First let me say this: I have seen multiple calculations of the nidavellir feat and none were accurate, they all ignored that the star was an old star that had gone out and had to be reignited. In this calc i will use the actual temperature of the star that is accurate to the visuals of the movie and not the temperature of a young real life neutron star.
Here i scaled the size difference between the star and the beams diameter. The size i assumed (2000 meters) is close to the one you get via pixel scaling but it doesn't matter as the result of this calc solely on the size radio of the beam and the star, the surface area of Thor that is exposed to the beam and the temperature of the star.
Surface area of the star = 4 x 3,1415 x 1000^2 = 12.556.000 m^2.
Going by color temperature of the star it is around 5000 Kelvin.
P = εAσT4
Going by this formula of black body radiation the energy output of the star is: 1 (emissivity of the star) x 12 556 000 (surface area) x σ=5.67 × 10-8 W/m2•K^4 (Stefan-Boltzmann constant) x 5000^4 = 4.45307625E+14 watts.
Surface area of the beam = ~396 m^2 going by 3,1415 x r^2.
Heat intensity of the beam = 4.45307625E+14 / 396 = 1.124.514.204.545 watts/m^2.
Thor had to hold it open for a few minutes. Assuming he couldn't do a small part of the process thus no handle, 2 minutes is a safe bet.
60 x 2 (timeframe in seconds) x 1.124.514.204.545 = 134.941.704.545.448 Joules per m^2. So approximately 135 TJ or ~32 Kilotons of TNT since exposed part perpendicular to the beam including the armor should be ~1 m^2.
Correction edit: star itself appears to be fompletely white so 6600 K is more fitting. So 1,35200000E+15 / 396 x 120 = 409.696.969.696.969 joules or ~410 TJ or 98 Kilotons of TNT.