The death of a Star: White Dwarfs and Supernovae
How Stars End Their Lives
What happens to a star at the end of its cycle depends on its mass. If the mass of the star core, (i.e., what remains in the center after it has shed the outer envelope in the giant phase) is greater than 1.4 solar masses (the Chandrasekhar limit), then a neutron star will be born; otherwise the star will continue its life as a white dwarf. Now, there is often some confusion around the value of the mass of the star that separates an evolution to a white white dwarf from the one to a black hole or a neutron star. What counts is not the mass of the star while it is on the main sequence, but the mass of the star after it has shed its envelope during the giant phase. The Sun is expected to lose 40 % of its mass during the giant phase and thus it will end up as a white dwarf. Other stars might lose more or less. For example, a bright B star of eight solar masses might shed 80% of its mass and remain with 1.2 solar masses (still below the Chandrasekhar limit). A bigger star will end up, presumably, with more than 1.4 solar masses at its core. In that case, the star becomes either a neutron star or a black hole. This final stage of evolution of such a star is preceded by an explosion, called a supernova explosion. When the star explodes, its brightness momentarily increases a million-fold and more. During this time elements heavier than iron are synthesized. Recall that no star can produce elements heavier than iron in its interior because iron is a very stable element and it would necessitate quite some more energy to fuse iron to the next element (such a condition never occurs in a star). As a result of the explosion, a shock wave moves away from the star. If there are surrounding gas clouds, the interaction of the shock wave with cloud material destroys dust molecules and dust grains in the clouds, and makes new ones as well. In 1987, a star from the Large Magellanic Cloud (a galaxy rotating around our own and located 50,000 pc away) exploded. Neutrinos produced in that explosion were detected on Earth, giving valuable clues about the physics of supernovae. The interaction of the blast wave with the surrounding medium created three rings of dust and gas. This is the first time that we were able to observe such processes in real time. This will help us understand not only supernova explosions, but also the making of elements in supernovas and the consequences on the interstellar medium. The material produced in supernovae is then recycled in stars. A star like the Sun should have only hydrogen and helium, but in reality it has also other elements; these were ejected by some previous supernovae and have been incorporated in the solar nebula at the time the Sun formed. Let us see what happen in some detail.The fusion of hydrogen in the core of a star produces energy and He atoms. The Sun is in this hydrogen burning phase. When the hydrogen in the core is almost burned up, the inner core starts to collapse inward and its temperature rises. Hydrogen atoms outside the core starts to burn because of the increased temperature and the star increases in size -red giant phase. The Sun, when it will reach its red-giant phase, will increase its current size all the way until its outer perimeter will reach the Earth's orbit. In the meantime, the core of the red giant will contract until the temperature will be high enough to start the fusion of helium into carbon. Helium will burn quickly (helium flash). As helium is converted into carbon, the core will grow smaller and denser. In a star of the size of the Sun, gravity is not strong enough to overcome the strong repulsion forces which prevent matter to come too close. (During fusion, the high temperatures and pressures allow matter to come close so as to produce fusion.) Eventually the size will shrink to a small fraction of the original size of the star. The Sun, for example, will reach the size of the Earth. At this point the star has reached the stage of a white dwarf and it will slowly cool off.
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For stars 40 % more massive than the Sun, the process of fusion will go on further. Because of higher temperatures reached inside the core, carbon and helium will fuse into oxygen. The cycle continues until all the core has fused into iron (Fe), and then the process stops. Iron has a rather stable nucleus, and no further fusion occurs. As the contraction continues, electrons and protons will be expelled together with the outer layers in a supernova explosion. The core will become a neutron star . The shock waves produced by the explosion will compress matter to such high densities than elements heavier than Fe will be produced. These elements will be returned to the gas and dust clouds where, eventually, new planets and stars will form.
At the beginning, neutron stars will spew off charged particles; the particles still in the core will produce intense magnetic fields which can be detected from Earth. These neutron stars that send rapidly varying signals are called pulsars. If a star is rather massive, more than 30-50 times the mass of the Sun, it is predicted that the gravitational force will compress matter so tightly that a black hole will form.
For more information about this Hubble space telescope image, click here.
Images and animations of supernovas can be found at this NASA site.