baftos wrote:i'm pretty sure the OP is not looking for the memory used by the Thread itself, but the total of all the Objects "owned" by the thread.
Write a small program that starts as many do-nothing-in-a-loop threads and see what happens. You may well find out that the limit is not the heap size, but some OS imposed limit on the number of threads per process.
By the way, the most memory allocated for a do-nothing thread is the thread stack, which is not in the heap. The heap will only contain the Thread object and whatever it allocates internally to be able to function, which should be very little (tenths of bytes, I guess).