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Star Formation and Main Sequence Evolution Condensation Theory Dark Nebulae Raw material for star and planet formation! Interstellar clouds are normally stable! Because the inward force of gravity is balanced by the outward force of pressure! 1 This can happen when clouds are compressed externally! To form a star and planetary system we need the cloud to become unstable: gravity > pressure causing the cloud to collapse under gravity size ↓ temp ↑ spin ↑ This can be caused by a nearby supernova explosion! Cloud Fragmentation The cloud does not fragment into equal-sized pieces but fragments into clumps with a range of masses Formation of Binary Stars The Conservation of Angular Momentum As a rotating object gets smaller it spins faster! 2 Particles of gas and dust stick together within the disk Similarly, as a cloud collapses, it spins faster forming a rotating protoplanetary disk (proplyd) around a central clump which will eventually become a star A process called accretion….. Leading to the formation…. of a family of planets…. Protostars Collapsing clumps of gas on there way to becoming stars are known as protostars as they have not yet started nuclear fusion! 3 Protostars are large cool and luminous! Found in the same region of the H-R diagram as red giants but they are not the same! Kelvin-Helmholtz Contraction F = GMAMB/d2 d2 < d1 so d ↓F ↑a ↑T↑ where: d = distance F = gravity a =acceleration T = temperature Conversion of gravity into heat! How can a collapsing clump of gas produce more energy than the Sun when it has not yet started nuclear fusion? Why does a collapsing clump of gas heat up anyway? Instability and Mass Loss Protostars transfer energy from their hot interiors to their cool surfaces via convection This makes their surfaces very unstable As they heat up they eventually start to eject their outer layers into space leading to mass loss Up 50% of the original mass of the clump can be lost in this way Mass Loss from Protostars Evolutionary Tracks As protostars collapse and heat up they move on the H-R diagram towards the main sequence Mass loss can only occur perpendicular to the protoplanetary disc leading to the formation of a bipolar outflow 4 A Star is Born A Stable Main Sequence Star Eventually nuclear fusion begins in the core when it reaches a temperature of around 10 million K Once nuclear fusion begins the star attains hydrostatic and thermal equilibrium producing a newborn stable zero-age main sequence star The Sun took around 20 million years to form Formation of Stars of Different Masses Final position on main sequence determined by mass luminosity relation End result: a cluster of newborn stars with a range of masses Formation of High Mass Stars 1. Form much faster due to stronger gravitational attraction 2. Move horizontally rather than diagonally onto the main sequence 3. Produce high luminosity stars at the top of the main sequence (mass-luminosity relation) 5 Distribution of newborn star masses If a young cluster contains one or more hot, massive O- or B-type stars an emission nebula will be produced Low mass stars are much more common! The Orion Nebula Observational Evidence? A star formation region 1500 ly away Protostars and Protoplanety disks are seen inside interstellar clouds! Disks of gas and dust are seen around other, mature solar-type stars! Infrared (IR) Image 6 Extrasolar Planets Main Sequence Evolution Stellar Adulthood A star spends more than 90% of its total lifetime on the main sequence This this the most stable phase of a star’s life similar to adulthood in humans Definition of a Main Sequence Star: Core hydrogen to helium fusion In hydrostatic and thermal equilibrium When the Sun formed its core contained roughly 75% H and 25% He by mass During the Main Sequence: hydrogen → helium so time ↑ hydrogen ↓ helium ↑ Eventually the hydrogen fuel runs out in the core! After 4.6 billion years of fusion the amount of hydrogen in the core has decreased 7 Main Sequence Evolution of the Sun Main Sequence Lifetime The Sun is gradually heating up and expanding Expect: High mass stars will live longer since they have more fuel! Depends on: 1. The amount of nuclear fuel = mass 2. Rate which fuel is consumed = luminosity t~M/L where: M = mass and L = luminosity Mass-luminosity relation: Find: L ~ M3.5 so t ~ M / L ~ M / M3.5 ~ 1 / M2.5 Inverse relation! mass ↑ lifetime ↓ 8 High mass stars have shorter lifetimes even though they have more fuel to fuse! Why? They have much higher central temperatures (and hence luminosities) so they burn through their greater amount of fuel in a shorter amount of time! Mass and Main Sequence Lifetime Analogy A hybrid car with a small fuel tank and good fuel economy can typically drive more miles than an SUV with a large fuel tank but poor fuel economy Low mass stars are like hybrids while high mass stars are like SUV’s! Energy Transport in Main Sequence Stars 9