Ultimate space simulation software

 
User avatar
Mr. Missed Her
Astronaut
Astronaut
Posts: 66
Joined: 09 May 2017
Location: Cistuvaert, Verge Vendor

Science and Astronomy Questions

08 Jan 2018 09:06

Are we certain that gravity is a property of space? Gravity is still listed as a fundamental force, and I don't think gravitons have been ruled out. If gravitons are the case, there's no such thing as spatial distortion; that part of general relativity just gives you the same end result. Also, a finite number of gravity-carrying particles doesn't have an absolute grip on objects, so it's easy to see why black holes leak hawking radiation.
Still, the relativistic description of gravity seems more likely. For one, I'm not quite sure how time dilatation would work if gravity is a force. And if the gravitons from the Sun apply enough energy to keep its planets in orbit, shouldn't it be losing magnitudes of order more energy to the gravitons that don't hit a planet? I guess we'll only be sure when Quatum Gravity theory's finally a thing.
Space is very spacious.
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 1121
Joined: 06 Sep 2016
Location: Bellingham, WA

Science and Astronomy Questions

08 Jan 2018 22:06

Gravitons may be a thing, but having them acting as the exchange particles would not contradict gravitation as a curvature of space-time.  They are different aspects or models of the same phenomenon.   Much like how electromagnetism is a classical field theory, yet also described as an interaction mediated by virtual photons.

Gravitation as space-time curvature (general relativity) makes many predictions that are very well supported by experiment, so I think we can say that we are very sure that modelling it that way works. :)  Some examples of these experimental tests include:

-Gravitational lensing (light moves along locally straight lines, but these lines lie in a curved space-time)
-Gravitational redshift / time dilation (a test of Schwarzschild's metric)
-Precession of orbits (curved space-time leads to different orbits than from Newton's Laws)
-Gravitational waves (violent events ripple space-time)
-Frame dragging (spinning masses drag space-time around them)
-Longer travel time for light near masses (the Shapiro delay, which can be thought of as a consequence of the space curvature described in the previous post.  That is, the speed of light is always locally c, but near the mass there is more space to travel through!)

Mr. Missed Her wrote:
Source of the post If gravitons are the case, there's no such thing as spatial distortion; that part of general relativity just gives you the same end result.

What more natural way is there than curvature to describe how the directly measured distance between shells of reduced circumference 3.000km and 2.999km is 8 meters instead of the 1 meter you would expect in flat space? :)  If gravitons are real they must cause an effect exactly equivalent to this distortion of geometry.  (They must also change the rate clocks tick, as you mention with the time dilation).

Mr. Missed Her wrote:
Source of the post Also, a finite number of gravity-carrying particles doesn't have an absolute grip on objects, so it's easy to see why black holes leak hawking radiation.

Hawking radiation is not a leakage, though.  Nothing escapes the black hole's interior!  The Hawking radiation comes from the space outside the horizon.

I think it's also useful to know how general relativity describes a black holes "grip" -- or rather, how it doesn't.  Falling into one, never do you feel gripped by anything!  Mass attracts mass not by directly exerting a force, but by distorting the geometry of space-time.  That distortion of the geometry then gives the marching orders for other masses in the vicinity.

"Matter tells space-time how to curve, and curved space-time tells matter how to move."  --John Wheeler

Those marching orders are as simple as could possibly be:  "Always go straight!"  But in curved space-time, straight paths deviate, just like lines of longitude (which are straight and start out parallel to each other at the equator) deviate on the surface of the Earth (they converge at the poles).  Almost as if the poles exert some attractive force on them!  Yet nobody walking on a longitude line feels a force toward the pole.  Curvature again becomes the natural explanation for this weird behavior of straight lines on the surface of a sphere.

Mr. Missed Her wrote:
Source of the post And if the gravitons from the Sun apply enough energy to keep its planets in orbit, shouldn't it be losing magnitudes of order more energy to the gravitons that don't hit a planet?

Graviton emission does not carry away energy, so the gravitons that don't interact may as well not exist.  You can think of them as virtual particles, whose existence comes "for free".

For analogy, think of two electrons repelling each other by exchanging virtual photons.  Emitting virtual photons does not change the mass or charge of those electrons.  The emission is "for free".  The virtual particles that are exchanged is what causes the acceleration -- "the source of the force" between the two.  We could also say that the emission of virtual photons is what distorts the electromagnetic field, in exactly the way classical electromagnetic theory describes.

