Celestia Forums

Hi all,

by Selden and Cham I was yesterday pointed to this interesting catalog with > 1500 pulsars,

http://www.atnf.csiro.au/research/pulsar/psrcat

The catalog seems to be well maintained.
Selden made a nice add-on already quite some time ago, which I can recommend. Selden's add-on used version 1.2, meanwhile we have version 1.18.

Cham is very interested in pulsar visualizations including various details, notably jet emissions from the poles, some surface textures, possibly magnetic field line displays etc.

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As to myself, I am very interested to include these scientific catalog data in some form into the Celestia
distribution. This in turn implies a number of restrictions for possible visualizations that I fully support:
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We should only render features that are tabulated in some published catalog! Be it the mentioned pulsar catalog or some other respectable, supplementary source.

Unfortunately, we don't have directional data for jet display, neither color classes or reliable radius information, let alone texture information.

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So the purpose of this thread is to stimulate a creative discussion about possibilities that might nevertheless allow a display of these pulsars in a proper (i.e. scientifically correct) manner.
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Note, I am NOT talking about add-ons in which I am usually not interested for various reasons.

While many pulsar properties are lacking, we do know quite a few, nevertheless. Here is an initial subset.

1) It is peanuts for me to extract a Celestia-readable catalog with any desired parameter information from the original ATNF Pulsar catalog using PERL. Here you find the complete list of provided parameters, i.e. pulsar properties:

http://www.atnf.csiro.au/research/pulsa ... l#par_list

2) Pulsar is the general term for neutron stars that emit directed pulses of radiation towards us at regular intervals due to their strong magnetic fields.

A neutron star is one of the few possible endpoints of stellar evolution. A neutron star is formed from the collapsed remnant of a massive star after a Type II, Type Ib, or Type Ic supernova.

A typical neutron star has a mass between 1.35 to about 2.1 solar masses, with a corresponding tiny radius between 20 and 10 km.

Pulsars are observable neutron stars, typically through their characteristic radio emissions.

3) They rotate extremely rapidly after their creation due to conservation of angular momentum. Typically several times/second initially! When they orbit a companion star and are able to accrete matter from it, they can increase this to several thousand times per second, distorting into an oblate spheroid shape despite their own immense gravity

There are all sorts of anomalies (e.g. glitches)

4) neutron stars have a HUGE surface gravity 2x10^11 to 3x10^12 times stronger than that of Earth! Similarly, there are magnetic fields ~10^12 times stronger than Earth's. All of this induces also strong distortions of light not unlike black holes!

Here is a (rigorous) simulation for the light distorions seen from the surface of a neutron star. Probably some of you know this video already (500KB).
http://www.celestiaproject.net/~t00fri/images/surfns.mpg

That sort of thing I was always interested to do much better in Celestia ...

5) The catalog contains also a number of pulsars that are part of binary systems. Most orbit data are given!

The complete list of types is


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Let me stop here for now, waiting for peoples ideas how we could do a rendering of these pulsars while still "keeping to the rules" of scientific rigor.
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Bye Fridger

t00fri wrote:4) neutron stars have a HUGE surface gravity 2x10^11 to 3x10^12 times stronger than that of Earth! Similarly, there are magnetic fields ~10^12 times stronger than Earth's. All of this induces also strong distortions of light not unlike black holes!

Here is a (rigorous) simulation for the light distorions seen from the surface of a neutron star. Probably some of you know this video already (500KB).
http://www.celestiaproject.net/~t00fri/images/surfns.mpg

That sort of thing I was always interested to do much better in Celestia ...

I mentioned this elsewhere, but how are we supposed to simulate gravitational bending/distortion of light if there's no way in Celestia to specify an object's mass? If Celestia doesn't support Mass then we'd end up having to do some horrible fudge to simulate it (eg not use mass but specify some "gravitational distortion coefficient" or something), which is inelegant and will undoubtedly cause complications down the road.

And if we're simulating one mass-based property then there's no reason to avoid simulating others. I can see us using so many other 'fudge terms' to simulate other properties of Mass that it would have been a lot easier to just use Mass to start with.

