Gases In Outer Space:

The outer space was once believed to be a perfect vacuum. Though, this is not the situation. There is plenty of material out there, and much of it is hydrogen and helium gas. (There are also trace quantities of heavier gases and some solid rocks and ice chunks also.) All the atoms in outer space interact gravitationally with all others. This may be hard to visualize at first, though if you think about it, there is no escaping it. Even a single atom of hydrogen applies a gravitational pull on the other atom 1 million km away.

The motion of atoms in outer space is almost arbitrary but not quite. The slightest perturbation in the arbitrariness of the motion gives gravitation a chance to cause the gas to clump into huge clouds. Once this procedure starts, it can continue until a globe of gas forms in which the central particle density is important the figure is as shown below. As gravitation continues to pull the atoms in toward the center, the mutual attraction between the atoms there becomes greater and greater. When the gas cloud has some spin, it flattens into an oblate spherical shape and ultimately into a disk with a bulge at the center. A vicious circle ensues, and the density in the central area skyrockets. The gas pressure in the center increases, and this causes it to heat up. Eventually, it gets too hot that nuclear fusion starts, and a star is born. Similar events among the atoms of the gas on a smaller scale can outcome in the formation of planets, asteroids, and planetary moons.

Figure: (a) Distribution of gas inside a container. (b) Distribution of gas about a planet with an atmosphere. (c) Distribution of gas in a star is forming. Darkest shading indicates highest concentration.