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KCiC Physics 4 Cosmic Engine

copyright 2009 keep it simple science

www.keepitsimplescience.com.au

Slide1

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Key Concepts in Colour

Preliminary Physics Topic 4

The Cosmic EngineUsage & copying is permitted according to the following

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Preliminary Physics Topic 4

The Cosmic Engine

First, Some Revision:The Struct ure of t he Univ erseThe EARTH is a PLANET. The Earthand 7 other planets (plusdwarf planets, moons,asteroids, comets, etc)are in orbit around

the Sun. The SUNand all thesethings in orbitaround it, makeup our "SOLARSYSTEM".

Everything stays

in orbit aroundthe Sun becauseof gravity.

Our understandingof the Universe is veryrecent. Just 100 yearsago we had no idea of thevastness of the Universe orwhat was going on inside the Sun.We had no knowledge of the life and death of a star.

Were st ill learning...

The SUN is a STAR. Energy is being produced

inside it, due to NUCLEAR REACTIONS. The Sun isone of over 100 billion stars that make up ourGALAXY. Each star in the night sky is another "Sun"within our galaxy, the "MILKY WAY". Our Sun andthe other stars of the Milky Way are orbiting aroundthe galaxys centre because of gravity.

Beyond our galaxy are billions of other galaxies.The distances involved are immense andunimaginable!

We have good reason to believe that the entireUniverse is EXPANDING, with the space betweengalaxies increasing.

This topic begins with a little history, then givesyou an introduction to modern Astronomy.

Along the way you ll learn about nuclear

reactionsand finishup with a

look atthe Sun,its many

radiationsand how

theyaffect the

Earth.

Other,distantgalaxies

OurGalaxy,theMilkyWay

Sun

Mercury

Venus

Mars

Earth

Pluto

Neptune

Uranus

Jupiter

Saturn

PositionofourSuninthe

galaxy

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Hertzsprung-RussellDiagram

Big BangTheory

Radiation

from the Sun

Temperature &Colour of Stars

Energyin a Star

Nuclear

Reactions.Radioactivity

Evidence for theBig Bang

Formation ofStars & Galaxies

Impacts &Effects

on Earth

Stages in a

Stars Life

Brightness &Distance.

Inverse-Square Law

Geocentricor

Heliocentric?

THE COSMICENGINE

A LittleHistory How the

Universe Began

Life-Cyclesof the StarsEnergy from

the Sun


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Different Models of the Universe

First, be aware that our understanding of galaxies and the true extent of the Universe was only discoveredwithin the last 100 years. Prior to that, any theory or model of the Universe really only dealt with our SolarSystem. The stars were thought to be part of the Solar System, or only just beyond it ..

Over the centuries there have been TWO main models of the Universe competing for acceptance.

1. HISTORY OF OUR UNDERSTANDING OF THE UNIVERSE

Heliocentric Models correctly place the Sunat the centre of the Solar System. ("Helios" = Sun)

Heliocentric models require that the Earth rotates onits axis so that everything in the sky appears to goaround us. However, we can't feel that the Earth isspinning, so this idea is harder to accept on the basis

of common sense, even though it is correct.

Only the Moon truly orbits the Earth.

Geocentric Models

Many early Astronomers believed that the EARTH isat the centre of the Universe.

("Geos" = Earth, centric = at the centre)

Geocentric models easily explain why the Sun, Moon,planets and stars all appear to move across the sky.Common sense suggests that everything revolvesaround the Earth once per day. Also, we cannot feel

that the Earth is spinning, so this model makescommon sense, even though it is wrong!

Earth

Sun

Sun

Fixed

Stars

Earth


Planets

Moon

FixedStars

Moon

Planets

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Aristotle ~330 BC Geocentric Theory

This great thinker of ancient Greece thought that theSun, Moon, planets & stars are carried on invisiblecrystal spheres rotating around the Earth.

This basic concept was believed for about 2,000years.

Aristarchus ~240BC

Heliocentric Theory

Another Greek, Aristarchusrealised that it was possiblethat the Sun is in the centrewith everything orbiting

around it. For this to work theEarth must rotate on its axis,so it appears that everythingmoves around us.

This idea was not accepted because "parallax"could not bedetected at thistime.

