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Astronomers Solve Magnetic Fields Mystery
An anonymous reader writes "It is a long-standing and unsolved mystery why 80% of all planetary nebulae are not spherical. Theories suggest that magnetic fields play a role in shaping planetary nebulae. A team of astronomers from Germany has now discovered the first direct clue that magnetic fields might indeed create these remarkable shapes. Planetary nebulae are expanding gas shells that are ejected by Sun-like stars at the end of their lifetimes."
If 80% aren't spherical one must ask why the other 20% are NOT.
"The First Direct Clue" While this may seem monumental, there will be many, many more clues and each will most likely lead the researches to a completely different conclusion.
The crab is a supernova remnant, not a planetary nebula.
Museum guy: Because you touch yourself at night!
> A couple in the Hamptons has asked the same group of scientists to determine why socks dissapear in the dryer.
They disappear into the electronic equivalent of a black hole, and re-appear on the internet as sock puppets.
Sheesh, evil *and* a jerk. -- Jade
I'm confused. The title suggests they've solved the mystery, but didn't they just find a huge clue? I mean, I can't come upon a murder, find a footprint, and say I finished. There's much more to it than that. Yes, this is a huge step, but no, everything isn't "solved." In fact, they could be completely wrong...
Whatever would iron have to do with this? This is about plasma movement in a magnetic field. Or did you think star explosions ejected nice, neutral iron atoms? Here's a clue: the magnetic moment of iron is caused by its outer electrons.
Uh, no. The iron has nothing to do with it--or, at least very little. Ferromagnetism really has nothing to do with it. It's all about plasma effects: charged particles can't travel transverse to a strong magnetic field since the v-cross-B force bends the path--think cyclotron.
This is a fairly nifty result--they're combining existing technique (Zeeman splitting measurements have been established for quite awhile as the means of measuring the field of sunspots) with some pretty serious equipment, and likely a lot of patience, to verify that the fields are strong enough to determine the shape of the plasma. Not a surprising result but a good piece of work just the same.
Most (all?) photos you are likely to see of nebula are enhanced, and thus the colors generally vary depending on what the 'artist' (astronomer) was studying; colors that highlight differences in density will be used by the astronomer studying gas density, colors that accentuate gas temperature by the astronmer studying gas dynamics, etc.
That isn't to say your argument is wrong in anyway. I would agree with your hypothesis; however am not an expert in nebula dynamics in any way shape or form. I will state with great certainty that IF there are significate magnetic forces within a nebula, you WILL see higher gas densities along the magnetic lines of force -- the same idea as when you have iron filings on a sheet of paper and put a magnet under it: those filings will align with the field lines of the magnet.
This could be an interesting topic (the whole tread). I hope some good answers come out of it!
It doesn't turn into a single spinning ball because as the constituents collide, they sometimes stick. The more the empty spaces between them grow, the less often they collide. (Collision rate scales as the third power of the mean free path.) In a mature planetary system or galaxy, collisions between major bodies are so rare that they're nearly unheard of.
(IANA astromoner, just a physicist)
You have to consider where they got the angular momentum to begin with:
A solar system isn't a bunch of objects that happen to be in the same place. It was originally a gas cloud (perhaps a nebula), which had a little bit of rotation (from whatever source: nova, magnetic fields, or the like). The gas particles, while very dilute from our standards, still interact enough to equalize their (average) velocities. As it collapses, conservation of angular momentum makes it spin faster, until it's dense enough for objects (asteroids, planets, sun) to condense. And since they all condensed out of that same cloud, they're all approximately aligned to the same orbital plane that the original cloud had. (The same explanation applies to why the axes of rotation are also mostly aligned.)
On a solar system scale, the spin of the central body plays a large role in this, but it is still a kind of a game of chase-the-tail.
When the whole system is still gas, something starts it spinning -- a simple thing like a star passing nearby gives objects (the gas particles) a bit of angular moment, which is thus transferred to the system as a whole over eons of time through collsions, gravity, magnetic forces, etc.
Now, if a LARGE object passed by in the XY plane, and a SMALL object passed by in the YZ plane, you will end up with a spin *mostly* in the XY plane, but the *WHOLE SYSTEM* will balance out with a single plane of spin somewhere in between.
Eventually the smaller objects become larger objects, which collide less, thus distributing the angular moment less efficiently. There may be one central body spinning in the XY plane, but a few of the large objects can have a wildly different orbital plane. But not many objects will HAVE this wildly different orbital plane, because back when the system was being formed, the angular moment transfer WAS very efficient.
Also, 'circular' orbits, like the earths or mars or Jupiters, are fairly rare on a random scale of things; and if you have a bunch of objects orbiting in different planes with highly ellipical orbits, they have a much higher chance of smacking into each other (or some larger object, like jupiter) than the same object would if it were in a more circular orbit which happened to be in a different plane than that of the central masses spin. Don't forget the time scales in question here!
Now, finally, in systems like that of the Earth and its huge moon, you get tidal interactions; while the moon will never shift in its orbit enough to be in an equatorial orbit, it *does* shift more closely to one every day, thanks to the 'gravity drag' between itself and Earth. Really what is happening is that the Earths spin is accelerated in the direction of the moons travel (really, this is slowing our spin rate down, think acceleration in the physics sense). Earth has already done this to the moon; hence the 'tidal lock' which has the moon presenting the same side to Earth at all times.
Were you to watch the Earth moon system forever, eventually what you would see is two bodies rotating about a central point, both with the axis of spin of each body being parallel to the axis of rotation about said central point (hope you can visualise that!). In reality this won't occur in any amount of time, the influence of the sun, and the fact that the moon would actually leave earths gravitation influence before alignment could occur prevent it. (The orbit of the moon gets larger as it steals earths rotational momentum).
That was fun.
Well, it's partly that, but keep in mind, planetary nebula are very much three-dimensional objects. For example, the Ring Nebula is actually, in all probability, more or a barrel shape like the Butteryfly Nebula. However, because we're seeing it edge-on, we see it as a ring, rather than it's true shape. And the result is color concentration on the edges.
Similarly, some of the perceived complexity in objects like the Ant Nebula may be due to perspective, as we see it from an angle.
And speaking of the Ant Nebula, as is mentioned in the APOD article, another likely contributing factor to nebular complexity is the presence of other bodies orbiting the new white dwarf, such as a companion star or planetary body. These objects likely manipulate the shape of the nebula via gravitational or electromagnetic forces.