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Properties of Stars II • The Hurtzprung-Russell Diagram • How long do stars live? • Star clusters. How stars age (evolve) • Suppose we want to study how people change over their life time. • How could we do this. • We could follow a person from birth to death. • That would take a long time. • But there are a lot of people in the world so …we could study people of different ages so we don’t have to wait a lifetime for our research to finish. • For obvious reasons we cannot wait for stars to change . The Hertzsprung-Russell Diagram Luminosity An H-R diagram plots the luminosities and temperatures of stars. THIS IS PROBABLY THE MOST IMPORTANT DIAGRAM IN ASTRONOMY. Temperature Most stars fall somewhere on the main sequence of the H-R diagram. large radius Stars with lower T and higher L than main-sequence stars must have larger radii: giants and supergiants These stars have no H to He fusion going on in their core (as they have run out of H fuel). They are fusing elements heavy than H in their cores (for example He to C). They are also fusing H to He in a shell outside their core but not in the core. Stars with higher T and lower L than mainsequence stars must have smaller radii: white dwarfs These are dead stars that have no nuclear fusion of any kind. small radius H-R diagram depicts: Temperature Luminosity Color Spectral type Luminosity Radius Temperature C B D A Which star is the hottest? C B D A Which star is the most luminous? C B D A Which star is a mainsequence star? C B D A Which star has the largest radius? What is the significance of the main sequence? Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun. Luminous mainsequence stars are hot (blue). Less luminous ones are cooler (yellow or red). High-mass stars Low-mass stars Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones. The mass of a star is its High-mass stars key property. A star’s mass determines where it will rest on the main sequence. Its temperature, radius, luminosity and lifetime on the main sequence are all determined by its mass only. Low-mass stars The reason for this is that a star’s mass determines the rate of H to He fusion. The core temperature of a higher-mass star needs to be higher in order to balance gravity. Hydrostatic Equilibrium A higher core temperature boosts the fusion rate, leading to greater luminosity. Stellar Properties Review Luminosity: from brightness and distance 10-4LSun– 106LSun Temperature: from color and spectral type 3000 K – 50,000 K Mass: from period (p) and average separation (a) of binary-star orbit 0.08MSun – 100MSun Radius: from blackbody radiation the L = constant * T4 * R2 (we will not use this formula or discuss this method on the course). 0.1RSun – 10RSun (on the main sequence) Stellar Properties Review Luminosity: from brightness and distance -4L 6L 10 – 10 (0.08MSun) Sun Sun (100MSun) Temperature: from color and spectral type (0.08MSun) 3000 K–50,000 K (100MSun) Mass: from period (p) and average separation (a) of binary-star orbit 0.08MSun–100MSun Mass and Lifetime Sun’s life expectancy: 10 billion years Mass and Lifetime Sun’s life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Mass and Lifetime Sun’s life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Life expectancy of a 10MSun star: 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years × 10/104 Mass and Lifetime Sun’s life expectancy: 10 billion years Life expectancy of a 10MSun star: Until core hydrogen (10% of total) is used up 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years × 10/104 Life expectancy of a 0.1MSun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years × 0.1/0.01 Given the age of the Universe is 14 billion years no star of 0.1MSun has ever die of old age. Main-Sequence Star Summary High-mass: High luminosity Short-lived Large radius Blue Low-mass: Low luminosity Long-lived Small radius Red What are giants, supergiants, and white dwarfs? These are Off the Main Sequence • Off the main sequence stellar properties depend on both mass and age. • These stars have finished fusing H to He in their cores are no longer on the main sequence. • They may be fusing He to Carbon in their core or fusing H to He in shell outside the core … but there is no H to He fusion in the core. • All stars become larger and redder after exhausting their core hydrogen fuel: giants and supergiants. • Most stars end up small and white after all fusion has ceased: white dwarfs. • The white dwarf stage is the final stage for most stars. C B D A Which star is most like our Sun? C B D A Which of these stars will have changed the least 10 billion years from now? C B D A Which of these stars can be no more than 10 million years old? Star Clusters Our goals for learning: • What are the two types of star clusters? • How do we measure the age of a star cluster? 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. To measure a star clusters age we make one important assumption, ALL STARS IN A CLUSTER ARE BORN AT THE SAME TIME. Visual Representation of a Star Cluster Evolving How do we measure the age of a star cluster? Main-sequence turnoff Pleiades now has no stars with a life expectancy less than around 100 million years. The mainsequence turnoff point of a cluster tells us its age. To determine accurate ages, we compare models of stellar evolution to the cluster data. Using the H-R Diagram to Determine the Age of a Star Cluster Globular cluster are very old. Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old. • Now do the lecture tutorial section on HR Diagram. • When you have finished do the lecture tutorial section on Star Formation and Lifetimes.