The SLT A58 has the so-called "long exposure noise reduction" function which performs the dark frame subtraction mentioned in Micheal's Clark's answer. You should choose the shutter speed a lot less to prevent start trails (A few seconds at most, depending on the focal length) and crank up the ISO instead. That will dramatically increase the noise, but you deal with that using image stacking mentioned in Micheal's Clark's answer.
In general, it is better to work with higher ISO images that are more noisy to do image stacking with than lower ISO underexposed images. In the end you'll have to multiply the result in the later case to get to the same result in the former case, but multiplication in post processing typically yields worse results than having started out with the correspondingly higher ISO in the first place.
You should also set the high ISO noise reduction to low, make sure you shoot raw images or raw + jpg. Then with Sony's Image Data Converter convert the raw files to 16 bit tiff files and switch noise reduction off when processing the files. You really do not want to do any noise reduction on the individual images, as you'll then lose the signal from faint stars that are hidden in the noise. In practice, the way the images are processed in camera and with Image Data Converter, even with all the noise reduction setting set to minimal or off, will still lead to some noise reduction being performed, so you'll want to minimize this.