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Activity for HDE 226868‭

Type On... Excerpt Status Date
Answer A: Could a civilization engineer supernovae?
I had written up a long spiel that was going to be part of a long answer to another question, but it looks like it'll do all right here. Just so you don't think that I whipped it up in 20 minutes. Type Ia supernova Let me take a detour to look at this. In a Type Ia supernova, we would need a second...
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over 8 years ago
Answer A: Tidal locking timescale assumption
As Wikipedia notes, $$t{\text{lock}} \approx \frac{\omega a^6IQ}{3Gmp^2k2R^5}$$ where $a$ is the semi-major axis, $mp$ is the mass of the primary, $\omega$ is the initial angular speed, and the other variables describe the secondary planet. Clearly, these properties will vary based on the different ...
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over 8 years ago
Answer A: What else is involved in "silicon based" life?
The term I've seen used to describe the necessary solvents for silicon-based life is cryosolvent. In Planets and Life: The Emerging Science of Astrobiology, by Sullivan & Ross (relevant excerpt here), the authors cite the work of Bains (2004) on various alternate solvents to form the basis for variou...
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over 8 years ago
Answer A: Can life be powered by thermal conduction?
Awesome question. To test the feasibility of your idea, we start with the heat equation, which in our case is (from these notes) $$\frac{\partial T}{\partial t}=DH\frac{\partial^2T}{\partial z^2}\tag{1}$$ where $T$ is temperature, $t$ is time, $DH$ is the thermal diffusivity, and $z$ is elevation. $D...
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over 8 years ago
Answer A: Starbuilding: What is lacking in the logic behind Cosmos 2 star system generation algorithm?
This is essentially a partial answer, insofar as it's a series of loosely chained together critiques. Sometime in the future, I'll revise it so it's more organized, but for now, it's a work in progress. I apologize for any and all problems regarding readability. On page 14, Peoples gives some dice...
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over 8 years ago
Answer A: Exoplanetary Review: Rock Storms
The planet The radius you have is perfectly fine. It is true that chthonian planets may have masses of 30-100 times that of the Earth, but this is merely because they are extremely dense. Mocquet et al. (2014) put together mass-radius curves for several different compositions, with the lowest one be...
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over 8 years ago
Answer A: How can you make a stable configuration including a moon that has a moon?
I'm going to make some assumptions in this answer: Two moons have masses $m1$ and $m2$ and orbit a planet of mass $mp$, where $m1, m2\ll mp$. The planet is far enough away from the star that any gravitational/tidal effects from that star are negligible. There are no other planets capable of destabil...
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over 8 years ago
Answer A: Where in the galaxy would one find the most heavier elements?
Research suggests that heavier elements can be found in greater quantities closer to the center of a spiral or elliptical galaxy. Henry & Worthy (1999) summarized several measurements of ratios of heavy elements to hydrogen. For example, here's a collection of graphs of oxygen-to-hydrogen ($\text{O/H...
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over 8 years ago
Answer A: How far could a planet be from its star and still be kept habitable by intense greenhouse gases?
I agree with Tim B's assessment that this is a complicated question. However, I disagree on just why this is the case. The characteristics of the star are well defined, given that it's a solar analog. From this, we immediately have mass, luminosity, and other characteristics. However, the reason I th...
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over 8 years ago
Answer A: What natural processes can cause a sidereal day to be significantly longer than a sidereal year?
The classic example here is Venus, with a sidereal day of 245 Earth days and a sidereal year of 224.7 Earth days - clearly less than its sidereal day. I wrote an answer related to this on Astronomy Stack Exchange that explains Venus's slow rotation (and why it has retrograde rotation). The sequence o...
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over 8 years ago
Answer A: The Earth's core consumes a human?
I have two main points. Compressibility. Your idea of his body forming an indentation rests on the assumption that the Earth's core is incompressible. This is not true. While it is thought to be solid, this does not mean that it can't be compressed. I find it highly unlikely that any sizable indent...
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over 8 years ago
Answer A: 61 Virginis Mission Design: Long Term High Gee Acceleration and the Human Body
One thing we have to take into account is that you can only accelerate for the first half of the trip. You then have to decelerate for the second half; otherwise, you won't be able to stop unless you crash into your target! So the proper time observed by someone onboard a ship for the first half of t...
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over 8 years ago
Answer A: Can a situation exist where there is one pole that is hot and another cold on a planet?
