Activity for HDE 226868
| Type | On... | Excerpt | Status | Date |
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A: How to figure out layers of the atmosphere? The five primary layers of the atmosphere are, with increasing, altitude, the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The corresponding boundaries are the tropopause, stratopause, mesopause, and thermopause. Here's how those boundaries are defined: Tropopause: The lapse ... (more) |
— | over 5 years ago |
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A: Hiding a solar system in a nebula We have plenty of examples where stars have been hidden by nebulae - and not just newborn stars. Typically, the gas and dust comes from mass loss from one of the stars in the system. Examples include LL Pegasi, a binary system containing a carbon star that is sloughing off large amounts of dust as ... (more) |
— | over 5 years ago |
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A: What element would make up a creature if it used the weak nuclear force during its metabolic processes? TL;DR I'd propose that weak force life has a tiny change of existing in environments where particles travel at high speeds. A possible example is the jets produced by an active galactic nucleus. At the high energies (and high speeds) particles reach in these jets, the range of the weak force could b... (more) |
— | over 5 years ago |
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A: What would the consequences be of a high number of solar systems being within close proximity to one another? Your environment is quite similar to that in a globular cluster. At its densest, a globular cluster may see peak stellar number densities of $\sim1000$ stars per cubic parsec, which implies a mean separation of about 20,000 AU. This leads us to conclude that many, if not most, planets will be strippe... (more) |
— | over 5 years ago |
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A: What light would my cloudy planet receive from a red dwarf star? TL;DR As most of the other answers say, the plants on this world would likely be purple-ish, using photosynthetic pigments that operate at the same wavelengths as bacteriochlorophylls. Chlorophyll a and chlorophyll b wouldn't receive as much light in the visible part of the spectrum as they do from ... (more) |
— | over 5 years ago |
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A: What would be the effects on a planet orbiting a shedding red giant? Atmosphere loss As you've suggested in your question, once a Sun-like star leaves the main sequence, it begins losing mass through a strong stellar wind, a stream of charged particles driven by photons. For a few hundred million years, it's a true red giant, expanding a bit and reaching luminosities... (more) |
— | over 5 years ago |
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A: Could life in a planemo be sustained by cosmic background radiation? Not unless life evolves extremely quickly. There are two conditions for cosmic background radiation to be able to support life: It's partially composed of photons at wavelengths required by photosynthetic pigments. It's bright enough to transmit a useful amount of energy to any life out there. I... (more) |
— | over 5 years ago |
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A: Appropriate liquid/solvent for life in my underground environment on Venus Supercritical carbon dioxide Once upon a time, Venus may have had seas of supercritical $\text{CO}2$ ($\text{scCO}2$) thanks to a higher surface temperature (by a few hundred Kelvin) and surface pressures (by a factor of 3 or so). However, now that the atmospheric pressure has dropped to about 9.3 M... (more) |
— | over 5 years ago |
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A: Effect of slow rotation on winds Super-rotation As I wrote in an answer to another question about winds, studies of slow-rotating planets like Venus have yielded information about what we should expect for planets like this in general: The troposphere may exhibit something called super-rotation, meaning the atmosphere rotates fas... (more) |
— | over 5 years ago |
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A: What would the minimum mass of a water world need to be to form ice Vll, due to pressure, at its core? What about ice X, ice Xl, and higher? Summary It turns out that even relatively low-mass ocean planets are capable of forming some of the exotic ices you name in their cores. Ice VII appears to form at the centers of planets of $0.015M{\oplus}$ (Earth masses), while ice X forms at the centers of planets of $1.256M{\oplus}$. Interestingl... (more) |
— | over 5 years ago |
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A: How could a civilization detect tachyons? Tachyons are detectable. Fortunately, I believe your question is based on a mistaken premise. Tachyons, if they exist, would likely indeed be detectable. In fact, since they were initially theorized, there have been several experimental searches for tachyons, though very few in recent years. I'll ta... (more) |
— | over 5 years ago |
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A: What is the largest I can make my water planet, while remaining within Earth-like parameters? Models of planetary structure indicate that ocean planets should reach peak size at a few hundred to a thousand Earth masses, reaching maximum radii of perhaps 4-5 Earth radii. Add in some iron and silicates and you decrease the size slightly. Indeed, this mass range features peak radii for planets o... (more) |
— | over 5 years ago |
