The Sun

THE SUN

Martin Crow Crayford Manor House Astronomical Society

The Solar System :


The Solar System : The Sun and Earth Moon system

Last time:

The history of our understanding of the Solar system.


The Solar System : The Sun and Earth Moon system

This week:

The formation of the Sun (and Solar system).

About the Sun

The Sun’s effect on the Earth.

The Earth Moon system.

The Sun


The formation of the Sun (and Solar system)

The Sun along with the rest of the solar system formed 4.567 x10⁹ years ago.

The age has been determined from radiometric dating of meteoritic material.


The Solar system formed from the gravitational collapse of a Giant MolecularCloud GMC.

GMCs are many tens of light years in size and have masses between 10⁴ - 10⁶ solar masses.

They have an average density of 100 – 1000 particles per cubic centimetre.This is similar to the very best vacuums created in the laboratory. The average density in the solar vicinity is one particle per cubic centimetre.

Ideas regarding the formation of the Solar system were first formulated by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century.This was called the ‘Nebula formation’ of the Solar system.

This idea fell in and out of favour, largely due to issues surrounding the Sun’s angular momentum, but it is now known that they were broadly on the right track.

In the air around us there are 10¹⁹ particles per cubic centimetre.


The gravitational collapse is thought to have been triggered by shock wavesfrom nearby supernovae events.Studies of meteorites containing pristine solar nebular material indicate five separate events. Dr Caroline Smith

The high mass stars that gave rise to these supernovae are likely to have formed in the same GMC that formed the Sun around 5 x 10⁶ years earlier.

GMC was mostly Hydrogen with around 2% consisting of heavier elements.Anything heavier than Hydrogen is described by astronomers as metals.


So what makes the Sun shine?

The Sun has been shining for a very long time and how it did this was a mysteryuntil the 20th century.

Solutions that had been put forward were:

In an influential lecture in 1854, Hermann von Helmholtz proposed thatthe Sun’s enormous radiated energy was caused be gravitational contraction.

Chemical burning would only account for 3000 years of heat, whereas,Gravitational contraction could account for 20 x 10⁶ years.This was at odds with geological evidence for the Earth beinghundreds of millions of years old.

In 1905 a way forward appeared when Einstein derived his famous relation between mass and energy, E = mc2.

F.W. Aston discovered in 1920 that four hydrogen nuclei were heavier than a helium nucleus.


So what makes the Sun shine?

So the Sun shines by converting Hydrogen into Helium, the so calledProton – Proton reaction which accounts for nearly all of the energy release.

This reaction will only occur at temperatures of 10⁶ - 14⁶ K.

The reaction converts 0.7% of the mass into energy (26.73 MeV) in the form of gamma rays and neutrinos.

The core temperature of the Sun is 15.7 x 10⁶ K.

Another reaction called the CNO cycle will work above 13⁶ K and accounts for1.7% of the ⁴He produced.

It can take up to 10⁵ years for the light produced in the core to reach the surfaceBut the neutrinos escape immediately.


The life cycle of the Sun

About the Sun

0–100,000 years - Pre-solar nebula forms and begins to collapse. Sun begins to form.

10⁵ – 50 x 10⁶ years - Sun is a T Tauri protostar.

50 x 10⁶ years - Sun becomes a main sequence star.

Sun remains a main sequence star, continually growing warmer and brighter by ~10% every 1 billion years.

The habitable zone?



10 x 10⁹ - 12 x 10⁹ years - Sun starts burning hydrogen in a shell surrounding its core, ending its main sequence life. Sun begins to ascend the red giant branch of the Hertzsprung–Russell diagram, growing dramatically more luminous (by a factor of up to 2,700), larger (by a factor of up to 250 in radius), and cooler (down to 2600 K): Sun is now a red giant. Mercury and possibly Venus and Earth are swallowed. Saturn's moon Titan may become habitable.



~ 12 x 10⁹ years - Sun passes through helium-burning horizontal branch and asymptotic giant branch phases, losing a total of ~30% of its mass in all post-main sequence phases. Asymptotic giant branch phase ends with the ejection of a planetary nebula, leaving the core of the Sun behind as a white dwarf.

The Helix Nebula Credit: NASA, ESA, and C.R. O'Dell



>12 x 10⁹ years - The white dwarf Sun, no longer producing energy, begins to cool and dim continuously; this continues for trillions of years, eventually reaching a black dwarfstate.

~ 10 x 1015 years - Sun cools to 5 K. Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.


About the Sun

Distance – 1.496 x 10⁸ km or 8.32 light minutes.

Visual brightness – mag -26.74

Diameter – 1.392 x 10⁶ km or 109 x Earth

The Sun accounts for 99.86% of the mass of the Solar system.

Mass – 1.989 x 10³⁰ kg or 333,000 x Earth

Surface temperature – 5778 K (5505° C)

75% of the Sun is Hydrogen and nearly all of the rest is Helium.


About the Sun

The centre of mass of the solar system (barycentre) is not at the middle of the Sun.


About the Sun

The Sun does not rotate as a solid object.


The structure of the Sun.


Granulation


Photosphere including sunspotwith plages


Chromosphere and prominence


Filament


Chromosphere, spicules and sunspot


The Solar cycle

Sunspot polarity reverseswith each cycle.


Corona - the solar wind

The Sun

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