Spheres of incandescent gas

Everybody knows the stars. Even in the skies above our cities, where there is so much light interference from other sources, we can spot some of them. And probably we are able to distinguish them by their color and brightness. In fact there are many kinds of stars and in order to include them all in a single category, something must be said about their intrinsic proprieties.
Stars are gigantic balls of incandescent gas suspended that have their own light. This is not exactly the definition we will find on any Italian vocabulary, but probably it helps us to focus on the real nature of these small bright spots that have always fascinated human beings.
As we said stars are gigantic spheres of incandescent gasses.
In fact all stars are spheroid or semispheric because of gravity forces. All matter found in the universe generates a force of attraction simply because of its mass. If the distribution of matter is uniform, such as for example in a cloud of gas, around the gravity center the mass tends to accumulate in an identical way from every direction, thus forming spheroid shaped celestial bodies. However, because gravity is a weak force, we only see its effects when masses are very large. This is why stars have very large masses. The sun’s diameter is 1,4 million km long, 100 times more than the earth’s. But the sun is an average star. Star diameters range from a few hundredths to hundreds of times the solar one.
Yet even star dimensions, no matter how large, are small compared to the distances between them. Proxima Centauri, the closest star to our Solar System, is 250 thousand times farther from Earth than the sun. Even at a speed of 300 thousand kilometers per second, Proxima’s light takes four years to reach us.
Now we know that stars are enormous spheres scatterred in the empty parts of interstellar space. What are they made of? We haven’t spoken yet about their composition. Stars are made of high temperature gasses. Even though there are many kinds of stars, by analyzing the light that they emit, we know that they are composed mainly of hydrogen (70%) and helium (less than 30%), the most simple and abundant substances in the Universe, in addition to minimal percentages of more complex elements such as, for instance, carbon, oxygen, nitrogen and metal.
Stellar gas has a very high temperature. The sun’s surface is around 6000 degrees, but some stars can be almost ten times hotter. All bodies surrounded by a cooler environment tend to release their inner energy irradiating it in the form of light and heat. This is why stars emit light; the hotter they are the brighter they shine. The power released by the sun as light and heat is equal to ten thousand billion atomic bombs the size of the one dropped on  Nagasaki. And there are stars that are one million times brighter than the sun!
But this is not all. Stars have colors. For instance, if you observe Orion’s big constellation on a winter night, you can notice the left shoulder of the hunter  is distinctly red, while the right foot is definitely blue. A star’s color gives astronomers precious information on the energy with which most of its radiation is released. Since the way that a star emits light depends solely on its surface temperature, thus color becomes and indicator of its temperature. Hotter stars whose surface can reach 40 thousand degrees emit a blue light, while the coldest ones, reaching “only” 2000 – 3000 degrees release red radiation. The sun at 6000 degrees is yellow. Therefore we have proceed to create a star classification based on this characteristic which includes all the main groups  O, B, A, F, G, K, M from the hotter to the colder ones.

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