Money is a constant source of consternation for graduate students. Here are some things to consider:

There is currently a facilities fee and a student activities fee that is charged to every graduate student at Columbia University. While some departments pay these fees for their students, currently our department does not. These bills come due at the beginning of each semester and are subject to late fees if not paid on time, though generally asking for amnesty on late payments will get the late-fees waived (though not the facilities and student activities fees themselves). There has been constant discussion about whether these fees will be going away. Though their amount has been reduced, for some reason the GSAS hasn't been able to dispense with them completely. Complain loudly and often about this and try to get GSAC to change this.

It is calculated by the university that roughly $1000 per year in expenses is not covered by fellowships and stipends. This means that you are allowed to borrow on ​unsubsidized student loans up to this amount per year. These loans are administered through the ​Financial aid office of GSAS located in 107 Low Library, but generally charge higher interest rates than subsidized loans.

Additionally, it is possible if you are in a temporary financial bind to take out an emergency loan of up to $1000. Post-dating a check to Columbia University for the amount you borrow will give you access to immediate cash.

There is a list of physics tutors maintained by the ​Physics Department in 704 Pupin if you would like to tutor students in non-astronomy physics classes for extra income. There are also tutoring companies that you can join which will pay the better part of $100 an hour for your services depending on your level of competance. Normally, science and mathematics tutors are in extremely high-demand. If you're very good, you can go private and charge even more money. Realize that it is departmental policy that you should not tutor in the department itself. It is also forbidden for you to tutor any student in Columbia or Barnard Astronomy.