Historical Summary up until about 1700 AD

Claudius Ptolemy ~120AD

Geocentric Model with "Epicycles"

Based on the best (naked eye) measurements of thetime, Ptolemy developed a model which could predictthe motion of planets & the times of eclipses.

Although we now know it was wrong, itwas a practical, working model used for

1,400 years.

The "epicycles" were needed to explainthe "retrograde" motion of the planets.

Ptolemys model was accepted for such

a long time that it became part of thebelief system of the Middle Ages, and

was even adopted as the off icialreligious explanation of the Universe.

So, when new ideas and new discoveries emergedaround 1500 AD, they were seen as dangerous andheretical, and were punishable by torture and death.

See Further Explanationsat the end of this section

See Further Explanationsat the end of this sectionWhich is

Spinning

Around theOther?

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Tycho Brahe 1546-1601

Accurate ObservationsTycho used the most advanced observatory of that time togather outstandingly accurate data (accurate for naked eyemeasurement) of planetary movements. He favoured thegeocentric model and hoped his observations would proveCopernicus wrong.

He jealously guarded his data from others, but when hedied it went to his student Kepler.

Johannes Kepler 1571-1630

Heliocentric Model, with elliptical orbits

Kepler tried to f it Brahe's extremely accurate data to theCopernicus model. Finally, he found it only fitted if the

orbits were ellipses, not circles.

Eventually he proposed 3 "Laws of Planetary Motion".These described the orbits accurately, but could give no

explanation of how or why the Earth and planetscould orbit around the Sun.

The Heliocentric idea was still NOT accepted widely.

Historical Summary continued...

Nicholas Copernicus 1473 - 1543 ADHeliocentric Theory

As measurements improved, Ptolemy's modelneeded more & more adjustments and epicyclesto stay accurate in its description of theheavens. It got so complicated that Copernicusdecided there must be a simpler explanation. He

decided that perhaps Aristarchus had beencorrect after all, and the Sun was in the centre.

Copericuss new model still relied on crystalspheres to carry planets and stars in circularorbits, but it was Heliocentric... Sun centred.

The accuracy of predicted motions remainedmuch the same as Ptolemys, but this model was

much simpler in its explanations.

This model was NOT immediately accepted atthe time.

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Sir Isaac Newton1642-1727

Mathematical Theory of Gravity

Newtons Theory of Universal Gravitation providedthe explanation for things to be in orbit , and did away with theclumsy crystal spheres ofprevious models.

From his equation for Gravity,Newton could prove Kepler's Lawsmathematically... this proved thatthe Heliocentric Model wascorrect.

Since the time of Newton, the Heliocentric model hasbeen accepted as the scientifically correct description ofthe Universe, but it took another 200 years to discoverthe full story of stars, galaxies and distances.

SirIsaacNewton

Historical Summary continued...

Galileo Galilei 1564-1642

Telescope ObservationsGalileo was the first to use a TELESCOPE to view the

heavens.

His observations conflictedwith the model of Ptolemy,

and supported theHeliocentric idea of

Copernicus.

He observed that the planetJupiter has moons orbiting

around i t. (Only the Earth wassupposed to have things go

around it!)

He saw that the planet Venus showed phases like theMoon. (This was only explainable if Venus orbi ted theSun, not Earth!)

The Significance of Telescopes in AstronomyAl l of the theories unti l the time of Gali leo, were l imited by the lack of the TELESCOPE.Without telescopes, all measurements and observations were made by naked-eye, andwere of limited accuracy.

If telescopes had been available earlier, then PARALLAX might have beenobserved in nearby stars, and greater accuracy would have been possible inmeasuring planetary posit ions and movements. This would have led to rejection of theclumsy and complicated "epicycles" of Ptolemy and perhaps the correct Heliocentricmodel would have been accepted earlier.


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ParallaxParallax is the apparent movement of an object againsta more distant background, when viewed from adifferent angle.

Opponents of any Heliocentric model throughout

history could argue (correctly) that if Earth was orbi tingthe Sun, then the stars should show some parallaxmovements relative to other stars, when viewed fromone part of our orbit compared to another.

A Simple Example of ParallaxHold up one finger and view it wi th one eye

against a distant tree or post. Hold the finger still

while switching to view it with your other eye.

