In 2005, the first hypervelocity star (HVS) was discovered by Smithsonian astronomers. These objects are defined as stars with very high velocities compared to normal star velocities in a galaxy. Some of them have velocities that exceed the escape velocity of the galaxy. They are thought to originate from encounters of binary stars with the supermassive black hole at the center of the Milky Way Galaxy. One of the partners of the binary star is “captured” by the black hole, entering orbit around it, while the other partner escapes with a high velocity to become an HVS. In addition to the black hole origin of HVSs, it is also thought that HVSs can be induced by supernovas. In this case, a HVS is ejected from a binary system when the other partner undergoes a supernova explosion.
As of 2014, there are 20 known HVSs. These HVSs all have masses a few times that of the Sun, though dwarf HVS candidates have also been discovered. There are so few confirmed HVSs because they are pretty difficult to track down. It’s not easy to search through a billion stars and find a couple that are moving abnormally. In order to locate them successfully, astronomers have used a specific type of telescope to focus on a large group of stars on the edges of the Milky Way’s black hole that had traveled a notable distance. They then were able to narrow down this large group to stars that were travelling at speeds consistent with ejection from the center of the Milky Way. Once we learn more about HVSs, we can use information about them to learn about stars that form in the centers of galaxies and the sizes of black holes at the centers of galaxies.
This link from Harvard contains a complete collection of links to general information, podcasts, scientific publications, and other links about hypervelocity stars. This article serves as a good introduction to the topic.