A Lindblad resonance, named for the Swedish galactic astronomer

Bertil Lindblad Bertil Lindblad (Örebro, 26 November 1895 – Saltsjöbaden, outside Stockholm, 25 June 1965) was a Swedish astronomer. After finishing his secondary education at Örebro högre allmänna läroverk, Lindblad matriculated at Uppsala Univer ...

, is an

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...

in which an object's

epicyclic frequency In astrophysics, particularly the study of accretion disks, the epicyclic frequency is the frequency at which a radially displaced fluid parcel will oscillate. It can be referred to as a " Rayleigh discriminant". When considering an astrophysica ...

(the rate at which one

periapse An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary (astronomy), primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two ...

follows another) is a simple multiple of some forcing frequency. Resonances of this kind tend to increase the object's

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...

and to cause its longitude of periapse to line up in phase with the forcing. Lindblad resonances drive spiral density waves both in

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

(where stars are subject to forcing by the spiral arms themselves) and in

Saturn's rings The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...

(where ring particles are subject to forcing by

Saturn's moons The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan (moon), Titan, which is larger than the planet Mercury (planet), Mercury. Saturn has 83 natural satellite, moons with confirmed ...

). Lindblad resonances affect stars at such distances from a

disc galaxy A disc galaxy (or disk galaxy) is a galaxy characterized by a galactic disc, a flattened circular volume of stars. These galaxies may or may not include a central non-disc-like region (a galactic bulge). Disc galaxy types include: * Spiral ga ...

's centre where the natural frequency of the radial component of a star's orbital velocity is close to the frequency of the gravitational potential maxima encountered during its course through the spiral arms. If a star's orbital speed around the galactic centre is greater than that of the part of the spiral arm through which it is passing, then an inner Lindblad resonance occurs—if smaller, then an outer Lindblad resonance. At an inner resonance, a star's orbital speed is increased, moving the star outwards, and decreased for an outer resonance causing inward movement.

References

External links

Three-Dimensional Waves Generated At Lindblad Resonances In Thermally Stratified Disks – Lubow & Ogilvie
Astrophysics Stellar dynamics Orbital perturbations Orbital resonance {{scattering-stub

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Further reading

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