Astronomy 100 Tuesday Thursday 2 30 3 45 pm Tom Burbine tburbine mtholyoke edu www xanga com astronomy100 Schedule Today Class Tuesday Class Wednesday Review Session Hasbrouck 134 from 7 8 pm Thursday Exam 3 Chapters 15 16 17 18 Homework Assignment Due Today Make up a test question Multiple Choice A E possible answers 1 point for handing it in 1 point for me using it on test The question needs to be on material that will be on the 3rd exam 15 people got extra HW credit for me using their question or inspiring a question OWL assignment Due Today There is be an OWL assignment due on Thursday March 31 at 11 59 pm There are 15 questions and a perfect score will give you 2 homework points OWL assignment Due Tuesday There is be an OWL assignment due on Tuesday April 5 at 11 59 pm There are 15 questions and a perfect score will give you 2 homework points Things to remember 90 of classified stars are on main sequence Main sequence stars are young stars If a star is leaving the main sequence it is at the end of its lifespan of burning hydrogen into helium Remember Largest stars on main sequence are O stars Largest stars that can exist are supergiants You need to know stellar classifications O B A F G K M A0 A1 A2 A9 in the order from the hottest to the coolest wd white dwarfs Classifications Sun is a G2 V Betelgeuse is a M2 I Vega is a A0 V Sirius is a A1 V Arcturus is a K3 III Binaries About half of stars orbit a companion 3 classes of binaries Visual Binary We can see the stars rotating around each other with a telescope Eclipsing Binary The light from a star system drops as a star goes in front and behind another star Spectroscopic Binary The spectral lines of a star can be seen to be moving to shorter wavelengths and also to longer wavelengths Importance of Binaries It allows you to possibly determine a star s mass if you know the orbital period and the separation of the two stars Life of a Star A star forming cloud is called a molecular cloud because low temperatures allow Hydrogen to form Hydrogen molecules H2 Temperatures like 10 30 K Denser than surrounding regions Region is approximately 50 light years across Condensing Molecular clouds tends to be lumpy These lumps tend to condense into stars That is why stars tend to be found in clusters Protostar The dense cloud fragment gets hotter as it contracts The cloud becomes denser and radiation cannot escape The thermal pressure and gas temperature start to rise and rise The dense cloud fragment becomes a protostar When does a protostar become a star When the core temperatures reaches 10 million K hydrogen fusion can start occurring 3 Basic Groups of Stars Low mass stars born with less than 2 Solar Masses Intermediate mass stars born between 2 and 8 solar masses High mass stars born with masses greater than 8 solar masses Things you need to know Fusion rate increases with increasing temperature There is a relation between thermal pressure and gravity Animation Death sequence of the Sun Sun ends it time on the main sequence When the core hydrogen is depleted nuclear fusion stops The core pressure can no longer resist the crush of gravity Core shrinks Why does the star expand The core is made of helium The surrounding layers are made of hydrogen And Gravity shrinks the inert helium core and surrounding shell of hydrogen The shell of hydrogen becomes hot for fusion This is called hydrogen shell burning And The shell becomes so hot that its fusion rate is higher than the original core This energy can not be transported fast enough to surface Thermal pressure builds up and the star expands And More helium is being created Mass of core increases Increases its gravitational pull Increasing the density and pressure of this region When When helium core reaches 100 million Kelvin Helium can fuse into a Carbon nucleus Helium Flash The rising temperature in the core causes the helium fusion rate to rocket upward Creates a lot of new energy However The core expands Which pushes the hydrogen burning shell outwards Lowering the hydrogen burning shell s temperature And Less energy is produced Star starts to contract Now In the core Helium becomes Carbon Star contracts Helium fusion occurs in a shell surrounding the carbon core Hydrogen shell can fuse above the Helium shell Inner regions become hotter Star expands Can Carbon undergo fusion Yes but can t reach the needed temperature 600 million Kelvin in a low mass star Carbon on Earth is produced in the cores of stars Planetary Nebulae There is a carbon core and outer layers are ejected into space The core is still hot and that ionizes the expanding gas Planetary Nebulae White Dwarf The remaining core becomes a white dwarf PRS Question At the end of the Sun s lifespan when it becomes a white dwarf the white dwarf will be mostly composed of A Hydrogen B Helium C Carbon D Oxygen E Iron PRS Question At the end of the Sun s lifespan and it becomes a white dwarf the white dwarf will be mostly composed of A Hydrogen B Helium C Carbon D Oxygen E Iron High Mass Stars The importance of high mass stars is that they make elements heavier than carbon You need really hot temperatures which only occur with the weight of a very high mass star Stages of High Mass Star s Life Similar to low mass star s Except a high mass star can continue to fuse elements When the fusion ceases the star becomes a supernova Supernova is a huge explosion Fusion in High Mass stars Besides fusion of Hydrogen into Helium The high temperatures allow Carbon Nitrogen and Oxygen to be catalysts for converting Hydrogen into Helium CNO cycle Fusion The temperatures of high mass stars in its latestage of life can reach temperatures above 600 million Kelvin Can fuse Carbon and heavier elements Helium Capture can also occur where Helium can be fused into heavy elements Supernovas We will discuss next class PRS for making it through class Questions