Activity for HDE 226868
| Type | On... | Excerpt | Status | Date |
|---|---|---|---|---|
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A: Dark energy in a planet? The simple answer is a no. We've gotten some pretty good constraints from the Planck satellite; their final results indicated a dark energy density of $\rho\approx6\times10^{-30}\text{ g cm}^{-3}$. Within a planet of about twice the radius of Earth, you'd find a total of 51 milligrams of dark energy ... (more) |
— | over 4 years ago |
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A: I can't seem to find correct starbuilding equations that work together anywhere and am generally confused. Which ones do you use? In reality, if you want to build a star, you need to specify a mass and a chemical composition, and then use the equations of stellar structure. This requires some numerical integration, and it's far from simple. People have making careers out of it for generations. (One upshot of this, of course, is... (more) |
— | over 4 years ago |
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A: What does it look like inside a transparent glowing gas? I agree largely with Matthew's answer; this is intended to put everything on a more quantitative footing. The answer to your question primarily depends on three things: the mass of molecules in the gas, the density of the gas and the opacity. These will play the key roles in determining how the gas ... (more) |
— | over 4 years ago |
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A: Could triangular DNA exist? Sort of. A structure similar to the one you describe can in fact form. Triple-stranded DNA can be stable under certain conditions. Two bases bond via slightly different structures, and a third base bonds via something called a Hoogsteen hydrogen bond. In this manner, three different strands of DNA c... (more) |
— | over 4 years ago |
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A: Could habitable planets form in an ultracompact galaxy? Sure. Not a whole lot, but you'll get a decent number. Beer et al. 2004 present a formula for calculating the mean time before a star passes within a distance $b{\text{min}}$ of another star: $$\tau=7\times10^8\left(\frac{n}{10^5\text{ pc}^{-3}}\right)^{-1}\left(\frac{b{\text{min}}}{\text{AU}}\righ... (more) |
— | over 4 years ago |
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A: Could it be possible to quantum entangle particles on a mass scale? Just reduce the rate at which you lose entanglement (The paper, for anyone wanting to read it, is Humphreys et al. 2018.) The hey problem here isn't entangling particles, per se - the problem is keeping them entangled. The authors make the point that what we're interested in isn't just the rate at ... (more) |
— | over 4 years ago |
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A: Roughly how long could an 'Oumuamua type object get? Based on the current state of thinking, somewhere in the vicinity of a couple hundred kilometers. This particular formation theory (Zhang & Lin 2020) is a variant of an idea that's been kicked around for a couple of years. The basic principle is that early in the history of a planetary system, newly... (more) |
— | over 4 years ago |
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A: Solar expansion and habitability A decent proxy for habitability and long-term colonizability is the effective temperature of the planet - essentially the surface temperature. A planet's effective temperature scales as $T\propto (L/r^2)^{1/4}$, where $L$ is the luminosity of the star and $r$ is the planet's orbital radius. We want o... (more) |
— | over 4 years ago |
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A: How would one detect a black hole when there are no more light sources for it to refract? Try microlensing other evaporating black holes Folks have suggested Hawking radiation; I don't think that's a particularly good idea. If you run the numbers, a black hole would need to have a mass of $M\sim10^3\text{ kg}$ before it reached a luminosity of $1L{\odot}$, and would have only about 10 na... (more) |
— | over 4 years ago |
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A: What, visually, would a hole bored in the moon look like? It would be uniform. The crust of the Moon is, on average, about 50 km thick. There is indeed some stratification in its composition, with upper layers composed largely of feldspar and a lower layer composed of more mafic rock (Arai et al. 2008). The upper feldspathic layers appear to be $\sim$10 km... (more) |
— | over 4 years ago |
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A: How long and where could an antimatter object survive in space? A cosmic void. Your best bet would be in a cosmic void. While not entirely empty - they do contain small numbers of galaxies and clouds of gas - they are substantially rarefied compared to your average patch of the universe. Although I've been unable to find concrete numbers for the low-density gas ... (more) |
— | over 4 years ago |
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A: How Would These Planets Affect Earth's Eccentricity? The best tools for this job, I think, are perturbation theory and Laplace's planetary equations. You might know that the orbit of a planet can be described by six osculating elements $(a,e,i,\omega,\Omega,\bar{M0})$. These are, respectively, the semimajor axis, eccentricity, inclination, argument of ... (more) |
— | over 4 years ago |
