Zvezdana rotacija

Ova ilustracija prikazuje spljošten izgled zvezde Ahernar uzrokovan brzom rotacijom.

Zvezdana rotacija je ugaono kretanje zvezde oko svoje ose. Brzina rotacije može se meriti iz spektra zvezde, ili praćenjem kretanja aktivnih karakteristika na površini.

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. Thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.

The magnetic field of a star interacts with the stellar wind. As the wind moves away from the star its rate of angular velocity slows. The magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, and over time this gradually slows the star's rate of rotation.

Merenje

Unless a star is being observed from the direction of its pole, sections of the surface have some amount of movement toward or away from the observer. The component of movement that is in the direction of the observer is called the radial velocity. For the portion of the surface with a radial velocity component toward the observer, the radiation is shifted to a higher frequency because of Doppler shift. Likewise the region that has a component moving away from the observer is shifted to a lower frequency. When the absorption lines of a star are observed, this shift at each end of the spectrum causes the line to broaden. However, this broadening must be carefully separated from other effects that can increase the line width.

Ova zvezda ima nagib i prema vidnom polju posmatrača na Zemlji i rotacionu brzinu v_e na ekvatoru.

The component of the radial velocity observed through line broadening depends on the inclination of the star's pole to the line of sight. The derived value is given as $v_{e}\cdot \sin i$, where v_e is the rotational velocity at the equator and i is the inclination. However, i is not always known, so the result gives a minimum value for the star's rotational velocity. That is, if i is not a right angle, then the actual velocity is greater than $v_{e}\cdot \sin i$. This is sometimes referred to as the projected rotational velocity. In fast rotating stars polarimetry offers a method of recovering the actual velocity rather than just the rotational velocity; this technique has so far been applied only to Regulus.

For giant stars, the atmospheric microturbulence can result in line broadening that is much larger than effects of rotational, effectively drowning out the signal. However, an alternate approach can be employed that makes use of gravitational microlensing events. These occur when a massive object passes in front of the more distant star and functions like a lens, briefly magnifying the image. The more detailed information gathered by this means allows the effects of microturbulence to be distinguished from rotation.

If a star displays magnetic surface activity such as starspots, then these features can be tracked to estimate the rotation rate. However, such features can form at locations other than equator and can migrate across latitudes over the course of their life span, so differential rotation of a star can produce varying measurements. Stellar magnetic activity is often associated with rapid rotation, so this technique can be used for measurement of such stars. Observation of starspots has shown that these features can actually vary the rotation rate of a star, as the magnetic fields modify the flow of gases in the star.

Literatura

• Hadhazy, Adam. „Fact or Fiction: The Days (and Nights) Are Getting Longer”. Scientific American. Приступљено 5. 12. 2011. CS1 одржавање: Формат датума (веза)
• Iorio, L. (2011). „Perturbed stellar motions around the rotating black hole in Sgr A* for a generic orientation of its spin axis”. Physical Review D. 84 (12): 124001. Bibcode:2011PhRvD..84l4001I. arXiv:1107.2916 . doi:10.1103/PhysRevD.84.124001. 
• Renzetti, G. (2013). „Satellite Orbital Precessions Caused by the Octupolar Mass Moment of a Non-Spherical Body Arbitrarily Oriented in Space”. Journal of Astrophysics and Astronomy. 34 (4): 341—348. Bibcode:2013JApA...34..341R. doi:10.1007/s12036-013-9186-4. 
• Renzetti, G. (2014). „Satellite orbital precessions caused by the first odd zonal J3 multipole of a non-spherical body arbitrarily oriented in space”. Astrophysics and Space Science. 352 (2): 493—496. Bibcode:2014Ap&SS.352..493R. doi:10.1007/s10509-014-1915-x. 
• King-Hele, D. G. (1961). „The Earth's Gravitational Potential, deduced from the Orbits of Artificial Satellites”. Geophysical Journal. 4 (1): 3—16. Bibcode:1961GeoJ....4....3K. doi:10.1111/j.1365-246X.1961.tb06801.x . 
• King-Hele, D. G. (1983). „Geophysical researches with the orbits of the first satellites”. Geophysical Journal. 74 (1): 7—23. Bibcode:1983GeoJ...74....7K. doi:10.1111/j.1365-246X.1983.tb01868.x . 
• Renzetti, G. (2012). „Are higher degree even zonals really harmful for the LARES/LAGEOS frame-dragging experiment?”. Canadian Journal of Physics. 90 (9): 883—888. Bibcode:2012CaJPh..90..883R. doi:10.1139/p2012-081. 

Spoljašnje veze

• Staff (28. 2. 2006). „Stellar Spots and Cyclic Activity: Detailed Results”. ETH Zürich. Архивирано из оригинала 25. 9. 2013. г. Приступљено 2008-03-16. CS1 одржавање: Формат датума (веза)