Please look @ this Astronomy Picture of the Day http://apod.nasa.gov/apod/ap110503.html The majority of stars are densely populated in center. How?
Astronomical observations of most galaxies show an increase in the density of stars with the area being viewed moving closer to the center of the galaxy. While the distance between stars far away from the center has to be measured in thousands or hundreds of light years, as one moves closer to the center of the galaxy the distance decreases exponentially and very close to the center the distance between stars can be measured in terms of light weeks or even light days.
This is primarily due to the fact that there is a black hole present at the center of most galaxies. As galaxies revolve the stars that constitute them are slowly moving towards the center where a point with an infinite gravitational potential lies. As the force of gravity between two masses is inversely proportional to the square of the distance between them even the rate at which they move closer increases as the distance between them decreases.
In the given example of the galaxy there exists a black hole at the center towards which all the stars are slowly being pulled. This leads to the increase in stellar density noticed at the center.