How to calculate magnetic field strength?

Calculating the strength of magnetic field?

• A main sequence star with the radius and mass of 𝑅=4𝑅⨀, 𝑀=6𝑀⨀ has an average magnetic field of 1×10^-4 T. Calculate the strength of the magnetic field of the star when it evolves to a neutron star with the radius of 20 𝑘𝑚.

• Answer:

  I intend going into the nature of 'magnetic field' (H), before getting into the Astronomy. Electric current (I) is the magnetomotive force. H = I /[2 π r] Amperes/metre "The magnetic field strength is thus defined by this relation in terms of 'current' that produces it and the geometry of the system." Here it is defined for a current (I) carrying wire. For n turns of the wire H = n I /[2 π r] Amperes/metre. Magnetic flux density B = μ H Tesla (or 'Weber / metre² ' in the old system) μ = permeability of the medium (in 'henry / metre'). For free space μ = 4π X 10^(-7) henry / metre . In the Astronomical environs that we deal normally it is (considered as) free space only. Now the questions we need to address: 1. But in those Plasma environments (star's interior) the equations are different ('μ' too). 2. How do we arrive at the value of 'current' depending upon the gyrating charged plasma. This data was not provided in the question. However given B= 1X 10^(-4) Tesla, if we can know the μ , H (magnetic field strength) can be calculated. For free space H = B/μ = 10^-4 / 4π X 10^-7 = 12,566.38 A/m