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Magnetic Field of Stars

Page history last edited by Peilin 15 years, 1 month ago

Introd

A magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of matters. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without significant gain in the star density. As a result the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. (As you can see in the picture)

NOAA image

How it generate

Stellar magnetic fields are generally believed to be caused within the star’s convective zone. The convective circulation of the conducting plasma functions like a dynamo. This activity destroys the star's primordial magnetic field, then it generates a dipolar magnetic field. As the star undergoes differential rotation—rotating at different rates for various latitudes—the magnetism is wound into a toroidal field of "flux ropes" that become wrapped around the star. The fields can become highly concentrated, producing activity when they emerge on the surface.

The magnetic field of a rotating body of conductive gas or liquid develops self-amplifying electric currents, and thus a self-generated magnetic field, due to a combination of differential rotation, then Coriolis forces and induction. (Example of Coriolis force below)The distribution of currents can be quite complicated, with numerous open and closed loops, and thus the magnetic field of these currents in their immediate vicinity is also quite multitwisted. At large distances, however, the magnetic fields of currents flowing in opposite directions cancel out and only a net dipole field survives, slowly diminishing with distance.

Figure 1: In the inertial frame of reference (upper part of the picture), the black object moves in a straight line. However, the observer (red dot) who is standing in the rotating frame of reference (lower part of the picture) sees the object as following a curved path.

The magnetic fields of all celestial bodies are often aligned with the direction of rotation, with notable exceptions such as certain pulsars. Another feature of this dynamo model is that the currents are AC rather than DC. Their direction, and thus the direction of the magnetic field they generate, alternates more or less periodically, changing amplitude and reversing direction, although still more or less aligned with the axis of rotation.

Starspots are regions of intense magnetic activity on the surface of a star. These form a visible component of magnetic flux tubes that are formed within a star's convection zone. Due to the differential rotation of the star, the tube becomes curled up and stretched, inhibiting convection and producing zones of lower than normal temperature. Coronal loops often form above starspots, forming from magnetic field lines that stretch out into the corona.

A computer reconstruction shows the surface of the young star AB Doradus to be mottled by the gaint starspots that betray its twisting rotation.

M. Jardine and K. Wood, University of St Andrews

Magnetar

A magnetar is a neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays.

Example of magnetstar

SGR 1806-20 located 50,000 light-years from Earth on the far side of our Milky Way galaxy in the constellation of Sagittarius.
SGR 1900+14 located 20,000 light-years away in the constellation Aquila.
1E 1048.1-5937, located 9,000 light-years away in the constellation Carina. The original star, from which the magnetar formed, had a mass 30 to 40 times that of the Sun.

Little is known about the physical structure of a magnetar because none are close to Earth. Magnetars are around 20 kilometres in diameter but substantially more massive than the Sun. A magnetar is so compressed that a thimbleful of its material would weigh over 100 million tons. Most known magnetars rotate very rapidly—at least several times per second. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more.

A magnetic field of 10 gigateslas is enormous. Earth has a geomagnetic field of 30–60 microteslas, and a neodymium based rare earth magnet has a field of about 1 tesla, with a magnetic energy density of 4.0×10⁵ J/m³. A 10 gigatesla field, by contrast, has an energy density of 4.0×10²⁵ J/m³, with an E/c² mass density >10⁴ times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1000 km, tearing tissues due to the diamagnetism of water. It has even been said that at a distance halfway to the moon, a magnetar could strip information from a credit card on Earth.

Formation

When, in a supernova, a star collapses to a neutron star, its magnetic field increases dramatically in strength (halving a linear dimension increases the magnetic field fourfold). Duncan and Thompson calculated that the magnetic field of a neutron star, normally an already enormous 10⁸ teslas could, through the dynamo mechanism grow even larger, to more than 10¹¹ teslas.

In order for such large stars (10 to 30 solar masses) not to collapse straight into a black hole, they have to shed a larger proportion of their mass.

It is estimated that about 1 in 10 supernova explosions results in a magnetar.

www.scienceimage.csiro.au