I am looking for a lens for my Canon 550D for astrophotography. What lens would be suitable for it? Thank you
Look for a wide-angle lens like the Rokinon 14mm f2.8. It will let you capture a bigger section of the sky. F2.8 or smaller takes in more light in a shorter time so that you don't get star trails.
What matters is the absolute aperture, not the aperture ratio (the "f/" value).
A 50mm f/5 (absolute aperture: 10mm) will give you more stars than a 14mm f/2.8 (absolute aperture: 5mm).
It is obvious after all: it has a bigger hole through which more light passes through.
This for more information, read the whole series of articles and you'll know enough :)
There is a site listing lens characteristics for astrophotography use.
And try here: http://www.lonelyspeck.com
There should be a link there to a spreadsheet with the work done for you calculating absolute apertures and lens width to give an index of that ought to work for you.