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Science and Astronomy Questions
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| neutronium76 | Date: Thursday, 02.02.2012, 22:27 | Message # 31 |
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| Are there any "white dwarf'' or ''blue dwarf'' type stars in the universe? I can't seem to find any in SE 
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| Talisman | Date: Friday, 03.02.2012, 00:17 | Message # 32 |
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| You can find white dwarfs easily, usually they are in supernova remnants or planetary nebulae. 
I don't know about blue dwarfs though.
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| neutronium76 | Date: Friday, 17.02.2012, 20:15 | Message # 33 |
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| How do we define the border of a star system? What characteristic do we take into account? Is it the star's gravitaional field of influence? Is it its solar wind termination effect ie its heliopause or starpause? Is it a combination of the above or something else?
Also how close can 2 stars be so that they are still moving independent from each othe (and not causing gravitaional disruptions on each others orbiting planets)? Is there a relation between star's size/mass and minimum distance from another star/star system?
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| SpaceEngineer | Date: Saturday, 18.02.2012, 14:36 | Message # 34 |
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| There are a few different definitions - by solar wind, by gravitationl influence, etc. In SE solar systems have no borders - the system is rendered if its size on screen is greater than 1 pixel (the distance depends on FOV angle and screen resolution).
Quote (neutronium76) Also how close can 2 stars be so that they are still moving independent from each othe (and not causing gravitaional disruptions on each others orbiting planets)? Is there a relation between star's size/mass and minimum distance from another star/star system?
I can't find the formula now, but yes, it depends on a star's mass and an external planet's semimajor axis.
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| HarbingerDawn | Date: Saturday, 18.02.2012, 18:29 | Message # 35 |
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| Quote (neutronium76) How do we define the border of a star system?
Like SpaceEngineer said, there are a few different ways. From an astrophysical standpoint, the edge of a star system is where the stellar wind from that system gives way to the interstellar environment. This depends on the strength of the star's magnetic field and stellar wind (e.g. the nearby star Epsilon Eridani is over 30 times more active than Sol, and so its interstellar boundary is much larger than ours is). By this definition, the Voyager 1 spacecraft has nearly left the solar system, and likely will within the next few months/years.
Perhaps the more conventional (and useful for travel maps  ) definition is by where the star's gravitational influence is dominant. This will nearly always be a larger space than by the stellar wind definition, and strongly depends upon how densely populated the region is (e.g. a star near the galactic center or in a globular cluster will have a smaller region of influence than a star in the Sun's neighborhood). A commonly cited physical "boundary" to our solar system, and other star systems, is the outer extent of its Oort cloud, which is a theorized region over one light-year from the Sun where a spherical "shell" of trillions of comets sit. This is thought to be the source of long period comets. When the Sun passes by another star system the gravitational perturbations may send comets plunging toward the Sun (others may be captured by the passing star or entirely lost to interstellar space). The outer solar system object called Sedna is thought to be a member of the inner region of the Oort cloud; its orbit takes it as far as 900+ AU from the Sun, about 1-2% the suspected distance to the outermost reaches of the cloud.
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Edited by HarbingerDawn - Saturday, 18.02.2012, 18:32 |
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| HarbingerDawn | Date: Saturday, 18.02.2012, 18:42 | Message # 36 |
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| Quote (HarbingerDawn) Are there any "white dwarf'' or ''blue dwarf'' type stars in the universe? I can't seem to find any in SE
White dwarfs are the stellar corpses left behind after the death of a sun-like star. You should find them in the middle of planetary nebulae. For a cataloged one, go to Sirius B.
Blue dwarfs are a hypothetical class of star that could be the next stage in evolution of a red dwarf near the end of its life (similar to the red giant phase of a sun-like star). After a blue dwarf exhausts all of its available fusion fuel it evolves directly into a white dwarf. However, since the universe is not nearly old enough for these kinds of blue dwarfs to exist, they will remain a hypothetical classification, possibly forever.
