An interesting story on hypervelocity stars. Around 16 know since 2005, one of them being the HE 0437-5439 which travells at 2.5 million kilometers / hour. The puzzle about this is that it is too young to have travelled so far from the Wilky Way (200 000 light-years over the disk plane).
The story (?):
“A hundred million years ago, a triple-star system was traveling through the bustling center of our Milky Way galaxy when it made a life-changing misstep. The trio wandered too close to the galaxy’s giant black hole, which captured one of the stars and hurled the other two out of the Milky Way. Adding to the stellar game of musical chairs, the two outbound stars merged to form a super-hot, blue star.”
Hubble NewsCenter release & the ApJ Lett paper / Warren R. B. et al, A galactic origin for HE 0437–5439, the hypervelocity star near the Large Magellanic Cloud, ApJ, 719, L23, 2010.