ASTR 1120: Fall 2005ASTR 1120Questions:• What does the Hertzprung-Russelldiagram mean?• How do stars form?Phil Armitage, standing in today for Andrew HamiltonChapter 16 & start of Chapter 17: `Star Stuff’ASTR 1120: Fall 2005luminosity of the startemperatureWhat properties canwe measure forstars?(1) color - recall thistells us about the temperature at thesurface of the star(2) how bright the star appears to usIf we know the distance (hard!) we can convertthe apparent brightness to a luminosity (powerin watts) and plot luminosity vs temperature for a collection of starsASTR 1120: Spring 2005Ejnar HertzprungHenry RussellASTR 1120: Spring 2005increasing surface temperatureluminosityMainsequenceGiant andsupergiantstarsDead stars,white dwarfsASTR 1120: Spring 2005Clicker Q: suppose you observe a binary starwhere the 2 stars (assume they are identical)are too close to resolve in the telescope (theyappear as a single point of light). Where doesthe binary appear:A: on the main sequenceB: above the main sequenceC: below the main sequenceD: as a supergiantASTR 1120: Spring 2005Clicker Q: suppose you observe a binary starwhere the 2 stars (assume they are identical)are too close to resolve in the telescope (theyappear as a single point of light). Where doesthe binary appear:A: on the main sequenceB: above the main sequenceC: below the main sequenceD: as a supergiantASTR 1120: Fall 2005Small amount abovethe main sequence: - twice as much light(luminosity) from the2 stars-same color (since thestars are the same)ASTR 1120: Fall 2005What does the HR diagram represent?1) An evolutionary sequence - perhapsstars start out hot and bright and becomecool and dim, moving along the main sequence as they go?2) A sequence of stars of different mass - each mass star has `its place’ in the diagramand remains there throughout its (main-sequence) life?ASTR 1120: Fall 2005Almost always, stars evolve very slowly, so evenif stars did move along the main sequence wewouldn’t observe thatVery beautiful exception - Eta CarinaASTR 1120: Fall 2005Debate was resolvedby measuring themasses of stars at different points alongthe main sequenceEven harder than measuring distances,get mass from motion in binary star systemse.g. the binary Sirius A/BASTR 1120: Spring 2005gravitypressuregradientMain sequenceMain sequenceUnderstand the main sequence as stars of different masses fusing hydrogen into heliumHigher mass - larger inwardforce of gravityTo balance this, need largecentral pressure - high central temperatureHigh T means faster nuclearreactions, generating moreluminosityASTR 1120: Spring 2005increasing surface temperatureluminosityWith higher mass, thesurface temperature increases and the luminosity increasesrapidlyRoughly, the luminosityof a main sequence starscales as the fourth power of the mass:† L µ M4ASTR 1120: Spring 2005Main sequence lifetimeMain sequence lifetimeMore massive stars have larger reservoir of fuel(hydrogen) for nuclear fusionBUT they burn it much much fasterMassive stars have shorter livesMassive stars have shorter livesthan low mass starsthan low mass starsQuantitatively:† Fuel µ MassLuminosity µ Mass4† Lifetime µFuel supplyLuminosityµ1M3e.g. a 2 Solar mass star lives for roughly 1 / 23(one eighth) as long as the SunASTR 1120: Spring 2005Main sequence lifetimeMain sequence lifetimeSome numbers from computer models of stellarevolution:0.25 Solar masses: 1000 billion (a trillion!) years1 Solar mass: 10 billion years10 Solar masses: 10 million yearsShort lifetimes of massive stars are crucial to the existence of the Earth - allow products of nuclear burning to be recycled into the Galaxy and form new generations of starsASTR 1120: Spring 2005Star clustersStar clustersObservational clues to what happens at the endof stars’ main sequence lives come from star clustersMost stars form in such clusters - e.g. in the OrionnebulaASTR 1120: Spring 2005Star formation:a large cloud ofgas (thousandsof Solar masses)collapses undergravity to forma star clusterASTR 1120: Spring 2005Star clusters come in 2 types:Open clustersModest groupingsof stars - up to several thousandExample: thePleiades clusterin TaurusASTR 1120: Spring 2005Star clusters come in 2 types:Globular clustersCan contain a millionstars in a region ~100light years acrossIn our galaxy, globularclusters are all old,whereas new open clusters are being formed nowASTR 1120: Spring 2005Why clusters are importantWhy clusters are importantFor both open and globular clusters:• all the stars are at about the same distancefrom the Earth• all the stars formed at about the same timeDetermining the distance and age to a globularcluster is much easier than trying to find the distances and ages of a million random stars!Laboratories for understanding howstars of different masses evolveASTR 1120: Fall 2005What happens when a star, fusing hydrogeninto helium on the main sequence, exhauststhe hydrogen in the core?• hydrogen burning stops, star loses energy• core contracts, and gets hotter• higher temperature allows new nuclearreactions to start that are very slowat the ~15 million K temperature maintained by hydrogen fusionStar leaves the main sequence, becomes ared giant!ASTR 1120: Spring 2005increasing surface temperatureluminosityIn a H-R diagramstar: - moves to theright - i.e. a lowersurface T and a redder color- moves upward,to much higherluminosityASTR 1120: Spring 2005Atmospheres of the largest giants can (just) beresolved with HSTASTR 1120: Fall 2005Can date a star cluster by looking for the mostmassive star that is still on the main sequence:YoungOldASTR 1120: Fall 2005Point where stars are leaving the main sequence is called main sequence turnoff,it moves to cooler (lower mass) stars with