Re: How the stars are attracting towards the black hole?

Well, I'm not even sure that theories espousing "gravitons"
per se are even in vogue any more.  An easier question to answer
might simply be "if nothing can escape a black hole, then how can
gravity?", since that gets us away from quantum field theory (which
is hard to explain even for people who actually understand it - and
I don't).

Perhaps the most straightforward answer is "gravity begets gravity".
You are probably familiar with the fact that any object with mass
generates a gravitational field; and in fact, in the theory of
General Relativity all sources of energy and momentum ultimately
contribute to the gravitational field.  Further, in General Relativity
the gravitational field itself generates a gravitational field, i.e.,
"some gravity here makes more gravity over there".  If you are familiar
with Newton's theory of gravity, then you know that Newton tells us that
the gravitational field from and object drops off like one divided by
the square of the distance from that object.  However, the more modern
theory of General Relativity predicts that the gravitational field
drops off more slowly, in fact, because the gravity near the object
actually makes more gravity farther away.  This extra effect is weak
in most situations, but measurable.  For instance, we find that the
orbit of the planet Mercury around the sun is not precisely an ellipse
as predicted by Newton, but rather more complicated because of the
effect described above.

So back to black holes.  We know, of course, that it is the intense
gravity of the black hole is what makes them "black", i.e., i.e.,
objects attracted by gravity, even light, cannot escape once they
pass a certain point (the event horizon).  But in General Relativity,
gravity behaves differently than "objects".  Because "gravity makes
gravity", the fact there is gravity inside the black hole means that
there is also gravity outside.  One way to think of this is as follows:
we might imagine a scenario where the gravitational field inside the
black hole is so strong that it can "suck in" the gravitational field
outside.  But of course, we know that the gravity inside makes more
gravity outside than would simply be there due to the black holes mass,
so we would need more gravity inside to "suck in" the extra gravity
outside, but that would make even more gravity outside, etc.

So it turns out that it isn't really true that "nothing can escape
a black hole", because gravity can.  Often people phrase this as
"no information about what's happening inside a black hole can escape
to the outside", but there are some important qualifications.  One
of these is that we can know the total mass inside the black hole,
simply by measuring its gravitational field at some point.  If
some object falls into the black hole, you can actually know its
mass as well by the change in the gravitational field.  You may not
know anything else about the object (was it a book? a bottle of milk?),
but you can know its mass.  There are other physical quantities, such
as electric charge and angular momentum, for which this is also true.