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Julian
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29 Jun 2019 20:44

Julian wrote:
FastFourierTransform wrote:
By the way, Whatsisname's diagram explicitly answers your question, look which axis the jets follow.

I wanted to make sure that the areas of radio emission always coincide with the jets, but thanks for the explanation. I can understand why Vladimir hasn't been able to implement astrophysical jets in SE yet, because there are certainly a lot of issues to work out, like how to model the distortion in the accretion disc when the jets are off-axis, and ensuring that the magnetic axes of known pulsars are oriented so that their jets can point toward Earth.

A followup on this discussion about pulsar jets that I started last year: When observing a pulsar from Earth, can we determine what the angle between the rotation axis and magnetic axis is?
 
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04 Jul 2019 02:10

Julian wrote:
Source of the post When observing a pulsar from Earth, can we determine what the angle between the rotation axis and magnetic axis is?

A late answer, but the answer is yes, this can in fact be done!  The method is outlined in section 2 of the paper On the Evolution of Pulsar Beams by Tauris and Manchester (1998), where the angle between the rotation axis and magnetic axis is labelled α:
Image

The details are fairly complicated, but the basic idea is straightforward.  To determine the angle, we're fitting a model of the pulsar's sweeping beams of emission to the signal we observe.  What we need are measurements of the period of the signal, the width of the pulse (how wide is the segment of the beam that crosses us), and its polarization.  The polarization reveals information about the magnetic fields the radiation passed through -- that is, what part of the pulsar beam are we seeing?  Combining that with the period and pulse width reveal how far that beam is from the spin axis.

This isn't foolproof and it may not work with all pulsars.  Some pulsars "glitch" which can throw off everything.  The results may also depend on model assumptions, sometimes by 20-30 degrees.  That being said, it has been accurate enough to reveal a lot of interesting information about pulsar evolution.  For example, for the first few thousand years the magnetic axis tends to migrate away from the spin axis until they are nearly orthogonal!  Then for the next millions of years a magnetic braking effect slows the pulsar's spin rate and also brings the magnetic axis back towards the spin axis, until they are closely aligned.  The pulsar's beam also becomes narrower.  Pulsar magnetic alignment and the pulsewidth–age relation.
 
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Julian
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06 Jul 2019 20:35

Is there a list of pulsars with known values of α and ζ? The ramifications for Space Engine would require neutron stars to have values of NorthLat, NorthLon, SouthLat and SouthLon like auroras on planets do, and to generate those values randomly if they're not known. Given the RA and Dec of a pulsar, defining the rotation axis with a quarternion and determining latitude and longitude coordinates for the magnetic poles such that one jet will sweep across Earth would be an interesting math problem, but it's beyond my ability.
 
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09 Aug 2019 10:42

Watsisname wrote:
Stellarator wrote:
Source of the post We on the forum have talked about this many times, and I always have to remind people that known physics DOES NOT FORBID FTL. Hell, things in our universe are traveling faster the light right 'now' beyond the Cosmic Event Horizon (it's important to note that this is from our perspective. It's all relative). 

A big difference though: nobody directly measures such FTL motion of galaxies in the universe, or objects passing below a black hole's horizon, or other such instances where we might say velocities are faster than c.  There are no actual causality issues with those motions.  But with an Alcubierre drive, someone would measure that FTL motion, and it is trivial to construct a causal paradox with it.  Just imagine someone making a round trip FTL journey.  Then we're back to the grandfather paradox of special relativity.  In my view this is a pretty powerful reason for concluding the Alcubierre style of warp drive is not possible, even aside from the exotic energy requirements.  (We could alternatively say the exotic energy requirements are a symptom of this being a nonphysical solution of general relativity's equations).

It could only be possible if we could resolve the paradox in some other way (like MWT or Deutsch's model.)
 
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10 Aug 2019 14:31

What does a sea that is blue on earth look like on a planet under a white / blue sun?
JackDole's Universe: http://forum.spaceengine.org/viewtopic.php?f=3&t=71
JackDole's Archive: http://forum.spaceengine.org/viewtopic.php?f=3&t=419
JackDole: Mega structures ... http://old.spaceengine.org/forum/17-3252-1 (Old forum)
 
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10 Aug 2019 20:19

So I have a roleplay character, that has the following addictions. I believe that her body would build up an immunity to these addictions, or would it lead to a premature death? Would this addiction get rid of her sense of smell and effect her nervous system? I believe it would, but I'm curious to see other people's thoughts on what would happen to someone that has these addictions over a long term basis, and potential cures if harm does occur. 

