Stellar Evolution

The stellar evolution involves two opposite forces: on one side, the star’s mass produces the force of gravity, which leads to a contraction, on the other side the nuclear forces in the core produce expansion.

BIRTH OF A STAR Stars were born from enourmous cloud of gas and dust. The explosion of a near supernova or a collision between two nebulas marks the beginning of the force of gravity: a protostar is forming.

These objects aren’t real stars, because their inner temperature isn’t enough to prime nuclear fusion. After a long process of condensation, if the mass of the protostar is less than 1/10 of solar mass, the protostar isn’t able to reach the temperature which permits the nuclear fusion and it starts to cool, very slowly. Jupiter, in particolar, but the Earth too, are examples of protostars which continue to be… protostars, or, as we say, planets. When the mass of a protostar is more than 1/10 of solar mass, temperature in the core reach 10 millions kelvin: nuclear fusion starts. Than stars live a period of instability: contractions and expansions try to sorpass each other, till they come to a balance. Here is the star.

Astronomers use to classify stars on the basis of temperature and magnitude, in what they call the H-R Diagram (h-R from hertzsprung-Russel)

Here we see the H-R diagram. In abscissa the surface temperature of the star, in kelvin, and the corrispondent spectral class are riported. As you can see, from low to high temperature, they are O, B, A, F, G,K, M. Here is a trick to remember them: they are the initials of this sentence: Oh Be A Fine Girl, Kiss Me. In the y-axis, the absolute magnitude of the stars (not to be confused with the apparent magnitude) is reported, with, on the other side of the diagram, the luminosity compared to the Sun. So, the hottest, brightest stars are at the top left while the coolest, faintest stars are at the bottom right. The diagonal band of stars running from the upper left to lower right is known as the Main Sequence and includes those stars which are converting hydrongen into helium in their cores under stable conditions (90% of all stars known). Red Giants or red Supergiants represent the second step of the life of a star, as we’ll see. Then, on most occasions, white dwarf are the lastest period of a common star’s life.

Maturity Stars live the 90% of their life in the main sequence, placed according to their mass. At a certain point in their life the hydrogen in the core finishes. As a conseguence, the nuclear fusion finishes too, and so the star starts to contract under the pressure of its own mass. Then, the destiny of the star depends on its mass:

Stars with mass less than 0.5 solar masses: in this case the core’s temperature doesn’t reach sufficient values to prime the nuclear fusion of helium. This star is going to die in a white dwarf. These are little stars, very hot initially, which cool slowly till they swich off completely, in black dwarf. If a white dwarf is part of a bynar system, for example with a red giant, the first one can steal some of the red giant’s mass and prime the fusion of hydrogen in the external layers. This cause a a big explosion which can be seen from the Earth. These stars are called Novae.

Stars with mass more than 0.5 solar masses: in this case, the contraction provokes the core’s temperature of 100 million kelvin, which is enough to prime the helium fusion. In the region around the core, instead, temperature is more than 10 million kelvin, and so here the hydrogen fusion starts, causing the expansion of external layers: the Red Giant. The red giant’s life is short, because the helium is less than hydrogen and because the energy produced by helium is less than that produced by hydrogen.

The subsequent phases of the life of a star depends, once again, on its mass: Stars with mass less than 1,44 solar masses: this star can’t have the nuclear fusion of carbon, and lives a period of instability, in which it expells the external layers, made of carbon and oxygen, and it becomes a white dwarf. What we see is a Planetary Nebula.

Stars with mass more than 1,44 solar masses: the star starts the process of nuclear fusion which leads to the formation of heavier and heavier elements. The star results as a composition of layers of different density, the lighter ones on the top and the heavier ones in the core. When the reactions get to iron, things change: the fusion of iron doesn’t trasform mass in Energy, but Energy in mass. Because of that, the star collapses into itself, and esplodes violently: it’s a Supernova, that can be visible from the Earth during the daylight.

Heaviest elements If the heaviest elements producted by stars is iron, how about all the others? In this phase, part of the espelled matter creates a bow wave that produce condensation among elements which forms new heavier elements. It’s the only way to produce the heavy elements we can find in nature. That’s why we are called sons of stars.

Then the future of the core of the star depends again to its mass. Core with mass less than 3-4 solar masses: it become a neutron star. It’s a little star in which all the protons and electrons have lost their individuality, and have fused into neutrons. The neutron star has a strong magnetic field and quick rotation. For that reason this type of objects are also known as Pulsar.

Core with mass more than 4 solar masses: the contraction goes on till unimaginable density and, at the end, the star becomes a Black Hole, one of the most mysterious objects in the universe.