star formation
A fundamental question in astrophysics is the formation of stars. Young stars are embedded in regions of dust and gas - the so-called molecular clouds. Because the gas and dust absorb most of the light emitted by these stars, only recent observations have revealed the presence of stars in molecular clouds. The details of the formation process are still scarcely understood, but the general outline is as follows. High-density regions in the molecular clouds collapse due to gravity to form a protostar. The protostar then accretes mass and undergoes further gravitational contraction untill the temperature in the core is high enough to ignite hydrogen fusion. At that time, a star is born!
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hot stars
These stars (usually classified as O, B or Wolf-Rayet stars) are hot, massive and extremely luminous. Although they are rare and live only for a short time - a factor of a thousand less than the Sun - they have an enormous influence on their direct environment. Their most direct impact is due to the outflow of stellar material - the so-called stellar wind - into the interstellar medium (ISM). Due to this wind, a hot star can lose more than half of its initial mass, thereby influencing its own stellar evolution and the evolution of the galaxy in which it resides. When these stars finally die in a supernova explosion, even more material is deposited in the ISM. From this material, new stars (and planets) are formed. Even we ourselves consist of elements produced in the interior of stars.
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