Pulsar masses are qualitatively known from various considerations.

Recently there are also quantitative measurements that are VERY interesting. The trick is to determine the mass of the Pulsar through optical spectroscopy of its (white dwarf) companion! Since the ApJ original papers are not accessible to the public (I do have access) here is a popular summary:

http://www.mpe.mpg.de/363-Heraeus-Semin ... er/p11.pdf

There is quite a lot going on as to mass determinations of pulsars.

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Here is an interesting 2006 review talk about recent methods to determine pulsar masses.
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http://home.physics.ucla.edu/calendar/c ... ingrid.pdf

Still in doubt?

About the rendering, what could we do whithout wavelenght filters? Shall we find (for now) a preliminary representation just to incorporate the catalogue? or do we go for wavelenghts for 1.6?

ElChristou wrote:About the rendering, what could we do whithout wavelenght filters? Shall we find (for now) a preliminary representation just to incorporate the catalogue? or do we go for wavelenghts for 1.6?

That's my question to you!

Well, the radius of pulsars is really small (~ 10 KM! ) so we might consider to just use a mark for it's position in form of a "sprite bubble" as I used for galaxies, for example. I mean just some abstract shiny mark that is less disturbing than a red diamond . Just as in case of our galaxies, right now, people are not supposed to get too close to the pulsar.

I am reluctant to allow people to come too close to a neutron star (like Cham does in his add-on), since we have not yet incorporated the massive light distortions visible from close to the pulsar surface (see movie!)

Bye Fridger

Malenfant wrote:I mentioned this elsewhere, but how are we supposed to simulate gravitational bending/distortion of light if there's no way in Celestia to specify an object's mass? If Celestia doesn't support Mass then we'd end up having to do some horrible fudge to simulate it (eg not use mass but specify some "gravitational distortion coefficient" or something), which is inelegant and will undoubtedly cause complications down the road.

And if we're simulating one mass-based property then there's no reason to avoid simulating others. I can see us using so many other 'fudge terms' to simulate other properties of Mass that it would have been a lot easier to just use Mass to start with.

Celestia hasn't required a mass property so far. While mass does influence the ultimate appearance of objects, Celestia does not attempt to model these mass-derived properties because the derived quantities are the ones actually measured and known to greater precision. For example, the orbital period of a two body system can be measured to great precision, but there's a great deal more uncertainty in the individual masses of the system. Thus, it doesn't make any sense to have Celestia derive orbital period from a mass property.

It would be nice to show the selected object's mass in the selection info corner of the display, but up until now, there's been no other reason to specify object masses. If we want to model the bending of light by large concentrations of mass, then of course it does make sense to start using a mass property.

I want to emphasize how difficult it would be to model the visual effects of curved space. It wouldn't be too hard in very restricted cases, such as the a star field with a neutron star or black hole nearby. The stars can all be treated as being at effectively an infinite distance, greatly simplifying things. But the assumption that light will always travel in a straight line is fundamental for Celestia and probably every other interactive 3D application. To change it would require a nearly complete rewrite of Celestia's rendering code. Possibly, we could come up with some clever hacks that would a handle a few cases, but modifying Celestia to deal generally with non-Euclidean geometries seems out of reach to me.

None of this is mean to imply that we shouldn't incorporate the pulsar catalog. The data would be a wonderful addition to Celestia. But, in the short term (version 1.5.x) we should be thinking about how to show pulsars in a simplified universe where light travels in a straight line. If you have though of some tricks to visualize gravitational distortion, please suggest them. On the other hand, if you think you've solved the whole problem of incorporating the effects of general relativity into an OpenGL based visualization, then I'd wager that you don't really understand the problem. (But post anyway )

--Chris

Perhaps the equivalent of "Limit of Knowledge" filters, like those used for planet surface textures, would be appropriate in STC catalogs. I don't mean alternate surface textures specifically, just some kind of meta-indication of an unknown.

