Discovered by an American student named Gary Flandro in the mid-1960s, the slingshot manoeuvre usually involves spacecraft briefly coat-tailing a planet orbiting the Sun, extracting some of the orbital energy to gain a big boost in speed. While the Sun has a strong gravity field, its hopeless as a slingshot, because as Galileo pointed out its the stationary heart of the Solar System. As such, it has no orbital energy to pass on to the spacecraft.

Learn more here http://www.space.com/23771-comet-ison-sun-approach-soho-telescope.html