With the Sun emitting gravitons, those gravitons must interact with all masses around it to cause them to accelerate, in exactly the way described by the Newtonian gravitational potential, or more precisely by Schwarzschild's metric.  We can also think of it as the gravitons distorting the space-time, and then the distorted space-time tells the planets how to move.


In summary, it is entirely possible to describe gravity and general relativistic effects without using curvature -- the key is to call it an effect on measuring rods and clocks.  But these two descriptions must be exactly equivalent, and consistent with experimental tests.  I can draw one last connection to the question Midtskogen asked about quantum computers earlier.  Different philosophical interpretations of quantum mechanics must all be consistent with observation, and make the same predictions.  Provided that they do, there is no reason to favor one over another.  We should only care about what agrees with experiment, and what helps us to do calculation.
 
User avatar
An'shur
Astronaut
Astronaut
Posts: 54
Joined: 26 Dec 2016
Location: Universe

Science and Astronomy Questions

09 Jan 2018 13:36

Watsisname wrote:
Hawking radiation is not a leakage, though.  Nothing escapes the black hole's interior!  The Hawking radiation comes from the space outside the horizon.

Would this apply to gravitons too? Gravity propagates through space at the speed of light, does it not? What puzzles me is, how could gravitons as particles even get away from the black hole to spread the information about it's existence (and therefore curve the spacetime) since nothing can escape the black hole?
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 1121
Joined: 06 Sep 2016
Location: Bellingham, WA

Science and Astronomy Questions

09 Jan 2018 19:39

If the gravitons can't escape, then they don't generate a black hole, and so they escape.  If they escape, then they generate a black hole, and so they can't escape.

Paradox?  It sure seems that way!  Asking how gravitons escape a black hole is a bit like asking how gravity escapes itself -- a quick road into an endless loop.  How do we break the loop?  Abandon the notion that gravitons must escape from anything.  Say instead that they are what make the black hole!  Gravitons, or the fields they generate, are what give the marching orders to other masses in the vicinity.

Thinking in the context of general relativity (without gravitons) may help.  How does an object near a black hole "know" that there is mass hidden away inside the horizon?  It doesn't!  It only knows the simplest rule: "always move straight".  It moves on a locally straight line according to the space-time geometry of its immediate vicinity, and that geometry is warped.  Why is the geometry warped? 

The field equations of general relativity say that the singularity of the black hole distorts the geometry immediately around it, just like poking at a rubber sheet.  But that distortion can't be localized.  The singularity can't just cause a tiny dimple that suddenly cuts into flat geometry.  Just as how your poking into a rubber sheet distorts the whole sheet, the distortion around the singularity must further distort the geometry around that, reaching out until the whole geometry is warped in a smooth manner given by the Schwarzschild metric.  This is how the singularity extends its influence beyond the event horizon.  It doesn't violate the one-way rule of the horizon, it makes the one-way rule of the horizon, by generating the geometry.


A quick detour:
We now know that there are gravitational waves, and that those also move at the speed of light.  But gravitational waves definitely do not escape from inside a horizon.  This is because gravitational waves are self-propagating changes in the space-time, rather than the static geometry attributed to the source mass.  So they must follow the rules of the geometry that already exists around their source.  They can't climb out of a black hole if they were generated inside one -- they are trapped.

In fact this must happen anytime two black holes merge together.  Some of the gravitational waves will spiral around inside the resulting black hole and make a new, particularly nasty type of singularity -- one which not only has infinite curvature, but infinitely rapidly oscillating changes in curvature, like the function y=sin(1/x)/x.  Those who read Kip Thorne's Science of Interstellar book may have seen it as the "BKL singularity".
 
Terran
Space Tourist
Space Tourist
Posts: 34
Joined: 24 Nov 2016

Science and Astronomy Questions

10 Jan 2018 12:07

Terran wrote:
Source of the post What lines are being curved? If its space being curved then when I stand still next to a gravity well, nothing would happen

Watsisname wrote:
Source of the post Both the space and the time are curved.  A manifestation of the time curvature is that clocks at different altitudes tick at different rates.  The manifestation of the spatial curvature is a little less obvious:

lol, I was asking rhetorically, I then proceeded to answer with an extremely simplified version of what you responded with. Its ok though!  :lol:
Watsisname wrote:
Source of the post Having a sense of what life cannot be surely helps, but that still leaves a very large and almost completely unexplored region of possibility space

Hey, possibility space is possibility space, its definitely better than nothing, which inversely leaves infinite possibilities. What life can be can still be narrowed down though, from my perspective...
Terran wrote:
Source of the post alien life requires a dynamic process and a balance between entropic systems and low entropic systems. It has to change over time.

