ASTR100 (Spring 2008) Introduction to Astronomy Classifying StarsWhat is a Hertzsprung-Russell Diagram?PowerPoint PresentationSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19What is the significance of the main sequence?Slide 21Slide 22Slide 23Slide 24Mass & LifetimeSlide 26Main-sequence Star SummaryConcept CheckWhat are giants, supergiants, and white dwarfs?Off the Main SequenceSlide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38What are the two types of star clusters?Slide 40Slide 41How do we measure the age of a star cluster?Slide 43Slide 44Slide 45To determine accurate ages, we compare models of stellar evolution to the cluster data.Slide 47Surprise Quiz!! (10 points)Slide 49ASTR100 (Spring 2008) Introduction to AstronomyClassifying StarsProf. D.C. RichardsonSections 0101-0106What is a Hertzsprung-Russell Diagram?TemperatureLuminosityAn H-R diagram plots the luminosities and temperatures of stars.Most stars fall somewhere on the main sequence of the H-R diagram.Stars with lower T and higher L must have larger radius R:giants and supergiants.Large RadiusL = 4R2  T4Stars with higher T and lower L must have smaller radius R:white dwarfs.L = 4R2  T4Small RadiusGiants and SupergiantsWhite DwarfsAdd luminosity class to spectral class:I - supergiantII - bright giantIII - giantIV - subgiantV - main sequenceExamples: Sun – G2 VSirius – A1 VProxima Centauri – M5.5 VBetelgeuse – M2 IH-R diagram depicts: Temperature Color Spectral Type Luminosity RadiusTemperatureLuminosityTemperatureLuminosityABCDWhich star is the hottest?TemperatureWhich star is the hottest?ABCDALuminosityTemperatureWhich star is the most luminous?ABCDLuminosityTemperatureWhich star is the most luminous?ABCDCLuminosityTemperatureWhich star is a main-sequence star?ABCDLuminosityTemperatureWhich star is a main-sequence star?ABCDDLuminosityTemperatureWhich star has the largest radius?ABCDLuminosityTemperatureLuminosityWhich star has the largest radius?ABCDCWhat is the significance of the main sequence?Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun.Luminous main-sequence stars are hot (blue).Less luminous ones are cooler (yellow or red).Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones.Low MassHigh MassThe mass of a normal, hydrogen-burning star determines its luminosity and spectral type!Low MassHigh MassThe core pressure and temperature of a higher-mass star need to be higher in order to balance gravity.A higher core temperature boosts the fusion rate, leading to higher luminosity.Sun’s life expectancy: 10 billion years.Life expectancy of 10 MSun star:10 times as much fuel, uses it 104 times as fast. 10 million years.Until core hydrogen(10% of total) is used up.Mass & LifetimeSun’s life expectancy: 10 billion years.Life expectancy of 10 MSun star:10 times as much fuel, uses it 104 times as fast. 10 million years.Life expectancy of 0.1 MSun star:0.1 times as much fuel, uses it 0.01 times as fast. 100 billion years.Until core hydrogen(10% of total) is used up.Mass & LifetimeMain-sequence Star SummaryHigh mass:High luminosityShort-livedLarge radiusBlueLow mass:Low luminosityLong-livedSmall radiusRedConcept CheckTwo stars have the same surface temperature but different luminosities. How can that be?Answer: one is bigger than the other!Why?Thermal radiation law: objects at a given temperature emit a certain luminosity per unit surface area.Hence the more luminous star has a larger surface area, and so a larger radius.What are giants, supergiants, and white dwarfs?Off the Main SequenceStellar properties depend on both mass and age: stars that have finished fusing H to He in their cores are no longer on the main sequence.All stars become larger and redder after using up their core hydrogen: giants and supergiants.Most stars end up small and white after fusion has ceased: white dwarfs.Main-sequence stars (to scale)Giants, supergiants, white dwarfsTemperatureLuminosityWhich star is most like our Sun?ABCDTemperatureLuminosityWhich star is most like our Sun?ABCDBTemperatureLuminosityWhich star will have changed the least 10 billion years from now?ABCDTemperatureLuminosityWhich star will have changed the least 10 billion years from now?ABCDCTemperatureLuminosityWhich star can be no more than 10 million years old?ABCDTemperatureLuminosityWhich star can be no more than 10 million years old?ABCDAWhat are the two types of star clusters?Open cluster: A few thousand loosely packed stars.Globular cluster: Up to a million or more stars in a dense ball bound together by gravity.How do we measure the age of a star cluster?Massive blue stars die first, followed by white, yellow, orange, and red stars.The Pleiades cluster now has no stars with life expectancy less than around 100 million years.Main-sequenceturnoffThe main-sequence turnoff point of a cluster tells us its age.To determine accurate ages, we compare models of stellar evolution to the cluster data.Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old…Surprise Quiz!! (10 points)Take out a piece of paper, print your name and section number on it.Sketch an H-R diagram…1. Label the temperature & luminosity axes.2. Sketch the main sequence.3. Plot a point representing the Sun.4. Plot a main-sequence B star.5. Plot a main-sequence M star.6. Indicate where giants & supergiants are found.7. Indicate where white dwarfs are found.Giant and SupergiantsWhite DwarfsHere’s what your sketch should look like!Sun (G2 V)B Star M