In particular, it seems to me that there ought to be some way to indicate some of the things we do know without implying that the representation is exact in other ways -- like strong magnetic fields and high rotational rates, although we don't know the orientations of their magnetic poles or rotational axes.

chris wrote:...
If you have though of some tricks to visualize gravitational distortion, please suggest them. On the other hand, if you think you've solved the whole problem of incorporating the effects of general relativity into an OpenGL based visualization, then I'd wager that you don't really understand the problem. (But post anyway )

--Chris

Chris,

I am surprised to read this sort of argument from you...
But perhaps I simply didn't understand the problem

An implementation of GR effects on cosmological mass and distance scales would NOT AT ALL influence the propagation of light on the typical scales Celestia has been operating so far.

In other words describing the light distortions seen from the surface of a neutron star within its monster gravitational field is virtually a disjunct task from describing any phenomena in the solar system or even beyond within the "normal" Celestia mode.

That sort of argument also holds for Celestia's C++ code

Bye Fridger

selden wrote:Perhaps the equivalent of "Limit of Knowledge" filters, like those used for planet surface textures, would be appropriate in STC catalogs. I don't mean alternate surface textures specifically, just some kind of meta-indication of an unknown.

In particular, it seems to me that there ought to be some way to indicate some of the things we do know without implying that the representation is exact in other ways -- like strong magnetic fields and high rotational rates, although we don't know the orientations of their magnetic poles or rotational axes.

Selden,

I think this is the way I would like the discussion to go. I am sure we could device some such indicator for lack of knowledge. Like renouncing to any texture details of pulsars. Eg using a dark steel gray, or a sprite for example.

Bye Fridger

t00fri wrote:

chris wrote:...
If you have though of some tricks to visualize gravitational distortion, please suggest them. On the other hand, if you think you've solved the whole problem of incorporating the effects of general relativity into an OpenGL based visualization, then I'd wager that you don't really understand the problem. (But post anyway )

--Chris

Chris,

I am surprised to read this sort of argument from you...
But perhaps I simply didn't understand the problem

An implementation of GR effects on cosmological mass and distance scales would NOT AT ALL influence the propagation of light on the typical scales Celestia has been operating so far.

In other words describing the light distortions seen from the surface of a neutron star within its monster gravitational field is virtually a disjunct task from describing any phenomena in the solar system or even beyond within the "normal" Celestia mode.

But the distortion was very evident in the video you clip that you posted, yes? The view is from near the surface of a star with other stars visible in in the background--this is a familiar domain in Celestia, yet the positions of the stars seem visibly affected. Celestia's renderer would need to be changed to simulate this phenomenon.

That sort of argument also holds for Celestia's C++ code

Not sure that I understand . . .

--Chris

t00fri wrote:...so we might consider to just use a mark for it's position in form of a "sprite bubble" as I used for galaxies, for example...

Could be... color code for the types (AXP, binary, HE, etc)?

chris wrote:But the distortion was very evident in the video you clip that you posted, yes? The view is from near the surface of a star with other stars visible in in the background--this is a familiar domain in Celestia, yet the positions of the stars seem visibly affected. Celestia's renderer would need to be changed to simulate this phenomenon.

But this video showed specifically the distortions viewed from the surface of a neutron star with a monster gravitational field! On the surface of any other star the corresponding effects would be NOT perceivable, of course.

As to Celestia, 99% of the code would remain unaffected. Except in case of VERY LARGE masses being involved on the screen (lensing, neutron stars, black holes,...), we have to fork the render code appropriately.

That sort of argument also holds for Celestia's C++ code

Not sure that I understand . . .

That meant in short what I explained a few lines up


Bye Fridger

ElChristou wrote:

t00fri wrote:...so we might consider to just use a mark for it's position in form of a "sprite bubble" as I used for galaxies, for example...

Could be... color code for the types (AXP, binary, HE, etc)?

Certainly. Selden already used color codes in his add-on to mark different frequency groups of pulsars.

Bye Fridger

chris wrote:...The view is from near the surface of a star with other stars visible in in the background--this is a familiar domain in Celestia, yet the positions of the stars seem visibly affected. Celestia's renderer would need to be changed to simulate this phenomenon...