She enjoys the smell of Propane, Gasoline, Kerosene, Paint Thinner, Oil, Diesel, and has a massive addiction to the consumption of Diethyl Ether and Polyethylene glycol, aswell as various other fluids that are polyetheric and organic in nature.
 
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11 Aug 2019 15:21

JackDole wrote:
What does a sea that is blue on earth look like on a planet under a white / blue sun?


My guess, blue. I’m not sure if it reflects or refracts blue light but either way, its not absorbing it, and the sun is white, so that’s answered. Under a blue sun, probably more blue than on earth. BUT, if there was photosynthesizing organisms like cyano bacteria, algae, plankton, or some alien equivalent, then it could vary. With a blue sun, depending on the sensitivity of the organisms, they might want that blue light, so the seas might look black or dark orange-ish, but if the blue light is too intense, they might be blue. White/Blue would be in between. Someone might want to cross check me though.
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Terran
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11 Aug 2019 15:31

Is there such thing as a stable orbit, or do the laws of physics forbid this? As in, any two body system should lose energy to gravitational radiation right? Even if one of the bodies were anomalous, like a space craft stabilizing an orbit, since over infinite time, it would need infinite energy to stabilize an orbit. Or is there a geometry of spacetime that allows for 0 gravitational radiation in a system, or one that reabsorbs that loss and is then net 0, like a universe with looped ends or something?
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Watsisname
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13 Aug 2019 01:24

Terran wrote:
Source of the post Is there such thing as a stable orbit, or do the laws of physics forbid this? As in, any two body system should lose energy to gravitational radiation right?

Yep, you're completely right.  All orbital motion generates gravitational waves, which while for most situations is a really, really small effect, would nevertheless cause all objects to spiral together given enough time... assuming proton decay, cosmic expansion, or a Big Rip doesn't get in the way first.

You raise a fascinating question though with introducing a curvature to the space.  If the universe is positively curved then the gravitational waves could eventually come back together at the "opposite side" or antipode of the universe, just like how all lines of longitude radiating out from the North pole eventually converge at the South pole on the Earth.  And then they'll converge once again back where they started, at the system that was radiating them.

Could those waves somehow actually pump orbital energy back into that system, cancelling out their decay?  Well, we'd need to calculate the propagation of those gravitational waves through the positively curved 3D space, while that space is presumably expanding or contracting (unless we perfectly balance it with dark energy?), and then how the re-converging waves then affect those two orbiting bodies much later.  

As I contemplate the notion of doing such a calculation to see if it could work that way, the following meme comes to mind,

► Show Spoiler


and I conclude the answer is "it probably doesn't work that way".  :P

(Actually I am quite sure it could not work that way, because as time goes on the rate of energy radiated by the decaying orbits will increase, and so even if the system could perfectly re-absorb that energy (totally dubious) after it has traveled across the whole universe, by the time those waves come back they won't offset the current decay rate.)
 
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Watsisname
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13 Aug 2019 01:52

Slightly aside, if we consider quantum mechanical systems like electrons orbiting protons in atoms, then those orbits are stable against gravitational decay.  In fact they are also stable against electromagnetic decay.  A classical calculation would suggest that because an electron orbiting a nucleus is constantly accelerating, and accelerating charges radiate, the electron should very rapidly lose orbital energy and crash into the nucleus.  All atoms should collapse in a tiny fraction of a second!

This doesn't happen because the electron isn't really "orbiting".  Its energy is quantized (thus the different, discrete energy levels in the atom), and the electron can only jump between those levels by absorbing or emitting a specific wavelength of photon.

However, there's another phenomenon that prevents the electron "orbit" in an atom from being stable indefinitely, anyway: quantum tunneling.
 
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14 Aug 2019 00:28

What is the structure of the interstellar medium on a galactic scale? In maps I've seen, the interstellar medium typically forms a shell around the sun a few hundred lightyears away. (The maps tend to be inconsistent, I guess the exact shape is still being figured out?) What is the structure beyond our local bubble? Is the entire galactic disc swiss cheese'd with bubbles and holes? Does it get thicker in the spiral arms, or disappear in between the arms?
 
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FastFourierTransform
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15 Aug 2019 08:31

slimy wrote:
Source of the post What is the structure of the interstellar medium on a galactic scale? In maps I've seen, the interstellar medium typically forms a shell around the sun a few hundred lightyears away.

