1. Susan CartwrightOur Evolving Universe1 Understanding Stars n What do we know? n From observations of nearby stars: l l luminosity/absolute magnitude l l colour/spectral class/ surface temperature l l chemical composition n From observations of binary systems: l l mass n How do we use these to develop understanding? l look at relations between quantities (luminosity & temperature, mass & luminosity) l compare with expectations from theoretical models

2. Susan CartwrightOur Evolving Universe2 Luminosity & temperature: the Hertzsprung-Russell diagram n Plot absolute magnitude (or log L) against spectral class (or colour, or log T)  the Hertzsprung-Russell diagram l l for Ejnar Hertzsprung and Henry Norris Russell n key to understanding stellar evolution

3. Susan CartwrightOur Evolving Universe3 main sequence red giant branch white dwarfs Data from the Hipparcos parallax measuring satellite Most stars lie on the main sequence Note: faint stars are undercounted faint hot cool

4. Susan CartwrightOur Evolving Universe4 The sizes of stars n We know cooler objects are fainter (for given size) n But red giant branch contains cool objects which are very bright n Therefore these stars must be much larger than the faint, cool main- sequence stars

5. Susan CartwrightOur Evolving Universe5 Giants and dwarfs n Red giants are up to 60 times larger than the Sun (~orbit of Mercury) n Supergiants (e.g. Betelgeuse) are larger still n Red main-sequence stars (“red dwarfs”) are only 1/3 as large as the Sun n White dwarf stars are roughly Earth-sized

6. Susan CartwrightOur Evolving Universe6 Mass and luminosity n Luminosity of main sequence stars is determined by mass n Main sequence is a mass sequence (bright hot stars are more massive, cool faint stars are less massive) 10 times as massive 10000 times brighter! 1/10 as massive 1000 times fainter!

7. Susan CartwrightOur Evolving Universe7 The sizes of stars n Why are more massive stars larger and hotter? l l force of gravity pulls stellar material downwards l l steadily increasing pressure pushes upwards l l larger mass needs higher pressures è è higher temperatures (increasing downwards) è è central energy source

8. Susan CartwrightOur Evolving Universe8 What is the energy source? n Chemical reactions? l l no: not powerful enough n Gravity? l l suppose the Sun is still contracting… l l …would work for “only” 20 million years (remember Earth is 4.5 billion years old) n Nuclear power? l l YES: stars are fusion reactors E=mc 2

9. Susan CartwrightOur Evolving Universe9 Evidence for nuclear fusion as stellar power source n Theory: it works! l l Sun can be powered for 5 billion years by converting 5% of its hydrogen to helium n Theory: it produces the elements we observe l l discussed later n Experiment: we see solar neutrinos l l elementary particles created as by-product of fusion n Experiment: correct solar structure l l as measured by helioseismology

10. Susan CartwrightOur Evolving Universe10 Stellar lifetimes n A star 10 times as massive as the Sun has 10 times more hydrogen to power nuclear fusion n But it is 10000 times as bright n Therefore it should use up its fuel 1000 times more quickly è Massive stars are very short-lived n Can we test this? l cannot watch stars age l need sample of stars of different masses but the same age  stellar clusters  groups of stars bound together by gravity  formed together of the same material  ideal test bed for models of stellar evolution

11. Susan CartwrightOur Evolving Universe11 Stellar clusters NOAO/AURA Pleiades: bright blue main sequence stars; no red giants M67: many red giants; no bright blue main sequence stars

12. Susan CartwrightOur Evolving Universe12 Globular clusters n Much larger than open clusters like the Pleiades n All have no upper main sequence stars n All have spectra showing low or very low heavy element content n The oldest clusters in our Galaxy (and some of the oldest objects)

13. Susan CartwrightOur Evolving Universe13 What have we learned? n If we plot luminosity against temperature l l stars fall into well defined bands l l most stars are on the main sequence n If we plot luminosity against mass l l main sequence stars fall on one line l l more massive stars are brighter, bluer and have shorter lives n If we look at the Sun l stars must be powered by nuclear fusion  provides enough power  produces neutrinos n If we look at stellar clusters l clusters with short-lived bright blue stars have few red giants l clusters with many red giants have no short-lived blue main sequence stars è clues to stellar evolution…next lecture