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Astronomy 1PO2.docx

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Astronomy 1PO2Lecture 1 Star Formations Ch 9How and where are the stars formed Clue look at the surroundings of young starsWhat are the young starsHint Recall the HR DiagramLuminosity L vs Surface Temperature T graph for a group of stars For the Main sequence stars one has Eddingtons MassLuminosity recation 35LM 3525Lifetime of a starfuel availableenergy output MLMM1MMore massive stars live shorter Thus very hot and bright stars those at the upper left corner of the main sequence are the youngest stars that we could observe Observations show that such stars are surrounded by the clouds of interstellar material ie gas hydrogen 74 by mass helium 24 by mass and dust small solid particles of rocky material and iceWhen such clouds can be seen at visual wavelengths they are called nebulae Three basic types of nebulae 1 Dark nebulaelight from the stars opposite the observer is absorbed by the dust particles in the nebula Emits infrared energy The observations using infrared detectors confirm that dark nebulae emit in the infrared part of the spectrumLecture 2 The infrared radiation is observed throughout the plane of the Milky Way Hence the clouds of interstellar material IM are distributed throughout the plane of out galaxy By observing other spiral arm galaxies one finds that the density of IM is higher in the spiral arms than in the regions in between the arms 2 Reflections Nebulaebluish in colour They appear bluish because the gas and dust in the nebula scatters blue lights much more efficiently than the longer wavelengths As a result the star itself appears to be reddish so called interstellar reddening The sunsets appear red for the same reason 3 Emission Nebulaereddish in colour Very hot bright luminous star and produces a lot of light in ultraviolet UV radiation has very highenergy photons energy of the photon scales as 1wavelength Proton and electron can bind again to form hydrogen atom in highenergy state Our atmosphere is transparent mostly to red photons and hence such emission nebulae appears red Lecture 3Stars like our Sun produce energy by nuclear fusion of hydrogen H into helium He in their coresThis requires high temperatures at least 10 million degrees density and pressure in the core Thus it must be that a part of a cloud of inter stellar material starts contracting under its own gravityStars are born in socalled giant molecular clouds GMCThey are particular type of dark nebula Giant their mass ranges from a few hundred thousand solar masses M to a ofew million M their size is from about 50400 lightyears ly across o The dust in such cloud blocks the starlight from surrounding stars and the 00interior of the cloud is cold from 10 K or 263 C to 30 K 243 C and dense As a result the hydrogen atoms can bind into molecules Hhence 2molecular cloudThe observations show that such clouds contain clumpsregions of higher density see the picture of Eagle nebula or H16 What causes clumpsOne possibility is that a shockwave from a nearby Type 2 Supernova produces clumps The other possibility is that several clouds collide and compress each other which results in clumps Those clumps that are dense enough start contracting under their own gravity As the material falls in the core of the clump heats up The object that is being formed is called ProtostarThus during this protostar stage in star formation the energy comes from gravitational contractionThese protostars can be detected in the infrared radiation emitted by the surrounding gas and dust which is heated by the protostarIf the protostar reaches the temperature in its core of at least 10 million K it will start fusing hydrogen into helium and become a main sequence star More massive stars contract faster because of their larger gravity and spend less time in protostar stage than less massive stars Lecture4Evolutionary track of a protostar can be understood on the basis of Stefan24Boltzmann Law L constRT Increasing temperature to the fourth power more than compensates the decreasing radius to the second power and the luminosity increases
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