Terran wrote:
Source of the post it comes from survivability, potential, necessity, competition, and so on...

Alien life still would operate under the same universal laws, and use the same chemistry, albeit probably with different molecules. But we should be able to narrow down the possibilities drastically.
Watsisname wrote:
Source of the post Most are familiar with the time dilation, but space gets warped, too.  There is more space near a compact massive object than meets the eye!

Isn't time dilation also space dilation? If it takes longer to get from A to B, you could just say the distance between A and B is longer as you can find no example of the correlation not occurring at the same time.
Watsisname wrote:

:!: I cannot accept gravitons being a part of the model of the universe. If spacetime curves, there is no gravitational force, thus no need for a force carrier. Saying there is undermines that fact. Its like saying space-itons or time-itons, everything being from a particle is just as reasonable as there being no particles, and makes even less sense. If people want to know how mass interacts with spacetime, it seems fairly obvious, thats because mass is spacetime, or just exclusively time. Density cannot exist in spacetime without changing the mass in the formula, the fact that mass is constant over time and not space is a sheer indicator of its relationship. Anyways gotta go, have a nice day  :)
To reality, through nothing, from un-everything.
 
User avatar
FastFourierTransform
Explorer
Explorer
Posts: 166
Joined: 17 Nov 2016

Science and Astronomy Questions

10 Jan 2018 14:05

Terran wrote:
Source of the post Alien life still would operate under the same universal laws, and use the same chemistry, albeit probably with different molecules. But we should be able to narrow down the possibilities drastically.

Rattus from the old forum gave a very rigorous and precise response to this. You should check his arguments it is a very interesting topic. In fact many things can be narrowed down.

[By the way Rattus said that he was "Sergey Andrewschenko, i'm a molecular microbiologist and a research fellow of a small Russian Academy of Sciences institute. Some months ago I have volunteered to create an "optimistic-rigorous" concept of biogenesis for SE". Does anyone knows where this collaboration went?]


Terran wrote:
Source of the post I cannot accept gravitons being a part of the model of the universe. If spacetime curves, there is no gravitational force, thus no need for a force carrier. Saying there is undermines that fact.

I feel this is logical and reasonable. But feeling is not sufficient. When we talk about gravitons we are talking with very precise mathematical definitions in hand when we are talking about space-time we are dealing with the same strict and rigorous concepts. We all hear interesting metaphores in science communication. Is not that the metaphores are bad but you have to bear in mind that they are only aplicable to the educational scenario they were created for.

For example when the expanding universe is associated with a ballon been inflated. Yeah that's a good analogy but not for all the possible situations. The galaxies separate in the ballon for example but if you take this mataphore as a usefull and functional physical concept instead of a usefull educational tool you could surpass the limits of the metaphore and start asking things like Where is the center of the universe if the ballon has one? Is just like if you were describing humans with a snowman as a explanatory model; if people don't stick to the examples you are giving they could start asking if humans melt and if noses are preserved when their corpses are completly transformed into a human-liquid.

Here I think you are taking some analogies done for educational purpouses and transferring them to another realm where they are no more usefull. The real answer and the lack of contradiction is what you were pointing lies in the mathematical explanation, something I know nothing as to make a coherent answer but surelly Watsisname has the capabilities :D
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 1121
Joined: 06 Sep 2016
Location: Bellingham, WA

Science and Astronomy Questions

11 Jan 2018 02:20

Terran wrote:
Source of the post Isn't time dilation also space dilation? If it takes longer to get from A to B, you could just say the distance between A and B is longer as you can find no example of the correlation not occurring at the same time.

Generally no.  You are right that time dilation or length contraction can be used to explain the same phenomenon in special relativity, where the effects of relative motion in flat space-time are concerned.  But in general relativity, where the effects of curved space-times are concerned, this no longer works.  Trying to call it purely time dilation or purely space distortion will lead to the wrong predictions!

Example where this works in special relativity: 
If you travel to Alpha Centuari (about 4.4LY away) on a spaceship travelling at 99.9% the speed of light, the trip will take about 10 weeks as measured on your clock.  How is that so?  People on Earth see your clock ticking 22 times more slowly (time dilation).  Of course your trip seemed fast.  But you say your clock ticks just fine, and it is the distance from Earth to Alpha Centauri was 22 times shorter (length contraction).

So in special relativity, different observers have very different explanations for the trip, but they work out to the same result.

