Activity for HDE 226868
| Type | On... | Excerpt | Status | Date |
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A: A self-eclipsing orbital ring The setup and the equation Let's look at the geometry involved here. I created two diagrams: On the left, we have the star of radius $R$. On the right, we have a cross-section of the ring. The center of the ring is a distance $r$ from the star, and the ring has a diameter of $2s$ and a cross-sect... (more) |
— | over 6 years ago |
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A: Which part of the universe would make celestial navigation most difficult for a planet? A swiss cheese-style dark nebula Celestial navigation requires seeing other objects in the sky. But you can't do much of that if you can't see many celestial bodies. Therefore, you need to place your planet somewhere where it will be enshrouded in gas and dust. The solution is a nebula of some sort.... (more) |
— | over 6 years ago |
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A: Is there a habitability zone between the primary and secondary stars of a binary star system? Yes, there is a habitable zone. Known examples of possibly-habitable binaries I have to disagree with StephenG's answer; we have data that indicates that this is possible for similar, Sun-like stars. I talked about this in an answer I wrote a few months ago; searching this exoplanet catalog, I foun... (more) |
— | over 6 years ago |
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A: Building a planet with a primordial black hole core Mass The big issue here lies in determining the mass of these black holes. On the one hand, the traditional primordial black holes you mention - formed by density perturbations - occupy a relatively low-mass regime. Constraints from a variety of observations indicate a peak in the mass distribution ... (more) |
— | over 6 years ago |
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A: Tiny galaxies with exoplanets Galaxy size The first question is whether such a galaxy (less than 3000 stars) is possible. The clear answer is yes; Segue II already satisfies those requirements. Segue I and Willman I appear quite similar in terms of size, mass, and mass-to-light ratio; they're small and likely contain a lot of da... (more) |
— | over 6 years ago |
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A: Simulating gravity in a gravity-less universe TL;DR Your world will experience constant, uniform gravity. Everything is accelerating upward at the same rate - but this isn't the case in our universe, as gravity follows (approximately) an inverse-square law, and more specifically the framework of general relativity. On Earth, this isn't the cas... (more) |
— | over 6 years ago |
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A: How can the Hertzsprung-Russell diagram be used in star building? To accurately answer your question, you might need to use a stellar evolution code, either doing your own modeling or looking up existing data tables. I'd recommend the MESA code for the former approach, and the Geneva grids for the latter (see Eggenberger et al. (2008) for details). Numerical simula... (more) |
— | over 6 years ago |
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A: How can I find local stars that are 'gravitationally bound', i.e. moving together? TL;DR There are several important factors considered when trying to determine if a group of stars are actually from the same stellar cluster, moving group, or association: Kinematics. Do the stars all appear to be moving in roughly the same direction, with similar velocities? Age. Are the stars al... (more) |
— | over 6 years ago |
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A: Radiation Levels and Effects on Planet with 27 Suns Let's look at two key equations: $$Fi=\frac{Li}{4\pi ri^2},\quad \lambda{\text{max,i}}=\frac{b}{Ti},\tag{1, 2}$$ For a given star with luminosity $Li$ and surface temperature $Ti$ a distance $ri$ from the planet, these equations give you the flux from the star ($Fi$) and the wavelength at which its e... (more) |
— | over 6 years ago |
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How can I make a black dwarf star? In response to a question about an asteroid filled with degenerate matter, Ender Look suggested that using matter from a black dwarf would be better than using matter from a white dwarf since black dwarfs are cooler. A typical white dwarf's temperature might be $\sim$10,000 K, which is way too high. ... (more) |
— | over 6 years ago |
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A: 5km artificial planet, with same gravity as on Earth A first approximation Let's calculate surface gravity. Assuming an object of density $\rho$ and radius $R$, the surface gravity is $$g=\frac{4\pi}{3}G\rho R\tag{1}$$ For a white dwarf, $\rho\sim10^9$ kg/m$^3$. If we want $g=9.8$ m/s$^2$, we find an $R$ of 35 meters. If you scale that up to a radius ... (more) |
— | over 6 years ago |
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A: Relative Super Powers Greater endurance/stamina: A thicker atmosphere/higher oxygen levels Try a thicker atmosphere - or rather, a planet where the humans largely live at higher elevations. Altitude training is useful for athletes on Earth. By training somewhere with a lower partial pressure of oxygen, a person's red blo... (more) |
— | over 6 years ago |
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A: Chlorine Trifluoride-producing microorganisms This isn't going to happen, for a number of reasons (all of which boil down to "It's dangerous, and these bacteria will kill their hosts"). Organic materials may "spontaneously ignite" when they encounter chlorine trifluoride. This thing loves water, to the extent that there will be explosive react... (more) |
