Posts by HDE 226868
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 ca...
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 $\rho\approx6\times10^{-30}\text{ g cm}^{-3}$....
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 th...
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 bas...
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: $$...
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 ...
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...
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 (...
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 ...
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. Phy...
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 ...
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 a...
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 radi...
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 th...
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, spherical...
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 larg...
The five primary layers of the atmosphere are, with increasing, altitude, the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The corresponding boundaries are the tropopause, st...
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 inclu...
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. A...
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 ...
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...
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 ph...
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 phot...
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 -...
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...
