Magnetic Fields Have Key Influence on Star Formation
Written by Brian Ventrudo
Star forming region in M16
When a giant cloud of interstellar gas and dust collapses to form a new cluster of stars, only a small fraction of the cloud's mass ends up in stars. Scientists have never been sure why. But a new study provides insights into the role magnetic fields might play in star formation, and suggests more than the influence of gravity should be taken into account in computer models of stellar birth.
Gravity favors star formation by drawing material together, so if most material does not coalesce into stars, some additional force must hinder the process. Magnetic fields and turbulence are the two leading candidates. Magnetic fields channel flowing gas, making it hard to draw gas from all directions, while turbulence stirs the gas and induces an outward pressure that counteracts gravity.
"The relative importance of magnetic fields versus turbulence is a matter of much debate," said astronomer Hua-bai Li of the Harvard-Smithsonian Center for Astrophysics. "Our findings serve as the first observational constraint on this issue."
Li and his team studied 25 dense patches, or cloud cores, each one about a light-year in size. The cores, which act as seeds from which stars form, were located within molecular clouds as much as 6,500 light-years from Earth.
The degree of polarization of light from the clouds is influenced by the direction and strength of the local magnetic fields, so the researchers measured polarization to determine magnetic field strength. The fields within each cloud core were compared to the fields in the surrounding, tenuous nebula.
The magnetic fields tended to line up in the same direction, even though the relative size scales (1 light-year-sized cores versus 1000 light-year-sized nebulas) and densities were different by orders of magnitude. Since turbulence would tend to churn the nebula and mix up magnetic field directions, their findings show that magnetic fields dominate turbulence in influencing star birth.
"Our result shows that molecular cloud cores located near each other are connected not only by gravity but also by magnetic fields," said Li. "This shows that computer simulations modeling star formation must take strong magnetic fields into account."
In the broader picture, this discovery aids understanding of how stars and planets form and, therefore, how the universe has come to look the way it is today.
Source: Harvard-Smithsonian Center for Astrophysics
Filed under: Astronomy
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September 9th, 2009 at 9:06 pm
For your convenience, the relevant paper is available here (PDF).
September 10th, 2009 at 6:51 am
This got me wondering. Can magnetic fields be strong and expansive enough to lead to the formation of planets outside main protoplanetary discs?
September 10th, 2009 at 7:52 am
Jorge, I don't think so; the article states that magnetic fields hinder the process of star formation, so likewise with planetary formation. Also, like gravity, electromagnetic forces are subject to the inverse-square law — strength is inversely proportional to the square of the distance from the source of that radiated energy/force.
September 10th, 2009 at 8:55 am
I hate to bring it up on this forum, but if there's so many magnetic fields, surely there are electric currents? After all, Alfven himself warned against the adoption of frozen-in magnetic fields in plasma, and there aren't a bunch of bar-magnets hanging around out there are there?
September 10th, 2009 at 8:59 am
IVAN3MAN wrote:
Hmmm. The electromagnetic force being 39 orders of magnitude higher than gravity, it will of course be much stronger…
September 10th, 2009 at 10:18 am
Gordon Bennett! Here we go again…
davesmith_au:
Cum hoc ergo propter hoc (With this, therefore because of this) is a logical fallacy.
There are two kinds of magnetic sources: (1) motion of electric charges, such as electric currents, and (2) the intrinsic magnetism of elementary particles, such as electrons and protons.
That would only be the case if the entire cloud of interstellar gas was composed of protons or electrons, but since it is always a mixture of the two, the net charges cancel out; gravity, on the other hand, has no negating charge.
September 10th, 2009 at 10:30 am
Not necessarily. Gravity has only positive charge for deep reasons, while EM has the more usual situation of two polarities. Many systems are then neutral or bounded in dipoles. (For example, noble gas atoms or the ubiquitous ambipolar plasmas are neutral, water molecules are dipoles.) Dipole electric fields goes as the inverse-cube with distance, as the inverse-square component is balanced out.
