unit

kilometer per second

A unit of velocity equal to one kilometer per second, used for the star's speed.
13 related articles

Description

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.

History

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.

How it works

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.

In practice

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.

Fun fact

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.

Concept graph →

Related articles · 13

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Phantom Note: Why Technological Progress Hides Extraterrestrial Civilizations
Decades of radio astronomy have listened to space, hoping to catch narrowband radio transmissions — the technogenic 'voice' of distant exoplanets. But our own technological evolution reveals a paradox: loud analog signals, like 1960s TV broadcasts, are just a brief squawk of a newborn civilization.
Freo Doctor: How Wind Governs the Fate of Meteorites
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Speed Filter of the Skies: How Swift Visitors Elude Telescopes
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High Energy2026-04-242 minarXiv:2604.22975express
A Hot Cocktail in the Heart of Perseus
A giant cauldron of simmering soup stretches across millions of light-years. This is the atmosphere of the Perseus galaxy cluster — a cloud of scorching gas held together by gravity. Until recently, astronomers thought that in the center of this cauldron, the gas was calmly cooling, ready to condense into new stars.
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Solar Wind: Fast Streams Inside Slow Ones
The Sun's corona looks like a shaggy dandelion: bright petal-like loops hold hot gas near the surface, while long, tenuous jets escape into space between them. These jets, the heliospheric current sheet, stretch across the entire solar system.
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A Long-Lived Pulsar with a White Dwarf Companion
A spinning top that whirls so fast it completes 220 rotations in a single second: that's exactly what China's FAST radio telescope has caught. This is the pulsar PSR J1810-0623, a neutron star the size of a city but with more mass than the Sun.
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Laser Tape Measure with Micrometer Precision
A standard laser rangefinder works like a stopwatch on a 100-meter dash: it times how long light takes to run to the target and back. But such a stopwatch has a limit: it ticks in steps of tens of picoseconds (trillionths of a second). In that step, light travels about a centimeter, so finer details are invisible.
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A Laser Tape Measure That Can Hear Music
Two laser beams, like two pencils, draw an invisible scale on the wall. One tip moves slightly faster than the other, and by the way their strokes alternately coincide and diverge, you can calculate the distance to the wall with an accuracy down to the thickness of a spider's thread. In the new work, physicists made these two beams work continuously, without pauses.
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One Transit, One Year
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Quantum Link Through 62 Kilometers of Air
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Fast Stars as Probes of the Galaxy
The Solar System takes 210 million years to complete one orbit around the center of the Milky Way. The last time we were on this same side of the Galaxy, early dinosaurs were roaming the Earth. That number, 243.72 km/s, has now been refined thanks to rare stars that streak through the Galaxy like cannonballs.