Posts by HDE 226868
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 to...
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...
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 mag...
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 ba...
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. W...
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...
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 was...
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 r...
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 awa...
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 t...
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 emitt...
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 ind...
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 s...
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 st...
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 Ca...
A naive first calculation The formulas for orbital velocity and escape velocity are $$v_o=\sqrt{\frac{GM}{r}},\quad v_e=\sqrt{\frac{2GM}{r}}$$ I get $v_o=9.6\text{ km/s}$ for Saturn. For this to e...
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 se...
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, ro...
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 th...
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 sys...
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 suspende...
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 W...
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...
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, $F_...
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 y...
