Create directional emitter. For sparkle effects, add an opacity Per Particle attribute. Create a ramp connected to age for opacityPP. You can put a fractal texture as the color of the ramp and the particles will sparkle. You'll need to set Lifespan Mode to lifespanPP and give it a value with an expression (use a rand).
For example, if you want the tail to start bright then begin to sparkle as it ages, put a fractal texture in the middle color of the ramp while the bottom color is white and the top black (fade to 0). For a better look add a fourth color at the very bottom set to black and move the white color up to around 0.075 that way the particles fade in at the beginning. You then adjust the arrayMapper max value to adjust overall opacity.
Try setting type to Multi-Streak and tweak to your liking. Use Multi-Pass rendering with motion blur and you should have a basic comet.