Your finger appears to move relative tothe distant " landmark" .

This apparent movement is called "PARALLAX"

Further ExplanationsThis information may help your understanding. It is NOT a syllabus requirement to learn it.

Sun

Earth

Earth,

6monthslater

lineofobservation

Star

being

observed

More

distant

stars

Thepositionofthestarshouldchangeagainstthe

backgroundstars.

Parallax

This parallax motion could not be detected by nakedeye observations, even with the most accurateinstruments invented right up until the 17th century,so heliocentric theories tended to be rejected.

In fact, nearby stars DO show parallax movement, butyou need a telescope to detect it, because even thenearest stars are billions of ki lometres away.

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KCiC Physics 4 Cosmic Enginecopyright 2009 keep it simple science

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Retrograde Motion & EpicyclesEpicycles were a device invented by Ptolemy toexplain the "retrograde" motion of the planets.

Firstly you must know that, while the stars alwaysappear in exactly the same relative positions everynight , the planets do not. ("Planet" means"wanderer" in Greek.) If you observe a planet nightafter night, it seems to move slowly eastward

compared to the background of stars. However,sometimes the planet moves westward for a while.This was called "retrograde" (backwards) motion.

To explain i t, Ptolemy proposed that the planets werecarried on smaller crystal spheres (the epicycles)which rotated on the rim of the main spheres("deferents") surrounding the Earth.

Normalplanetarywanderings

Retrogrademotion

FixedStarsinbackground

Further ExplanationsThis information may help your understanding. It is NOT a syllabus requirement to learn it.

This "wheels-on-wheels" idea was able to explainretrograde motion adequately, if rather clumsily.

The real explanation for retrograde motion is that we

view the moving planets from a moving Earth. Atcertain parts of our orbit , we "overtake" other planetsand so they appear to move "backwards" for a while.Retrograde motion is easily explained by thegeometry of a Heliocentric model, with the Earth andother planets all orbiting the Sun.

Earth DeferentEpicycle

Planet

Planet

Eachplanetsmainorbitisarotatingglasssphere,called

thedeferent.ItrevolvesaroundtheEarth.

Theplanetiscarried

onasmaller

sphere,the

epicycle,

which

rotatesonthe

deferent.

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Activity 1The following activity might be completed by class discussion,

or your teacher may have paper copies for you to do.

Some History Student Name .................................

1. Outline the difference between a Heliocentric and Geocentric model.

2. The following people all proposed a model to describe the universe.

Aristotle Aristarchus Ptolemy Copernicus Kepler

Circle those whose model was Geocentric.

3. What was Tycho Brahes contribution to Astronomy?

4. What was the significance of Galileos observations?

5.a) What did Keplers Laws describe?b) In what way was this model different to any previous idea?

c) How, and by whom, were Keplers Laws confirmed to be correct?


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Outline of the Big Bang Theory1. The universe began about 13-14 billion years ago.

2. In the beginning, all the space, matterand energy o f the universe wasconcentrated in a "singularity"...one tiny point of incredible density andtemperature.

3. This exploded outwards in alldirections, becoming cooler and lessdense as it expanded very rapidly.

This expansion is still occurring today.Galaxies are moving further apart as space expands.

4. Within a galaxy, gravity attracts matter and holds stars andplanets together in their orbits around each other, so there is noapparent expansion in the "local" area of space.

This theory seems strange and unbelievable whendescribed in simple outline, so why is it accepted as

being correct? Simple! ...because the theory explainsand fits many observed facts about the universe.

2. HOW THE UNIVERSE BEGAN

What the

Big Bang Explains

The Universe is ExpandingThe main evidence is the "Red-Shift" of

the spectral lines of distant galaxies. Thiscan only be explained by a continuing

expansion of space. Expansion is thoughtto due to the original explosion.

Cosmic Background RadiationIt was discovered in 1965 that the entire

Universe seems to be fil led with

microwave radiation coming from everydirection. This is explained as being the"afterglow" of radiation from soon after

the Big Bang explosion.

What the Universe is Made FromThe observed chemical composition ofthe universe (almost entirely Hydrogen

and Helium) agrees with theoreticalpredictions of what should have

happened during the first secondsof the Big Bang.

Note: You must NOT think of this as if the matter explodedoutwards into the space surrounding it.