As do the magnetic field for the same reason, it has "two charges", "north" and "south", corresponding to electric field source (negative charge for historical reasons) and sink (positive charge). Higher polypole field vanish even faster. (For example residual van der Waal forces.)
And then you have EM coupling, where accelerated charges gives uncharged photons that dissipate energy. Here again you can have dipoles as antennas that gives a inverse-cube near field, but couples further away in the far field. Again a neutral charge force system, or nearly so, from a distance. (Instead you have radiation pressure forces that behaves according to the number of radiation lobes analogous to field polypoles. Oh, well.)
September 10th, 2009 at 10:32 am
Duh! Yes, the intrinsic magnetic moment is an even better example.
September 10th, 2009 at 11:07 pm
Thanks for the link, IVAN3MAN, even skipping the technical section it was interesting to learn about grain alignment.
I don't know what Alfvén said, but more to the point: after reading the paper one obvious objection to this is that the work tests, successfully, a frozen-in field model.
Also, while reaching way back to a never really used plasma physics course precisely because it was so oriented towards astrophysical situations, but also from the paper: frozen fields is a known effect from enough ionization. I would be very surprised if not astronomers have a handle on typical amounts of ionizations seen, it would be vital and so on?! Unfortunately, as per above, the paper didn't need to check for this.
September 11th, 2009 at 3:30 am
@davesmith_au: there certainly are some electric currents!
But so what? The inter-relationship of magnetic fields, electric fields, and electric currents – in plasmas – is part of standard plasma physics (a filed of science which Alfvén did much to develop). You may find this BAUT forum thread interesting (link below): "What comes first? Electric currents or magnetic fields". I could also recommend you some materials on how, and where, plasma physics is incorporated in contemporary astrophysics.
http://www.bautforum.com/science-technology/92961-what-comes-first-electric-currents-magnetic-fields.html
September 11th, 2009 at 3:32 am
That should read "… a field of science which Alfvén did much to develop …"
September 11th, 2009 at 3:49 am
Not to start an endless discussion again, but I also have a comment I need to get rid of
Einstein himself warned against the weird effects of quantum mechanics.
So, even great minds can err.
Frozen-in flux is a concept that relies on ideal magnetohydrodynamics. In fact, it needs the situation that there is no resistance to electric currents, in other words one needs very high states of ionization. On earth it is almost impossible to achieve such a condition. One will always find neutrals in earthly plasmas.
But conditions that are close to ideal MHD can be found in space. And the concept of frozen-in flux has been used to explain why we have so large magnetic fields. Because currents to produce so weak but so large fields are missing and I wonder how such a current should look like.
I guess, now almost everyone made the comment that was necessary
cu next time 
September 11th, 2009 at 11:12 am
As gas falls into a forming star, it ionizes. As the resulting plasma moves, it generates magnetic fields.If conditions are right, the magnetic field may pinch.
Per Carlqvist formulated the "Carlqvist Relation" that "describes the Bennett pinch, the Jean's criterion (for gravitational instability, in one and two dimensions), force-free magnetic fields, gravitationally balanced magnetic pressures, and continuous transitions between these states."
Basic plasma physics.
September 13th, 2009 at 1:53 pm
@ Ian Tresman
So bloody what? That's still no bloody proof that the Sun and other stars, in general, are "Electric Sun[s]", which is what you and your ilk are trying to sell!
September 13th, 2009 at 2:59 pm
Good catch IVAN3MAN.
@iantresman: although the words you write are unexceptional in and of themselves, in context they are at the very least extremely misleading.
Or did you, perhaps, think that the current which plays a crucial part in the Calrqvist Relation is intrinsic (i.e. that it arises naturally, and automatically, in the process of star formation, from the background magnetic field perhaps)?
September 16th, 2009 at 9:18 am
**CRICKETS**