Astronomy 111 – Lecture 18 Our Sun - An Overview

Astronomy 111 – Lecture 18

Our Sun

-

An Overview

Tuesday, November 30, 2004

Astronomy 111 - Lecture 18 2

Basic Concepts

• Size and Composition of Sun

• Age and Energy Problem

• Models of the Sun

• The Atmosphere of the Sun

• The Future of the Sun



Basic Sun Data

• Distance to Earth – Mean Distance : 1 AU = 149,598,000 km (light travel

time to Earth = 8.32 min)

– Maximum : 152,000,000 km

– Minimum : 147,000,000 km

• Mean angular diameter : 32 arcmin• Radius : 696,000 km = 109 Earth radii• Mass : 1.9891 x 1030 kg = 3.33 x 105 Earth Masses



Basic Sun Data

• Composition (by mass)– 74% Hydrogen, 25% Helium, 1% other elements

• Composition (by number of atoms)– 92.1% Hydrogen, 7.8% Helium, 0.1% other elements

• Mean Density : 1.4 g cm-3

• Mean Temperatures– Surface 5800 K, Centre 15.5 Million K (!)

• Total Luminosity : 3.86 x 1026 W (1 sec enough energy for 1 million years for humankind)



How old is the Sun ?

• Really problematic : Sun does not come with a time-stamp, shows no wrinkles etc…

• Start with Earth & Solar System– Date geological formations, rocks

• Various geochronometrical methods using radioactive decay properties >4.2 billion years

– Find oldest meteorites 4.5-4.7 billion years

• Conclusion : Sun at least that old !



What fuel does the Sun use ?

• A truly gigantic energy crisis emerges:– Sun is billions of years old.– Life on Earth as well. Sun must shine roughly like today for

already a long time.

• How does it create all the energy ?

• What keeps the Sun so hot ?



The Solar Energy Source

• First guesses (early 18th century) :– Chemical Processes (‘burning’ fuel)

• Maximum burning time : 10,000 years !

– Using gravitational energy (Kelvin-Helmholtz mechanism, mid-1800s)

• Slow contraction of Sun releases energy, heats gas up -> energy radiated into space

• Maximum contraction time : 25 million years !

• Only important at very early stage (birth of Sun)



A new Energy Source

• Problem : – Need more energy released per atom

• Solution :– Albert Einstein : E = mc2

• m = mass, c = speed of light, E = energy

• Mass and energy are equivalent.

• Why does that help ? – Speed of light is a large number !



Thermonuclear Fusion

• Turning mass into energy : – Transforming Hydrogen into Helium

– Mass balance : 4 1H atoms 6.693 x 10-27 kg

1 4He atom 6.645 x 10-27 kg

------------------------------------

Mass lost : 0.048 x 10-27 kg

• Extremely efficient process :– Fusing 1 kg of Hydrogen releases same energy as

burning 20,000 metric tons of coal !




• How much hydrogen must be converted to give solar luminosity ?– 600 million metric tons per second !– Sounds enormous, but…..– Sun has enough fuel for at least 9 billion years

• Solution to energy crisis !

• Problem : How does fusion work in detail ?




• ‘nuclear’ – regarding nuclei of atoms

• ‘fusion’ – putting together (NOT fission)

• ‘thermo’ – need enormous temperatures as nuclei do not fuse easily due to electric repulsion



• Introducing the Proton-Proton Chain


(twice) eHpp e2

(twice) HepH 32 ppHeHeHe 433

positron

neutrino

2H

positron

neutrino

2H

photon

3He

4He

photon

3He



Models of the Sun

• Solar surface temperature ~ 5800 K• Temperature required for fusion ~ 10 million K• Fusion can only occur at core of the Sun !• Can we understand and describe the conditions

inside the Sun ?• Use theoretical models !

– Start with well-known principles and laws of physics



Models of the Sun

• Observational Fact 1 :– The Sun does not change size over long periods

of time, keeps its shape quite well.

• Principle of Hydrostatic Equilibrium– Pressure and Gravity maintain a balance.

16Astronomy 111 - Lecture 18Tuesday, November 30, 2004

Hydrostatic Equilibrium

Gas Pressure

Gravity



Models of the Sun

• Conclusions :

• Pressure increases with depth

• Temperature increases with depth



Models of the Sun

• Observational Fact 2 :– The Sun does not heat up or cool down over

long periods of time, keeps its temperature quite well.

• Principle of Thermal Equilibrium– Energy Generation and Energy Transport

maintain a balance.



Models of the Sun

• Modes of Energy Transport :– Heat conduction very inefficient for gases– Convection : ‘circulation of fluids’, upwelling

of hot gases– Radiative diffusion : Photons carrying away

energy


Convection

cooler watersinks

Hot blob rises


Photon Random Walk



Models of the Sun



Models of the Sun

• Construct computer model to describe state of solar material from core to atmosphere

• How can we test those models ?

• Do they describe the interior well ?

• How can we ‘see’ into the Sun ?



Testing the Models

• Test 1 : Helioseismology– Sun vibrates (‘rings like a bell’)– Use waves to test interior structure– Same Principle as Geologist/Geophysicists with

Earthquakes– Nice analogy : Ripeness of melons



Astronomers probe the solar interior usingthe Sun’s own vibrations

• 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



Testing the Models

• Test 2 : Catching solar neutrinos– By-products of fusion– Unlike photons, neutrinos escape solar interior

very easily– ‘Ghost-like’ particles : very, VERY hard to

detect• 1014 solar neutrinos every second through 1m2 on

Earth



Catching Neutrinos



The Solar Atmosphere

• Core of Sun hidden because gases become opaque

• Outermost layers show remarkable structures (‘atmosphere’)

• Tiny and much less dense than interior

• Nonetheless most important for life on our planet



The Solar Atmosphere



The Photosphere•Almost all of the visible light emanates from that small layer of gas.

• Temperature decreases upwards in photosphere.

• Sun darker at the edge ?



The Photosphere



The Photosphere

• Cooler layers further out Notice absorption lines in spectrum !

• Cool ? – Still about 4400 K at the upper edge of the

atmosphere.– Comparison : Siberian winter night to hot

tropical day in Hawaii.



The Photosphere

• Solar Granulation : Convection cells of gas in photosphere

• 4 million granules cover the solar surface

• Each granule covers area ~ Texas & Oklahama combined







The Solar Chromosphere• Above the

photosphere is a layer of less dense but higher temperature gases called the chromosphere

• Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules



The Solar Chromosphere

• Spectrum : Emission lines ! Temperature must rise again :

– Top of chromosphere : 25,000 K

• Approximately 300,000 spicules exist at one time, each last about 15 minutes

• Covering ~ 1 % of solar surface

• Phenomenon related to Sun’s magnetic field



• The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density

• The solar corona blends into the solar wind at great distances from the Sun

The Solar Corona



The corona ejects mass into space to form the solar wind



Activity in the corona includes coronal mass ejections and coronal holes



Sunspots are low-temperature regions inthe photosphere







Sunspots are produced by a 22-year cyclein the Sun’s magnetic field





The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field





The future of the Sun

• Equilibrium holds due to energy generation

• What happens when Sun runs out of fuel ?

• Drama at the End of the Solar Life

• A story for another day….

• When will that happen ?– ~ 4.0 – 5.0 billion years from now– Puuuh, we are safe !



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Astronomy 111 – Lecture 18 Our Sun - An Overview