Solution to the Angular Momentum Puzzle

The angular momentum problem that defeated Kant and Laplace—why the planets have most of the solar system's angular momentum while the Sun has most of the mass—can now be approached in a cosmic context. All stars having masses that range from slightly above the mass of the Sun to the smallest known masses rotate more slowly than an extrapolation based on the rotation rate of stars of higher mass would predict. Accordingly, these sunlike stars show the same deficit in angular momentum as the Sun itself.

The answer to how this loss could have occurred seems to lie in the solar wind. The Sun and other stars of comparable mass have outer atmospheres that are slowly but steadily expanding into space. Stars of higher mass do not exhibit such stellar winds. The loss of angular momentum associated with this loss of mass to space is sufficient to reduce the rate of the Sun's rotation. Thus, the planets preserve the angular momentum that was in the original solar nebula, but the Sun has gradually slowed down in the 4.6 billion years since it formed.

The Sun

The Sun

At the centre of the solar system is the Sun, the star around which Earth and the other components revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides Earth with the light and heat necessary to support life.

The Sun is classified as a G2 V star, with G2 standing for the second hottest stars of the yellow G class and the V representing a main sequence, or dwarf, star, the typical star for this temperature class. (G stars are so called because of the prominence of a band of atomic and molecular spectral lines that the German physicist Joseph von Fraunhofer designated G.) The Sun exists in the outer part of the Milky Way Galaxy and was formed from material that had been processed inside a supernova. The Sun is not, as is often said, a small star. Although it falls midway between the biggest and smallest stars of its type, there are so many dwarf stars that the Sun falls in the top 5 percent of stars in the neighbourhood that immediately surrounds it.

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