Have you considered a Sitnikov planet? Image courtesy of Scholarpedia user Christoph Lhotka under the Creative Commons Attribution-ShareAlike 3.0 License. A Sitnikov planet moves on an axis, perpendicular to the orbital plane of two binary stars. It oscillates along this axis, gradually moving up...
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over 8 years ago
Question What's the quickest way to add oceans to a planet?
After writing this question about the ecology of a terraformed planet, I realized that I hadn't quite tied up all the loose ends of the terraforming process itself. I intended for a group of aliens scientists to turn a desolate hunk of rock into a somewhat Earth-like planet, but this isn't so simple....
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over 8 years ago
Question Can every animal eat most other animals?
An idea for a story I'm working on involves a far away world that was once inhospitable. Scientists - likely alien ones - terraformed it so it could support life (coincidentally, Earth-like life). They then introduced new species to the world, species that had been genetically engineered to have cert...
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over 8 years ago
Answer A: Is this planet possible in real life?
Doing a simple density calculation, I get $$\rho=\frac{0.09M{\oplus}}{\frac{4}{3}\pi R^3}\approx15.97\text{ grams/cm}^3$$ For comparison, here's a list of the average densities, $\bar{\rho}$, of the planets in the Solar System: $$\begin{array}{|c|c|c|} \hline \text{Planet} & \bar{\rho}\text{ (g/cm)}^...
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over 8 years ago
Answer A: Could life evolve in the degenerate era of the universe?
There are several possibilities - actually, quite a few - for the development of life in the Degenerate Era. Some have potential; some don't. The planet is a rogue planet. It has been proposed that rogue planets could retain heat and support life via geothermal energy from radioactive decay (see al...
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over 8 years ago
Answer A: Is this planetary system stable?
This is an exceedingly tricky question. Determining the stability of this system is akin to determining the stability of the Solar System, a difficult - and currently unfinished, as far as I know - task. Planetary systems with more than a couple planets are chaotic, meaning that on timescales of 107 ...
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almost 9 years ago
Answer A: At what distance from these planets should this moon be placed?
Is it possible? Absolutely. Pluto and Charon are tidally locked, and yet Pluto has four other moons1. Charon is relatively massive in comparison to Pluto - about one twelfth its mass. Indeed, the center of mass of the system lies outside Pluto. I see no reason why the stability should differ for two ...
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almost 9 years ago
Answer A: How do I calculate sea level on a planet with no seas?
Use a geoid. A geoid is, to quote Wikipedia the shape that the surface of the oceans would take under the influence of Earth's gravitation and rotation alone, in the absence of other influences such as winds and tides. It can be expressed in part using spherical harmonics. This might sound ove...
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almost 9 years ago
Answer A: How big a black hole is needed for this story?
I suppose I'll address the solar flares angle, since nobody appears to have tried that yet. My main objection here is that an external source will have a very hard time causing solar flares. Solar flares are, at their hearts, magnetic phenomena. While the exact mechanism behind them is unknown, one ...
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almost 9 years ago
Answer A: Gravity on a Minecraftian world?
If I may, I'll make a slightly simpler (I think) derivation of the answer. We start with Gauss's law for gravity. In its integral form, it is $$\oint{\partial V}\mathbf{g}\cdot \mathrm{d}\mathbf{A}=-4\pi GM$$ Here, $\mathbf{g}$ and $\mathrm{d}\mathbf{A}$ are vector quantities of the gravitational ac...
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almost 9 years ago
Answer A: A three foot cell?
Maybe. Giant cells actually exist. They occur when many cells fuse together, often to fight an infection. They typically reach a maximum size of 120 micrometers - nowhere near your size - but I suggest that it might be possible in an extreme case. Potential reasons for the formation of a giant cell...
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almost 9 years ago
Question Renewable energy in space
I'm working on a story set in a generation ship traveling through the stars. Its propulsion is a solar sail, which will accelerate it over many decades to a substantial fraction of the speed of light. However, its inhabitants need power for electricity, and don't want to use nuclear power (or radiois...
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almost 9 years ago
Answer A: Alternatives to DNA
Xenonucleic acids (XNAs) (see also Wikipedia) may be what you're looking for. XNAs are nucleic acids related to DNA, some of which can store information for organisms in the same way that DNA does for life as we know it. These six are HNA (anhydrohexitol nucleic acid) CeNA (cyclohexene nucleic aci...
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almost 9 years ago
Answer A: Would it be possible to ride a gravitational wave?