— | over 6 years ago |
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A: Multiple moons but only one of them is tidally locked to its planet? The time it takes a moon to become tidally locked to a planet depends on a number of factors. In particular, $$t{\text{lock}}\propto\frac{\omega a^6}{mp^{2}}$$ where $\omega$ is the moon's initial angular speed, $a$ is its semi-major axis, and $mp$ is the mass of the planet. There's a very strong dep... (more) |
— | over 6 years ago |
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A: How to estimate a star's heliopause? Deriving the radius The heliopause is a place of equilibrium, where the ram pressure from the solar wind is equal to the pressure of the interstellar medium (ISM). There are a number of sources of pressure in the ISM, but thermal pressure is the main one.1 The ram pressure from a wind with terminal ... (more) |
— | over 6 years ago |
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A: I've determined how many planets my solar system could plausibly have. How do I figure out what kind of planets they are? Massive objects beyond the frost line are likely giant planets. Let's talk about the frost line, which you helpfully specified (3.94 AU). For anyone not familiar with the term, the frost line is the distance beyond which - in the protoplanetary nebula, while the system was still forming - heavy elem... (more) |
— | over 6 years ago |
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A: Why would we use direct current instead of alternating current? Reverse the War of the Currents. In the late 19th century, Thomas Edison supported the use of direct current, and his company, Edison General Electric, attempted to use it on a large scale. A major competitor, George Westinghouse, supported the use of alternating current, with his competing company.... (more) |
— | over 6 years ago |
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Can I monitor animal movements from space? The Earth-like planet I'm working on is the culmination of years of labor by the galaxy's best scientists. It consists of fully artificial ecosystems, and is, for all intents and purposes, like Earth. The life-forms are carbon-based and have similar structures, metabolic processes, and habits as anim... (more) |
— | over 6 years ago |
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A: How do I maximize a planet's wave height (on average)? For waves that have small heights relative to the water depth ($H\ll a$), we can use Airy wave theory. This works well for waves that are far out at sea, and can include tsunamis, in deep water. I wrote about this in more detail here, but to make a long story short, the height of a wave is given by $... (more) |
— | over 6 years ago |
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A: Moon that changes color with phase The Moon can appear to be different colors, as seen from Earth: A red, orange, or yellow moon can appear when the moon is near the horizon, and light has to travel through more of the atmosphere, and more light is scattered A blue moon (not figuratively, literally) can appear if particles of size 5... (more) |
— | over 6 years ago |
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A: Can you have reduced visibility (due to fog) but an atmosphere that is still breathable? Optical depth Let's start by looking at the idea of optical depth. Optical depth is a quantity that describes how light is attenuated as it travels through a medium. There are two commonly used equations for optical depth, $\tau$. In a homogeneous1 medium, they are $$\tau=n\sigma x,\quad\tau=\kappa\... (more) |
— | over 6 years ago |
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A: Alternative elements for oxygen transport in an alien blood Coboglobin is one of the main iron- and copper- free proteins that can be used for oxygen transport. It was first synthesized by humans in 1970, where cobalt was intentionally substituted for iron in a hemoglobin-like protein. It also looks like it's been talked about before on Worldbuilding. To have... (more) |
— | over 6 years ago |
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A: Could the Sun be born again? I actually figured this out shortly after beginning the story, and I'm going to therefore post a self-answer, because it might be useful to others. Obviously, I'm still open to more ideas if this one is flawed or incomplete. TL;DR: Yes, it could work, on short enough timescales. So, the Sun will tr... (more) |
— | over 6 years ago |
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Could the Sun be born again? About 5 billion years from now, the Sun will begin to swell into a red giant. This will cause some problems, because life on a planet orbiting a red giant is hard. Even if Earth isn't engulfed by the expanding Sun, it's going to be scorched. We're looking at surface temperatures anywhere from 500$^\c... (more) |
— | over 6 years ago |
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A: Can a biological creature detect and absorb electricity from power sources? Yes. You want a hornet-like platypus in the rainforest. Hear me out. Water conducts electricity substantially better than air, salt water in particular. Therefore, electroreception - the ability to detect electric currents - is far more common in marine life than terrestrial creatures. Now, it's no... (more) |
— | over 6 years ago |
