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Susan CartwrightOur Evolving Universe1 Understanding Stars n What do we know? n From observations of nearby stars: l l luminosity/absolute magnitude l.

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Presentation on theme: "Susan CartwrightOur Evolving Universe1 Understanding Stars n What do we know? n From observations of nearby stars: l l luminosity/absolute magnitude l."— Presentation transcript:

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


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