Re: How long will the stars in consellations be there?

It's true that stars don't live forever. How long a star lives depends on
two things: how much fuel it has and how quickly it uses up this fuel.

Stars shine by converting hydrogen to helium in nuclear reactions, so a
star's total fuel supply is determined by the number of hydrogen atoms
inside it. Also, by studying nuclear reactions on Earth, we know how much
energy is released when four atoms of hydrogen combine to form one atom of
helium. So if we know the mass of a star, we know how many hydrogen atoms
are inside it and if we know that, then we know how much energy the star
can put out over it's entire lifetime.

Astronomers can measure how much energy a star is putting out every second.
This "energy per second" measure is also called power. You can think of it
in terms of lightbulbs. There are dim bulbs that are only releasing 40
Watts of energy per second and brighter bulbs that are pumping out 200
Watts. Some bulbs are brighter than others. Stars are like that too. What
kind of wattage a star has is determined by one thing only: the star's
initial mass. Massive stars burn brightly, while their less massive cousins
are dimmer. 

So if we know a star's mass, we know both how much fuel it has *and* how
quickly that fuel will be consumed. By dividing the first number by the
second, astronomers are able to calculate how long a given star will live. 
To give you an example, the Sun's total lifetime will be about 9 billion
years (right now it's about 4.5 billion years old -- middle age for the
Sun). But a more massive star like Vega won't live as long as the Sun will. 
Even though Vega has more fuel, it also consumes it much more quickly.

That's half of the answer to your question. The stars won't last forever,
the constellations won't look the same in a few billion years.

But there's something else! Stars also move through space at different
speeds. The movements of stars relative to each other is very slow. You'd
have to live thousands of years to be able to detect a change in the
constellations with your eyes alone. But precise measurements of stellar
positions using telescopes (a subdisipline known as "astrometry") have
allowed astronomers to catalogue these movements quite well. What they've
found is that long before the stars in constellations begin to exhaust
their fuel supplies and die out, the constellations themselves will
actually be twisted out of shape. Carl Sagan offers a great discussion of
this in his book "Cosmos". He notes that in as little as 25 000 years, the
Big Dipper will be pretty hard to recognize because the stars in it have
moved in their own separate ways.