Stars generate magnetic fields

Cosmic magnetic fields

In the middle of the last century, astronomers discovered that the galaxy was traversed by an extensive magnetic field. It is still largely unclear what role this cosmic magnetic field played and does in the development of the Milky Way system. In the World of Physics podcast, Rainer Beck from the Max Planck Institute for Radio Astronomy in Bonn spoke about research into such galactic magnetic fields.

A magnetic field can also be detected in the entire galaxy outside of star systems and far beyond other matter. No compass needle would align itself with it in space - it is a million times weaker than the earth's magnetic field. On the other hand, it extends over tens of thousands of light years.

Rainer Beck from the Max Planck Institute for Radio Astronomy in Bonn

Rainer Beck: “At the beginning of the 20th century, the engineer Karl Jansky first observed that the entire Milky Way is a strong source of radio radiation. However, at the time he did not know what he was actually observing. The German astronomer Karl-Otto Kiepenheuer in Freiburg made an important contribution to this question at the beginning of the 1950s: He explained radio radiation by the fact that charged particles move around magnetic fields and emit this very strong radio radiation. "

To capture these radio waves, Rainer Beck and his colleagues use the radio telescope in Bad Münstereifel-Effelsberg. From the strength of the radiation, you can directly infer the strength of the magnetic field. In addition, the direction of oscillation of the radiation, the so-called polarization, depends on the direction of the magnetic field. The higher the degree of polarization, the more orderly the magnetic field. A magnetic field in which all field lines point in the same direction would produce a degree of polarization of 75 percent.

“We receive radiation with a degree of polarization of up to fifty percent. This means that there are actually regions in the Milky Way system where the magnetic field is almost completely aligned. We made this discovery here at the Max Planck Institute for Radio Astronomy with our telescope in Effelsberg several years ago. That was completely unexpected at the time. Nobody had expected that such strongly aligned magnetic fields could even exist in a Milky Way system. "

However, if astronomers want to measure the structure of the cosmic magnetic field from the earth, they are faced with a problem. Because our solar system is in the middle of this magnetic field.

“It is almost completely hopeless to determine the structure of the magnetic field in our own Milky Way. So we have to look for galaxies that look something like our Milky Way and instead measure their magnetic structures. "

Observations show that the magnetic fields of other spiral galaxies are spiral. The so-called dynamo theory can explain this form.

Magnetic field lines of the galaxy M51

“The dynamo theory says that electromagnetic energy is branched off from the kinetic energy of a rotating Milky Way system. In concrete terms, this means that a significant proportion of the rotational energy is converted into magnetic energy. However, the structure of the magnetic field must be such that the so-called freedom from divergence is guaranteed: the magnetic field lines must be self-contained. This is the case, for example, with a ring-shaped magnetic field, but also with a spiral-shaped magnetic field. "

A spiral magnetic field in a spiral galaxy - can it be coincidence?

"The spiral structure is an inherent property of large-scale magnetic fields, which initially has nothing to do with the spiral arms in the Milky Way system."

The question remains, how such large-scale magnetic fields could arise at all. Because magnetic fields are generated by charged currents - and in no galaxy can electrical currents be observed that would be large enough to generate such extensive magnetic fields. However, there are many small, electrically charged gas flows in galaxies.

“Small electrical currents are generated by charged particles in plasma clouds. The particles rise, for example, from inside the sun - or from the Milky Way into the halo. This changes the pressure and these plasma clouds expand. The so-called Coriolis force then ensures the rotation of these plasma clouds and depends on the general rotation of the object. "

Countless small magnetic fields are superimposed to form a large, cosmic magnetic field. Once created, magnetic fields are practically indestructible and can last for billions of years. Although they are relatively weak, they have an impact on the evolution of their galaxy - for example, when it comes to star formation.

Magnetic field of the Andromeda Galaxy M31

“On the one hand, they hinder star formation because the dust and gas clouds from which stars develop are stabilized by magnetic fields. The magnetic fields prevent the gas clouds from collapsing and thus ensure that the rate of star formation in Milky Way systems is kept at a relatively low level. If it weren't for that, all Milky Way systems would have already converted their gas supplies into stars - and that is obviously not the case. "

On the other hand, the magnetic field can also promote star formation.

“But if a gas cloud becomes unstable, the magnetic field can also have a positive effect. Because when a gas cloud collapses, angular momentum has to be transported away, otherwise the gas cloud would rotate faster and faster. And at some point the centrifugal force would ensure that the collapse is stopped. "

The magnetic field can take over this removal of angular momentum by slowing down the rotation - and thus making it possible for a star to emerge in the first place. Scientists are currently discussing whether cosmic magnetic fields did not also play a role on much larger scales, for example in the entire evolution of galaxies. So far, computer models have mainly used gravity. Because it was far too computationally expensive to represent the cosmic magnetic fields in the simulations. In the meantime, however, supercomputers are so powerful that scientists will soon be able to get a more comprehensive picture of the formation of galaxies - including cosmic magnetic fields.