Example where this doesn't work in general relativity: 
A classic test of general relativity is the gravitational lensing of light.  The revelation is actually not that light rays bend, but that they bend twice as much!  The Newtonian calculation predicts half of the correct lensing angle.  Using general relativity and treating the effect as arising from time dilation also predicts half of the correct angle.  Einstein himself initially fell for this trap when predicting the bending of light around the Sun (and was lucky to fix this before the observation was made!)  The correct calculation requires the complete Schwarzschild metric where both time and space are distorted, and demonstrates that the spatial geometry is not Euclidean.

 
Another test is the Shapiro delay, which was measured using radar signals transmitted through the solar system.  The delay occurs as if the speed of light is slowed in a gravitational field.  But if we try to calculate this effect as being caused by time dilation, we will get the wrong answer.  Again this is because it is not only time being dilated, but also space being distorted. They are different and we must account for both!

Combined, the effect looks like this.  The image on the left side shows propagation of rays in flat space-time, while the right side shows the same propagation but with a black hole in the center.  Notice the light's apparent speed is not direction-independent, as it would be if it was simply a time dilation effect.  The spatial curvature gives an additional effect which depends on whether motion is radial or tangential.  Radial motion is slowed even more.

Image

Terran wrote:
Source of the post I cannot accept gravitons being a part of the model of the universe. If spacetime curves, there is no gravitational force, thus no need for a force carrier. Saying there is undermines that fact.


I am a big fan of the geometric approach to understanding gravitation.  It works, and it is beautiful. :)  However, this is also only a model.  Models attempt to describe reality.  They do not dictate what reality is!  The same phenomenon can sometimes be described effectively by very different models.

There is no clear indication that gravitons can not be part of a successful theory (though a successful one has not yet been worked out).  The motivation for gravitons is to bring together quantum mechanics and general relativity using principles that did work very well for the other interactions.  If it does work here, then gravitons must generate the effects we call space-time curvature.  And it may explain some new things, like what happens under conditions close to singularity (an outstanding problem that general relativity does not answer!)

So I don't think it's good to be so married to a model as to avoid the potential for better ones.  A better model describes a wider range of phenomena more accurately.  Newton's gravitational force was updated with general relativity's curvature, yet general relativity simplifies to Newton's Laws in the appropriate limits.  And general relativity may not be the final picture -- a newer model could be developed to extend it, and it could have a different description for how gravity works.
 
User avatar
Mr. Missed Her
Astronaut
Astronaut
Posts: 66
Joined: 09 May 2017
Location: Cistuvaert, Verge Vendor

Science and Astronomy Questions

11 Jan 2018 09:58

Watsisname wrote:
Mr. Missed Her wrote:
Source of the post And if the gravitons from the Sun apply enough energy to keep its planets in orbit, shouldn't it be losing magnitudes of order more energy to the gravitons that don't hit a planet?

Graviton emission does not carry away energy, so the gravitons that don't interact may as well not exist.  You can think of them as virtual particles, whose existence comes "for free".

For analogy, think of two electrons repelling each other by exchanging virtual photons.  Emitting virtual photons does not change the mass or charge of those electrons.  The emission is "for free".  The virtual particles that are exchanged is what causes the acceleration -- "the source of the force" between the two.  We could also say that the emission of virtual photons is what distorts the electromagnetic field, in exactly the way classical electromagnetic theory describes.

With the Sun emitting gravitons, those gravitons must interact with all masses around it to cause them to accelerate, in exactly the way described by the Newtonian gravitational potential, or more precisely by Schwarzschild's metric.  We can also think of it as the gravitons distorting the space-time, and then the distorted space-time tells the planets how to move.

Ah, your explanation here gives me a lot of answers. I would have asked the same question about virtual photons if we were talking about the electromagnetic force. But gravitons do change the Sun's energy- its relative energy, in the form of momentum. The gravitons emitted on a side of the Sun have their effect canceled out by the gravitons emitted on the other side. Any object can emit any virtual particles, and lose only virtual energy.

Great explaining, guys!
Space is very spacious.
 
User avatar
SpacePioneer
Observer
Observer
Posts: 13
Joined: 12 May 2017
Contact:

Science and Astronomy Questions

14 Jan 2018 11:43

Could someone show the formula for the maximum height of mountains on a planet with, I'm guessing, some variables related to density, possibly rotational period, etc...I've seen it talked about either on here or on the old forum at some point so I knew I could come here to ask but I want to make some planets in Terragen and not have the mountains be impossibly high based on the mass, radius, materials, etc I assign it.