No can do. I was able to find the answer here, written by LIGO scientist Dr. Amber Stuver: How valid is the wave-like-in-water analogy? Can we "surf" these waves? Are there gravity "peaks" like there are "wells"? Stuver: Because gravitational waves can travel through matter unchanged, there...
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almost 9 years ago
Answer A: Would a dragon be useful in a space program?
Splashdown recovery! Currently, NASA does not have any manned spacecraft in use (astronauts use the Russia Soyuz capsules). However, in the past, NASA capsules used a splashdown landing in the ocean. A helicopter would come to bring back the astronauts (and then the capsule) to an aircraft carrier d...
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almost 9 years ago
Answer A: Close the door on your way out - Life lit by a blue dwarf star
Yes, it can happen. For this to be possible, you first have to put the planet far enough away that it can become totally icy. Then, after the temperature rises as the red dwarf transitions into a blue dwarf, the habitable zone must encompass the planet's orbit, melting the ice and making it a better...
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almost 9 years ago
Answer A: Would 'cheap' FTL make powerful telescopes obsolete?
Not quite. Possibly.1 Advantages of faster-than-light travel for astronomy: You can see events happening in the present. Astronomers stuck on Earth can only observe objects in the past. Even our observations of the closest star system Alpha Centauri, are four years out of date. Sometimes, the fini...
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almost 9 years ago
Answer A: Can you add a mini moon to Earth?
Note: If anyone can double-check my numbers and result at the end of this answer, that would be much appreciated. This looks like a question that can be broken down as per your bullet points, so I think I'll do it like that. Is earth's gravity strong enough to have a second moon further from us ...
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almost 9 years ago
Answer A: How much energy would it take to ignite a brown dwarf?
$2.55\times10^{45}\text{ Joules}$. But it probably won't work. There are a few things we need to get straight here, namely, the differences between brown dwarfs and giant planets. Here are two of the most important:[1] Mass. Brown dwarfs are, at the lower mass end, greater than 13 Jupiter masses, ...
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almost 9 years ago
Answer A: How could state borders equal major watersheds?
Lots and lots and lots and lots and lots and lots of mountains. Prior to the invention of flight, mountains were often more than a little bit annoying to cross. Passes make traversing a mountain range easier, but the journey can be treacherous if the roads aren't good. In a society in a more medieva...
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almost 9 years ago
Answer A: How close could we be to a stealth planetarysystem?
This is a tough question to answer because any planetary system with more than one planet is tough to analyze. When two planetary systems collide, the result is overwhelmingly difficult to analyze, even for computers. It's safe to say that we would notice effects before the planets even got near each...
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almost 9 years ago
Answer A: Can stars that are not powered by nuclear fusion exist?
Note: This answer is not even close to being finished. I'm putting it out there as a sort of sanity-check, so I can get some input as to whether or not my idea is totally crazy or not. Links and more numbers will be coming. Introduction When I wrote this question, I thought that the Kelvin-Helmho...
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almost 9 years ago
Answer A: How would the night sky appear from the inside of a star cluster?
I wrote a blog post about this for Worldbuilding Stack Exchange's blog, Universe Factory, so I'll work off of that. First, the good stuff: a picture of what you're looking for. It's from Alexey Bobrick's answer to the same question on Astronomy Stack Exchange: Image from this article. That big b...
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almost 9 years ago
Answer A: Considering life to be a matter modifier - what would be the final state?
Work towards the heat death of the universe. Your idea sounds a lot like the heat death of the universe, a hypothetical event where the universe reaches maximum entropy and no more work can be done. This will be the end of the dark era, when just photons, neutrinos, and other small subatomic particl...
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almost 9 years ago
Answer A: Could it be possible for two planets to collide and not kill everything?
Everyone will burn to death. This NASA page shows temperature changes during the collision between Earth and Theia in the Giant Impact Hypothesis. The temperature is on the x-axis, in Kelvin: Your people are going to die at temperatures worse than hellish. This will be an inferno never before see...
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almost 9 years ago
Answer A: What happens when one planet "lands" on another?
Your planet is screwed. As was mentioned in comments, the Giant Impact Hypothesis details a very similar situation, as Earth collides with another body of slightly smaller size at some angle of roughly 45°, so neither of the two are totally destroyed. In a more direct impact, the centers of both b...
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almost 9 years ago
Answer A: Can a star be so distant/isolated that its 'Earth' can't see other stars?
