understanding stars - university of sheffield · 9/24/2008 6 susan cartwright our evolving universe...

9/24/2008

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Susan Cartwright Our Evolving Universe 1

Understanding Stars

� What do we know?

� From observations of nearby stars:� luminosity/absolute magnitude

� colour/spectral class/ surface temperature

� chemical composition

� From observations of binary systems:� mass

� How do we use these to develop understanding?� look at relations between quantities (luminosity & temperature, mass & luminosity)

� compare with expectations from theoretical models


Luminosity & temperature: the

Hertzsprung-Russell diagram

� Plot absolute magnitude (or log L) against spectral class (or colour, or log T)

� the Hertzsprung-Russell

diagram

� for Ejnar Hertzsprung and Henry Norris Russell

� key to understanding stellar evolution

The Nearest and Brightest Stars

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Colour Index

The Nearest and Brightest Stars

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Colour Index

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Structure of the HR DiagramStructure of the HR DiagramStructure of the HR Diagram

main sequence

red giant branch

white dwarfs

Data from the Hipparcosparallax measuring satellite

Most stars lie on the main sequence

Note: faint stars are undercounted

brightbrightbright

faint

coolcool


The sizes of stars

� We know cooler objects are fainter (for given size)

� But red giant branch contains cool objects which are very bright

� Therefore these stars must be much larger than the faint, cool main-sequence stars

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1,000

10,000

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wavelength (nm)

3600

4600

5800

7000

10000

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Giants and dwarfs

� Red giants are up to 60 times larger than the Sun (~orbit of Mercury)

� Supergiants (e.g. Betelgeuse) are larger still

� Red main-sequence stars (“red dwarfs”) are only 1/3 as large as the Sun

� White dwarf stars are roughly Earth-sized

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33.544.55

log T (K)


Mass and luminosity

� Luminosity of main sequence stars is determined by mass

� Main sequence is a mass sequence(bright hot stars are more massive,cool faint stars are less massive)

y = 2.2735x - 0.567

y = 3.8971x + 0.0781

y = 3.4557x + 0.4708

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log(mass)

y = 2.2735x - 0.567

y = 3.8971x + 0.0781

y = 3.4557x + 0.4708

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log(mass)

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B - V

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B - V

10 times as massive10 times as massive10000 times brighter!10000 times brighter!

1/10 as massive1/10 as massive1000 times fainter!1000 times fainter!

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The sizes of stars

� Why are more massive stars larger and hotter?� force of gravity pulls stellar material downwards

� steadily increasing pressure pushes upwards

� larger mass needs higher pressures

� higher temperatures (increasing downwards)

� central energy source

pressure

gravity


What is the energy source?

� Chemical reactions?� no: not powerful enough

� Gravity?� suppose the Sun is still contracting…

� …would work for “only” 20 million years (remember Earth is 4.5 billion years old)

� Nuclear power?� YES: stars are fusion reactors

0.998

0.999

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1.001

1.002

1.003

1.004

1.005

1.006

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0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 560.998

0.999

1

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 56

E=mc2

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Evidence for nuclear fusion as

stellar power source

� Theory: it works!� Sun can be powered for 5 billion years by converting 5% of its hydrogen to helium

� Theory: it produces the elements we observe� discussed later

� Experiment: we see solar neutrinos� elementary particles created as by-product of fusion

� Experiment: correct solar structure� as measured by helioseismology


Stellar lifetimes

� A star 10 times as massive as the Sun has 10 times more hydrogen to power nuclear fusion

� But it is 10000 times as bright

� Therefore it should use up its fuel 1000 times more quickly

�Massive stars are very

short-lived

� Can we test this?� cannot watch stars age

� 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

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Stellar clusters

M67

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B-V

V

Pleiades

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B-V

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NOAO/AURA

Pleiades: bright blue main sequence stars; no red giants

M67: many red giants; no bright blue main sequence stars


Globular clusters

� Much larger than open clusters like the Pleiades

� All have no upper main sequence stars

� All have spectra showing low or very low heavy element content

� The oldestclusters in ourGalaxy (and someof the oldestobjects)

M3

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-0.3 0 0.3 0.6 0.9 1.2

B-V

V

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What have we learned?

� If we plot luminosity against temperature� stars fall into well defined bands

� most stars are on the main sequence

� If we plot luminosity against mass� main sequence stars fall on one line

� more massive stars are brighter, bluer and have shorter lives

� If we look at the Sun� stars must be powered by nuclear fusion

� provides enough power

� produces neutrinos

� If we look at stellar clusters� clusters with short-lived bright blue stars have few red giants

� clusters with many red giants have no short-lived blue main sequence stars

� clues to stellar evolution…next lecture

understanding stars - university of sheffield · 9/24/2008 6 susan cartwright our evolving universe...

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