# Tag Info

37

It depends on what object it's acting on. There are many objects, including stars, that have magnetic fields where Lorentz forces on charged particles like electrons and protons are stronger than the gravitational force on them. Also remember that the strength of the Lorentz force depends on the speed of the particle moving through it, so a fast enough ...

30

There is very likely to be a random scatter. Unlike planets orbiting the Sun in the Solar System, most of the stars in the Galaxy did not form at the same time as the Galaxy itself. There is therefore no strong reason to suspect that the angular momentum vectors would be aligned for similar reasons. On the other hand, the Galactic gravitational potential ...

17

Let's look at the proper magnetic force (as opposed to the Lorentz force on a moving, charged object described in @KenG's answer) on a specimen $S$ of magnetized material with mass $M_S$ as a way to try to compare. Let's arbitrarily assume it has a fixed, permanent magnetic moment $m_S$. We can't use iron because it will saturate too easily. Then let's ...

12

Answering in the spirit of the question, I think he's asking if there is ever a chaotic 3 body system that's long term stable, or, to put it another way, a 3 body system without a standard hierarchy where it's stable. The answer is no. 3 body systems without Heirarchy are never stable for very long. They can certainly exist for a while, but they ...

12

The boundedness or otherwise of clusters remains to be established in most cases. The vast majority of clusters become unbound and disperse at a much younger age than the Pleiades. Or they may be born unbound as you suggest. The stars in a cluster have a distribution of velocities and there will always be a tail of high speed stars that will be able to ...

9

It isn't impossible, but the short answer is "no". A gravitational field will accelerate all matter and energy equally while a magnetic field will only accelerate moving electric charges (other magnets). The force due to gravity is proportional to the inverse square of the distance, and the force due to magnetism asymptotically approaches the inverse cube ...

8

Stellar clusters around supermassive black holes are systems in which relativity likely plays a role. Currently, only bright stars can be seen in our own galactic center because there is a ton of neutral gas between us and the galactic center that obscures it. As a result, we only have a few "test particles" out of the many stars that actually orbit the ...

8

The galactic disk, as Riley Jacob wrote, has a definite thickness. It's actually composed of a thin disk $\sim0.3\text{ kpc}$ thick and a thick disk $\sim1\text{ kpc}$ thick, at least (McMillan (2011) has models with data from the Sloan Digital Sky Survey). There's also a central bulge that is even thicker, as the following diagram (from here) shows: Pollux ...

8

The three body problem is a theoretical problem in Newtonian mechanics. It is possible to solve, exactly, the two body problem: Both bodies move in conic sections, typically ellipses, relative to their common centre of mass. The general three body problem cannot be solved exactly. There are special cases that can be solved exactly, of greater interest are ...

7

I'll turn this around for you. The brightest star that is classed as a "halo" or metal-poor population II star is HD 140283, with a visual magnitude of 7.2 and not even visible to the naked eye. Even this is only a halo object in the sense of being a high velocity object whose trajectory will take it back into the halo in the future - it is presently only ...

7

Some classical Cepheids pulsate simultaneously in two or even three modes. Their lightcurves can be explained as a overposition of fundamental plus overtone modes. The terminology you used is also found in this paper (where you have also other references): "the discovery of many double-mode Cepheids (DMCs) pulsating in both the fundamental and first-...

6

Such a three-body system is in principle expected to show a chaotic behavior sooner or later. Nope. Hierchical multiple systems (such as this), where the semi-major axes differ by a factor ten or larger may well be stable for ever (never become chaotic), in particular if the eccentricities are low and if the most massive object is in a tight binary. An ...

6

What you're looking for is stars with "High Proper Motion". A Google search of "High proper motion stars" will provide a number of such lists. A quick look at Wikipedia gives this link: https://en.wikipedia.org/wiki/Proper_motion#Stars_with_high_proper_motion You can follow the links to individual stars if you wish to determine their latitudes.

6

The convective overturn time is the typical timescale for a convective cell to rise in a gas. Imagine a "lava lamp" - it's the time for one of the blobs to rise from its lowest to highest point. I am most familiar with its use in stars, where convective energy transport is modelled using a mixing length. This posits that the typical height travelled by an ...

