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UW ASTR 101 - Lecture Notes

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Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•How does the HR diagram of main sequence stars change with time?•Why do stars leave the main sequence?•What conditions are required for elements besides Hydrogen to fuse, and why?•Why are red giants giant?•Why can high mass stars use more varieties of fuel over their lives, compared to low mass stars?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWAdministrative Stuff• Midterm a week from Wednesday!• 50 multiple choice questions in 50 minutes.• Bring a scantron form!• PDFs of old midterms are now posted on the syllabus, under 2/10• Remember all those office hours? Great for reviewing old midterms!• CLUE review session 2/9 in the evening.• Will cover everything we cover this week -- keep in mind that we’re 1 lecture ahead of the syllabus2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Upper Right: “Red Giants” “Supergiants”Lower Left: “White Dwarfs”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4The H-R diagram for a group of stars is not constant!1. Massive stars use up the Hydrogen in their cores.2. They evolve off the main sequence.3. Evolving stars turn into red giants, red & blue supergiants, and white dwarfs.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5Stars cannot stay on the main sequence foreverLuminosity!"TemperatureThese stars are still forming, and haven’t had time to reach the Main Sequence yet!These stars have already used up their Hydrogen, and are doing weird things…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6Stars leave the Main Sequence and do weird things!Luminosity!"TemperatureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7Stars leave the Main Sequence and do weird things!Luminosity!"TemperatureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8Stars leave the Main Sequence and do weird things!Luminosity!"TemperatureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9Stars leave the Main Sequence and do weird things!Luminosity!"TemperatureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Clusters of stars of different ages. With increasing age, the tip of the main sequence shifts to fainter, redder stars, and more bright red stars show up.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXClusters of stars of different ages. The only reason we don’t see the white dwarfs is that they’re too faint to detect in these particular observations.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11We can represent the locations of stars in these different clusters as lines on the HR diagram showing the location on which stars would lie.“isochrones”: latin for “same time”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12"An HR diagram of all the stars for which we have parallax distances.Q: Why does the MS extend bluer than the turnoff?Color, in funny units (RED!)Absolute Magnitude (BRIGHTER!)Results from the Hipparcos satelliteAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13Low mass main sequence stars and white dwarfs are the most common!Note: the stars for which we have parallaxes are not representative of the whole population!The universe prefers to make small thingsAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14Which star(s):Color, in funny units (RED!)Absolute Magnitude (BRIGHTER!)1.Is the most massive?2.Is the lowest mass main sequence star?3.Is definitely young?4.Are not experiencing Hydrogen fusion in their cores?BlueGreenPurpleYellowPinkAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15Which pairs of stars:Color, in funny units (RED!)Absolute Magnitude (BRIGHTER!)1.Have the same luminosity?2.Have the same temperature?3.Are likely to have the same size?BlueGreenPurpleYellowPinkAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16Where do all the non-main sequence stars come from?Color, in funny units (RED!)Absolute Magnitude (BRIGHTER!)•Must be due to lack of Hydrogen in the core.•What happens when fusion stops?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Rules for the Day•When Stuff falls inwards, it heats up.– Gains Kinetic Energy– Collides with other stuff, turning “ordered” motion into random “thermal” motion.•When Stuff falls inwards, it gets denser too.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXAn example: The main sequence brightens with time.•4 H ! 1 He means fewer particles.• Fewer particles = less pressure• Gravity wins a little more all the time!• Stuff falls in (i.e. star contracts)• Core gets hotter!• Fusion happens faster!• More energy is released per secondAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18Rules for the Day•When Stuff gets hotter and denser, heavier elements can fuse together.–Heavier elements have more nuclear charge, and thus more repulsion!–Takes more energy to force them together.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXHappy Star on the Main Sequence.Nice Hot Dense region with lots-o-hydrogen to burn.La la la la…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19Remember: Stars initially can’t burn Hydrogen outside the core! • Too cool! • Too low density!This.Not this.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20As time goes on, Hydrogen in the core gets used up!(PS. Keep in mind that the mass in the sun remains almost the same. It’s just that the mass is in a form that it can’t burn…for now!)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Happy Star on the Main Sequence.Nice Hot Dense region with lots-o-hydrogen to burn.La la la la…Becomes…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22Nervous Star ending its time on the Main Sequence.Running out of Hydrogen!Energy output is going down!Um…errr…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23Nervous Star ending its time on the Main Sequence.Without fusion, the temperature of the core will:Um…errr…A. Cool downB. Heat upAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24Nervous Star ending its time on the Main Sequence.The pressure in the core will:Um…errr…A. DecreaseB. IncreaseThe core will:A. ShrinkB. ExpandAstronomy 101, Winter


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UW ASTR 101 - Lecture Notes

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