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A: Could scientists test a theory of everything? First, I suppose I should define what a "theory of everything" actually is. I'd describe it as a mathematical model that predicts the behavior of any object under any given set of conditions. It should be valid in all situations, and should, experimentally, match previous observations of the universe... (more) |
— | over 6 years ago |
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A: Physiological adaptation of life on a planet orbiting a red giant. Temperature and luminosity Let's start with some calculations. For the sake of argument, I'll assume that we're talking about a planet that's identical to Earth orbiting a star that's identical to the Sun. To be as generous as possible, I'll assume that this red giant expands to only about $\sim$200... (more) |
— | over 6 years ago |
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A: Why would anyone build robots when they have human rights? Robots can do jobs humans can't. In fact, they can do a lot of jobs humans can't. They can Explore and clean up after a nuclear accident Go into a volcano to explore Travel the universe without worrying about food or water None of these are factory jobs, but they should make the point that robots... (more) |
— | over 6 years ago |
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A: Is it possible to have a Earth-like planet with a red moon, similar to the color of Mars? The redness of Mars' surface comes from the large quantities of iron in its crust. Other terrestrial planets in the Solar System also contain iron, but not at their surfaces. Mars in particular has several things going for it that allowed it to keep much of that iron in the crust: Low surface gravi... (more) |
— | over 6 years ago |
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A: Aircraft that can endure high winds and storms Avoid helicopters and airships. Try the WC-130 or WP-3D. First, airships are not fantastic choices for maneuvering in high winds. Helicopters are bad enough, but airships are large and slow, meaning that there's even more drag on them from high winds. Adding stabilizers can help, but if wind directi... (more) |
— | over 6 years ago |
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A: How can I get my continent placement and latitude lines right to make my climates work? Look at Earth's biomes! When it comes to life and everything that comes along with it, we only have one data point, our own planet. Climate modeling is . . . hard. Really hard. When we're talking about where biomes will arise, sometimes it's easier to just look at the existing case study. Here, for ... (more) |
— | over 6 years ago |
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A: Can two stars orbit each other but each have their own planets in stable orbits? Yes, and the planets can orbit in the habitable zones. This page gives a long list of planets in these S-type orbits, and states For a few systems (2 so far), there are planets orbiting each member of the binary. In this case, the system is divided into two "binaries", one where the first star i... (more) |
— | over 6 years ago |
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A: Could a huge pole that's moved back and forth be a method of FTL communication? The answer to your question, regrettably, is no, and the counterargument is a classic one. The pole is composed of atoms and molecules and particles; it isn't one giant indivisible object. Those atoms and molecules and particles can move about, to some extent, even if the pole isn't easily deformab... (more) |
— | over 6 years ago |
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A: How can I make a planet covered in ice with an ocean beneath the surface? 1. Orbit a high-mass planet In the Solar System, two moons are covered with ice and have subsurface oceans: Europa and Enceladus. Europa orbits Jupiter; Enceladus orbits Saturn. In the case of the latter, jets of water vapor were observed, indicating an underground source. For both moons, the oceans... (more) |
— | over 6 years ago |
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A: Can a fission satellite duplicate the radiance frequencies of a star? Option 1: Continuous spectrum Your first choice would be to recreate the black body spectrum of the star. For a black body at a temperature $T$, the intensity $J(\nu)$ is $$J(\nu)=\frac{2h\nu^3}{c^2}\frac{1}{e^{h\nu/kT}-1}$$ where $h$, $c$ and $k$ are constants. For low-energy photons, $J(\nu)\appro... (more) |
— | almost 7 years ago |
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A: How do I design the solar transit of a hot Jupiter? Why are transits so rare? Essentially, you want a low relative orbital inclination. A body's orbital inclination is the angular difference between its orbital plane and a reference plane. In the Solar System, this reference plane is the the ecliptic, such that Earth's orbit is contained in this pla... (more) |
— | almost 7 years ago |
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A: Could modern sharks adapt to live in deep water? Yes. There are sharks that live quite deep in the ocean; the goblin shark and frilled shark, for instance, have been known to live over 1,000 meters below sea level (Wikipedia claims great whites have been found at this depth, too). This is off the edge of the continental shelf. Your depths (180 me... (more) |
— | almost 7 years ago |
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A: Conjoined Planets? Not for a planet, but for a smaller body. Massive bodies are in hydrostatic equilibrium. Gravity pulls them into spheres, eliminating most non-negligible variations from perfect roundness. Obviously, protrusions like mountains exist, but they're minute compared to the size of a planet. That said, s... (more) |
