Angular Momentum of the Sun
From SkepticWiki
[edit] Introduction
The puzzle of the Sun's angular momentum is stated by one Creationist website (Answers in Genesis) as follows:
- One major problem can be shown by accomplished skaters spinning on ice. As skaters pull their arms in, they spin faster. This effect is due to what physicists call the Law of Conservation of Angular Momentum. Angular momentum = mass x velocity x distance from the centre of mass, and always stays constant in an isolated system. When the skaters pull their arms in, the distance from the centre decreases, so they spin faster or else angular momentum would not stay constant. In the formation of our sun from a nebula in space, the same effect would have occurred as the gases allegedly contracted into the centre to form the sun. This would have caused the sun to spin very rapidly. Actually, our sun spins very slowly, while the planets move very rapidly around the sun. In fact, although the sun has over 99% of the mass of the solar system, it has only 2% of the angular momentum. This pattern is directly opposite to the pattern predicted for the nebular hypothesis. [1]
This was once a genuine puzzle in science, and for creationists to pretend that a puzzle in science is somehow proof of fiat creation is one of their oldest traditions (see the main article on the God of the Gaps Fallacy); Although the puzzle was solved over three decades ago, this news has reached lamentably few creationists.
[edit] Discussion
The puzzle arose from an incomplete model of the solar system, in which only gravity was taken into consideration. In models which incorporate electromagnetism as well as mechanics, the spin-up of the collapsing core of the disc during the formation of the solar system, and the spin of the Sun after formation, are countered by magnetic braking [2][3][4]
The idea of magnetic braking is simple enough: the Sun is known to emit charged particles in the solar wind; and the Sun's magnetic field is twisted around its axis of rotation. The result is that the Sun's magnetic field drags the particles round with it, which requires a loss of angular momentum from the Sun in order that the net angular momentum of the system is conserved. The smallness of the Sun's angular momentum does not defy the conservation of angular momentum: it is caused by it.
One obvious way of testing this model is that if it holds good, then stars which are older, as verified by such methods as chromospheric dating, should spin slower than younger stars, as is the case:
- Stellar rotation is now known to change systematically, even predictably, on the main sequence ... Furthermore, the specific form of the rotational spindown of stars is such that initial variations in the rotation rates of young stars appear to become increasingly unimportant with the passage of time, leading to an almost unique relationship between rotation period and age for a star of a given mass. Finally, rotation is a property we can now measure to great precision; rotation periods for late-type stars are sometimes determined today to better than one part in ten thousand.[5]
