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Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge due to centrifugal force. As stars are not solid bodies, they can also undergo differential rotation.
These distortions will move across spectral line profiles due to the stellar rotation. The technique to reconstruct these structures on the stellar surface is called Doppler-imaging, often based on the maximum entropy image reconstruction to find the stellar image. This technique gives the smoothest and simplest image that is consistent with ...
In astrophysics, Zeeman–Doppler imaging is a tomographic technique dedicated to the cartography of stellar magnetic fields, as well as surface brightness or spots and temperature distributions. This method makes use of the ability of magnetic fields to polarize the light emitted (or absorbed) in spectral lines formed in the stellar atmosphere ...
The slightly longer stellar period is measured as the Earth rotation angle (ERA), formerly the stellar angle. [4] An increase of 360° in the ERA is a full rotation of the Earth. A sidereal day on Earth is approximately 86164.0905 seconds (23 h 56 min 4.0905 s or 23.9344696 h).
Exposure times must be short (under a minute) to avoid having the stars point image become an elongated line due to the Earth's rotation. Camera lens focal lengths are usually short, as longer lenses will show image trailing in a matter of seconds. A rule of thumb called the 500 rule states that, to keep stars point-like,
The Sun's stellar-wind bubble, the heliosphere, a region of space dominated by the Sun, has its boundary at the termination shock. Based on the Sun's peculiar motion relative to the local standard of rest , this boundary is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. [ 225 ]
The pure mathematical quantum mechanics and classical mechanical models of stellar processes enable the Hertzsprung–Russell diagram to be annotated with known conventional paths known as stellar sequences—there continue to be added rarer and more anomalous examples as more stars are analysed and mathematical models considered.
The rotational velocity of Beta Pictoris has been measured to be at least 130 km/s. [11] Since this value is derived by measuring radial velocities , this is a lower limit on the true rotational velocity: the quantity measured is actually v sin ( i ), where i represents the inclination of the star's axis of rotation to the line-of-sight .