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How soon can the first stars form? Introduction In our universe, the cosmic microwave background was formed approximately 400,000 years after the Big Bang. It was hot, but within a few million years after the Big Bang, it would no longer have consisted significantly of visible light. The first stars formed about 100 million years lat... (more) |
— | over 5 years ago |
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A: Pressure inside an infinite ocean? Equations of state The "ocean" should be described by an equation of state that relates thermodynamic variables (e.g. density $\rho$, temperature $T$, pressure $p$, etc.) to one another. Different equations of state are valid in different regimes; some are valid at high temperatures and pressure, wh... (more) |
— | over 5 years ago |
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A: Implications of cigar-shaped bodies having rings? Yup! This is possible, and a number of small bodies in the Solar System have rings: Haumea, a dwarf planet in the outer Solar System, was recently discovered to have rings, which lie inside its Roche limit. Chariklo, a very large asteroid, has two known rings. Chiron, another minor planet, is suspe... (more) |
— | over 5 years ago |
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A: Can you make a star from other gases, and how long would they last? What can a star be made of? A star's composition is limited by the elements that exist in significant quantities in the universe. These include primordial elements - hydrogen, helium and lithium - as well as heavier elements formed through nucleosynthesis in stars, supernovae and certain rare proces... (more) |
— | over 5 years ago |
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A: Navigating storm fronts in space The answer to your question depends strongly on the supernova rate in the galaxy. The Milky Way currently is not an active galaxy - the supermassive black hole at its center is relatively quiescent - and is not undergoing dramatic interactions with any of its neighbors. Energetic events like gamma-ra... (more) |
— | over 5 years ago |
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A: Can a single, larger black hole be split into multiple smaller black holes? Alexander's answer is completely correct; there is no way to split one black hole into smaller ones. I think, though, that it might be worth explaining why this is the case, particularly because there are two independent lines of reasoning that come to the same conclusion. If you want to split a blac... (more) |
— | over 5 years ago |
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A: Can the event horizon of a black hole be distorted or destabilized by an extreme spin rate The situation you're considering involves a rotating black hole characterized by the parameters $M$ and $J$, the mass and angular momentum of the black hole. The two are encapsulated in something called the Kerr parameter $a$, given by $a\equiv cJ/GM$ where $c$ and $G$ are the speed of light and the ... (more) |
— | over 5 years ago |
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A: Will winds always be the same? No, wind patterns are not always going to be the same. Venus is a key counterexample here. Its Hadley cells extend to 60 degrees above and below the equator - double the size of Earth's Hadley cells. In other words, they extend above the subtropics and through what would be considered, on Earth, tem... (more) |
— | over 5 years ago |
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Can a planet be tidally unlocked? In my answer to another question, I suggested that the Super-Earth in question be tidally locked to its host star for a period of time while part of its surface experienced a bombardment. After that's finished, however, I'd want the planet to rotate normally, ideally with a rotation period similar to... (more) |
— | almost 6 years ago |
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A: How to explain one side of Super Earth is smoother than the other side? I'm reminded strongly of Iapetus, a moon of Saturn that has a dramatic two-tone coloring. One hemisphere is quite light, while the other is dark. It almost looks like you dunked half of the moon in chocolate: Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute. Pub... (more) |
— | almost 6 years ago |
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A: Solar recycling or: How to keep your star from dying A star's life ends when it can no longer undergo fusion at its core. For massive stars, this often happens when the core is made largely of iron, which can be fused (and is) in small amounts, but only endothermically. The products of nuclear fusion are, at this point, like carbon monoxide: it's not t... (more) |
— | almost 6 years ago |
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A: How far out can a pre-telescope human society (naked eye observations only) detect planets? With the naked eye, humans can see approximately 6th-magnitude objects. We can compute the apparent magnitude of a planet at a given distance, and find the distance corresponding to an apparent magnitude of +6. The formula is $$mp=Mp+5\log\left(\frac{d}{10\text{ pc}}\right)$$ where $mp$ and $Mp$ are ... (more) |
— | almost 6 years ago |
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A: Is it possible to place a permanent probe on Uranus? Pressure is important here. It's both a problem and a solution. As expected in giant planets, the atmospheric pressure and temperature change with altitude and depth. By the time you get to the base of the troposphere, pressures are at about 100 bars, which is slightly higher than the atmospheric pr... (more) |
