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In astronomy, the local standard of rest or LSR is a reference frame which follows the mean motion of material in the Milky Way in the neighborhood of the Sun (stars in radius 100 pc from the Sun), [1] on average sharing the same velocity around the Milky Way as the Sun. [2] The path of this material is not precisely circular. [3]
In galactic astronomy, peculiar motion refers to the motion of an object (usually a star) relative to a Galactic rest frame. Local objects are commonly examined as to their vectors of position angle and radial velocity. These can be combined through vector addition to state the object's motion relative to the Sun.
Figure 1: Geometry of the Oort constants derivation, with a field star close to the Sun in the midplane of the Galaxy. Consider a star in the midplane of the Galactic disk with Galactic longitude at a distance from the Sun. Assume that both the star and the Sun have circular orbits around the center of the Galaxy at radii of and from the Galactic Center and rotational velocities of and ...
For the Milky Way, the speed of disk stars is = and an RMS (Root mean square) velocity relative to this speed of = . For bulge population stars, the velocities are randomly oriented with a larger relative RMS velocity of V R M S = 150 k m s − 1 {\displaystyle \mathrm {V_{RMS}} =150~\mathrm {km} ~\mathrm {s} ^{-1}} and no net circular velocity ...
Speed is the magnitude of velocity (a vector), which indicates additionally the direction of motion. Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph).
The comoving distance from an observer to a distant object (e.g. galaxy) can be computed by the following formula (derived using the Friedmann–Lemaître–Robertson–Walker metric): = ′ (′) where a(t′) is the scale factor, t e is the time of emission of the photons detected by the observer, t is the present time, and c is the speed of ...
A galaxy's recessional velocity is typically determined by measuring its redshift, a shift in the frequency of light emitted by the galaxy. The discovery of Hubble's law is attributed to work published by Edwin Hubble in 1929, [ 2 ] [ 3 ] [ 4 ] but the notion of the universe expanding at a calculable rate was first derived from general ...
This motion is caused by the movement of the stars relative to the Sun and Solar System. The Sun travels in a nearly circular orbit (the solar circle ) about the center of the galaxy at a speed of about 220 km/s at a radius of 8,000 parsecs (26,000 ly) from Sagittarius A* [ 5 ] [ 6 ] which can be taken as the rate of rotation of the Milky Way ...