Also, a question I just thought up because of you guy's current discussion, if quantum field theory is true in that particles are only excitations in fields that permeate the universe, then why doesn't general relativity work well with it? Relativity is fairly similar with the concept that at each point in the universe spacetime is curved this much or that based on the matter and forces that can bend it, and why do we need a quantized version of general relativity to fit it together with basically any quantum mechanical theory if the quantized nature of particles overrides any possibility of an open, "any value goes" outlook GR gives? Its like as a teenager how you might ask one parent (GR) if you can go to a party this saturday and they say that yeah, thats fine, but then you ask the other parent just to make sure they agree (QM) and they say that no, they're not going to let you because you need to get caught up on homework or study or something. Well, the first parent said you could, but does that mean you will? Well, realistically in a lot of cases yeah but this is a metaphor for physics and math, so no. 
Search the skies for a new home
As the old one burns
In ashen skies and firey night
The void my only savior
I, am the lone, space pioneer.
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 1121
Joined: 06 Sep 2016
Location: Bellingham, WA

Science and Astronomy Questions

14 Jan 2018 23:25

SpacePioneer wrote:
Source of the post Could someone show the formula for the maximum height of mountains on a planet with, I'm guessing, some variables related to density, possibly rotational period, etc..

The physics behind the maximum height of terrain on a planet gets quite complex and is a big part of a course in planetary surface processes, but it is possible to come up with a simple formula that gets approximate answers.  We can say that the maximum height of terrain supportable on a planet is roughly

  Image

where Fc is the compressive strength of the material, ρ is the density of the material, and g is the surface gravity of the planet.  Alternatively, expressing g in terms of the planet's mass M and radius R (g=GM/R^2), it becomes

Image

^I like putting it in this form because it distinguishes the properties of the planet (R and M) and the properties of the material (Fc and ρ).  And you mentioned using Mass and Radius anyway. :)

How could one come up with this formula from scratch?  In fact I do not have this formula in my memory, so if you're interested, this is how I figure it out.  The technique is "dimensional analysis", which is very powerful, and I think worth spending an in-depth post about.  Much insight into difficult problems of physics was gained through this method, from figuring out how atmospheric scattering works to estimating the yield of the first nuclear tests.

► Dimensional Analysis



A cautionary note:
Remember this formula captures only a part of the physics of terrain support.  It is not a precision tool, it is meant to give an intuition and a practical way to get an estimate.  A more careful calculation should consider that terrain is supported by the underlying rock or mantle, where changes in density, composition and temperature may be important.  (A mountain of width w probes the strength of the mantle up to a depth of about w/3 beneath it.)  Then we must consider mechanisms that both build and erode terrain.  But accounting for all of this gets very complicated.  (The physics of planets is hard!)  


Applying the formula to Earth:
The densities and compressive strengths of many common substances can be found online.  Granite has a strength of about 200Mpa and a density of about 3000kg/m3.  The Earth's surface gravity is about 10m/s2.  (Or use mass = 6x1024kg and radius 6371km).  Plugging these numbers in (and be sure to convert the units correctly; Wolfram Alpha can do this automatically) gives a max height of about 7km.  Fairly good agreement with the highest mountains on Earth. 

Also, consider that most mountain elevations do not correspond to how much they actually rise above their surroundings.  A better measure, common in mountaineering, is their prominence.  Mount Evans for example has an elevation of 4350m, but a prominence of only 840m, because it rises up from the Colorado plateau.  Whereas Mount Rainier, a large volcano, has an elevation of 4390m and a prominence of 4026m, since it rises up from nearly sea level.  

Mount Everest has a prominence equal to its elevation by the definition (a weakness of how prominence is defined), but in terms of how much it rises from the immediate surroundings is more like 4000 meters.


Applying the formula to other objects:
Mars has a much lower surface gravity than Earth, of 3.7m/s2, and all else being equal this formula implies the max supportable terrain there should be about 18km.  This agrees fairly well with Olympus Mons, which rises around 20km above the surroundings.

For Venus it predicts a max height comparable to Earth.  Yet we observe mountains on Venus that rise upwards of 13km!  This is very surprising.  In fact planetary scientists are still not certain exactly how the crust and mantle of Venus is able to support such topography.  (A common assumption is that it involves the lack of water.)


Where might this formula break down?  It is particularly bad for smaller bodies (moons and asteroids).  Applying the formula to the Moon (surface gravity 1.6m/s2, density 3000kg/m3 and strength about 150Mpa) yields a max height of around 31km.  This is pretty crazy high, and of course we do not see terrain like that (the highest mountains on the Moon are similar to Earth's).  Applying to asteroids gives even crazier results.  At this scale we're basically hitting the limit where objects crush themselves into spheres (the Potato Radius).