— | almost 6 years ago |
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A: The running-backwards Olympics Backwards running will almost certainly never be as fast or efficient as regular running. A study back in 2011 showed that it should takes about 30% more energy to run backwards at a given speed. Why? It depends on how foot muscles respond during both types of strides. In normal running, landing on t... (more) |
— | almost 6 years ago |
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A: Explaining a Low-Mass Brown Dwarf 7110 Earth masses and a surface temperature of 1700 K aren't unreasonable for a brown dwarf. The lower mass limit is thought to be around 13 Jupiter masses (or 4100 Earth masses) (see e.g. Spiegel et al. 2010), and we see temperatures as low as 500 K in certain Y-class brown dwarfs. 12 Earth radii al... (more) |
— | almost 6 years ago |
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A: A supermassive black hole is coming our way. When's the latest that we would notice? This sort of scenario is quite possible, and would likely be the result of the merger of two supermassive black holes during the collision of the galaxies. We have evidence of this in the quasar 3C 186 (see Chiaberge et al. 2017). Over the course of about two billion years, two supermassive black hol... (more) |
— | almost 6 years ago |
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A: Would gas giants work as waste disposal sites? Yes, but for a different reason. This is basically what happened with the Cassini probe, which was sent to crash into Saturn in 2017. However, the reason the probe was successfully disposed of wasn't because Saturn's a gas giant; it's because the spacecraft burned up upon entering Saturn's atmospher... (more) |
— | about 6 years ago |
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A: Is a solar system around a black hole possible? Problem 1: The supernova The first concern I have is one that Zeiss Ikon's answer discusses. To form a black hole, you need some sort of energetic event, likely a supernova. However, a supernova releases three extremely problematic sources of energy: High-energy photons, like gamma rays, that have... (more) |
— | about 6 years ago |
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A: Planetary cave: Gravity inside a non-concentric shell This is a classic problem in electrostatics - that is, in an analogous situation where we care about calculating the electric force on an object inside some cavity. The same solution technique applies for Newtonian gravity, and it relies on something called superposition. Effectively, the cavity is l... (more) |
— | about 6 years ago |
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A: Effects of Hot Jupiters on habitable planets Orbits Conventional wisdom says that terrestrial planets and hot Jupiters are not likely companions. These gas giants in close orbits aren't thought to form in situ, but rather to form further away from their stars and then migrate inwards. Two main mechanisms have been proposed: Type II gas disk ... (more) |
— | about 6 years ago |
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A: Purple plants on a planet orbiting a green star? To determine the color of the plants most likely to arise on a planet orbiting the star, we have to consider the wavelengths of peak absorption of different photosynthetic pigments. I would argue that different star types do indeed influence pigment choices. Chlorophylls respond well to light around... (more) |
— | about 6 years ago |
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A: What is the minimum size for the Sun? Black body radiation The Sun is, approximately, a black body. That means that the light it emits follows a particular spectrum according to Planck's law, with the shape of the spectrum determined solely by the Sun's surface temperature. In particular, the wavelength of peak emission can be found thr... (more) |
— | about 6 years ago |
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A: How is infra-vision any good? Let me address your second paragraph: Upon thinking more about this, sight in the infrared band seems like it would come with several problems. This vision would be relying on the radiant emitted by blackbody radiation, according to their temperature. But, the frequency band of this emission reli... (more) |
— | about 6 years ago |
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A: Could the TRAPPIST-1 planets have moons? M. A. Golding's answer is correct, I think, but it's also a bit incomplete, because the exoplanets in the TRAPPIST-1 system are not equally unsuitable for hosting exomoons. In particular, their individual orbital radii (as well as masses and compositions) play an important role in determining what so... (more) |
— | about 6 years ago |
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A: How does the n-body-problem affect a star system around a red dwarf? The thing about planetary systems - and many $N$-body systems in general - is that they are fundamentally chaotic. That is, small changes grow over time, eventually creating wildly divergent results. A way to quantify this is the Lyapunov exponent $\lambda$ and the Lyapunov time, $\tau=1/\lambda$, wh... (more) |
— | about 6 years ago |