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| neutronium76 | Date: Sunday, 19.02.2012, 09:34 | Message # 37 |
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| Quote (HarbingerDawn) Perhaps the more conventional (and useful for travel maps ) definition is by where the star's gravitational influence is dominant. This will nearly always be a larger space than by the stellar wind definition, and strongly depends upon how densely populated the region is (e.g. a star near the galactic center or in a globular cluster will have a smaller region of influence than a star in the Sun's neighborhood). A commonly cited physical "boundary" to our solar system, and other star systems, is the outer extent of its Oort cloud, which is a theorized region over one light-year from the Sun where a spherical "shell" of trillions of comets sit. This is thought to be the source of long period comets. When the Sun passes by another star system the gravitational perturbations may send comets plunging toward the Sun (others may be captured by the passing star or entirely lost to interstellar space). The outer solar system object called Sedna is thought to be a member of the inner region of the Oort cloud; its orbit takes it as far as 900+ AU from the Sun, about 1-2% the suspected distance to the outermost reaches of the cloud.
So the outer limit of this Oort Cloud is where gravitational effect from parent star becomes minimal i.e it is almost a lagrance point with the rest of the galaxy's gravitational effect. Am I correct or did I say something really stupid? Also this point is not always 1 light year - it depends on the mass/size of the parent star. Right?
Quote (HarbingerDawn) This will nearly always be a larger space than by the stellar wind definition
Huh? I though that Electromagnetic Forces were stronger than Gravitational forces 
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| SpaceEngineer | Date: Sunday, 19.02.2012, 10:12 | Message # 38 |
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| Quote (neutronium76) So the outer limit of this Oort Cloud is where gravitational effect from parent star becomes minimal i.e it is almost a lagrance point with the rest of the galaxy's gravitational effect. Am I correct or did I say something really stupid? Also this point is not always 1 light year - it depends on the mass/size of the parent star. Right?
No. The border between two neighbooring systems is a flat plane where gravitational forses from two stars is equivalent. It looks like 3D Voronoi diagram, but taking into account the masses of stars.
But for real bodies you should define maximum radius of stable orbit. It is much smaller than size of this voronoi cells. For Example, for our Sun it is about 1 light year, while "voronoi wall" between Sun and Alpha Centauri is near 1.4 light years away from Sun (distance to Alpha Centauri is 4.2 ly, but Alpha Centauri system is 2 times massive as Sun, so wall is located in 4.2 / 3 = 1.4 ly).
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| HarbingerDawn | Date: Sunday, 19.02.2012, 16:57 | Message # 39 |
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| Quote (neutronium76) Huh? I though that Electromagnetic Forces were stronger than Gravitational forces
They are, but most large (macroscopic) objects in the universe contain roughly equal numbers of positively and negatively charged particles, so they don't exert a strong net EM force over a large distance. This is why we tend to say that EM is stronger than gravity (almost 10^38 times stronger!) but acts over smaller distances, because the net charges are small compared to the size of the object. Gravity though is only attractive, and everything with mass attracts everything else via gravity. Since there is no "anti-gravity" to cancel it out, gravity just gets more and more influential on larger and larger scales.
But this is not why heliopause is smaller than sun's gravitational dominance. It is because the strength of the interstellar wind and galactic magnetic field overwhelms the sun's as you get farther from the sun. It is like blowing air on your hand on a windy day. You will only notice the effect of your breath a short distance away before it gets lost in the breeze.
Quote (neutronium76) So the outer limit of this Oort Cloud is where gravitational effect from parent star becomes minimal i.e it is almost a lagrance point with the rest of the galaxy's gravitational effect.
No. Like SpaceEngineer explained, stable orbits lie inside of the edge of a region of gravitational dominance, not on it.
And unlike with the solar wind, the Sun's gravity isn't fighting with the rest of the galaxy's gravity, but with the stars immediately around itself. So for a star's region of gravitational dominance, you compare it to it's neighboring star systems in the manner than SpaceEngineer describes above.
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| SpaceEngineer | Date: Sunday, 19.02.2012, 19:36 | Message # 40 |
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| Quote (HarbingerDawn) But this is not why heliopause is smaller than sun's gravitational dominance. It is because the strength of the interstellar wind and galactic magnetic field overwhelms the sun's as you get farther from the sun. It is like blowing air on your hand on a windy day. You will only notice the effect of your breath a short distance away before it gets lost in the breeze.
