Northern Lights? You must be going somewhere really cold I suppose? If that is the case, you should try to go for a camera that handles well even with gloves on, have full manual controls, decent low light performance and can operate in the environmental conditions you will be in.
There are many cameras that fit the bill, so some information about what kind of photos of the northern lights you want to take would help. For the average tourist, sometimes even the phone camera is sufficient.
Camera aside, there are also other gear such as lenses and tripods to consider. It may be a hefty investment depending on how much you are willing to spend. Plus, there are some key techniques that one should learn as there is some level of skill involved in shooting the Northern Lights (just like shooting pictures of stars).