The Origin of the Solar System (Stellar Formation).

Stars start out as interstellar clouds of gas and dust. If you were inside such a cloud, you probably couldn't tell that there was anything at all there, because the gases which make up the clouds are incredibly thin. Each cubic inch of these clouds contains only a few dozen to a few hundred atoms, while each cubic inch of our atmosphere contains almost a billion trillion atoms. If you had to expand a single cubic inch of our atmosphere until it was as thin as the gases in an interstellar cloud, it would be almost 200 miles on a side.

Although the clouds are incredibly rarefied, they are also incredibly big, stretching for trillions of miles in all directions. Because of their huge size, even though there is practically nothing at any given place within them, the huge extent of practically nothing adds up to substantial masses, hundreds of thousands of times greater than the mass of the Earth, like that of the Sun.

Because the material of the cloud is spread out over such a huge volume of space, the gravity caused by its mass is incredibly small, and under normal circumstances, it cannot force the cloud to contract to a smaller size. But under some circumstances, the clouds ARE forced to contract to smaller sizes. During such contractions, gravity gradually increases, and if its force becomes large enough, the thin gases within the cloud will not be able to exert enough outward pressure to prevent the gravitational pull from contracting the cloud to still smaller sizes, and so the cloud will continue to contract.

Although gravity is trying to make the cloud smaller, the pressure of the gases within the cloud is trying to stop the contraction. At first the pressure is negligible, since the gases are so incredibly thin, but as the cloud gets smaller, the gases become denser and hotter. The inward pull of gravity tries to make the gases move inward with greater and greater speeds, but random collisions between the atoms of the gas tend to convert this inward motion into a random sub-microscopic movement, which we perceive as heat. As the cloud contracts, greater and greater amounts of inward movement are converted into faster and faster microscopic motions, or greater and greater amounts of heat. The greater temperatures which result, combined with the greater density of the gas, create a continually increasing pressure, which fights against gravity.

As the heat generated by the contraction of the gases increases, pressure gradually rises, until it equals the gravitational forces, stopping the inward motion of the cloud. If this occurs while the cloud is still very large, and not very warm, no further contraction will occur. But if this balance does not occur until the cloud has contracted a long way, and has therefore generated a large amount of heat, some of the heat will be radiated away in the form of infrared light. The heat lost in this way reduces the ability of the gas to hold up against the pull of gravity, and causes a slow, semi-equilibrium contraction of the cloud. At each stage of the contraction, pressure and gravity are in balance, and if no more heat were radiated away, the contraction would stop, but the continual radiation of infrared light at the outside of the gradually warming cloud prevents this, and allows gravity to have a slow but steady victory over pressure.

Although the loss of heat at the outside of the cloud forces it to contract, there is a limit to how far the contraction can go. As the cloud continues to contract, temperatures within the cloud continue to rise. By the time that the cloud is as small as a star like the Sun, the central temperatures have risen to many millions of degrees, and a conversion of hydrogen to helium begins, in a process known as thermonuclear fusion. At first, this conversion is slow, and produces only a small amount of energy, but as the star continues to contract, the rate of nuclear fusion increases, producing more and more energy. The heat generated by this fusion helps replace the heat being lost at the outside of the star, slowing the rate of contraction. The closer the star gets to a stable size, the closer the core approaches an equilibrium temperature at which the nuclear reactions produce exactly as much heat as is being lost on the outside. When the star reaches that temperature, there is no longer any net loss of heat, and so the star's contraction finally ends.

Дата добавления: 2015-09-04