A star is born when enough material, usually from vast clouds of gas and dust in space, gets in close enough proximity to coalesce by gravity into a compact form. When the force of gravity at the center of the mass becomes powerful enough to crush atomic nuclei together into larger nuclei, against the repulsive force of the like-charged protons, the leftover binding energy is released and the star begins to shine. An equilibrium forms between this outward radiation of energy and the inward pull of gravity.
But if all that is required to make a star is enough mass, then why does not all of the matter in the galaxy just form one big star?
Bodies in space invariably undergo rotation. The earth, obviously, rotates in one day. The sun rotates in about a month, and the galaxy in about two hundred million years. This universal rotational tendency is still somewhat mysterious, my string theory on the cosmology blog explains it as the strings in multi-dimensional space wrapping around one another, but that is somewhat beyond the scope of our discussion here.
When a body such as a star rotates, it creates centrifugal force. The larger the rotating body, the more difference there is in velocity between a point on the inside of the body and a point on the outside. The concept is similar to that in a CD player. There is a groove pattern in the tracks of the CD, which varies in wavelength from the beginning of the track to the end. This pattern is called "ATIP", and it tells the motor of the CD how fast to turn. This is because the CD has to turn faster when the outer tracks are being played, and slower for the inner tracks.
The difference in the speed on the surface of a spherical body results in the banding effect that can be seen in the different colors (colours) of the cloud that form at different latitudes on the four large planets of the Solar System; Jupiter, Saturn, Uranus and, Neptune.
The outward centrifugal force opposes the inward force of gravity. I once read somewhere that if the earth rotated sixteen times as fast as it does now, objects on the equator would be weightless because the resulting centrifugal force would balance that of gravity. Indeed, we can see that the earth's equatorial diameter is greater than it's polar diameter because of this centrifugal force.
This means that within the body of any rotating object, the force of gravity holding the object toether must be stronger than the outward centrifugal force. Any mass in the outer portion of the object for which it is not will be ejected out into space. The conclusion is that there is a certain maximum rotational energy that a rotating body, such as a star, can have. If this limit is exceeded, some of the outer mass of the star, which has the most rotational energy, will be thrown out into space and thus bringing the rotational energy of the star back within the limit.
Exactly the same principle is applied when a governor is installed on a rotating axle, as a safety device to precent the axle from spinning too fast. The governor is simply weights which can move away from the axle on an arm mechanism as it spins faster. When the weights are further from the axle, they require more rotational energy. In drawing rotational energy from the axle itself, the weights force it to rotate more slowly.
Our galaxy, a so-called spiral galaxy shaped like a pinwheel, holds together because individual stars in the spiral arms of the galaxy are free to revolve around the center at their own rates. These rates of rotation vary according to the distance of the star from the galactic center.
This rule that, in a rotating body, the outward centrifugal force cannot exceed the inward force of gravity, limits the sizes of planets as well as stars. But with planets, the scarcity of material is usually a more important factor. But we can definitely say that rotation maintains the universe of multitudes of stars and planets as we know it.
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