Spheres suspended

Let’s go back to the initial definition that we have given of a star. First of all let us stop and think: have we ever seen a gas take on a definite shape, such as a sphere, without beong enclosed in a container? The answer obviously is no, because gasses tend to spread and occupy all the available space. Then how can it be that the gasses of the stars are somehow confined and don’t dispel into space? The explanation can be found once again in gas behavior: when compressed, a gas warms becomes warmer. Stars have  a hydrostatic equilibrium thanks to the balance between two equal forces pulling in opposite directions: gravity, that tends to make matter collapse inwards toward the center, and pressure caused by the expansion of the hot gas, pulling outwards Astronomers estimate that temperatures at the center of the sun reach 15 million degrees Celsius and that density is about a dozen times the one of lead. Nevertheless the center of the sun is still gaseous because gas at such high temperatures is in a particular state called plasma where electrons and nuclei, are no longer bound by their classical atomic structure so they generate clouds of free-floating electrically charged particles; in this state, matter is highly compressible thus remaining in a gaseous state.
This eternal clash between forces lasts throughout the star’s long life. Star longevity has been one of the main problems for astrophysics to solve in the past. Stars in fact appear eternal and immutable compared to our life time. Let’s take the sun for instance: because Earth cannot exist without its star , we know that the sun is at least as old as our Planet which is about 4,5 billion years old. And this is not all: terrestrial fossil discoveries let us know that during all this time, the sun has continued to shine more or less as it does nowadays. The age problem is strictly related to the production system of the energy released. In fact, this energy could come solely from gravity: as the sun contracts it heats up and becomes brighter. Estimates on the gravitational energy available to fuel the process prove, however, that the sun cannot survive beyond thirty million years. Therefore there must be an alternative energy source to fuel the longevity that we have observed.

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