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E4N2_Nov8[1] - Protostar growth and Development Since gas...

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Unformatted text preview: Protostar growth and Development Since gas clouds are usually Since slowly rotating, a protostar forms from a rotating disk. The protostar emerges from the The center of the disk. Planets may well form from the Planets disk itself. Figure 11.4 Figure Protostar Development by Mass The initial mass of a The protostar determines where on the main sequence a star will lie. There is a relation There between initial mass and contraction time. A one solar mass star one requires 30 million years to reach the main sequence for the protostar stage. Late Stages for Protostars It is difficult to It observe a protostar collapse. We observe other We late protostar stages. Herbig-Haro Herbig objects such as HH30 is a newly formed star at the center of a dense disk of dusty gas. Associations of Young Stars There are many observations such as this one showing early stage There stars which form in groups called associations . For example T Tauri stars are in a T association. For These stars are very young. These Star Clusters The compression of the The initial gas cloud is probably due to a supernova explosion. Here we see two star Here clusters. The yellow one is made up The of 50 million year old stars. The white one is made up The of 4 million year old stars. The cluster began forming The about 50 million years ago due to a supernova. Structure and Evolution of Stars • What are the basic facts we observe about the Sun, and what does this tell us? • Throughout our memory, our parents and grandparents memories and all of recorded history, the sun has remained the same. – Radius remains the same! – Temperature remains the same! – Luminosity remains the same! Hydrostatic Equilibrium •Radius constant=> A dynamically stable star is in hydrostatic equilibrium. •The gravity of the sun is balanced by outward gas pressure. •Each “layer” of star supports its on weight plus those of the layer above so pressure increases inward. •Punishment is swift when balance is Figure 11.13 Figure disturbed: it takes only one hour for the sun adjust in pressure to restore equilibrium. This is the timescale for flares or prominences. Mechanical Properties of Stars • Gravity- holds the star together from mutual attraction of all particles • Gas Pressure – keeps the star from collapsing in on itself - Boyle’s Law: P proportional to density Blowing up a balloon - Charles’ Law: P proportional to T heat can, close lid => implosion - Compressing gas causes it heat up tube of bicycle pump warms as you pump CO2 cartridge, spray can cool as they discharge gas Kinetic Theory of Gases (James Clerk Maxwell) • Kinetic theory of gases: “tiny elastic particles (atoms) in constant motion” • With this idea, we can understand – Random thermal motions: Pressure Collisions – Temperature is measure of kinetic energy – Charles’ Law – Boyle’s Law – Hydrostatic Equilibrium! Thermal Equilibrium • Heat in = heat out – Energy flows from Hot => cold • Temperature profile in a star can be determined from the condition that the temperature at the surface is constant Energy Transport Energy can be transported from the star's core in several ways. Energy Conduction, radiation or convection. Conduction, These three modes are the main energy transport system within a These star. Energy transport in our star … ..\animations\TheSun.swf Opacity • Absorption of radiation by matter is called opacity • In interior of Sun it is electron scattering that is important opacity: it takes radiation about 1 million years to reach the surface • Opacity in a region of a star determines the rate of energy flow through it, if it becomes too large (partial ionization zone) we get convection Convection in a Star The core radiates The its energy outward. Partial Ionization Partial increases opacity The upper regions The of the star form a convection zone. Energy is Energy transported to the photosphere. Granules are the Granules visible evidence of convection. Mass Luminosity Relation (Sir Arthur Eddington) • Now we can explain it: As Mass increases, gravity increases, so Pressure (P) must increase to maintain HSE; Charles’ Law then says if P increases, Temperature (T) increases; S-B says if T increases, Luminosity increases and we are done! • Eddington worked out the details and showed quantitatively that Luminosity should be proportional to Mass (to the 3.5 power), in agreement with our observational result! Why is a Star Hot? • Independent of the nature of the energy source, stars radiate because they are hot • We must ask, Why are stars hot? – Because they have an energy source… • What is the source of energy? ...
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