This is a fascinating and very complicated topic. I wanted to create a separate topic in the forum since last year, called "Atlas of the Solar Neighbourhood", where we can do the detailed insight this thing merits. I will launch it given enought time (and study).
For now let me adress at least the concern of the maps been inconsistent. Making good visualizations and maps of what essentially is just a gas is very difficult. Even if you know the actual shape and density distribution of it you have to realize that a few things make it very hard to visualize. There are several issues related to this:

1) When we represent a 3D structure in a 2D map we lose a lot of information. We can tackle this by making different 2D maps from different perspectives. For example one that uses the Solar to Galactic Center line as the x-axis and the Solar - North Galactic Pole line as the y-axis (perpendicular to the galactic disc), the so called  UW plane (to note the vertical structure of the galactic disc), another that uses the same x-axis but lies on the galactic plane, with the y-axis pointing in the same direction as the Sun moves, the so called UV plane (to better visualize the disc structure around us), and many other possible projections of the actual 3D structure. We can also make an equirectangular projection of what we see from Earth radially. But as I say, translating 3D objets to 2D representations (or any lowering in dimensions) always comes to a cost in the amount of information.
Image
A very simple example is this object that can be projected linearly into a circle, a square or a triangle (all very different shapes with very different qualitative descriptions) depending only on your choice of perspective.
The Interstellar Medium is shaped in a very intricate way so you have to choose what information is the one you want to prioritize in your representation. Looking for the Gould Belt for example (a ring of blue stars that surrounds the Solar System with 1500 ly in radius and that is almost parallel to the galactic plane) from the UW plane perspective you would only see a line and all the visual information about the ring shape is lost by squishing. On the other hand, the UV plane perspective gives the ring information but since you squish everything vertically in the galactic disc you no longer have a reference as to how inclined is this plane with respect to the disc. These decisions make interstellar structure maps often complicated to interpret.
In fact there are features that no shadow-casted perspective allows for a good visualization into a 2D map! For this reason data scientists have invented PCA and other dimensionality reduction algorithms that transform a 3D system into a 2D map in such a complicated way as to allow the specified feature to be clearly separated from the mess but with the downside that axes are no longer equally measured nor perpendicularly oriented between themselves and can even be distorted in non-linear ways to reach that goal (which makes the intuition about the actual 3D distribution of the entire ensamble go insane even if now you can clearly see some specific part of it). You might have encountered these representations in scientific articles.
They are no inconsistent, they are only different perspectives depending on the focus of the research. The actual 3D distribution is actually fairly modeled.

2) There is no solid surface but a solid volume so we have to make slices. Since the gas is a volumetric object it is not enought to represent it as a solid blob with opaque exterior. Sometimes you want to get some insight on what is inside of it. So to do that you need to make a cut and dissect the object. By doing so you lose information about the enclosure but gain information about the internal structure. We all realize how missleading a slice of a human body is to understand what a person looks like from any conceivable angle but how usefull is to make these kind of representations to help visualize the shape of different organs and how they are connected. The same goes for the interstellar medium, there are artistic representations that show the view from the outside but several shells and clouds overlap and you lose the details of the ensamble, and there are representations that show things as you would see them if you made a cut somewhere.
Image
For example, this is a representation of the 815 ly around the sun as viewed from the UV plane perspective (top-down view from above the galactic disc). But is just a specific slice of it, the UV plane that crosses the Sun. Here you can see a more or less consistent region of low density medium (the Local Chimney) surrounded by denser walls (and huge gaps called tunnels).
This is not a closed irregular cavity but is opened from above and below the galactic disk. You can see that from the UW perspective and sliced so that the sun is contained in the plane:
Image
I'm not going to explain in detail here but the chimney is the result of several overlaping cavities generated by supernovas a few million years ago in this region of the disc (the sun is unrelated but we have been traveling inside this structure for some ten million years now and we are currently just in the middle). These, more or less spherical, cavities have been compressed by the pressure of the gas in the disc, squished until they have poped into the less pressurized region above and below the disk, creating this cilindrical lower-density medium.
If we didn't sliced the first representation we would have seen gas everytwhere since the chimney has it's axis inclined with respect to the perpendicular of the galactic disc and thus there are parts of the upper exit that would obstruct the view of the inner part of the chimney and the same goes for the lower exit. A good perspective would be the view from a certain angle so you can peer through the chimney to the other side, but since it has an hourglass shape (the chimney gets thinner in the middle and opens at the exits) you would always miss information about the walls of the opening of the other side. So you really need to make a cut in the structure and visualize it with slices.
The UV cut made to contain the Solar System might not even be very helpfull for other purposes. Another UV plane sliced at another hight (paralell to the first UV plane) might be much more interesting. Maybe we have lost a tunnel in intergalatic medium connecting the chimney in the side to other structures nearby just because the tunnel was higher up in the chimney and we decided to make the cut at solar hight. These are one of the many problems in trying to represent these features.
Just so you have an actual example of this, here they've made many different slices, all aligned y-axis to the North Galactic pole and rotated 15º degrees each (all the slices centered on the solar system). You can see how the apparent shape of the local chimney changes as we move around.
Image
There is no inconsistency in the mappings is just a matter of perspective and good slicing. Different but complementary visualizations that yield different information about the real structure.

