This is a difficult question. I am willing to wager that there are a lot more biochemists than astronomers ( 6500 in the USA ). This means that the turnover from retirement and the choices of places to work ( industry vs academe) are probably more numerous in biochemistry. Both subjects are very exciting, so it would be difficult to make a choice based on this factor alone. You need to determine whether you will need to get an advanced degree or not. In astronomy, a PhD is absolutely required. In biochemistry, I suspect there is a broader range of jobs that can be filled. A Phd is hard work. Don't let anyone tell you otherwise. You will work long hours during your mid - late 20's, at very low pay, and with almost no perceptible social life to get your PhD. You better decide early on whether this added effort will really get you a better chance for long term employment.
Sad to say, your goal is to do whatever you can NOW to insure that you will be able to maintain your career for the long term. Post doctoral jobs are a dime a dozen and last 1- 2 years. At the age of 30 you deserve better than that, but we all go through this phase after the PhD because it is our only hope for bringing down that 5- 15 year, or tenured job. Astronomy is highly competitive, but so is biochemistry I'm sure. The choice you make now must be based not just on your personal interests, but a harsh and realistic self-appraisal of your strengths. This is a very hard thing to do while an undergraduate. I didn't do it, and most of my colleagues didn't either, but in today's job situation, I think it makes good sense to know who you are and what your talents really are. If you are a wiz at physics and math, then physical science and astronomy may be a good match for you. If physics and math have always been a struggle, but you like chemistry and biology and have done well in these subjects, then biochemistry may be a better match.