Ability of a substance to rotate the plane of polarization of a beam of light passed through it, either as crystals or in solution. Clockwise rotation as one faces the light source is “positive,” or dextrorotary; counterclockwise rotation “negative,” or levorotary. Louis Pasteur was the first to recognize that molecules with optical activity are stereoisomers (see isomerism). Optical isomers occur in pairs that are nonsuperimposable mirror images of one another. They have the same physical properties except for their effect on polarized light; in chemical properties they differ only in their interactions with other stereoisomers (see asymmetric synthesis).

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Successive cultivation of different crops in a specified order on the same fields. Some rotations are designed for high immediate returns, with little regard for basic resources. Others are planned for high continuing returns while protecting resources. A typical scheme selects rotation crops from three classifications: cultivated row crops (e.g., corn, potatoes), close-growing grains (e.g., oats, wheat), and sod-forming, or rest, crops (e.g., clover, clover-timothy). In general, cropping systems should include deep-rooting legumes. In addition to the many beneficial effects on soils and crops, well-planned crop rotations make the farm a more effective year-round enterprise by providing more efficient handling of labour, power, and equipment, reduction in weather and market risks, and improved ability to meet livestock requirements.

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Rotation-powered pulsar is one of the major classes of pulsars. A Rotation-powered pulsar is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced intervals, or pulses. It differs from other types of pulsars in that the source of power for the production of radiation is the loss of rotational energy.

Name

As the first type of pulsars to be discovered, rotation-powered pulsars were originally known simply as pulsars, a term coined by a Daily Telegraph journalist as a contraction of "pulsating star". Although it was soon learned that the pulses were related to rotation rather than to physical expansion and contraction, as in true pulsating variable stars, the term stuck. After the discovery of accretion-powered x-ray pulsars, rotation-powered pulsars were known as radio pulsars. Since rotation-powered pulsars are now known that emit x-rays but not radio waves, the term "rotation-powered pulsar" is preferred.

Theory

There is general agreement that what we observe as a pulse is what happens when a beam of radiation points in our direction, once for every rotation of the neutron star. The origin of the beam is related to the misalignment of the rotation axis and the axis of the magnetic field of the star. The beam is emitted from the poles of the neutron star's magnetic field, which may be offset from the rotational poles by a wide angle. The source of energy of the beam is the rotational energy of the neutron star. The rotation slows down over time as the energy is emitted.

Millisecond pulsars are thought to have been spun up to high rotational speed by infalling matter pulled off a companion star.

Of interest to the study of the state of the matter in a neutron stars are the glitches observed in the rotation velocity of the neutron star. This velocity is decreasing slowly but steadily, except by sudden variations. One model put forward to explain these glitches is that they are the result of "starquakes" that adjust the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly superconducting interior of the star have also been advanced. In both cases, the star's moment of inertia changes, but its angular momentum doesn't, resulting in a change in rotation rate.

In 2003 observations of the Crab nebula pulsar's signal revealed "sub-pulses" within the main signal with durations of only nanoseconds. It is thought that these nanosecond pulses are emitted by regions on the pulsar's surface 60cm in diameter or smaller, making them the smallest structures outside the solar system to be measured.

Importance

The discovery of pulsars allowed astronomers to study an object never observed before, the neutron star. This kind of object is the only place where the behaviour of matter at nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed a test of general relativity in conditions of an intense gravitational field.

Significant pulsars

• The first radio pulsar, CP 1919 (now known as PSR B1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967. A drawing of this pulsar's radio waves was used as the cover of British rock band Joy Division's debut album, "Unknown Pleasures".
• The first binary pulsar, PSR B1913+16, confirming general relativity and proving the existence of gravitational waves
• The first millisecond pulsar, PSR B1937+21
• The first pulsar with planets, PSR B1257+12
• The first double pulsar binary system, PSR J0737-3039
• The longest period pulsar, PSR J2144-3933

See also

• Neutron star
• Pulsar
• Radio pulsar
• X-ray pulsar
• Magnetar
• Millisecond pulsar
• Pulsar planets
• Rotating radio transient