It's totally possible (sort of)! Such stars are called - not surprisingly - intergalactic stars. These stars are often referred to as hypervelocity stars, because in order to reach galactic escape velocity, they have to be traveling really quickly. The current theory is that a pair of binary stars ...
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almost 9 years ago
Answer A: How would the sudden destruction of Mars affect Earth?
The effect of Mars on Earth is negligible. The total force between the planets is &mdash; at their closest approach &mdash; $$F{\text{max}}=\frac{GM{\text{Earth}}M{\text{Mars}}}{r{\text{min}}^2}\approx8.6\times10^{16}\text{ Newtons}$$ This produces an acceleration of . . . well, pretty much zero. Ma...
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almost 9 years ago
Answer A: Possibility of a civilization on (or around) Planet Nine
The major problem here is that, while we don't know for sure what this planet is like - after all, the paper (Batygin & Brown (2015)) was based on simulations of the movements of Kuiper Belt Objects that matched observations - Batygin and Brown have implied that it may be the core of a 5th gas giant....
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almost 9 years ago
Answer A: Could humans survive on a planet with only deuterium 'heavy water' made with hydrogen isotopes?
It depends. Heavy water proves toxic to human cells at levels 20% of a human's body weight (although other sources give 25%). At lower levels, however, it's harmless. Given that 2/3 of a human's body weight is water, you'd have to have much higher ratios of D2O to H2O than you have in this scenario....
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almost 9 years ago
Answer A: Significant day-to-day impact of having eyes on palms of hands?
Hands are now useless. We use our hands to carry lots of things. This generally involves curling our fingers around something, which in turn blocks the palms of our hands. If we had eyes on our palms, they would be blocked whenever we need to use our hands for something. Meaning that we wouldn't be...
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almost 9 years ago
Answer A: Can there be planets with extremely strong magnetic fields?
Wikipedia states that the strength of the magnetic field near a kitchen magnet is approximately 5 mT ($5 \times 10^{-3}$ Tesla), roughly 1,000 times as strong as Earth's magnetic field in that same kitchen. Note that this is measuring the $\mathbf{B}$ field of the magnet. The magnitude of a magnetic...
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almost 9 years ago
Answer A: What common chemical/physical reactions would, in an atmosphere composed of ~20% Oxygen and ~80% Argon, behave differently than on Earth?
Argon is inert, meaning that in doesn't really interact much with anything. Like the other noble gases, it is much less reactive than the major components of the atmosphere. Therefore, to look at changes in this atmosphere, it's important to look at what we'd lose if the nitrogen was this substantial...
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almost 9 years ago
Answer A: Multiple star system near Sun: better choices
Xi Boötis Basic statistics: Distance from the Solar System: 22 light-years Spectral types: G8 Ve and K4 Ve Masses: 0.86 and 0.70 solar masses Semi-major axis: 33.14 AU Maximum semi-major axis for planetary orbit around Xi Boötis A: 3.8 AU Maximum semi-major axis for planetary orbit around Xi BoÃ...
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almost 9 years ago
Answer A: What Is The Minimum Size Of A Planet To Harbor Human Life?
The retention of gases such as oxygen, carbon dioxide, and water depends on the mass and radius of the planet, via its surface gravity. It also depends on the effective temperature, something we can calculate easily. I'd like to quote from an earlier answer of mine: The planetary equilibrium temp...
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almost 9 years ago
Answer A: Attenuation of a laser in space?
The equation for the power emitted by a laser beam at a distance in a circle of radius $r$ at a distance $z$ where the beam diameter is $w(z)$ is $$P(r,z)=\frac{1}{2}\pi I0w0^2\left(1-e^{-2r^2/w^2(z)}\right)$$ where $I0$ is the initial intensity and $w0$ is the initial beam diameter (see these course...
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almost 9 years ago
Answer A: What is the best way to use my two portals?
Congratulations! You've solved the energy crisis! Place Portal B some distance above Portal A. Now place a turbine blade in between the two of them. Slowly insert a steady stream of water, stopping when you reach a certain amount. As the water travels through Portal A, it goes to Portal B. It then ...
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almost 9 years ago
Answer A: Would the death of 50% of the human population drastically reduce carbon dioxide levels?
Some basic statistics: Number of humans on Earth: 7.3 billion people CO2 output per day per human: 1.0 kilograms CO2 output per year by vertebrates: 220 gigatonnes (=220 trillion kilograms) This means that humans annually breathe out 2.66 trillion kilograms of CO2 per year - less than 1% of the t...
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almost 9 years ago