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This mass of hydrogen, a nascent galaxy, spins as it forms, and centrifugal force spreads it into a wheel-like disk ten lightyears* thick and 60,000 light-years in diameter. At this early stage the whole mass is dark, as in the Biblical account of the first day of Creation,† but "gravitational instability" is still on the job. Gas clots form, pack denser and denser; they also grow hot as gravitational energy (the energy of matter falling toward a center) turns into heat. When a gas cloud has contracted to something like one-millionth of its original diameter, its center gets hot enough to start the nuclear reaction which turns hydrogen into helium with a great release of energy.
Such a glowing, reacting mass is an ordinary star—like the sun. But it does not contract indefinitely. As soon as the energy generated within it balances the radiation escaping from its surface, the star becomes stable. If left to itself it could continue for many billions of years, slowly "burning" its hydrogen.
Growth of Stars. Most stars are not left to themselves, at least not all the time. A good part of the gas that forms the young galaxy remains as thin gas. Both stars and gas move in swirls like the eddies and surges in flowing water, and these motions frequently carry the stars through the clouds of gas.
What happens then was worked out chiefly by Lyttleton. The easiest way to understand it is to imagine a moving gas cloud passing a stationary star. As the individual particles in the cloud come under the influence of the star's gravitation, they are pulled into curving paths that lead to a line of points directly behind the star (see diagram). There they collide with one another, and their energy of motion is turned into heat. Robbed in this way of the speed which might have carried them safely past the star, many of them are captured by it, falling into it in a mighty stream.
Cloud Tunnels. How many particles are captured can be calculated mathematically. It depends on their speed. If the particles are passing the star at as much as 30,000 miles an hour, few are captured. Their curves are rather flat; they collide far away from the star and rarely fall into it. But if the speed is low (around 5,000 miles an hour), the particles curve sharply, collide close to the star and fall into it in great numbers. Behind the star an empty "tunnel" is left in the ravaged dust cloud. The tunnel is fat if the speed is low, thin if it is high.
At present, says Hoyle, the sun is not catching much material; it is moving too fast through too thin a cloud. Some time in the past, however, it must have tunneled a dense cloud. One proof of this hypothesis is the sun's spectacular bevy of comets. These "bagsful of nothing" (probably loose aggregations of small particles and gas) plunge toward the sun on long, elliptical orbits. They whip around the sun and streak out again into cold, dark space. They are mementos, says Lyttleton, of a time when the sun and its brood of planets were passing through a dense cloud of interstellar matter. Loose blobs of captured material falling toward the sun were deflected by the pull of the larger planets. Once having missed the sun, they were condemned by the laws of celestial mechanics to swing wearily around it.