Stars are thought to originate from clouds of mostly hydrogen gas that are present throughout the universe. A gas cloud contracts in response to self-gravitation, and as the gas contracts its kinetic energy - and therefore its temperature - increases. When the temperature in the center of this "proto-star" is sufficiently high, hydrogen undergoes fusion to form helium in a process known as "hydrogen burning." After this fusion begins, the proto-star is a full-fledged star. The energy from fusion increases the star's interior pressure, and as a result the gravitational contraction ceases. The total power radiated by a star is represented by its luminosity. Studies of nearby stars show that a star's mass is related to its luminosity: the more massive a star, the greater its luminosity. The Hertzsprung-Russell diagram plots a star's luminosity versus its surface temperature. This is used for classifying stars. Most stars, like our Sun, fall along the "main sequence" of the plot. Stars appear on this curve a few tens of millions of years after their birth. Those similiar in mass to the Sun remain there for about 10 billion years. The Sun has been there for nearly 5 billion years. Where would a main-sequence star that is much more massive than the Sun be found on the Hertzsprung-Russell diagram? ... As hydrogen-burning in a star progresses, a core of helium forms. Thereafter, the intensity of hydrogen-burning decreases, and the outer region of the star begins to collapse on the core. The resulting temperature increase in the shell around the helium core initiates more hydrogen-burning. This, in turn, causes a significant expansion of the outer region of the star. By this process, a star similar in mass to the Sun becomes a "red giant." Next, the helium core of the star contracts, and the higher temperature initiates helium-burning. Meanwhile, hydrogen-burning continues in the shell around the core. After only a few tens of millions of years after leaving the main sequence stage, the star runs out of fuel. It collapses to become a "white dwarf" about the size of Earth. The outer shell blows off as a a 'planetary nebula'. The white dwarf radiates its remnant energy until, at length, it becomes a cold, dark hulk, known as a "black dwarf." On the other hand, a main sequence star that is much more massive, with several times the mass of the Sun, will evolve to a "super giant." It initially follows the helium-burning sequence previously discussed. However, because of its massive size, fusions involving heavier nuclei occur and produce elements as heavy as iron and nickel. Thus, a massive super giant star develops an "onion shell" structure, with layers of progressively heavier elements toward the core. As a massive star runs out of fuel, it rapidly collapses. This contraction results in extremely high temperature and pressure in the core. Heavy iron and nickel nuclei are broken apart, and free protons and electrons are forced together to form neutrons. So much energy is released in the final collapse of the core that the star blows off its outer region in a massive explosion known as a "supernova." The star's remaining core forms an extremely dense "neutron star." A neutron star with a diameter of only 10 km has a mass greater than that of the sun. If the neutron star has a mass several times that of the Sun, it contracts to an even smaller diameter, and becomes a "black hole." A black hole is so dense that even light is unable to escape its gravitational field. It can only be viewed indirectly from the material surrounding and 'feeding' it, and gas 'jets' propelled away from it.

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