— | almost 7 years ago |
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A: Inducing a Solar Flare Let's first think about what causes solar flares. The processes behind them are not substantially well-understood, but we do have some ideas. Magnetic reconnection is one possibility. Essentially, the magnetic field of the Sun quickly changes in shape and topology, releasing a substantial amount of e... (more) |
— | almost 7 years ago |
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A: What would happen if the black hole at the center of a galaxy was removed? The answers so far have assumed that the galaxy in question is a spiral galaxy - and if we're talking about the Milky Way, then that's all well and good. But galaxies are pretty diverse, both in shape, size, mass and composition. Most look nothing like our own. It turns out that if you're willing to ... (more) |
— | almost 7 years ago |
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A: How long would it take a civilization to see and understand a Dyson ring? Anywhere from ancient times to the present. Question 1: Is there anything there? (answered pretty quickly) Sunspots, for instance, were first observed by the Chinese in 364 B.C., two millenia before telescopes or camera obscura were first used to study the Sun! While I don't know what the Chinese u... (more) |
— | almost 7 years ago |
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A: How can I make manure without cows? Use chicken manure. Other animals, besides livestock, also produce good manure, with different qualities. Now, not all manures are created equal; they differ in composition, volume, and production rate. If we define one "animal unit" as 1,000 pounds of animal, then one dairy cow unit produces 15 ton... (more) |
— | almost 7 years ago |
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A: What could cause the world to be almost completely submerged in water, naturally or man-made? The idea of an ocean planet isn't too far-fetched. There are several moons in the Solar System - Enceladus and Europa, for instance - that have subsurface oceans. If the ice covering their surfaces melted, they'd be just what you're looking for. Extrapolating that to a larger, planetary-mass body isn... (more) |
— | almost 7 years ago |
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A: How will birds be different on a <1g world? There will be little difference. The lower atmospheres are the same. Let's assume that the atmospheric pressure, $P$, follows a simple exponential scale height model: $$P=P0\exp\left(-\frac{z}{H}\right)$$ where $P0$ is the pressure at ground level, $z$ is altitude, and $H$ is the scale height, given... (more) |
— | almost 7 years ago |
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A: How can my dragon convert heat to usable energy? Try thermosynthesis. Thermosynthesis is a hypothetical mechanism, usually applied to the RNA world theory for the rise of life on Earth. In a nutshell, it states that an organism could function as a heat engine, where thermal energy is gained from a heat reservoir - in this case, a volcano. Thermos... (more) |
— | almost 7 years ago |
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A: Could you float a boat on a gas giant? Let's look at some key Jovian atmospheric characteristics: Density at $P=1\text{ bar}$ (i.e. the surface): $\rhoJ=0.16\text{ kg m}^{-3}$ Temperature at $P=1\text{ bar}$: $T=165\text{ K}$ Mean molecular weight: $\mu=2.22$ Primary atmospheric constituents: H (89.8%), He (10.2%) In other words, if y... (more) |
— | almost 7 years ago |
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A: How could a natural road form across a sea? You could use a large mid-ocean ridge, like the Mid-Atlantic ridge, and then play around with sea levels. Mid-ocean ridges occur at (divergent) plate boundaries, and can be thousands of kilometers long. They can rise high above the sea floor, at times breaking through into islands. So, here's how yo... (more) |
— | almost 7 years ago |
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Under what conditions would humanoids evolve eyes in the backs of their heads? Many creatures - humans among them - have binocular vision, where two eyes side by side allow good depth perception. There are quite a few other advantages over one eye, including a larger field of view, as well as redundancy (an injury leading to blindness in one eye won't cripple the creature's sig... (more) |
— | almost 7 years ago |
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A: What conditions could create a planet that has higher amounts of antimatter in its radiation belt? I read through the discovery paper (Adriani et al. (2011)) about antimatter in Earth's Van Allen belts, and I just want to lay out a few points before we begin: The antimatter in the Van Allen belts consists mostly of antiprotons, as you stated. Antineutrons may be initially produced, but free neut... (more) |
— | almost 7 years ago |
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A: How would a colliding binary affect its planetary system? Basic characteristics The system you're describing here may be a contact binary, where the two stars have come close enough to actually touch. These systems may actually be stable for millions (or even billions, in extreme cases) of years; the lifetime of the system as a whole - outside the contact ... (more) |
— | almost 7 years ago |