Question on the difficulty of Astanga as a beginner

I wanted to try Astanga yoga, however I have heard it is more advanced than Hatha. I have been to Bikram yoga a number of times and find Hatha fine, but have only recently got back into yoga. I do remember all the poses from Bikram though and understand it is based on Hatha. Is Astanga really different and that much harder than Bikram and seeing as I have only been going back a couple of weeks, would it be too dynamic for me to gain the full benefit? Thanks for your advice.

I tried Astanga today, my mind told me to try it and i followed what i felt. I felt very relaxed after but felt very challenged. The heat came from within, whereas with Bikram it comes from the outside. I am so glad I have connected with Yoga again. It seems that I am drawn to it because of the calming mind aspect that it gives. I am becoming very interested in energy flows and spirtual healing. Thank you for helping me decide for myself.