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. Nuclear fusion no longer occurs in this type of star; gravity’s opponent is no longer the expansion pressure of high temperature gas, but the pressure generated by gas being compressed into a very small volume. On a white dwarf a teaspoonful of matter can weigh as much as a car! In this state the body gradually loses its residual energy, cooling off and gradually fading until it becomes a translucent, dark lifeless body in the cold interstellar space.
In stars with masses comparable to the sun, the nucleus stops collapsing only when the inner temperature reaches above 100 million degrees, the threshold temperature necessary to start up nuclear fusions again. This time helium atoms fuse with carbon atoms releasing the necessary energy to regain the lost balance; however with hydrogen fusion, stability can last tens of billions of years, while the one obtained with helium fusion is not so long lasting and runs out within a billion years. When helium runs out as well, this balance is lost again and the force of gravity takes over. The sun’s fate is the same as the red dwarfs’, besides helium’s brief action, the red giant phase will be followed by the white dwarf surrounded by planetary nebula one.
However, the destiny of stars with a mass 3 to 4 times larger than the sun’s is very different. In these stars the balance is lost rather frequently and at progressively shorter intervals. Each time that fuel for the nuclear reaction runs out, the process that we have just described is repeated over and over but every time the elements involved are heavier thus providing an increasingly shorter period of stability. When a star is left with an iron nucleus, the nuclear fusion reaction ends forever. In the absence of an opposing force, gravity causes the nucleus to collapse suddenly thus releasing stored energy: the star explodes and becomes a supernova, so bright that for a couple of months it will obscure even the galaxy it belongs to. However the explosion does not completely destroy the star; the nucleus survives and, once again, its fate will depend on its mass. In the case of nuclei up to 2 to 3 times the size of the sun, a neutron star will be created, in other words a body which is made exclusively of this type of atomic particles. Here gravitational collapse is opposed once again by the pressure coming from the matter’s extremely high density. Thus, on a neutron star, a teaspoonful of matter can weigh as much as 100 million cars. It is as if the entire solar mass was compressed into a sphere with a ten km radius, just slightly larger than a medium sized city.
Sometimes neutron stars revolve at high speeds around their axis. In this case the body is called a pulsar, because its light is channeled in the direction of its magnetic field, which is 1000 billion times more powerful than earth’s. This phenomenon produces a luminous pulsation, similar to the beam of a light house which we can see only when it shines in our direction. Recently we have discovered the fastest pulsar ever, which rotates at the astonishing speed of about 1100 revolutions per second!
If the star is even more massive, we will be able to witness the ultimate triumph of gravity; in fact, their collapse generates the notorious black holes, which are bodies so dense and compact that not even light is able to escape from their surface. Because the only source of information that we have in astronomy is the one brought to us by light, for decades black holes have been the mere result of theoretical calculations. Their existence has been proven only in the past 50 years and this proof is obtained indirectly from the gravitational effects that they have on their immediate surroundings…

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Facts

  • 17 May 2011

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