AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekHow do we use the Hertzsprung-Russell diagram to makesense of the temperatures and luminosities of stars?Describe the life stages of a low-mass star, like the Sun.Use the HR diagram to show the evolution of a low-massstar graphically.What happens inside of a star that makes it change from amain-sequence star into a red giant, then a planetarynebula and a white dwarf?How do the life stages of a high-mass star differ, and why?Describe the two types of supernovaeTemperature-Luminosity diagramsAstronomers measure the temperatures and luminosities ofmany stars and plot them on a diagram called theHertzsprung-Russell (or H-R) diagram.The horizontal axis is surface temperature and the verticalaxis is luminosity.Each star is a dot on the diagram.For historical reasons they plot temperatures increasing tothe left (not right) and luminosities increasing upward.They also sometimes plot colors instead of temperaturesand absolute magnitudes instead of luminosities.They find that stars cluster in 3 groups.Star survey resultsMany stars fall on a diagonal line running from the upperleft (hot and luminous) to the lower right (cool and faint).This group is called the main sequence.The Sun is one of these stars.But some fall in the upper right (cool and luminous) andsome fall toward the bottom of the diagram (faint).What can we say about the stars in the upper right?What can we say about the stars toward the bottom?If all stars had the same size, what pattern would theymake on the diagram?Mass – Luminosity DiagramWe can plot the masses and luminosities of stars on adiagram like the H-R diagram.Red giant and white dwarf stars follow no pattern, but mainsequence stars fall along a line with luminosityincreasing with mass.Masses of starson the H-RdiagramGroups of fourChoose a discussion leaderand a scribe.Read the graph:What is the luminosity in solarluminosities of the Sun?What is the luminosity of a 10solar mass main sequencestar? (Make an estimate.)What is the relation betweenmass and luminosity?L α MxWhat is x? (Make anestimate.)Equilibrium in starsMain-sequence stars are in equilibrium in two ways:Hydrostatic equilibrium means that the pressure of the gasinside of a star (which tries to make it expand) balancesthe force of gravity (which pulls one side of the startoward the other, and so tries to make it contract).The result is that the size of the star doesn’t change.Thermal equilibrium means that the loss of energy byradiation of light balances the generation of energy bynuclear fusion.The result is that the temperature inside the star doesn’tchange.Stable and unstable equilibriumAn equilibrium is stable if a small change in one of thebalanced influences will change the situation by only asmall amount.For example, heating a balloon will make the pressure ofthe air in it increase. As a result, the balloon will begin toexpand. But that will cause the pressure to drop, so itwill stop expanding.An equilibrium is unstable if a small change in oneinfluence causes a large change in the situation.A stick of dynamite is unstable.Are stars stable or unstable?What would happen if some gas were added to a star so itspressure increased?It would be out of hydrostatic equilibrium and would expand.But when a gas expands its pressure decreases, so afterexpanding a little bit it would again be in equilibrium.What would happen to a star if the rate of nuclear fusionincreased so it was generating energy faster than it wasradiating it from its surface?You would expect it to get hotter.That would make it radiate more, but it would also makenuclear fusion go faster, and fusion would increase morethan radiation, so the star would get even hotter.QuizIf a spaceship is orbiting the Sun and it is given moreenergy, what happens?A. It goes into a smaller orbit in which it goes faster.B. It goes into a smaller orbit in which it goes slower.C. It goes into a bigger orbit and speeds up.D. It goes into a bigger orbit and ends up going slower.QuizIf a spaceship is orbiting the Sun and it is given moreenergy, what happens?Like a spaceship, the atoms in the Sun are held in bygravity, but don’t fall to the center because they aremoving.If the atoms in the Sun were given more energy, the Sunwould expand and the atoms would move more slowly.Adding energy to the Sun would make it bigger and cooler.Removing energy from the Sun would make it contract andheat up.Thermal Equilibrium in StarsProtostars are not in thermal equilibrium.They lose energy by radiation from their surfaces, but theyaren’t hot enough inside to ignite nuclear fusion toreplace the lost energy.As a result, they contract and heat up.Once they are hot enough inside (about 107 K) fusion canreplace the energy they are losing.They are then in a stable thermal equilibrium; if fusionslowed down, they would contract and heat up causingfusion to speed back up until it balances the energy theyare losing.Because of this stable equilibrium, the Sun will hardlychange for 1010 years, until it uses up all of the hydrogenin its core.QuizIf a star (or a part of a star) radiates more energy than itgenerates, it will …A. contract and heat upB. contract and cool offC. expand and heat upD. expand and cool offBecoming a Red Giant(The complete explanation for how a main-sequence starbecomes a red giant is complicated, and I’m not reallygiving you the whole story. But the conclusion is right.Don’t worry if you don’t follow all of the explanation.)When all of the hydrogen in the core of a main-sequencestar is all turned into helium, fusion will stop in the core,and the core will contract and heat up.Fusion will continue in a shell around the helium core, andwill generate more energy than fusion in the core did.The extra energy going out from the core+shell will makethe envelope expand and cool off.Red GiantsWhen the Sun becomes a red giant, its radius will increaseto about ½ AU, and it will become more than 100 timesmore luminous than it is now.Life will not be pleasant on Earth.The core of the Sun will be mostly helium, and will continueto contract and heat up.When the temperature in the core reaches about 108 K(about 1 million years after the Sun leaves
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