Chapter 16 Notes
Chapter 16 Notes ASTR-1020-90
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This 3 page Class Notes was uploaded by Hannah Moore on Monday February 15, 2016. The Class Notes belongs to ASTR-1020-90 at Clayton State University taught by Dr. Campbell in Winter 2016. Since its upload, it has received 18 views. For similar materials see Stellar and Galactic Astronomy in Science at Clayton State University.
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Date Created: 02/15/16
Steller and Galactic Astronomy Notes: 2/16/2016 Chapter 16: Star Birth Stellar Nurseries o Where do Stars form? Stars form in dark clouds of dusty gas in interstellar space. The gas between the stars is called the interstellar medium. It is high in density (300 molecules per cubic centimeter) and cold(10-30K). The clouds are made of 70% H for2, 28% He and 2% other stuff. Most of the matter is in the form of molecules. Tiny solid particles of interstellar dust block our view to stars behind it. Long wavelength infrared light passes the clouds easily. Observing the infrared light from a cloud can reveal the newborn star embedded inside it. This will let us see new stars as they are being born. Brighter regions have more stars forming o Why do Stars form? Gravity must overcome thermal energy to cause the stars to collapse into a star. A molecular cloud must contain at least a few 100 solar masses for gravity to overcome pressure Emissions from molecules in a cloud prevent a pressure buildup A cloud must have even more mass to begin collapsing of there are additional; forces opposing gravity Magnetic fields, and turbulent gas resist gravity Gravity within a contracting cloud becomes stronger as the cloud becomes denser. Gravity can overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments. Fragments can go on to form a star. Isolated Star Formation If small cloud is unusually dense, a star could form. Elements like carbon and oxygen had not been formed so the first stars formed had to be much warmer that today’s molecular clouds. They were much larger than today’s stars Stages of Star Birth o What slows the contraction of stars? As contraction packs molecules and dust particles closer together, it becomes harder for thermal energy to escape Thermal energy builds up inside and internal pressure increases Contraction slows down and a protostar is formed. Matter will continue it fall into protostar until the star blows it away or a neighboring star blows it away. The rotation speed will increase as the cloud begins to contract. Around every star there should be a disk. It will flatten in time as it spins faster. The collision of particles will cause the flattening Jets of charged particles are released as the disk flattens. Protostar looks star like after surrounding gas is blown away , but its thermal energy comes from gravitational contraction Contraction must continue until the core can begin the fusion process Contraction will stop when the protostar reaches an energy balance. Birth Stages on a Life Track Assembly of a protostar o Luminosity and temp grow as matter collects into a protostar Convection Contraction o Surface temp remains near 3000 K Radiative Contraction o Luminosity remains nearly constant, while radiation is transporting energy thought star Self-Sustaining Fusion o Core temp continues to rise until star reaches the main-sequence High mass stars form faster. Masses Of Newborn Stars o What is the Smallest mass of a new born star? Fusion will not begin if a force stops contraction before core temp rises above 10 K7 Brown Dwarfs o less than 8% mass of the sun o Hot, but not hot enough to be a star o Star like, but not stars. o There are a lot of them, but hard to see because they are so dim. o What is the greatest mass a newborn star can have? Photons exert a small amount of pressure when they strike matter. Radiation pressure will blow stars above 150M sunapart. o What are the typical masses of newborn stars? More low mass stars than high mass stars.