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A: Life on planets near quasars AGN structure and emission Let's talk about the structure of an active galactic nucleus like a quasar, and the types of emission we see from it. The classic unified model of an AGN consists of a supermassive black hole (of perhaps $\sim10^8\text{-}10^9M{\odot}$) surrounded by an accretion disk about... (more) |
— | about 6 years ago |
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A: Are there any plausible 'alternative' methods of planetary formation? Gravitational instabilities There's actually a second idea for planet formation that's been around for some time (see Kuiper 1951). It requires the nebula hypothesis, like accretion, but it's a top-down process, not a bottom-up one, involving the fragmentation of accretion disks around young stars t... (more) |
— | about 6 years ago |
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A: How do you non-catastrophically reduce the mass of the Sun by half? There are a number of ways a star can lose mass, and I think it's worth talking about them: A normal coronal mass ejection may contain $\sim10^{-18}M{\odot}$, which is also extremely low. Eta Carinae's Great Eruption averaged about $1M{\odot}\text{ yr}^{-1}$, but this is not an expected event in Su... (more) |
— | about 6 years ago |
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A: Can the Sun lose enough mass that Saturn's current velocity becomes escape velocity? A naive first calculation The formulas for orbital velocity and escape velocity are $$vo=\sqrt{\frac{GM}{r}},\quad ve=\sqrt{\frac{2GM}{r}}$$ I get $vo=9.6\text{ km/s}$ for Saturn. For this to equal $ve$, the Sun's new mass would have to be $0.5M{\odot}$, if we neglect the mass of the ejected gas. By... (more) |
— | about 6 years ago |
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A: Detecting objects around other stars Transit simulation I simulated the transit of your spaceship in front of a Sun-like star. Besides the dimensions of your ship, I made the following assumptions: A roughly circular orbit, with semi-major axis $a=0.1\text{ AU}$. A limb darkening coefficient of $u=0.6$, and a linear limb-darkening la... (more) |
— | about 6 years ago |
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A: Increased Luminosity in Stars I would recommend looking at pre-existing numerical models, rather than computing your own. This has a couple of advantages: You don't need to use any approximations. Factors like metallicity, rotation and composition have already been taken into account. You just need to look up the values in a ta... (more) |
— | about 6 years ago |
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A: How big can a nebula be? TL;DR: About 2150 light-years Here's the gist of my answer, for simplicity: The largest nebulae are HII regions, clouds of gas ionized by young hot stars forming inside them. We can calculate the radius of a sphere corresponding to the maximum distance at which neutral hydrogen gas can be ionized ... (more) |
— | about 6 years ago |
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A: A blue flamestorm blasting through a system of big yellow caves? Cave formation This setup reminds me of rhizomes, which are essentially underground root systems that can spread out underground, sending up new seedlings at various intervals. Rhizomatic root systems make colonies of aspen possible, and colonies can be both massive and long-lived. If one tree dies,... (more) |
— | over 6 years ago |
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A: Would planets be cubic in my "cuboverse"? TMM;DR (Too Much Math, Didn't Read): For anyone who doesn't want to go through the derivations and calculations below, here are the important points from my answer: We're not working with the same space as normal, friendly, Euclidean space. This means that while we can still integrate and differen... (more) |
— | over 6 years ago |
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Can my spaceship figure out its position using Cepheid Variables? In my story, reasonably far in the future, an intrepid group of explorers are on the first manned mission to the Andromeda galaxy, travelling close to the speed of light. They slumbered in suspended animation for thousands of years, in their frame of reference (to a stationary observer, it would have... (more) |
— | over 6 years ago |
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A: Determination of reasonable volcanic island height from area? For Hawaii, $H\propto\sqrt{A}$. I looked at the islands on the Hawaiian-Emperor Seamount Chain with surface volcanoes, and plotted their surface area vs. their maximum elevation, using data from Wikipedia. I checked that the topography of each island is indeed dominated by the central volcano or vol... (more) |
— | over 6 years ago |
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A: Can I produce a true 3D orbit? Try the Kozai mechanism The basis for the Kozai mechanism is that in a binary system, a third orbiting body - with a much lower mass than the other two - has a quantity that doesn't change in time: $$Lz=\sqrt{1-e^2}\cos i$$ where $e$ is the orbital eccentricity and $i$ is the orbital inclination - t... (more) |
— | over 6 years ago |
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A: How do I calculate the amount of sunlight a planet gets? Calculating flux I think a slightly more helpful quantity to calculate is the flux received by the planet - the power per unit area from the star. The mean flux on Earth is the solar constant, $Fe=1.36\times10^3\text{ W m}^{-2}$. If a planet orbits a star of luminosity $L$ at a distance $rp$, the fl... (more) |
— | over 6 years ago |