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.

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 and pressure.

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Spheres that shine
To discover the system which is able to heat so much gas and for such a long time we have to dive into the microscopic world of atomic nuclei. Atoms have a precise structure: they have a nucleus formed by particles called protons and neutrons, around which orbits a cloud of smaller particles called electrons. We are in the extremely small world: take a millimeter, divide it by a million and then again by ten and we will have the size of an atom.

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Star groups
Constellations are groups of stars which form certain familiar shapes in the sky. Nevertheless, the celestial sphere is simply a two-dimensional projection of the universe that surrounds us centred on our planet. Thus when considering the third dimension, which is depth, stars belonging to the same constellation are not bound together in any way, in fact, they are often considerably far from one another. Therefore the belief that certain constellations may influence people’s lives is apparently groundless.

Yet this doesn’t mean that stars live alone, on the contrary. Often stars form complex systems with two, three or more components bound together by their mutual gravitational attraction.

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The distance problem
One of the principal problems in astronomy is the measurement of stellar distance. We have already seen that all objects are “squashed” on a spherical projection, known as celestial sphere, at the centre of which we have Earth. A lack of depth obviously brings to misled calculations of luminosity and distances among the objects. The sun for instance is a medium sized object, yet because it is also the star which is closer to us, it seems larger and brighter than many other stars that, even if they are much brighter, they seem smaller and weaker because of the distance.

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