Some tips
You are using a 135mm lens, that reduces the exposure time drastically for having pinpoint stars in the image.
Find out what that time is and use it.
Use the highest ISO available, even if you get noise.
Stop down the lens so that you won't get ant flares (comet like coma) on the stars close to the corners (sagittal coma that is) or other lend abberations. This is lens dependant so you might be able to use your lens at full aperture.
Take several shots (5-10 or as much as you like). You might need to adjust the frame as the Earth rotates, but it doesn't have to be a perfect re-framing.
Shoot RAW, use the RAW converter of your choice, adjust the image to your likeness and save the results as 16bit TIF files. But you can also use the RAW files for the next step.
Use Deep Sky Stacker (freeware) to align and stack the images. It's rather easy to use.
Additionally, Deep Sky Stacker software above can work with dark frames (put the cap on the lens and take several shots with the same ISO and exposure time as the ones with the stars), flat frames (use the same ISO and exposure, either defocus the lens to the other end or target the lens to a flat and even colored surface (dark white/grey)) and bias frames (use the same ISO and the shortest exposure time the camera can deliver).
Dark frames are used to 'map out' noise and hot/dead pixels.
Flat frames are used to adjust for eventual vignetting and such.
Bias frames are used to reduce any inherent problems with the sensor (hot/dead pixels and such).
Take all the shots described above in the same sitting, 2-3 of each, before and after the stars photo session
If you-re really prone to learning astro-photo, you can use software as Iris, Registax, K3CCD Tools that give you some more control on the post processing.
For a 1 second shot at ISO 6400 you have a lot of information in the picture there, and more can be pulled out by post processing as described above.