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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
apenpaapDate: Sunday, 04.01.2015, 00:56 | Message # 316
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Quote Watsisname ()
This is also one of the reasons why gravitational collapse must lead to a black hole, and not some solid body with a stable radius smaller than the Schwarzschild radius. Speed of sound cannot exceed c, so a body crushed within its Schwarzschild radius cannot be supported by pressure forces. Collapse must continue.


Ooh, I never realised this, though it makes complete sense.

Added (03.01.2015, 23:56)
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Does the Sunlight's radiation pressure have any destabilising effect on the planets' orbits whatsoever? I know the effect is tiny, but considering that tiny force has been working on the planets for billions of year, I wonder if it doesn't add up to something significant.




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WatsisnameDate: Sunday, 04.01.2015, 03:19 | Message # 317
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The effect of radiation pressure on a planet is nonzero, but so small as to be dwarfed by other phenomena, like tidal evolution, star's mass loss, and perturbations from other planets.

To be quantitative, pressure is force per area (P=F/A), and F=ma. The acceleration, then, is a=AP/m. A is proportional to r^2, while m is proportional to r^3, so acceleration of a body due to radiation pressure is inversely proportional to its size. A planet, being thousands of kilometers across, is billions to trillions of times less affected by pressure than dust grains, which are millimeter to micron sized.





 
Billy_MayesDate: Sunday, 04.01.2015, 20:46 | Message # 318
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Just wondering, why is there usually a huge hurricane on one side of a tidally locked planet or moon? Is it because of the heat?




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Edited by Billy_Mayes - Sunday, 04.01.2015, 20:46
 
WatsisnameDate: Sunday, 04.01.2015, 23:49 | Message # 319
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Basically yes. It's a consequence of the differential heating of the planet surface, with one "hot pole" on the sun-lit side.

On planets with low tilt angles and moderate rotation, like Earth, heating is greatest at low latitudes. Air there rises and flows toward the poles, then sinks, and forms a return flow along the surface. In the simplest case this would form two great circuits (one for each hemisphere), but rotation breaks it up into separate cells.

http://en.wikipedia.org/wiki/Atmospheric_circulation

On a tidally locked world, the heating is strongest not on an equatorial band, but at a single point which is fairly well fixed. So there is one hot pole and one cold pole. The circulation then takes the form of rising air (low pressure) at the hot pole, and return flow from the dark side of the planet.

Why does this circulation spin around the hot pole? Because the planet is rotating (same rate at which it orbits its star), and so there are Coriolis forces. So the hot pole tends to form a giant, cyclonic low pressure system similar to a hurricane. These features appear in atmospheric models of warm tidally locked planets, and that's what you're seeing in SE. smile





 
Billy_MayesDate: Tuesday, 06.01.2015, 15:02 | Message # 320
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Very interesting, thank you for the answer Watsisname. smile




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FastFourierTransformDate: Tuesday, 06.01.2015, 22:06 | Message # 321
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Quote Watsisname ()
Why does this circulation spin around the hot pole? Because the planet is rotating (same rate at which it orbits its star), and so there are Coriolis forces. So the hot pole tends to form a giant, cyclonic low pressure system similar to a hurricane. These features appear in atmospheric models of warm tidally locked planets, and that's what you're seeing in SE. smile


I think that all of this things shoul be explained in, maybe, the manual when the version 1 gets released smile things like this and things like the shape of supergiants are something that, when explained, gain a lot of apreciation and makes SE quite astonishing
 
WatsisnameDate: Wednesday, 07.01.2015, 08:46 | Message # 322
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I think this is a grand idea, and would further enhance SE as an educational resource. Also, it would be an efficient place to direct people with the common questions, instead of having repeated forum threads about why giant stars look so strange. biggrin




 
NiakoDate: Friday, 16.01.2015, 16:56 | Message # 323
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The other day while exploring some gas giants, i found some black rings, and it remind me to the Horsehead nebulae and i thought:
Could gas form rings in some gas planets in a stable orbit?
 
WatsisnameDate: Friday, 16.01.2015, 17:36 | Message # 324
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Gas molecules have too much thermal motion and not enough collisional settling to form rings around a planet in the manner that solid dust and ice particles can. Gas instead forms ring structures on a different level of scale -- accretion disks (e.g. around black holes), protoplanetary disks, and galaxies.




 
midtskogenDate: Friday, 16.01.2015, 18:16 | Message # 325
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Quote Watsisname ()
Gas instead forms ring structures on a different level of scale -- accretion disks (e.g. around black holes), protoplanetary disks, and galaxies.

Are these disks truly gas more or less evenly distributed, or will the gas in such disks first accumulate into "lumps" which by their great numbers make up the disk? At an even greater scale, galaxies tend to be disks, and one could argue that they're mostly made up of gas, but describing them as gas disks is misleading.





NIL DIFFICILE VOLENTI
 
WatsisnameDate: Saturday, 17.01.2015, 01:38 | Message # 326
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Depends on the disk. Accretion disks of black holes are extremely uniform since the velocity gradient shears it so strongly that no meaningful structure remains intact. (Magnetic fields might play a competing role in this, though.)

Galactic disks on the other hand are obviously not very uniform, as well as contain dust and stars and whatnot. Protoplanetary disks have irregularities (and dust, ices, etc) as well, e.g. by gravitational instabilities and perturbations from forming planets. Very dense disks may even have regions that directly collapse to form gas giant planets.

Planetary rings are often not very uniform either. smile





 
ProteusDate: Saturday, 17.01.2015, 18:04 | Message # 327
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What is behind the choice of letters in the MK spectral temperature class system? (Why does it use the letters O, B, A, F, G, K, and M?) Do they each stand for a term that is associated with what they represent?




 
WatsisnameDate: Sunday, 18.01.2015, 04:30 | Message # 328
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The letters A through Q were associated with the stellar spectra while attempting to arrange them according to their general appearance, before we understood the physics behind them. After more rigorous study we realized the sequence could be rearranged by a physical property (the surface temperature), which also made a lot of the letters in the scale redundant. Thus the OBAFGKM we know today. So the letters themselves don't mean much of anything, really. They're rather arbitrary. smile

If you look at the spectra charts for the sequence now you can kind of get an idea for how they were arranged originally, with spectra of increasing complexity (roughly speaking) attributed toward later letters.



Interestingly, parts of the old sequence are preserved (like AFGKM), with exception of O's and B's which are hotter than A's. So early intuition of sequencing by eye was not far from the mark since the physical appearance of the lines actually is largely related to the temperature.





 
pzampellaDate: Friday, 30.01.2015, 23:41 | Message # 329
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Are the Magellanic Clouds really flat like in SE? Because I would expect them to be like clouds, more width.
 
HandbananaDate: Sunday, 01.02.2015, 07:30 | Message # 330
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Why is it impossible for high mass terrestrial planets to exist? In other words, why are the massive planets always gaseous and never have solid surfaces?




Tonight... you.

Edited by Handbanana - Sunday, 01.02.2015, 07:31
 
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