The mysterious hypervelocity stars jet through space at mind-bogging speeds. The fastest known one is S5-HVS1, and it travels at 0.58% of the speed of light. This is how they can achieve such incredible velocities.
- Stars, which survived supernova explosions of their binary partners, can be accelerated to up to 0.375% of the speed of light.
- Our galaxy has about 59 known satellite galaxies. Some pierce the disk of its spiral if they are close enough and their orbit is vertical. During this process, they accelerate some stars to escape velocity speeds. Some are only accelerated to speeds giving them elongated orbits, or they can become halo stars that are gravitationally bound to our galaxy but are not located within the disk of the spiral.
- Similarly, stars can acquire escape velocity speeds from our galaxy by approaching each other too closely. It almost never happens in the outer regions of the galaxy where we are located but is more frequent in the central bulge of our galaxy and in some dense clusters of stars, where distances between suns are much shorter.
- Between 8 and 11 billion years ago, our galaxy merged with the Gaia-Enceladus galaxy, which increased its number of stars by about 50 billion to the 200–400 hundred billion, it now contains. It caused the ejection of many systems from the merged galaxy. In 4.5 billion years, the Milky Way will collide and merge with the Andromeda Galaxy. It will eject about 12% of systems from the outer regions of the spiral where we are located. They will acquire escape velocity speeds.
- By far, the fastest stars acquired their velocities when they were a part of a multi-star system that encountered massive black holes, especially the supermassive black holes in the centers of galaxies. The ones that are freed from their system via this process can acquire up to an incredible one-third of the speed of light, but most would be accelerated to much more modest velocities.
Another type of fast star orbits massive, and especially supermassive black holes in the center of galaxies. The fastest known one, S2, orbits the supermassive black hole of our Milky Way Galaxy at an eye-popping 16% of the speed of light. Likewise, some binary stars orbit the common center of mass between the two, the barycenter, at very high speeds.
