ASTR 1120General Astronomy:Stars & Galaxies• Homework #3 due today, by 5pm• Homework #4 on Mastering Astronomy, due on Thursday of next week, 10/08, by 5pm• Next Extra Credit Observing Night:–Thursday, 10/08 at Sommers-BauschLast Time• The Hertzsprung-Russell Diagram– Main Sequence• Masses• Lifetimes (M/L)– Luminosity Classes• Supergiants, Giants, MS– Star Clusters• Open Clusters• Globular ClustersStar Birth We start with cloudsof cold, interstellargas:• Molecular clouds– cold enough to formmolecules; T=10-30K• Often dusty• Collapses under itsown gravity Recurring theme:conservation of energy1. Collapse due to gravity increases thetemperature (gravitational energy !thermal energy). If thermal energy canescape via radiation (glowing gas),collapse continues2. If thermal energy is contained, or moreenergy is generated due to fusion,collapse is slowed (by thermal pressure)Collapse from Cloud to Protostar• First collapse from very large, cold molecularclouds - cloud is turbulent and clumpy• Fragments into star-sized masses• Temperature increases in each fragment as itcontinues to collapseMultiple Protostars Can FormFrom a Single CloudStarting point: turbulent gas cloud 1.2 ly across and with 50 Msun of gasDusty, darkmolecular cloudregionsProtostars start out relatively cooland dark. On the HR diagram, aprotostar appears at the:A. Upper left cornerB. Upper right cornerC. Lower right cornerD. Lower left cornerE. Confused and lost cornerClicker QuestionProtostars start out relatively cooland dark. On the HR diagram, aprotostar appears at the:A. Upper left cornerB. Upper right cornerC. Lower right cornerD. Lower left cornerE. Confused and lost cornerClicker Question1. Our cloud collapses to form one or more protostars,heating up as it shrinks.2. Collapse continues, temperature stabilizes asconvection circulates energy outwards• On HR Diagram, moves slightly left, downwardsCollapsing cloudspins up, formsstar, disk and jetHST: actual edge-on disk, jetProtostars and PlanetsProtoplanetary DisksThis is where planets arebornFormation process takes 50million years for Sun;compare with 10 billionyear lifetimeEventually the diskfragments and dissipatesor is blown away3. As core temperaturesreach millions K,fusion begins but ourprotostar is still notcompletely stable– Our protostar is almosta STAR!• Collapse slows butdoesn’t stopOn HR diagram,movement is slightlydownward4. With fusionsupporting the core,our protostar finallysheds its cocoon ofgas and settles on themain sequence• Stable Hydrogen !helium in the core• Stellar thermostatkeeps luminosity andtemperature stable forbillions of yearsWhat is the energy source thatheats a contracting protostar?A. Friction of the gas molecules rubbing against eachotherB. Pressure, as the gas and dust are compressedC. Gravitational energy that is released as the cloudcompressesD. FusionE. Kinetic energy of all the material falling faster andfasterClicker QuestionWhat is the energy source thatheats a contracting protostar?A. Friction of the gas molecules rubbing against eachotherB. Pressure, as the gas and dust are compressedC. Gravitational energy that is released as the cloudcompressesD. FusionE. Kinetic energy of all the material falling faster andfasterClicker QuestionStarbirthand ColorFor every massiveO-star that is born,there are ~200low-mass M-starsalso born !-Most stars in thegalaxy are low-mass, mainsequence starsOGMStar forming regions-NEBULAE = CLOUDY• Note: hot new bluemain sequencestars• Pink hydrogen gas• Black sooty dust• Blue nebulae arereflections ofstarlight frommassive blue stars(blue light reflects moreeasily than red- this isalso why our sky isblue)Eagle Nebula: cold dark cloudsare eroded by intense starlightMolecules Observed in Radio Light:We Can See Through the Dust!Carbon Monoxide Doppler Shifts show motionWhat Happens If the CloudCollapses but Never Gets HotEnough for Fusion?• Brown Dwarfs– Objects thatnever quitemade it tostardomFailed Stars: Brown Dwarfs• If masses are smaller then0.08 solar masses (80Jupiters), core collapsestopped by “degeneracypressure”• Temperatures in the core arestill not enough for fusion• Balls of warm hydrogen ->cool and faintGliese 229B; brown dwarf in orbit around a star©Hubble Space TelescopeHow Big CanStars Get?• As stars start tocollapse, their windsblow off their outerlayers– If they are toomassive, they willblow themselvesapart• Theoretical limit of~150 MSunEta Carina © Hubble Space TelescopeEvolution of Low Mass Stars(less than 2x Sun’s mass)Protostars ! MainsequenceMost of its life onMain sequence(billions of years)What happens whenit runs out ofhydrogen?When The Core Runs Out Of Hydrogen,All That is Left in the Center is Helium• But the temperature is nothot enough to fusehelium.• With fusion no longeroccurring in the core,gravity causes corecollapse " key theme– Core temperature starts toheat up• Now Hydrogen fusion hasmoved to shellssurrounding the core– Pushes outer layers of thestar out.RED GIANTRed Giants• As core collapses,hydrogen SHELL burnsfaster and faster – moreenergy created• Luminosity increases, liftsouter parts of star• Star becomes brighter,larger and cooler!!• All the while, the core iscontinuing to shrink andis heating up.What happens to the Earth?• Red giants havesizes up to 100 x theSun’s radius, 1000times the luminosity• Sun will swallowMercury, Venus…EARTH!!!In 5-7 billion years, wewill be toast.Putting Doom into Perspective• 65 million years ago, thedinosaurs died• Present-day mammals (likeus) evolved from smallrodents alive at that time• In the next 5 billion years, wehave about 80 equal sizedtime intervals - enough timeto re-evolve over and overagain if necessaryEventually, The Core Is HotEnough To Burn Helium• At Temp >108 K –helium flash occursand helium ignites.• Hydrostaticequilibrium has beenrestored and the coreis now balancedagain… happilyburning helium tocarbon as a HorizontalBranch StarElectron Degeneracy PressureCauses The Helium Flash• Typical Conditions:– Temperature rise of gas corresponds to pressure rise(SOLAR THERMOSTAT)• As pressure goes up, gas expands and temperature drops backdown.– Nuclear fusion is kept at a constant rate• Degenerate conditions (extremely high pressures):– As temperature rises pressure does not change.• No expansion, no cooling, no stabilization…RUNAWAY FUSION» (lasts for only a