High-velocity star

A high-velocity star is a star moving faster than 65  km/s to 100 km/s (definitions vary) relative to the average motion of the stars in the Sun's neighbourhood. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium.

There are three types of high-velocity stars:
*Runaway stars, which were ejected with high velocity from a binary system or a supernova event.
*The halo stars. These stars are members of the galactic halo and moving in highly elliptical orbits around the galactic centre.
*Hypervelocity stars. These are stars that move faster than the escape velocity of the galaxy and typically originate from the galactic center.

Halo stars

High-velocity stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the centre of the Galaxy. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Galaxy. Although their orbital velocities in the Galaxy may be no faster than the Sun’s, their different paths result in the high relative velocities.

Typical examples are the halo stars passing through the disk of the galaxy at steep angles.

One of the nearest 45 stars, called Kapteyn's star, is an example of the high-velocity stars that lie near the Sun. Its observed radial velocity is -245 km/s, and the components of its space velocity are "U" = 19 km/s, "V" = -288 km/s, and "W" = -52 km/s.