3) We are talking about a gas not any volumetric object. Gradients are tricky. What is the wall of the local chimney? This might seem a simple question but since we are dealing with gases and they tend to be distributed in continuous gradients of density the way we establish the boundary comes to a decision that can make for very different visual representations. If we take density as the feature to be taken into account then before visually representing the structure we need to mark an equal-density surface (a threshold to signal the transition between the lower densities under consideration and the higher ones).

Just so you see the extreme differences caused by different iso-density surfaces selection take a look at this. Here they've added countour lines to the interior of the local chimney as seen in the UW plane cointaining the Solar System. The countour lines are nothing else than the result of slicing the iso-density surfaces.

Image

As you can see the lower density region is not only smaller but completely different in shape as other boundaries marking higher densities. Adding more countour lines helps to grasp the density gradient of the gas, but can be a mess to have it in a detailed map. So maps have also to choose the threshold to which they determine the boundaries of different structures in the interstellar medium. That is another reason for apparent discrepancy in visual representations.

Look at this depiction of the local chimney

Image

Again, we are not getting information of what is inside and how it's structured. So a solution is to create 3D renders on which the equal-density surfaces are many and semitransparent so you can see the others. This is what they did in this animation of the entire solar neighbourhood:



As you can see it is still very difficult to extract a meaninful description of what you see in here. Also video and 3D rederings are not a good for printed articles.


4) Fail to explain the structures involved and confusion. Scientists not always explain these structures in a orderly, schematic and clear way when it comes to communicating to the public. The thing is that the stellar neighbourhood cosmography is almost like a Matryoshka doll. The solar system is embeded inside the Local Interstellar Cloud (LIC) which is interacting with another gaseous medium called the G-cloud (which contains the alpha centauri system and other nearby stars in that direction). Both the LIC and G-cloud are less than 10 pc in size, and both reside inside the Local Bubble which is around 300 pc in size, but the Local Bubble is just an undifferentiated part of what is now known as the Local Chimney which is in the 800 pc regime. All of this is part of a region called the Orion Spur, which is a branch of the Orion spiral arm of the Milky way. Inside the Local Bubble we know of around 15 clouds (like the LIC and G-cloud). They deform and interact one with the other and are subjected to stellar wind forces from our closest neighbours. These clouds have filamentary structures at the scale of parsecs due to the magnetic field of the galaxy shaping them, in a similar way as the iron filings behave when exposed to a bar magnet, following the magnetic field lines.

Image

The Local Bubble at another scale is also not alone. Connected to it by interstellar tunnels in the gas there is the Loop I bubble and other structures of hundreths of parsec in size.

Image

They are not filamentary like the much smaller interstellar clouds inside the bubbles, they are shaped like spherical and cilindrical cavities (results of supernova shells overlapping toghether). It is indeed like cheese in the sense that there are lots of tunnels connecting these "voids" traversing the more dense walls of the galactic interstellar medium.

Besides all of this, many do not fully realize the fact that these clouds and bubbles are essentially vacuum. The thing is that clouds like the LIC are 0.3 atoms per cm3 dense, that the bubble is floats inside (the Local Bubble) is empty in comparison with just 0.05 atoms per cm3 and that these bubbles are pressed against by the interstellar medium of the rest of the galaxy which is almost as ethereal as the clouds inside the bubbles with just 0.5 atoms per cm3. But 0.3, 0.05 and 0.5 atoms/cm3 is essentially the same as a vacuum. In fact all of them have lower densities than the vast majority of vacuum chambers on Earth (and still they are clouds, and material mediums that interact). When viewed from the scale of a galactic sized creature these are indeed dense gasses with clear differences, from our perspective it is just a huge void filled with stars.