The explosion and expansion was of space itself.Before the explosion there was no space or time.

The RED SHIFT isexplained in the next slide

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In 1922, the Russian Alexander

Friedmann predicted that the universewas expanding.

His prediction arose from working onthe equations of Einstein's "GeneralTheory of Relativity". This was a braveprediction at the time, since othergalaxies beyond ours had not been

discovered, and there was no knownevidence of expansion.

During the 1920's new, biggertelescopes led to the discovery of otherdistant galaxies. The American, EdwinHubble, analysed the spectral linesfrom distant galaxies and discoveredthe "cosmological red-shift" .

What is the "RED-SHIFT"?The "Red-Shift" is when the lines in agalaxy's light spectrum have shifter tolonger wavelengths... nearer to the redend of the visible light spectrum.

Discovery of the Expanding UniverseThis is due to the Doppler Effect:

The waves emitted by a stationary object spread out evenly in alldirections, with the same wavelength.

However, when the object is moving, the waves in front get bunched up and their wavelength is shortened. The waves behindget stretched and the wavelength is lengthened.

The Red-Shift in the light from distant galaxies seems to be causedby them moving away from us as the universe expands. Thewavelength of light gets longer (redder). If they were approaching,we would see a blue shift in the light.

All dis tant galaxies show a red-shift.It seems to be a fact that space is expanding.

Wavesspreadingoutevenlyfromastationary

object

InFront,wavelength

shortened

LightBluer

Behind,

wavelength

lengthened

Lightredder

LightWavesSpreadingOutFromaMovingGalaxy

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In 1915,Albert Einstein had deduced his

famous equation

E = energy, m = massc = the speed of l ight = 3x108 ms -1

The equation predicts that matter andenergy are equivalent and inter-changeable.

Because the c term in the equation is a very largenumber, it follows that a very small amount of matteris equivalent to a large amount of energy

For example, during a nuclear explosion a smallamount of matter "disappears". It has been convertedinto the energy of the explosion. In the Sun, as in allstars, energy is constantly being released from the

conversion of matter to energy.

The reverse happened during the Big Bang.Originally there was only energy. The matter andmass of the universe was formed from this energy,according to Einstein's equation. Obviously it musthave taken large amounts of energy to form each tinyparticle of matter.

E = mc

How the Matter of the Universe was Formed

In the first split second of the Big Bang

explosion, all the "substance" of the universewas radiation energy. It was too hot for matter toform, or rather, any matter that formed wasinstantly torn apart again.

As the fi reball expanded, however, it cooledrapidly until particles of matter (protons,electrons & neutrons) were "condensed" from the

energy according to E=mc.

After further cooling, some protons & neutrons wereable to combine into simple atomic nuclei.

After approximately 300,000 years it became coolenough for electrons to combine with nuclei to formatoms of (mainly) hydrogen and helium, with a traceof lithium.

Theatomsformedwerenearlyallhydrogen,

withasmallamountofheliumandatrace

oflithium

p

e

e

e

p

n n

p

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keepit simplescienceThe Formation of Stars and

Galaxies

Gravity was able to attract thematter within each " lump" ofgas and cause it to col lapsein on itself. Eventually, eachseparate "lump" of matterbecame a galaxy. Further

"accretion" of " lumps" within each galaxy led to the formationof stars. Later, the debris ofexploded stars, containing

heavier elements, accreted toform solar systems like ours.

As the early universe (now made up of large

amounts of atoms) continued to expand, italso cooled further. At this time the entireuniverse may be pictured as a single, hotcloud of mostly hydrogen gas, stillexpanding as space itself grows.

Expansion of a gas causes it to cool, so thetemperature of the fireball must have fallenas the cloud expanded. Since temperature

is really a measure of the Kinetic Energy(i.e. speed) of the partic les, it fol lows thatthe KE of the atoms must have droppedalso.

Eventually, the particles became coolenough (and slow enough) for gravity tohave an effect.

If the atoms in the cloud had beenperfectly evenly distributed, thengravitational attractions would havebeen equal in every direction andcancelled out . However, it seemsthat random fluctuations within thecloud had caused a degree of "lumpiness".

Roughly 13 billionyears later, here weare on a planet, in a

solar system,orbi ting a star.

