Newborn Stars

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Alexandra Pope

Small Mass Newborn Stars Fusion and Contraction ● Fusion will not begin in a contracting cloud if a force stops contraction before the core temperature rises to about 10 million K. ● Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation. ● Degeneracy Pressure: the laws of quantum mechanics prohibit two elections from occupying the same state in same place. ● Thermal Pressure: ○ Depends on heat content ○ Is the main form of pressure in most stars ● Degeneracy Pressure ○ Particles can’t be in same state in the same place. ○ Doesn’t depend on heat content. Brown Dwarfs ● Degeneracy pressure halts the contraction of objects with <0.08Msun before core temperature becomes hot enough for fusion. ● Starlike objects not massive enough to star fusion are brown dwarfs. ● Abrown dwarf emits infrared light because of heat left over from contraction. ● Its luminosity gradually declines with time as it loses thermal energy. Brown Dwarfs in Orion ● Infrared observations reveal recently formed brown dwarfs because they are still relatively warm and luminous. Large Mass Newborn Stars Radiation Pressure ● Photons exert a slight amount of pressure when they strike matter. ● Very massive stars are so luminous that the collective pressure of photons drives their matter into space. Upper Limit on a Star’s Mass ● Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart. ● Maximum thought to be around 150Msun, but new observations indicate some may be even larger. Limits ● Stars more massive than 150Msun would blow apart. ● Stars less massive than 0.08Msun can’t sustain fusion. Demographics of Stars ● Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars. REVIEW ● Degeneracy pressure stops the contraction of objects <0.08Msun before fusion starts. ● Stars greater than 150Msun would be so luminous that radiation pressure would blow them apart. ● Star formation makes many more low-mass stars than high mass stars. Chapter 17: Star Stuff Stellar Mass and Fusion ● The mass of a main-sequence star determines its core pressure and temperature. ● Stars of higher mass have a higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived. ● Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes. Types of Stars ● High mass stars > 8Msun ● Intermediate Mass Stars ● Low-Mass Stars 0.08 - 2 Msun ● Brown Dwarfs Star Clusters and Stellar Lives ● Our knowledge of life stories of stars comes from comparing mathematical models of stars with observations. ● Star clusters are particularly useful because they contain stars of different mass that were born about
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