
A unit of velocity equal to one kilometer per second, used for the star's speed.The kilometer per second (km/s) is a unit of velocity commonly used in astronomy and astrophysics to express the speeds of celestial objects. It is defined as the distance traveled in one kilometer over the duration of one second. While everyday speeds are often measured in kilometers per hour or meters per second, cosmic velocities are so large that the kilometer per second provides a convenient scale: the Earth orbits the Sun at about 30 km/s, and the Sun orbits the Milky Way's center at roughly 220 km/s.
The unit became standard in the early 20th century as spectroscopic measurements of stellar velocities became routine, particularly after Vesto Slipher's observations of galaxy redshifts in the 1910s, which were reported in kilometers per second.
The unit is a straightforward ratio of distance to time, derived from the International System of Units (SI) base units. In practice, astronomers measure a star's speed by analyzing the Doppler shift of its spectral lines: a shift toward the blue indicates motion toward the observer, and a shift toward the red indicates motion away. The magnitude of the shift, combined with the known rest wavelengths, yields the radial velocity in km/s. For transverse motion, proper motion measurements across the sky are combined with distance to obtain a tangential speed, also expressed in km/s.
It is used universally in stellar kinematics, galaxy rotation curves, and cosmological expansion measurements, such as the Hubble constant, which is expressed in km/s per megaparsec.
The speed of light is about 299,792 km/s, so a star moving at 1 km/s is traveling at roughly 0.0003% of light speed.