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A: How fast would strange matter conversion go? The behavior of strange matter is not well understood - least of all under the conditions we're used to on Earth! Most theoretical treatments focus on places in which strange matter is likely to be produced or remain stable, like at the centers of neutron stars. If we put our mathematician hats on, w... (more) |
— | over 4 years ago |
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A: What biological trait could make (certain members of) an insectoid species good at astronomy? At least on Earth, astronomical instruments are extremely precise and sensitive compared to the innate biological abilities of an astronomer! Sure, I can look up at the sky and see that Betelgeuse is a bit dimmer than normal, but I can only guesstimate how drastic the fainting is. With a decent teles... (more) |
— | almost 5 years ago |
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A: What would happen to a gas giant during a supernova? Simulations of the dynamics of planets close to massive stars during a supernova (Veras et al. 2011) indicate that a planet in a reasonably tight orbit ($\sim2\text{ AU}$) around all but the lowest-mass supernova progenitors is almost certainly going to be ejected from the system. The cases where a p... (more) |
— | almost 5 years ago |
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A: An anti matter planet behaving like a star The scenario you describe - accreting matter being expelled by radiation pressure - will occur if the object exceeds the Eddington luminosity, a limit derived from hydrostatic equilibrium based on the balance between gravity and radiation pressure. The Eddington luminosity $L{\mathrm{Edd}}$ is propor... (more) |
— | almost 5 years ago |
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A: Plausible reason not to notice a planet A simple answer is that the planet is on an orbit with a high inclination relative to our line of sight. The other planets in the system may appear, from our perspective, to be in line with us and the star; we could then discover them through transits. This new planet, on the other hand, would never ... (more) |
— | almost 5 years ago |
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A: How to calculate the thickness of the ice layer of a frozen ocean planet/moon? There are a whole bunch of different ways to determine the thickness of a planet's ice sheets; over the decade, dozens have been tried on Europa and other icy bodies. Broadly speaking, as we're working from a purely theoretical perspective, our method will have to be thermodynamic in nature - we can'... (more) |
— | almost 5 years ago |
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A: Can an atmosphere be thicker at higher altitudes? The density profile of a planet's atmosphere arises from two laws of physics: hydrostatic equilibrium and the ideal gas law. Put together, they require that the density $\rho(z)$ be a function of the form $$\rho(z)=\rho0e^{-z/H}$$ where $H$ is the scale height, determined by the planet's surface grav... (more) |
— | almost 5 years ago |
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A: Could there be something like the Higgs field which gives particles their energy in a similar way to how the Higgs boson gives particles their mass? In most quantum field theories$^{\dagger}$, we have a quantity called the Lagrangian, from which we can derive information about the behavior of our system. It consists of a number of terms representing different quantum fields. Now, we are mathematically forbidden from naively adding mass terms by s... (more) |
— | almost 5 years ago |
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What would the geologic record look like on a planet in the galactic halo? Imagine an Earth-like planet orbiting a Sun-like star in the inner halo of the Milky Way. As a halo star, it will likely be somewhat metal-poor, having formed early in the life of the galaxy, but other than that, we can assume the planet and star to be like our own. The system lies about 4 kpc from t... (more) |
— | almost 5 years ago |
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A: Randomly generating plausible star types for a synthetic galaxy? This has actually been an area of intense research for decades now. Astronomers are quite interested in the distribution of stellar masses in a variety of different galaxies and clusters. The precise mix you're going to get of course depends on the environment you choose; galaxies with higher metal c... (more) |
— | almost 5 years ago |
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A: Assuming a flat world and no obstacles, how far could you see? Here's a first-order approximation based on a fundamental limit: diffraction and angular resolution. How far someone can see of course depends on the size of the object they're looking at, because the main limitation we have is one of angular size. The issue comes down to something called the Raylei... (more) |
— | almost 5 years ago |
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A: How would the strong magnetic field of a white dwarf affect humans inhabiting a planet that orbits such a star? It wouldn't. The field of a magnetic dipole has a strong radial dependence; it falls off proportional to $r^{-3}$, where $r$ is the distance to the dipole. The values you list are the strengths of the white dwarfs' magnetic fields at their surfaces. The field strength at the orbit of a planet would ... (more) |
— | almost 5 years ago |
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A: Popping "vacuum bubbles" loudness The collapse of bubbles on various scales has actually been an area of research for quite some time. Analyses are typically numerical, and rely on something known as the Rayleigh-Plesset equation, which tells you how the bubble's radius varies as it oscillates or collapses. If you know the pressure i... (more) |
