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The Stars

HNRT 227 Chapter 1422 October 2015

Great Idea:The Sun and other stars use nuclear fusion

reactions to convert mass into energy. Eventually, when a star’s nuclear fuel is

depleted, the star must burn out.

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Chapter Outline

• The Nature of Stars• The Anatomy of Stars• The Variety of Stars• The Life Cycles of Stars

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iClicker Question

• How often do you look up at the night time sky?– A Always (when possible)– B Often– C Sometimes– D Never

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iClicker Question

• Do you enjoy looking at the stars?– A yes– B no

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The Nature of Stars

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The Nature of Stars

• Astronomy– Oldest science (?)

• Star– Ball of gas– Fusion reactor

• All stars have a beginning and an ending

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Measuring the Stars with Telescopes and Satellites

• Electromagnetic radiation• Measurement of photons

– Wavelength– Intensity– Direction– Variation

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Telescopes

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Orbiting Observatories

• Great Observatories Program– Hubble Space Telescope– Spitzer Infrared Telescope– Chandra X-Ray Observatory

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iClicker Question

• What source of data is used to analyze stars?

– A chemical testing of actual samples of stellar matter

– B electromagnetic radiation– C spacecraft in orbit around

distant stars– D both B and C above– E no data is used

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iClicker Question

• Telescopes are devices that:– A focus and concentrate

radiation– B magnify only visible light– C all use mirrors– D both B and C above

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The Anatomy of Stars

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The Structure of the Sun

• Structure– Stellar core– Radiative zone– Convection zone– Photosphere– Chromosphere– Corona

• Solar Wind– Stream of

particles

14Our Sun

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More On Solar Structure

• Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius

• The core is surrounded by a radiative zone extending to about 0.71 solar radius– In this zone, energy

travels outward through radiative diffusion

• The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure– In this zone, energy

travels outward primarily through convection

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How do we know the Sun’s interior?

• Helioseismology is the study of how the Sun vibrates

• These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun

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Magnetic Fields

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Northern Lights (Aurora Borealis)

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The Sun’s Energy Source: Fusion

• Sun’s Energy Source– Historical– Current

• hydrogen

• Fusion– 3-steps-hydrogen burning

1) P + P D + e+ + neutrino + energy2) D + P 3He + photon + energy3) 3He + 3He 4He + 2protons + photon + energy

• Life expectancy– 11-12 billion years

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The Variety of Stars• Differences

– Color– Brightness

• Distance • Absolute

brightness– Energy output– luminosity

• Apparent brightness

• Behavior– Total mass– age

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The Cosmic Distance Ladder

• Distance– Light-years

• Measurement– Triangulation

(parallax)– Spectroscopic

parallax– Cepheid variable– Tully-Fisher– Supernovae Type Ia– Hubble’s Law

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The Hertzsprung-Russel Diagram

• Star Groupings– Main-sequence stars– Red giants– White dwarfs

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Step 1 to an H-R Diagram

• Plot for the 20 nearest and brightest stars to Earth

Not an H-R Diagram

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Step 2 to an H-R Diagram

• Reversing the y-axis

Getting Closer to an H-R Diagram

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Step 3 to an H-R Diagram

• Reversing the x-axis– Lowest

temperature to the right

Now it's looking Like an H-R Diagram

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A Standard H-R Diagram

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iClicker Question

• Compared with other stars, our Sun is:

– A an unusually large star– B not a star at all– C a rather ordinary star

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iClicker Question

• The outer part of the Sun, the part that actually emits most of the light we see, is called the:

– A chromosphere– B convective zone– C photosphere– D core– E radiative zone

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iClicker Question

• The solar wind is composed of:– A bits of neutral gas– B all kinds of organic

substances– C air– D charged particles including

hydrogen and helium ions

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iClicker Question

• Northern lights result from interactions of:

– A the Sun’s magnetic field with Earth’s gravity

– B UV light from the Sun with Earth’s ozone layer

– C the solar wind with the Earth’s magnetic field

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iClicker Question

• The Sun’s peak output of energy is in:

– A the ultraviolet portion of the electromagnetic spectrum

– B the visible portion of the electromagnetic spectrum

– C the infrared portion of the electromagnetic spectrum

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iClicker Question

• What is the Sun’s energy source?– A combustion of hydrogen rich

chemical fuels– B fusion of hydrogen– C fission of hydrogen– D radioactive decay– E gravitational collapse

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The Life Cycles of Stars

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The Birth of Stars

• Nebular Hypothesis– Laplace

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The Main Sequence and the Death of Stars

• Stars much less massive than the Sun– Glows 100 billion years

• No change in size, temperature, energy output

– Brown dwarfs• Some don’t consider

these stars

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The Main Sequence and the Death of Stars

• Stars about the mass of the sun– Hydrogen burning

at faster rate• Move off main

sequence

– Helium burning– Red giant– Begin collapse– White dwarf

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The Life Cycle of a Star Like the Sun

• Gas cloud• Fragmentation• Protostar• Kelvin-Helmholz

contraction• Hayashi Track• Ignition• Adjustment to Main

Sequence• Hydrogen Core

Depletion

• Hydrogen shell burning

• Helium flash• Helium core burning• Helium core depletion• Helium shell burning• Helium shell flashes• Planetary nebula• White Dwarf

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Sun’s Life Cycle on H-R Diagram

H-R Diagram with annotations for Stellar Stages

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CoreContraction

WhiteDwarf

SubGiant

HeliumFlash

HorizontalBranch

AsymptoticBranch

Planetary Nebula

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The Main Sequence and the Death of Stars

• Very Large Stars– Successive

collapses and burnings

– Iron core– Catastrophic

collapse• supernova

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Layers of Massive Star

Si -> Fe

Mg -> Si

Ne -> Mg

O -> Ne

C -> O

He -> CH -> He

Fe

Layers

of

Nuclear

Fusion

in

High

Mass

Stars

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Neutron Stars and Pulsars

• Neutron Star– Dense and small– High rotation rate– Little light

• Pulsar– Special neutron star– Electromagnetic

radiation– End state of supernova

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Black Holes• Black Hole

– Result of collapsed large star

– Nothing escapes from surface

– Cannot “see” them• See impact on

other stars, dust, etc.

• Detect x-rays, gamma rays

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iClicker Question

• It is estimated that the total lifetime of our Sun is 12 billion years. Roughly, how far is it through its hydrogen-burning phase now?

– A 10%– B 25%– C 50%– D 90%

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iClicker Question

• Triangulation and Cepheid variables are methods to measure:

– A distances to stars– B energy output of stars– C lifetime of stars– D composition of stars

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iClicker Question

• A Hertzsprung-Russell diagram plots a star’s temperature versus its:

– A color– B distance– C age– D size– E energy output

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iClicker Question

• Large dust and gas clouds are commonly found throughout space. They are called:

– A red giants– B galaxies– C nebulae– D supernovae– E white dwarfs

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iClicker Question

• The fusion process in very large stars produces chemical elements up to:

– A He– B C– C Fe– D U– E Pb

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iClicker Question

• All natural elements beyond iron are created in:

– A supernova explosions– B fusion of very large stars– C the big bang

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iClicker Question

• An object that is so dense and massive that nothing, including light can escape from its surface is called:

– A a supernova– B a red giant– C a white dwarf– D a black hole– E a neutron star

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iClicker Question

• Are you surprised that the chemical elements about you were made in a supernova?

– A Yes– B No