So that's the simple max mountain height formula in a nutshell, and how to find it.  Hopefully you find this helpful and interesting!
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 1121
Joined: 06 Sep 2016
Location: Bellingham, WA

Science and Astronomy Questions

15 Jan 2018 03:20

SpacePioneer wrote:
Source of the post if quantum field theory is true in that particles are only excitations in fields that permeate the universe, then why doesn't general relativity work well with it?

The conflict happens in the details of mathematical formulations of these theories, and much of it is beyond my understanding.  Part of the problem comes in "normalizing", or how to end up with answers that are not infinite.  That was already a challenging problem to solve in the development of quantum field theory (more on that in a moment), and so far, nobody has figured out how to do it for a quantum gravitation theory without it being either internally inconsistent or untestable.

For some good, deeper explanation I can recommend watching PBS Space-time's "Quantum Field Theory" series of videos, and in particular Part 3 ("Feynman's Infinite Quantum Paths") which describes how Feynman figured out how to deal with the infinities in regular quantum field theory.

 
 
User avatar
SpacePioneer
Observer
Observer
Posts: 13
Joined: 12 May 2017
Contact:

Science and Astronomy Questions

15 Jan 2018 18:12

Thanks for the answers Watsisname, very interesting and intriguing indeed  :) 
Search the skies for a new home
As the old one burns
In ashen skies and firey night
The void my only savior
I, am the lone, space pioneer.
 
User avatar
Ensabahnur
Astronaut
Astronaut
Posts: 51
Joined: 27 Nov 2016

Science and Astronomy Questions

20 Jan 2018 06:40

I'm not sure if this is the best place to ask this but here goes, I am looking for a list of all stars within 1000 LY from earth, does anyone know where can i find such a list or database or catalog?
 
User avatar
FastFourierTransform
Explorer
Explorer
Posts: 166
Joined: 17 Nov 2016

Science and Astronomy Questions

20 Jan 2018 10:17

Ensabahnur wrote:
Source of the post I'm not sure if this is the best place to ask this but here goes, I am looking for a list of all stars within 1000 LY from earth, does anyone know where can i find such a list or database or catalog?

There is no such a list yet (a list with all the stars in 1000 ly from the Sun). Why? because we haven't reached completeness in the solar neighbourhood. Sure, in 1000 ly O- and B-type stars are all known (they are easy to spot with our technology) but for A-type the percentage of known objects drops. The vast majority of G, K and M stars inside that sphere are unknown as far as we can extrapolate the density of those in our inmediate vicinity and see that the count is lower than expected when you compare with the G, K and M type stars that we currently know in that radius from the Sun.

With our current understanding of the solar neighbourhood the stellar density is around 0.004 stars per cubic light-year. A sphere with 1000 ly in radius has a volume of 4.2 billion cubic light-years. Therefore your list should contain roughly 16.8 million stars. Let's see how many of those we know (also roughly). We are searching for stars that have a parallax value larger than 3.26 milliarcseconds. Doing a search in Simbad (the biggest centralized astronomical database in existence) we have 473.813 stars closer than 1000 ly.

This means that we currently have confirmed the 2,8% of the stellar population within 1000 ly from home (the vast majority are indeed cold stars since we see a lack of them in our databases when we go further). Besides that there are several problems with this estimate I've done.

1.- Not all known stars have parallax measurements (so maybe there are more that are inside the sphere but our query can't show them since no distance estimate has been archived in this case).

2.- Parallaxes have uncertainties and those uncertainties yield larger distance uncertainties if the star is closer to the edge of the 1000 ly sphere. So many of the stars we counted could be outside the sphere (a big part of them in fact), and also many that are outside the sphere could be inside in reality.

3.- Due to Lutz-Kelker bias many of these stars (even with those parallaxes) are actually farther away than we think.
Factoring all of this I guess that the known stars in that 1000 ly radius is below 3% of the stars that really inhabit the region.
Also I made a crude assumption taking the stellar density of the solar neighbourhood as constante in that sphere. A ball of 2000 ly in diameter is as large as to include the limits of the Galactic disk in the upper and lower part, and since the stellar density decreases (exponentially in general) there are less stars and thus maybe we have explored some percent more of the actual population. But as you see this is a tiny bit also and humanity still has to wait to have that list.

