These are the system requirements:
Linux/Unix; Windows 7 and above; OS X 10.8.5 and above, 3D graphics card which supports OpenGL 3.3 and above, 1 GiB RAM or more, 1.5 GiB on disk.
Typically, a good video card is all that's necessary to run the software without lag, delay and visual downgrade. You don't need additional downloads to run the software, unless you want to improve the database which has nothing to do with functionality but with the enhancement part.
Official website: http://www.stellarium.org/en/