul 09 17, 11:25 PM |
| brm50diboll
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OK, black holes. Where to start? Theory of relativity? Observational data? Let's try this: It often has been said what goes up must come down. Not quite true, as with many things about the universe we live in. Imagine a super powered cannon imbedded in the ground pointed straight upwards with the end of the barrel at ground level. This is a special adjustable cannon where you can choose the velocity you want the cannon ball to emerge from the ground at. Say you set it at 50 miles per hour and fire it. What will the cannon ball do? It will emerge from the ground at 50 mph and go upwards, but slowing as it rises because of gravity and eventually reach some maximum height, then start falling back to the ground. If we set it for 100 mph, what will it do? Pretty much the same thing, except it will reach a higher maximum height before it starts to fall? Will it *always* reach a maximum height and start to fall no matter how fast we set the initial velocity? Actually, NO! If we set the velocity high enough, it will leave Earth permanently and never come back. The minimum velocity for that to happen is called the *escape velocity*. For Earth, the escape velocity is about 7 miles per second. Objects shot out slower than that will eventually fall back to Earth. Objects shot out faster than that will never return. Now the escape velocity depends on the mass and density of the celestial body from which you are trying to escape. The Moon has a lower escape velocity than Earth, so a lower velocity will escape the Moon's gravity. The Sun has a higher escape velocity than Earth. Now the maximum possible speed any object can travel is c, the speed of light in a vacuum, about 186,000 miles per second. Are there objects with escape velocities at c or higher? Yes there are. What does that mean? It means *nothing*, not even light, can escape such a body. These objects are known as black holes. The escape velocity decreases as one moves further from the center of mass of the body. In a black hole, all the mass is concentrated at a single point at the center known as the singularity. There the escape velocity is effectively infinite and unmeasurable. A certain distance away, the escape velocity may have dropped to, say, twice the speed of light. Imagine you were somehow able to stand at a pedestal at that point and point a flashlight directly upwards, away from the singularity. What would the light do? Amazingly, it would go up a bit, reach a maximum height, then *fall back down* like the cannon balls we discussed before. If we go even further away from the singularity (in our minds), there is a certain distance at which the escape velocity would exactly equal c. This distance is called the Schwartzchild radius, and forms an imaginary sphere around the singularity called the *event horizon*. For typical stellar mass black holes, the Schwartzchild radius is around 5 miles or so. Light outside the event horizon can escape the black hole, light inside the event horizon is trapped forever and can never escape, being pulled back in to the singularity. We cannot see what is inside the event horizon of a black hole for that reason. But black holes can still be detected by the effect their extremely powerful gravity has on nearby bodies outside their event horizons. Gas gets sucked into black holes, heating up as it spirals inward and radiating energy from this *accretion disc*. The first well-documented body to be recognized as a black hole is called Cygnus X-1, an X-ray emitter in the constellation Cygnus. In addition to stellar mass black holes created by collapse of extremely large supergiants, there are even larger ones at the centers of galaxies millions of times the mass of our Sun. These are called supermassive black holes, and the one at the center of our Galaxy, the Milky Way, is called Sagittarius A*. Other larger galaxies have supermassive black holes with masses many *billions* of times the mass of our Sun. Black holes represent a test of Einstein's General Theory of Relativity. Even bodies much less massive than a black hole, like our Sun, still *bend* light in accordance with Einstein's theory, even though they cannot trap light like a black hole. In fact, a total solar eclipse was used to test Einstein's Theory of General Relativity a few years after he proposed it, and the results were consistent with Einstein's theory. Light bent around the Sun slightly as it passed near from distant stars during the total eclipse, and the amount of bending fit the predictions of the theory. Reply #120. Jul 18 17, 8:02 PM |
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