— | almost 5 years ago |
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A: Orange Suns and Blue Jupiters You have two questions to consider here: Can compounds required for blue atmospheres form in significant amounts on this planet, and are the temperatures right for them to condense and form clouds? I talked about atmospheric composition and color in an answer to a related question. Essentially, the ... (more) |
— | almost 5 years ago |
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A: Feasibility of compressing matter to electron degeneracy Use low temperatures. For a given system, we can tell if degeneracy pressure is important by comparing the Fermi energy $EF$ to the thermal energy $kT$. if $EF\gg kT$, the gas is fully degenerate; even $EF\sim10kT$ will apparently lead to at least partial degeneracy. As the Fermi energy scales as $E... (more) |
— | almost 5 years ago |
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A: What could cause a spike in the natural uranium depletion rate? I agree that a possible solution would be to add in a source of high-energy ambient neutrons. Cosmic rays are a possible neutron source, at least at high enough altitudes and assuming the uranium does not have adequate shielding. A strong spike in cosmic rays could then provide such a neutron source;... (more) |
— | almost 5 years ago |
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A: When will the last white dwarfs become black dwarfs? About 100 trillion years from now. The answer to your question depends on several things: When the final low-mass star forms How long it takes that star to exit the main sequence and become a white dwarf The time needed for the white dwarf to cool to become a black dwarf Given a typical galaxy a... (more) |
— | almost 5 years ago |
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A: How to get a green gas giant? You need methane, ammonia and atmospheric temperatures of $∼$150 K. The color of a giant planet depends on the type of clouds dominating its upper atmosphere. These in turn depend on the temperature of the planet, as different molecules reach their gaseous phases at different temperatures. There a... (more) |
— | almost 5 years ago |
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A: Could bacteria survive on a star? Some simple molecules can exist in stars - but not the right ones. Contrary to popular belief, many different types of molecules can exist in stellar atmospheres, especially cool stars like red dwarfs. Some are even helpful spectral diagnostics; TiO bands are very common in stars cooler than 4000 K... (more) |
— | almost 5 years ago |
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A: How can I get a total solar eclipse to be a yearly event? The simplest way to achieve this is to have the Moon be on a perfectly circular orbit that lasts twelve months and lies in Earth's orbital plane; this would mean that at each new moon, we would get a solar eclipse. Let's say we keep Earth and the Moon at the same masses. We'd then need the Moon to or... (more) |
— | almost 5 years ago |
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How much of a star can be covered in starspots? Many stars, including the Sun, periodically display starspots, cooler areas of the surface associated with higher local concentrations of the stellar magnetic field. They can sometimes be a couple thousand Kelvin cooler than the surrounding regions of the stellar photosphere. My reasoning is that bec... (more) |
— | about 5 years ago |
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A: Are elliptical rings feasible? It's unlikely, on the whole, for a ring system to maintain a high eccentricity on significant timescales. Dissipative collisions tend to circularize the orbits of individual particles, even if the original constituent body traveled on a fairly eccentric orbit. Therefore, you need some external pertur... (more) |
— | about 5 years ago |
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A: Can adding mass to a ringed planet cause a huge chain reaction? It's an interesting scenario. The major problem is that ring systems tend to be quite low-mass in comparison to their parent bodies. For example, measurements by Cassini indicate that in the case of Saturn, the ratio of ring mass to planet mass is $MR/Mp\simeq2.7\times10^{-8}$ (Iess et al. 2019). Eve... (more) |
— | about 5 years ago |
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A: Anatomically Correct Quantum Slime Handwaving away some of the problems here, one basic one appears insurmountable: Decoherence. This is a process that involves the destruction of the superposition of a quantum system. The loss of quantum coherence is a major experimental problem in, among other things, quantum computing. Minute inter... (more) |
— | about 5 years ago |
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A: Could a binary planet system have a shared magnetic field? We can treat a planet as a magnetic dipole. In this case, the strength of the field scales as an inverse cube law (rather than the more familiar inverse square law), with some angular dependence. At the magnetic equator, we can write this as $$B(r)=BS\left(\frac{r}{Rp}\right)^{-3}$$ where $Rp$ is the... (more) |
— | about 5 years ago |
