Stars

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.

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.

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.

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.

Stars are born, live and die like living beings; the only difference is that they do it in such a large time frame as to appear eternal and immutable. Therefore, if we want to study their lifecycle the only thing that we can do is to assume that all stars have a similar evolutionary process and pick a vast number of samples at different stages of their life span. Rather like observing a newborn, an adult and an old person to study man’s cycle.

Why do stars evolve? All life long a star has to put up with a titanic battle between the two main forces that govern it. If some conditions are modified and one prevails over the other the balance is lost, triggering chain reactions that modify the structure of the star leading to a new balance. Therefore, the evolution of a star will go through long stable phases alternated by short periods of instability when main evolutionary changes occur. Development of a star depends above all on its mass. The greater the amount of matter, the greater the amount of pressure necessary to oppose the collapse and therefore the greater the amount of fuel burned. Consequently, larger stars are brighter than the ones with a smaller mass, but they also live much less.

Birth and maturity

Where are stars born? Interstellar space isn’t empty but is full of so called interstellar medium, a widespread evenly distributed gas and dust mix. Yet if we observe it on a reduced scale we notice that the matter tends to thicken and form gas clouds, for the most part hydrogen, and dust. These clouds have a dynamic and thermal balance at the extreme temperature of -270°C. For reasons beyond the clouds themselves, sometimes the substances they are made of will start to compress into a smaller volume and be affected by the mutual gravitational attraction.

Old Age

What happens when the main fuel is running out? The star has burnt almost all the available hydrogen, which accounts for about 10% of the total, in the nuclear fusion, and the nucleus is composed almost exclusively of inert helium. The energy production that counterbalanced the gravitational collapse is no longer sufficient to oppose it and the balance is lost. Even if at first no change is noticeable on the star’s surface, the nucleus starts to contract under the pressure of its mass, gradually increasing its density and temperature. The first consequence is that fusion reactions occur out of the nucleus which by now is spent involving the thin layer of surrounding hydrogen.

Death

And then? It all depends on the mass of the star. Stars that are smaller than the sun become unstable. As the star is no longer able to manage its entire mass, it expels the superficial layers in a gas puff, thus creating a planetary nebula. There is no planetary nebula the same as another: the expelled gas becomes of many different colors and takes on different shapes, creating one of the most fascinating shows in the sky. The center of the nebula contains the beating heart of the old star which is no larger than our Planet, but extremely hot: a white dwarf.

Compact objects

In the chapter on star evolution we said that both in white dwarfs as well as neutron stars the gravitational collapse is opposed by a pressure which no longer depends on gas temperature, but on its density. In astrophysics bodies of this kind are called compact objects and the matter of which they are made of is called degenerate matter. In order to explain this behaviour we must go from the extremely huge field to the infinitely small one: here is where modern quantum mechanics come to help us. However one must not be surprised by the leap from one scientific discipline to another.

Nuclear reactions

There are two reactions that involve an atom’s nucleus: fusion and fission. The first starts from simple elements to produce complex ones; the second acts in the opposite way by splitting nuclei of heavy elements into nuclei of lighter elements. In both cases there is a very high energy output. In the case of fusion for instance: the mass of the atomic nuclei that melt is greater than that of the new nucleus that will form. Since we know that mass and energy are equivalent as according to Einstein’s famous formula of E=mc2, the difference in mass is what is transformed into energy.