Our star is oneof billions, orbitingaround our galaxy.

Our galaxy is one ofbillions, all flyingapart from eachother as spaceitself continues

to expand.Note: we know this is true because the Cosmic Background Radiation (the afterglow ofthe Big Bang fireball) shows distinct patterns of unequal distribution.

Universeisasingularity

Particlesofmatter,thenatomsform.

Asspaceexpands,thegalaxiesgetfurtherapart.

Galaxiesform

Universeofpureenergy

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Beginning of the Universe Student Name .................................

1. The Big Bang Theory is accepted as our best explanation for the beginning ofthe Universe because it can explain certain observations. What are the 3 majorfacts about the Universe that Big Bang explains?

2.a) What is the cosmological red-shift?

b) What causes it?

3. In the early Universe there was no matter, only energy.Outline how matter is thought to have formed.

4. How can all parts of the Universe be moving away from all other parts?


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To understand the life of a star, you first need to know somebasics about the radiation of energy (e.g. light) from a hot objectsuch as a star.

Any hot object wi ll radiate energy (typical ly infra-red heat andlight ) from its surface. The hotter it gets, the more energy will beradiated. This energy wi ll be radiated at a variety of wavelengths,but for any given temperature there is a particular "peak"wavelength that dominates the emitted energy.

The graph shows the relationship.

At (relatively) low temperature, there isless energy being emitted, and the peakwavelength is longer.

At higher temperatures, there is more energy emitted and the peak wavelength gets shorter.

3. LIFE-CYCLES OF THE STARS

shorter longer

WavelengthofRadiation

veryhotobject

hotobjectpeak

wavelength

peak

wavelength

shorter

A

m

o

o

E

n

g

R

d

a

e

d

HOTBODY

RADIATION

CURVES

warm

object

peakwavelength

longer

Relationship of Temperature & Wavelength of Radiation from a Hot Object

For stars, this means

there is a relationship

between their

TEMPERATURE and

their COLOUR.

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Relatively cool stars (surface temp 3,000C) emit

radiation which peaks at long wave-lengths in the infra-red and red light part of the spectrum.

COOL STARS ARE RED

Hotter stars (our Sun's surface temp is about 5,700C)also emit a lot of inf ra-red and the whole range of

visible light, but the peak is yellow light rather than

red. (shorter wavelength)

Very hot stars (30,000C and more) have a peak emissionat the shorter wavelengths of blue light.

SpectralLines

The light spectrumfrom a star isnever a simple rainbow ofcolours. It alwayscontains many fine lines.

These lines are the fingerprints of particular typesof atoms. The spectral lines in starlight reveal whichchemical elements are present in the star. Each typeof atom absorbs or emits light at precise frequenciesto make its own unique spectral pattern.

The Study ofStar Light

You are familiar withthe way that a prismcan break whitelight up into the colours of therainbow byrefracting eachwavelength so that they separate.

A spectroscope is simply a more sophist icated version ofthe prism, and allows the intensity of each wavelength tobe measured. Measuring the peak wavelength of thespectrum of light from a star allows astronomers todetermine the stars surface temperature.

Temperature and Colour of StarsShort wavelengths of l ight are BLUE. Longer wavelengths are RED.


HOT STARS ARE BLUESome bright stars can be seen to be reddish or blue-ish to the naked eye, but generally the "peak" colourof a star can only be determined by using aSpectroscope to analyse the wavelengths of lightgathered via a telescope.

The spectrum of light from a star gives us a lot ofinformation, but the "peak" wavelength (i.e. the dominant

colour) tells astronomers the star's surface temperature.This turns out to be vitally connected to the star's life andultimate death.

whitelightisamixtureofwavelengths

different

wavelengths

spreadoutto

formaspectrum

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KCiC Physics 4 Cosmic Enginecopyright 2009 keep it simple sciencewww.keepitsimplescience.com.au



Some Definitions

"Luminosity" = amount of light energy emitted froma glowing object such as a star.

"Brightness" orIntensity = amount of light being received when

you look at it from a distance.

Obviously, how bright a star appears depends on how luminous

it is, AND how far away it is.

Example: A real ly luminous star (i.e. emitt ing a lot of light)could look quite dull (low brightness) if viewed from a hugedistance. A less luminous star could appear very bright ifviewed from close up.