5

Only one force counts on galactic scales: gravity. Stars like Alpha Centauri are orbiting in the galaxy, as is the sun. Both stars are moving in a similar direction and at roughly the same speed, relative to the galactic centre. The distance to such stars will vary slowly, but both stars will remain within the galaxy, and the stars will stay fairly near ...

5

Cepheid pulsations The basic description of the mechanism behind Cepheid pulsations is given here: The accepted explanation for the pulsation of Cepheids is called the Eddington valve,[38] or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly ionized helium (helium whose ...

5

We know that stellar radiation pressure balances the gravitational compressive forced of a star. We do not know that. Degeneracy pressure, thermal pressure, and radiation pressure are what collectively balance gravitation in a star. Degeneracy pressure dominates over the other two in white dwarfs and neutron stars. Thermal pressure dominates in low mass ...

5

The effective temperature $T_\mathrm{eff}$ of a star, which is presumably what's been plotted, is defined through its relationship with the star's radius $R$ and luminosity $L$ by $$L=4\pi R^2\sigma T_\mathrm{eff}^4$$ This comes from the assumption that the star radiates like a black body at the photosphere. While this isn't strictly true, it's quite ...

5

The mean density of the star is really only defined by the formula $\bar\rho=M/V=3M/4\pi R^3$. The radius of a star is a generally a very complicated function of a star's other properties. When we determine the radius in stellar models, it's only because we've solved equations that describe the structure of the whole star, and read off the value at what we ...

5

What you're really asking about is less to do with Astronomy and more to do with mathematics. You're basically asking if, given a system of differential equations, will a unique solution exist for all time? For an answer, you should check out the Existence and Uniqueness theorems of differential equations. You'd be better to ask questions like this on the ...

4

It is correct that the sun bobs "up and down" (relative to the plane of the galaxy as it orbits the galaxy, and it takes about 64 million years to complete a full cycle, so it does pass through the plane of the galaxy every 32 million years or so (there are quite large error bars on those numbers.) This idea is uncontroversial, but it gets caught up in ...

4

Not clear what the initial part of your question means. Objects can be in virial equilibrium without being in thermal equilibrium. A clear exception to the virial theorem would be any system that is gravitationally unbound. So you couldn't apply it to a supernova explosion or a dissolving cluster of stars. The examples you quote are not self-gravitating, ...

4

SIMBAD allows you to search for stars given various criteria. I searched for: Dec > -20 (easily visible from most locations in the Northern hemisphere) Vmag < 4 (easy naked eye objects) pm > 1000 (more than 1 arcsecond of proper motion per year. I got 8 results (including a double star counted twice) #| identifier |typ| coord1 (ICRS,...

4

There are many systems with 3 and more bodies, but more or less universally they are highly hierarchical systems that behave approximately like a bunch of two-body systems. For example: The solar system has 4 giant planets, 4 rocky planets, dozens of moons, and thousands of asteroids and Kuiper belt objects. Most bodies approximately behave like they are ...

4

No, this is not possible. During the stellar formation, some angular momentum will always be present. And any "braking" effects (magnetic, relativistic, tidal etc.) will become weaker as the rotation slows down. So the rotation will never completely stop, because any forces slowing down the rotation will weaken as well.

3

Yes, there are triple star systems, bound by gravity, that seem stable. See a list here: https://en.wikipedia.org/wiki/Category:Triple_star_systems There are also systems with 4 stars or more, although these are fewer, of course. One could argue that star clusters and galaxies are the ultimate N-body problem, for very large values of N.

3

The same way rotation does. If you look at each star, it is following some kind of orbit, so it has a motion. The only difference between a spiral and an elliptical is that the motions in a spiral are all in the same plane and the same direction, but in the elliptical, they are all over the place. So "random velocity" just means you look at a bunch of ...

3

Adding to @Guillochon's answer, there are even a number of general relativistic tests in our solar system, the most famous being the precession of the perihelion of Mercury. In short, the location of the point of closest approach to the Sun (perihelion) for the planet Mercury is a changing quantity. Essentially, given one full revolution, it doesn't trace ...

3

The analogy is rather weak and not really useful. So-called collisionless stellar systems (those for which relaxation by stellar encounters has no appreciable effect over their lifetime), such as galaxies, can be described by the collisionless Boltzman equation, but never settle into thermodynamic equilibrium (only into some dynamical or virial equilibrium)....

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