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Download 3. Stellar Formation and Evolution
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Formation of Stars • Stars are formed within extended regions of higher density in the interstellar medium. • These regions are called molecular clouds mainly composed of hydrogen plus helium. • As massive stars are formed from molecular clouds, they powerfully illuminate those clouds, ionizing hydrogen and creating an H II region. Protostar formation 1. The star forming clouds are initially in hydrostatic equilibrium (정수압 평형). 2. The formation of a star begins with a gravitational instability inside a molecular cloud 3. It is triggered by shock waves from supernovae or the collision of two galaxies. Stellar wind and radiation pressure from massive young stellar objects may compress interstellar medium. 4. Once a region reaches a sufficient density of matter when the internal gas pressure is not strong enough to prevent gravitational collapse (gravitational instaility), it begins to collapse under its own gravitational force. hydrostatic equilibrium • As the cloud collapses, dense dust and gas form 'Bok globules'. • As a globule collapses and the density increases, the gravitational energy is converted into heat. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. • These pre-main sequence stars are often surrounded by a protoplanetary disk (explain later). M 83, a barred spiral galaxy NGC 3603, an open cluster of stars surrounded by massive cloud The Antennae Galaxies, very high starburst galaxy occurring from the collision of two galaxies • Early stars of <2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. • These newly born stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig-Haro objects. • These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud in which the star was formed. Upper limit: 150 M Radiation pressure too great. Lower limit: 0.08 M Too cool for H-fusion to begin Brown dwarf (Jupiter mass = 0.001M) Main Sequence Main Sequence • Stars spend about 90% of their lifetime at this stage, fusing hydrogen to produce helium near the core. Such stars are said to be on the main sequence. • Once a star is born, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. For example, the Sun is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago. • Fusion process differs by mass. • For low mass stars, protonproton chain is dominant process for nuclear fusion at the core, whereas for high mass stars, carbon-nitrogenoxygen cycle is dominant. Electron Electron Proton Proton 1H 2H (Hydrogen) Proton Neutron Neutron 3H (Tritium) Neutron (Deuterium) Post-Main Sequence Massive stars process up to iron explode in Supernova events Low Mass stars stop before iron gently blow themselves to death forming planetary nebulas Red Giant • When stars > 0.4 M run out their hydrogen fuel in their core, their outer layers expand and cool to form a red giant. • In a red giant of up to 2 M, hydrogen fusion proceeds in a shell-layer surrounding the core. Eventually the core is compressed enough to start helium fusion. Stars shrinks in radius and increases its surface temperature. • After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. • Eventually the outer layers of the star will be shed, creating a planetary nebula, with only a white dwarf left behind. Planetary nebula Red Supergiant (High Mass Star) • After a helium-burning runs out of helium fuel in its core, the star's core starts to collapse and heat up. This causes the outer layers of the star to expand and cool, similar to the process that occurred after the star ran out of hydrogen fuel and left the main sequence. As the star becomes larger and larger, it eventually becomes a red supergiant. • Extremely massive supergiants can generate high enough pressure and temperature to fuse elements even heavier than carbon and oxygen. Near the end of the red supergiant phase, a high mass star will develop several "onion layers" of heavier and heavier elements. • Eventually stars this massive die … Death of High Mass Stars: Supernova (Type II) • The "Type II" supernovae are the result of a massive star consuming all of its nuclear fuel and then exploding. • These stars have large H-rich envelopes, hence the presence of H in the spectra • Elements heavier than Mg produced during explosion. Lighter elements produced during preceding stellar evolution Example spectra of Type Ia and Type II SNe H H No H or He H S Ca Typical Type II Supernova Si Typical Type Ia supernova The Sun • The Sun is a Population I, or heavy element-rich, star. – Population I: metal rich – Population II : metal poor – Population III: metal free, which is believed to form in the early universe • The formation of the Sun may have been triggered by shockwaves from nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as uranium, relative to the abundances of these elements in Population II stars. • These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation through neutron absorption inside a massive second-generation star. Photosphere (6000K) Core (13,600,000 K) with 0.25 solar radi: Thermonuclear reaction Summary Gas and Dust Main Sequence Large Mass Red Giant Medium Mass Red Giant Small Mass Red Giant Iron Core Iron Core Carbon Core Black Hole + Supernova Remnant Neutron Star + Supernova Remnant White Dwarf + Planetary Nebula