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Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine [email protected] 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