No way to create a some kind of auto modification of the FOV depending on the position of the observer at the surface of the bodie? (the kind of distortion in the video make me think in some extreme FOV...)

The main difficulty I can see would be to provide smooth displays in transitional regimes (as concerns masses and distances) between GR sensitive and normal Celestia modes.

One way of evading the accompanying complexity could be as follows: We might use frame transformations in interactive mode, --analogous to GOTO-- as the ONLY allowed mode of access to strong gravitational field objects on cosmological distance/mass/time scales.

Bye Fridger

ElChristou wrote:

chris wrote:...The view is from near the surface of a star with other stars visible in in the background--this is a familiar domain in Celestia, yet the positions of the stars seem visibly affected. Celestia's renderer would need to be changed to simulate this phenomenon...

No way to create a some kind of auto modification of the FOV depending on the position of the observer at the surface of the bodie? (the kind of distortion in the video make me think in some extreme FOV...)

The resemblance is only superficial though . . . The calculations required to reproduce the effects of gravitational deflection of light rays are much more involved.

--Chris

Here are a few more video sequences about ultracompact (neutron) stars

http://www.celestiaproject.net/~t00fri/images/surful.mpg
http://www.celestiaproject.net/~t00fri/images/gotoul.mpg
http://www.celestiaproject.net/~t00fri/images/circul.mpg

Enjoy,
Fridger

Here are the analogous videos about a black hole:

http://www.celestiaproject.net/~t00fri/images/circbh.mpg
http://www.celestiaproject.net/~t00fri/images/gotops.mpg
http://www.celestiaproject.net/~t00fri/images/lookup.mpg
http://www.celestiaproject.net/~t00fri/images/pscirc.mpg
http://www.celestiaproject.net/~t00fri/images/rsgrow.mpg

Since some are quite short you'd better set your player to 'repeat' mode first.

Enjoy,
Fridger

[quote="t00fri"]
As to Celestia, 99% of the code would remain unaffected. Except in case of VERY LARGE masses being involved on the screen (lensing, neutron stars, black holes,...), we have to fork the render code appropriately.
[quote]

Right . . . but how is this alternate rendering path going to work? It's a large and difficult project, and likely completely impractical except for a restricted set of objects (such as distant stars, which can be effectively treated as points.)

--Chris

I don't agree with Fridger about the texture representation. I think it isn't worst to use any reasonable texture than showing a dull gray ball. A gray ball is as much unrealistic as using a light blue texture with some noise. At least, we know (mostly from theory) that a real pulsar is VERY hot. So it should emit light in the blue-violet side of the visible spectrum. I then advocate the uses of a generic light-blue texture with some noise, exactly like what we already do for all the stars in Celestia.

We also know (partially) the magnetic field strenght of most pulsars. Magnetic fields are what makes pulsars so special, compared to normal stars. IMHO, any reasonable representation of a pulsar MUST say something about its magnetic field. The best reasonable guess we could make about the field configuration (acording to theory) is a dipolar magnetic field.

I advocate a GENERIC REPRESENTATION of the pulsars, with its "typical" magnetic field and jets, EVEN if we don't have any real data about them (jets shape and aspect, rotation axis orientation, etc). I'm emphasize the idea : GENERIC REPRESENTATION, not just a dull, boring one ("gray ball"). The real data that can be shown in Celestia are only the position in space and rotational frequency, and that's all. So at least, the generic representation could give some idea of the main features of a real pulsar. We simply need to show some kind of "warning" to the user, so she/he can't be confused and that she/he could learn it's just a generic representation, and not a realistic one (it will never be). This is already the case for all extrasolar objects in Celestia, anyway.

Personally, I think we need something about magnetic fields. Magnetic fields are extremelly important in astronomy (and cosmic rays too, by the way). So I think pulsars are a great opportunity to make something about them. I think vector fields are much easier to represent in Celestia than the effect of curved geometry on light propagation. Since magnetic fields obey the linear superposition law, we could define some "magnetic primitives" (dipolar and quadrupolar fields of various strenghts), and add the vector fields in some SSC. Celestia could then generate the superposition and represent the result with curves in space.