Another source of confusion is the fact that scientists consider different things clouds and bubbles in relation to their surrounding material. For example, if the LIC was located outside the Local Bubble in the interstellar medium of the galaxy then it wouldn't be considered a cloud but a void (or a small bubble), since it is less dense than it's surroundings. But since it is inside a less dense bubble which is encapsulated in a more dense medium we talk about is as a cloud. So this relative designation yields a lot of confusion when talking about other structures. Remeber, it's all relative to its inmediate surroundings.


-----------------------------------------------------------------------------------------------------------------

If you want to read about this I can reccomend some easy to follow explanations from the very basics. Also there are awesome maps made to explain this architecture in detail and not only to point some specific feature while obscuring others as research papers tend to do.

For the 100 pc to 600 pc surroundings of the Solar System:
Image


For the 600 pc to the 4000 pc we have this http://gruze.org/galaxymap/poster4/


Also I highly recommend this NASA video about the results from the IBEX mission which explores the smallest scale of all of this, the immediate surroundings of the Sun and the LIC and G-clouds:

 
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Cantra
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15 Aug 2019 12:04

If you had an Earth around a hotter star and it's in the habitable zone, will the star be the same (similar apparent brightness in the sky as the sun or not?
 
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Watsisname
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15 Aug 2019 19:33

Cantra wrote:
Source of the post If you had an Earth around a hotter star and it's in the habitable zone, will the star be the same (similar apparent brightness in the sky as the sun or not?

Similar, but not exactly the same.  In fact, if the star is either much hotter or colder than our Sun, then from the habitable zone its apparent magnitude will be slightly dimmer!  

To keep Earth's temperature the same, the amount of solar radiation hitting it must be the same.  We would measure the same number of watts per square meter in sunlight at the top of the atmosphere.  But if the star is hotter, then its spectrum will peak at shorter (bluer) wavelengths:

Image


And here's the key: our eyes are not equally sensitive to all colors of light, even across the visible spectrum:

Image


Our eyes see color via 3 types of cones, each of which is sensitive to different but overlapping parts of the visible spectrum..  Their combined response makes our eyes most sensitive to green light, and the sensitivity drops off rapidly towards red or violet.  This isn't obvious in everyday experience, but can be witnessed dramatically by shining lasers of different colors but equal intensity.  A 5mW green laser appears much brighter than a 5mW red or violet laser.  Yet they give off the same amount of light.

But just how big of a difference will this effect make for the appearance of our Sun, if its temperature were different but we move the Earth so that it stays in the habitable zone?  Using the process outlined here for calculating the apparent magnitude of a star using the sensitivity curves of a V filter (which most closely resembles our own eyesight), I find the following apparent visual magnitudes for stars of different temperatures from their habitable zones and plot them.  All are computed from a distance where the flux of sunlight is kept the same as what we get here on Earth (about 1368W/m2).  Our own Sun's temperature and apparent magnitude is marked with a + for reference.

Image


Our Sun is near the minimum of the curve, or the brightest apparent magnitude.  Coincidence? :)  A little bit, but also a product of evolution.  Our vision evolved to be sensitive to a window of wavelengths that the Earth's atmosphere transmits, which is also where the Sun's spectrum peaks.

But the true minimum is a little further to the right, at higher temperatures of about 6700 Kelvin, corresponding to F-type stars.  This is where we consider the star's color to be "white" instead of "yellow" like our Sun, and we would get a slightly stronger response from our eyes.  At 6700K the star would appear brighter from its habitable zone by about 0.05 magnitudes (too small a difference to be noticeable by any stretch of imagination).  For even higher temperatures, the star appears dimmer, because larger fractions of its light are made up of blue, violet, and even ultraviolet light that we are less sensitive to.  

Hot B-class stars with temperatures over 25,000K would appear 2 magnitudes dimmer from a distance that would feel comfortable (ignoring being fried by UV radiation).  On the other end, cool M-class stars with temperatures under 3000K would also appear more than 2 magnitudes dimmer, with most of their warmth coming through infrared.
 
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Cantra
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15 Aug 2019 19:40

I see. That's very interesting! Quite useful for worldbuilding purposes as I feared that planets around hotter stars in habitable zones would also be brighter, and thus an unpleasant place to live. Even the hottest stars, maybe 0 type stars would still have the same apparent brightness as our sun but appear as a pinprick of light in the sky. That'd be very interesting to see.

If we had an Earth around a hotter sun in a habitable, say an F, A, or O star, would we get sunburn easier? Would it be as pleasant or not so much as our Earth?

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