It's very difficult to kick a star out of the galaxy. In fact, the primary mechanism that astronomers have come up with that can give a star the two-million-plus mile-per-hour kick it takes requires a close encounter with the supermassive black hole at the galaxy's core.
Vanderbilt astronomers have identified nearly 700 rogue stars that appear to have been ejected from the Milky Way galaxy. When these stars received the powerful kick that knocked them out of the galaxy, they were small, yellow stars like the sun. But in the multi-million-year journey they evolved into red giant stars. (Credit: Michael Smelzer, Vanderbilt University)
Source: Science Daily: News & Articles in Science, Health, Environment & Technology