ASTR 1346 1st Edition Exam 2 Study Guide Lectures 9 13 Lecture 9 February 26 Interstellar Matter How do stars form Stars form from dust and gas and are still forming today Massive stars don t shine longer The birth of a star goes from protostar pre main sequence main sequence What is interstellar matter Interstellar matter is gas and dust that fills the universe It blocks light from stars and is not uniformly distributed At some places it s very rare and in others very dense Define interstellar medium Interstellar Medium is the matter between stars This contains at least 10 of all known mass in our galaxy and is made up of dust and gas Define obscuration Obscuration is caused by dust particles dust grains and makes the universe look redder Define extinction Extinction is the general dimming of starlight by interstellar matter Define reddening Reddening is when the blue part of the light from different stars is obscured and scattered more than the red so the universe appears redder What is gas Gas is the vast majority of the universe Gas is very cold and made up of individual atoms and small molecules 90 hydrogen 9 helium and 1 heavier elements Particles are spherical and invisible What is dust While most of the universe is gas there is a tiny bit of dust in there which helped make the planets Dust consists of clumps of atoms and molecules that have a coating of ice on them This helped them stick together to form bigger and bigger particles Particles are elongated but the large scale structure is more complex What are Giant Molecular Clouds GMC s Giant Molecular Clouds are very dark and huge and is where stars form They have a very low temperature a lot of matter and stability with just a little bit of instability About 6 000 GMC s are estimated to exist in our galaxy Part of the Orion Nebula is in a GMC Lecture 10 March 3 Nebulas and Stars What is a reflection nebula 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 What is an emission nebula 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 What is a dark nebula 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 What happens when a GMC collapses 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 What is Jean s Instability 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 How does a protostar form 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 What is an evolutionary track 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 How do high mass stars live 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 How do low mass stars live Low mass stars for slow stay on main sequence longer leave it slowly and die gently What are brown dwarfs 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 Lecture 11 March 17 Hydrostatic Equilibrium What is hydrostatic equilibrium Hydrostatic equilibrium is the balance between pressure and gravity An example is no contraction or expansion Pressure pushes outwards while gravity pushes inwards Most stars especially main sequence stars are in hydrostatic equilibrium but forming stars are NOT they are contracting and gravity is overcoming pressure What evolutionary events does a star go through Evolutionary events from protostar to main sequence take about 40 50 million years This is less than 1 of the star s lifetime A star lasts in the main sequence for about 10 billion years Our Sun is about 5 billion years old Post main sequence red giants white dwarf black dwarf Lecture 12 March 19 Deaths of Stars What are the mass ranges Less than 08 solar mass failed stars 08 4 solar mass extremely low mass star 4 8 low mass star 8 25 intermediate mass star 25 high mass star What occurs in a low mass star Our Sun is a low mass star In low mass stars Helium is convected upward and hydrogen downward The entire star becomes a Helium star This takes 100 s of billions of years and no stars have done this yet because our galaxy is too young These stars have a helium core hydrogen burning shell and non burning envelope Core is contracting and temperature increases turning helium into hydrogen which expands and pushes radiation pressure outward Temperature of the Sun decreases overall and as it gets bigger it gets brighter What happens as core contraction continues As core contraction continues the amount of inert helium increases shell burning adds more helium ash There is no energy generated in the core and the gravity contracts the core Temperature in core increases reaching 100 million K What is electron degeneracy Before Helium burning starts core