<< Graphics`;

h = 6.626*10^(-27);  c = 3*10^10;  
    k = 1.38*10^(-16);

E39a[lambda_, 
        t_] = (8*Pi*h*c/lambda^5)/(E^(h*c/lambda/k/t) - 
            1)

Lmp[te_] = 0.2898/te;  {Lmp[2.725], Lmp[300], Lmp[4300]}

LogLogPlot[{E39a[l, 2.725], E39a[l, 300], E39a[l, 4300]}, {l, 
        10^(-6), 10^(1)}, PlotDivision -> 5000000, 
      PlotRange -> {10^(-15), 10^5}, 
      PlotStyle -> {RGBColor[1, 0, 0], RGBColor[0, 1, 0], 
          RGBColor[0, 0, 1]}, ImageSize -> {800, Automatic}, 
      AxesLabel -> {"λ(cm)", "E(λ,T) \
(erg/cm^3/s)"}]