The fact that there is no completness in this 1000 ly around us can be illustrated by the low-right plot:
Image
https://www.handprint.com/ASTRO/galaxy.html#NEstars
This is the count of stars (splitted in different temperature classes) that can be seen with the naked eye against distance. If we where able to see the all the objects (not caring about the sensitivity of our eyes or telescopes) the count should increase with the cube of the distance (since the volume cointaining stars increases with the cube of the radius of the sphere). Instead we see that kind of increasing trend only in the beggining and then the fact that farther away stars are dimmer makes us miss more and more with increasing distance so the effect of our inhability to spot dimmer objetcs start to break the power 3 behaviour. When we reach 300 parsec (more or less you 1000 ly) we count stars very sporadically while distance still increases, those are there but we can't see them. The change in behaviour of those curves marks where the completeness of our sample starts declining. This diagram is made for naked eye stars, but a similar diagram appears if you take into accound the capabilities of the most sensitive telescopes in the world (just that the point where the curves change behaviour are much farther away than in this case).

But, if you agree to have only the known stars in that radius and not all the existing stars then there are some tips you can follow:

1.- Start with this list that spans the first 16.3 ly from the Sun. We believe that this has a 100% completeness (we expect no unkown stars in this region).This list is very well curated and expand to the 21 light years. Between the wikipedia and this list there are probably missmatches (keep wikipedia list as the more rigorous if you see some that are missing in the other list).

2.- For beyond the 21 light years we have a very good list. Lists for all the stars within 100 ly (Supergiants and subgiants, B, A, F, G, K and M lists). But bare in mind the completness here gets below the 20% of the actual expected stellar population (for each of those stars there are probably 4 more inside this 100 ly neighbourhood).

3.- Beyond 100 ly the only thing I can reccomend you is to go to Simbad's query configuration and select ASCII format and click in file output, then save the configuration, go to the criteria query tab and write plx>=3.26 as criteria. A file with the 473.813 stars closer than 1000 ly should be able for download. Then you should program some code (or do it manually until exhaustion) that sees if each star of the file has a van Leeuwen's Hipparcos reduction catalog (HIP) entry or a Tycho-Gaia astrometric solution entry (TGAS). In the case there is an entry in both you have to prioritize the data from TGAS over HIP. From parallax measurements you should get distances (the catalog issue is because Simbad doesn't put the best parallax measurements and is quite arbitrary, you have to search for them in HIP and TGAS since those are the best current references in distance measurements). Also keep in mind that the Hipparcos main catalogue is also non reliable because parallax measuremens were biased since there where attitude control errors and other problems in the mission (you always have to search for van Leeuwen's reduction). In principle don't worry, I talked to SpaceEngineer and in the next month I'm going to work very hard to make this possible (since all catalogue stars in SE come from hipparcos main catalogue and not from the currently accepted measurements). So if you wait I can do this work for you (but the completeness will be the ~3% expected, remember).

4.- You can wait until April of this year because ESA's Gaia mission will release the biggest astrometric database of all time (with the most accurate parallax measurements archived to date) so you could have a more complete and accurate list. All stars with apparent magnitude of 20 or below would be visible and the survey would be complete (but since the very abundant G, K and M stars can have apparent magnitudes larger than that in the 1000 ly radius don't expect this to get the completeness in the 1000 ly sphere to the 100% in any way).
 
User avatar
Ensabahnur
Astronaut
Astronaut
Posts: 51
Joined: 27 Nov 2016

Science and Astronomy Questions

20 Jan 2018 10:27

FastFourierTransform wrote:
Ensabahnur wrote:
Source of the post I'm not sure if this is the best place to ask this but here goes, I am looking for a list of all stars within 1000 LY from earth, does anyone know where can i find such a list or database or catalog?

There is no such a list yet (a list with all the stars in 1000 ly from the Sun). Why? because we haven't reached completeness in the solar neighbourhood. Sure, in 1000 ly O- and B-type stars are all known (they are easy to spot with our technology) but for A-type the percentage of known objects drops. The vast majority of G, K and M stars inside that sphere are unknown as far as we can extrapolate the density of those in our inmediate vicinity and see that the count is lower than expected when you compare with the G, K and M type stars that we currently know in that radius from the Sun.

With our current understanding of the solar neighbourhood the stellar density is around 0.004 stars per cubic light-year. A sphere with 1000 ly in radius has a volume of 4.2 billion cubic light-years. Therefore your list should contain roughly 16.8 million stars. Let's see how many of those we know (also roughly). We are searching for stars that have a parallax value larger than 3.26 milliarcseconds. Doing a search in Simbad (the biggest centralized astronomical database in existence) we have 473.813 stars closer than 1000 ly.

This means that we currently have confirmed the 2,8% of the stellar population within 1000 ly from home (the vast majority are indeed cold stars since we see a lack of them in our databases when we go further). Besides that there are several problems with this estimate I've done.