I'd add here that massive blue supergiant stars have a strong stellar wind and ultraviolet radiation, so it ionizes interstellar matter within 50-100 parsecs from the star! This is the so-called HII regions, that can be clearly seen on Hubble's photos of another galaxies (it often looks like red nebulae, but this is because these photos usually have false colors - HII regions have been photographed using a hydrogen line filter and then blended into an RGB image of a galaxy with a red hue). This huge region has millions of stars, with their own planetary systems, which of course is not being disturbed by supergiant gravity (it is only 10-100 times massive as other stars), and has their own heliosphere. But heliospheres of stars that are located close to a supergiant are disturbed by its stellar wind, like Earth's magnitosphere is disturbed by solar wind.
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| neutronium76 | Date: Monday, 20.02.2012, 18:32 | Message # 41 |
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| Wow guys... Plenty of info here.  Thanks & much appreciated  . So I am a bit confused: This voronoi diagram (if we imagine it in 3D as polyedrons in space) does it describe the gravitational area of dominance of a star? Cause I thought that this area is somehow spherical or eliptical in shape and that there are no fixed boundaries like in the diagram above - More like gravity "bubles" sort of to say.. Sorry for being so stupid 
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| HarbingerDawn | Date: Monday, 20.02.2012, 19:59 | Message # 42 |
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| Quote (neutronium76) Cause I thought that this area is somehow spherical or eliptical in shape and that there are no fixed boundaries
Space is like in the diagram, where some stars are closer than others to each other and their distribution is essentially random. Pick the Sun, for example. The closest star system is Alpha Centauri, with 2 solar masses, so the gravitational boundary will be between the two systems, and closer to the Sun because it is less massive. But go in another direction and you might find that the nearest system is much farther away, and less massive, so the boundary on that side would be farther away. So the Sun's gravity might be dominant for 1 ly in one direction, 3 ly in another, and 4 ly in another, just as an example. So imagine that there are bubbles extending out from every system, but they keep going until they meet another bubble, so the shapes of all these bubbles will be irregular. In addition, the bubbles from more massive systems will "push" harder, so it will compress its neighboring bubbles more.
That's the best way I can think of to describe the shapes of gravitational boundaries between stars. Basically they look just like rounder, 3D versions of the diagram SpaceEngineer posted.
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| SpaceEngineer | Date: Monday, 20.02.2012, 20:42 | Message # 43 |
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| This looks like the foam. Every bubble inside the piece of foam (the galaxy  ) has a form of polyhedron, not a sphere, because every bubble presses to its neighbor. The star masses here are equivalent to the amount of air enclosed in the bubble (i.e. its volume).
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| neutronium76 | Date: Monday, 20.02.2012, 23:37 | Message # 44 |
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| Ok so its like polyhedron but with rounded corners right? It would be great if SpaceEngineer could implement this in SE but I suspect it is very difficult  .
So this answers (or does it not ?) one of my oldest questions regarding astronomy: there are no areas between stars where gravity influence of the neigbouring stars is equal or lower than the total sum of gravity influence of the galaxy or galactic core?
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| HarbingerDawn | Date: Tuesday, 21.02.2012, 00:39 | Message # 45 |
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| Quote (neutronium76) there are no areas between stars where gravity influence of the neigbouring stars is equal or lower than the total sum of gravity influence of the galaxy or galactic core?
There could be, it depends on what the local environment is like, i.e. how many stars are nearby, how massive they are, etc. And even if you are in a region where no one star is dominant, that will change over thousands of years as you drift through space, like a comet coming into the inner solar system is in the influence of the sun (galaxy) but may be deflected or captured by a planet (star). So the dominant gravitational field you are in is not the only important thing, but your velocity and direction as well, because those determine what path you will take as time moves on. If you are moving fast enough to escape a star (like the Voyager spacecraft for example), then you can be very far inside its influence and you will still leave to orbit the galactic center. So everything in the galaxy ultimately orbits the galactic center (unless it's moving REALLY REALLY fast), it's just a question of whether you orbit something else at the same time, like Apollo orbited the Moon which orbits the Earth which orbits the Sun which orbits the galactic center.
Sorry, just kind of rambled for a bit
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