ASTR 1346 1st Edition Lecture 10 Outline of Last Lecture I Star Formation II Electromagnetic Spectrum III Interstellar Matter A Interstellar Medium B Obscuration C Extinction D Reddening IV Gas V Dust VI Giant Molecular Clouds Outline of Current Lecture I Nebulas A Reflection Nebula B Emission Nebula C Dark Nebula II GMC Collapses III Jean s Instability IV Formation of Protostar V Evolutionary Track A High Mass Stars B Low Mass Stars C Brown Dwarfs Current Lecture l A A reflection nebula is when a star illuminates gas and dust cloud Star light is reflected by dust and the dust grains scatter blue light from stars more effectively than any other color so the nebula looks bluish B An emission nebula is a hot O or B type star that illuminates gas clouds by exciting and ionizing gas Electrons combine falling back to the ground state producing emission lines These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute C A dark nebula is a dense cloud of gas and dust that absorbs light from stars behind they appear dark in front of a brighter background ll Stars form when GMC collapses under its own gravity and breaks up into pieces The collapse of a GMC is caused triggered by cloud cloud collisions supernova blast waves starlight shock waves by nearby young massive hot O or B type stars emission nebula and density waves in spiral arms of the galaxy Compression forms O and B type stars and triggers further star formation second generation stars When collapsing the cloud fragments into smaller clumps Depending on the condition the cloud may produce a few dozen stars much larger than our Sun This process takes a few million years lll Jean s Instability is when conditions of temperature pressure and density under which a fragment of cloud is forced to collapse under the influence of its own gravitational attraction This was named after British physicist Sir James Jeans who calculated these in 1902 lV A GMC contracts and forms a dense core of gas and dust called Bok globules As the protostar grows in mass its surface gets brighter while its core heats up When very little gas is left in the core the star becomes pre main sequence V The evolutionary track is the graphical representation of a star s life It represents changes in the star s temperature and its luminosity on HR Diagram not its motion in space This depends on mass G M and K stars before reaching main sequence shed lots of mass and show vigorous surface activity called T Tauri stars Depending on its mass stars take different times to reach main sequence some stay on the main sequence for different amounts of time and some die differently A High mass stars would be O and B type stars These stars form fast live fast burn hot leave the main sequence fast and die young and explosively B Low mass stars for slow stay on main sequence longer leave it slowly and die gently C A brown dwarf is a failed star It never began burning hydrogen because it was too small and didn t generate enough heat The minimum amount of gas needed to generate nuclear fusion is 08 solar mass which is 80 times the mass of Jupiter