Why would you want to!!! Seriously, you will need to take additional courses in quantum mechanics, statistics, advanced mathematics and do extremely well in them to get into a good graduate school with a program in astronomy. If you do not get into graduate school in astronomy, that's pretty much it for continuing in astronomy. Your best bet is to take advantage of your mechanical engineering background and get into the engineering side of designing satellites and observatories. There is a lot of action in these areas these days. Your likelyhood of finding a rewarding job in astronomy via the engineering avenue is far higher than finding a job as an astronomer, at least until some of us annoying baby-boomers retire!