1.- Not all known stars have parallax measurements (so maybe there are more that are inside the sphere but our query can't show them since no distance estimate has been archived in this case).

2.- Parallaxes have uncertainties and those uncertainties yield larger distance uncertainties if the star is closer to the edge of the 1000 ly sphere. So many of the stars we counted could be outside the sphere (a big part of them in fact), and also many that are outside the sphere could be inside in reality.

3.- Due to Lutz-Kelker bias many of these stars (even with those parallaxes) are actually farther away than we think.
Factoring all of this I guess that the known stars in that 1000 ly radius is below 3% of the stars that really inhabit the region.
Also I made a crude assumption taking the stellar density of the solar neighbourhood as constante in that sphere. A ball of 2000 ly in diameter is as large as to include the limits of the Galactic disk in the upper and lower part, and since the stellar density decreases (exponentially in general) there are less stars and thus maybe we have explored some percent more of the actual population. But as you see this is a tiny bit also and humanity still has to wait to have that list.

The fact that there is no completness in this 1000 ly around us can be illustrated by the low-right plot:
Image
https://www.handprint.com/ASTRO/galaxy.html#NEstars
This is the count of stars (splitted in different temperature classes) that can be seen with the naked eye against distance. If we where able to see the all the objects (not caring about the sensitivity of our eyes or telescopes) the count should increase with the cube of the distance (since the volume cointaining stars increases with the cube of the radius of the sphere). Instead we see that kind of increasing trend only in the beggining and then the fact that farther away stars are dimmer makes us miss more and more with increasing distance so the effect of our inhability to spot dimmer objetcs start to break the power 3 behaviour. When we reach 300 parsec (more or less you 1000 ly) we count stars very sporadically while distance still increases, those are there but we can't see them. The change in behaviour of those curves marks where the completeness of our sample starts declining. This diagram is made for naked eye stars, but a similar diagram appears if you take into accound the capabilities of the most sensitive telescopes in the world (just that the point where the curves change behaviour are much farther away than in this case).

But, if you agree to have only the known stars in that radius and not all the existing stars then there are some tips you can follow:

1.- Start with this list that spans the first 16.3 ly from the Sun. We believe that this has a 100% completeness (we expect no unkown stars in this region).This list is very well curated and expand to the 21 light years. Between the wikipedia and this list there are probably missmatches (keep wikipedia list as the more rigorous if you see some that are missing in the other list).

2.- For beyond the 21 light years we have a very good list. Lists for all the stars within 100 ly (Supergiants and subgiants, B, A, F, G, K and M lists). But bare in mind the completness here gets below the 20% of the actual expected stellar population (for each of those stars there are probably 4 more inside this 100 ly neighbourhood).

3.- Beyond 100 ly the only thing I can reccomend you is to go to Simbad's query configuration and select ASCII format and click in file output, then save the configuration, go to the criteria query tab and write plx>=3.26 as criteria. A file with the 473.813 stars closer than 1000 ly should be able for download. Then you should program some code (or do it manually until exhaustion) that sees if each star of the file has a van Leeuwen's Hipparcos reduction catalog (HIP) entry or a Tycho-Gaia astrometric solution entry (TGAS). In the case there is an entry in both you have to prioritize the data from TGAS over HIP. From parallax measurements you should get distances (the catalog issue is because Simbad doesn't put the best parallax measurements and is quite arbitrary, you have to search for them in HIP and TGAS since those are the best current references in distance measurements). Also keep in mind that the Hipparcos main catalogue is also non reliable because parallax measuremens were biased since there where attitude control errors and other problems in the mission (you always have to search for van Leeuwen's reduction). In principle don't worry, I talked to SpaceEngineer and in the next month I'm going to work very hard to make this possible (since all catalogue stars in SE come from hipparcos main catalogue and not from the currently accepted measurements). So if you wait I can do this work for you (but the completeness will be the ~3% expected, remember).

4.- You can wait until April of this year because ESA's Gaia mission will release the biggest astrometric database of all time (with the most accurate parallax measurements archived to date) so you could have a more complete and accurate list. All stars with apparent magnitude of 20 or below would be visible and the survey would be complete (but since the very abundant G, K and M stars can have apparent magnitudes larger than that in the 1000 ly radius don't expect this to get the completeness in the 1000 ly sphere to the 100% in any way).

Wow, that is an amazing answer, thank you very much for enlighting me sir! I believe I will wait for Gaia data release.

Who is online

Users browsing this forum: No registered users and 0 guests