Adding a lens flare to OpenGL

I want to create a lens flare in OpenGL C ++. I have referenced this video and star rendering .

double calculateGlowSize(double diameter, double temperature, double distance) {
    static const double DSUN = 1392684.0;
    static const double TSUN = 5778.0;

    // Georg magic formula
    double d = distance; // Distance
    double D = diameter * DSUN;
    double L = (D * D) * pow(temperature / TSUN, 4.0); // Luminosity
    return 0.016 * pow(L, 0.25) / pow(d, 0.5); // Size
}

      

        
        
        
      

    

but I want the result to be more like the image shown below.

How do I create a lens flare effect?