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A: What element, if any, would justify mining stars (financially)? No, there's no such element that would justify this sort of attempt. We have data on the composition of the Sun's photosphere, one of its outermost layers. By mass, the solar photosphere is 98.3% hydrogen and helium. Oxygen and carbon compose another 1%, followed by even smaller quantities of iron, ... (more) |
— | about 5 years ago |
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A: What elements would be created in a star composed entirely of gold? This star would not fuse gold. Fusion reactions producing elements beyond zinc-60 are not energetically favorable; they are endothermic, and so consume energy. Several elements heavier than iron are formed through this fusion chain and subsequent decay (cobalt, nickel, copper and zinc), but these ar... (more) |
— | about 5 years ago |
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A: What does and doesn't work with this planetary system around a red dwarf? I can see four key problems with the system as you've described it: Giant planets. The major thing that concerns me about the system is the presence of at least two gas giants. It's long been thought that low-mass stars like red dwarfs are unable to form giant planets, because the stars' protoplane... (more) |
— | about 5 years ago |
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A: What kind of mathematical disciplines would be most useful for physics? Let's assume that this student wants to begin by understanding the twin pillars of modern physics: quantum mechanics and general relativity. There are several major tools in the toolkit of anyone studying both of these theories at a basic level: Calculus (single-variable and multivariable) Differ... (more) |
— | about 5 years ago |
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A: How to find the density of a planet and its core taking into account the gravitational compression in them? The technique to do this is similar to that used in constructing stellar models. You know some of the properties of your object - in this case, it look like the mass and radius. You want to figure out the internal structure of the planet, including the central density and pressure, as well as the den... (more) |
— | over 5 years ago |
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Can my planet have a very thin atmosphere only at the poles? While thinking about Starfish Prime's answer to the question Algae using UV light from auroras for photosynthesis, I considered the possibility of an alternate Earth which has a normal, Earth-like atmosphere throughout most of the globe, but a thin atmosphere at the poles. This would allow for higher... (more) |
— | over 5 years ago |
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A: Algae using UV light from auroras for photosynthesis It might be possible. We've known for around a century (since at least 1933) that ultraviolet light can inhibit photosynthesis and possibly damage photosynthetic mechanisms inside an organism. Phytoplankton in particular have been recent targets of this sort of research, and it's been shown that the... (more) |
— | over 5 years ago |
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A: How long until two planets become one? We're talking hours to days. A good deal of work has been done on protoplanet-protoplanet collisions, mainly focused on testing the Giant Impact Hypothesis for the formation of the Moon. A number of fluid simulations (many smoothed-particle hydrodynamics) have been performed, for varying angles of a... (more) |
— | over 5 years ago |
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A: Life around Cepheid Variable stars Stars that become Cepheid variables stay in this phase of their lives for only a short period of time, and after they leave the main sequence. While their properties vary (in particular, Cepheids are divided into two groups, Type I (Classical) Cepheids and Type II Cepheids) there are a few characteri... (more) |
— | over 5 years ago |
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A: A planet illuminated by a black hole? This scenario is quite problematic for two main reasons: evaporation and peak wavelength. The black hole's lifetime is too short We can make a rough estimate of the properties of the Hawking radiation coming from the black hole. First, let's start with the luminosity. Since $L\propto M^{-2}$, where... (more) |
— | over 5 years ago |
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A: Designing (a specific kind of) dark matter As the sphere is self-gravitating, it must be in hydrostatic equilibrium; that is, there must be a non-zero pressure gradient to balance the force of gravity. For a fluid of uniform density, you can show that this pressure gradient is proportional to radius: $$\frac{dP}{dr}=-\frac{4\pi G}{3}r\implies... (more) |
— | over 5 years ago |
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A: Intragalactic velocity past the central black hole I don't have a solution; what I do have is a possible path to a numerical solution. For the sake of simplicity and sanity, I will consider the special case of a non-rotating, chargeless, spherically symmetric black hole. This black hole causes space to take a shape described by the Schwarzschild met... (more) |
— | over 5 years ago |
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A: What can my moons be made of? Carbon planets We typically expect a moon's composition to reflect the part of the protoplanetary disk it came from. If it's orbiting a planet close to the star, we'd expect it to be composed largely of silicates, like Earth or the Moon. If it orbits farther out, it would be reasonable to expect the... (more) |
— | over 5 years ago |