Mathematically, the relationship is that the apparent brightnessor intensity (I) is inversely proportional to the SQUARE of thedistance (d) from which i t is viewed.

This relationshipwas studied inan earlier topic

(The WorldCommunicates )

I 1 or I.d2 = constantd2

(The symbol means proportional to )

One way to understand this is explained in

the diagram.

If you start with the mathematicalrelationship:

I.d2 = constant,this means that no matter how far you arefrom a star the product (brightness xdistance squared) has the same value.

Therefore, at a posit ion A , IAdA2 = k

and at another position B , IBdB2 = k

therefore, IAdA2 = IBdB

2

Lightspreadingoutfromastar

Brightness & Distance: the Inverse Square Law

x

Star

distanced

distan

c

e2d

2xSquare

Area x2

Square withsides twice aslong.

Area = 4x2

Same amountof light fallson 4 times

the area

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Now we put together the Colour-Temperature relationship,and the Brightness-Distance relationship:

The Hertzsprung-Russell (H-R) diagram is a graphical plot ofthe Luminosity of stars against Temperature. It is namedafter the 2 astronomers who independently discovered therelationship.

HertzsprungandRusselfoundthatwhentheygraphedluminosity

againstsurfacetemperaturelike

this,thevastmajorityofstarsplottedinthisshaded

zone.

cool,dull,redstars

hot,bright,bluestars

Spectral O B A F G K MClasses

Colours Blue White Yellow Red

Temp. 30,000+ 10,000 5,000 2,500(oC)

L

u

m

i

n

t

y

n

e

n

(

A

o

u

e

M

a

t

u

+15

+

10

+5

0

-5

-10

ourSun

This zone is now called theMAIN SEQUENCE

To calculate a star's luminosity, astronomers mustmeasure the apparent brightness as seen from Earth,and measure (or estimate) the star's distance from us.

The luminosity can then be calculated usingIAdA

2 = IBdB2

Luminosity is often expressed on a numerical scale of"magnitudes" as shown on the graph. Our Sun has a

magnitude of +3 on this scale.

The temperature scale is often described by "spectral class" . This uses letters toclassify stars according to the peak wavelength, and colour, being emitted. For

example, our star (the Sun) is classified as spectral class "G" . This translates to apeak wavelength of yellow light and a surface temperature about 5,700C.

Note:Tempscaledecreases totheright

The Hertzsprung-Russell Diagram

To an astronomer,

the Sun is anaverage Main

Sequence star,classified G3 onthe H-R diagram.

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Temperature & Luminosity of StarsStudent Name .................................

1. Hot objects (like stars) emit radiation.As the object gets hotter, what happens to:a) the amount of radiation given off?b) the wavelength of the peak radiation?

2. What does a Spectroscope allow Astronomers to study?

3. Outline the relationship between the temperature and colour of stars.

4.a) What is the difference between the luminosity of a star and its brightness?

b) If 2 stars had the same luminosity, but one was twice as far away, how wouldtheir brightness values compare?

5. Two scientists independently discoveredthat if star data was graphed in a certain way,most stars would be grouped in a pattern.

a) Name the 2 scientists.b) Label the axes of the H-R graph.c) Sketch the zone where most stars will lie

when their data is graphed.


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Not all stars fit into the "main sequence". Some stars haveluminosity-temperature combinations that place them

elsewhere on the H-R grid.

"Red Giants" are very large(and therefore luminosity is quite high)

but relatively cool (therefore red in colour).

"White Dwarfs" are very small(therefore luminosity is low)

but relatively hot.

Astronomers have figured out that stars go through aseries of changes during their life. Most stars spend most

of their life on the Main Sequence, but later they will rapidlychange to become Red Giants, and end their life as a

White Dwarf. The H-R diagram shows what our

Sun is likely to do in the future.

So, what causes a star to change fromone type to another during its life?

To answer that, you must understandwhere the energy of a star comes from,

and that different types of star(at di fferent phases of their life)are powered by different energy

sources.

SUN

30,000 10,000 6,000 3,000blue green yellow red

TEMPERATURE(oC)&COLOUR

L

u

m

i

n

t

y

RedGiants

White

Dwarfs

MinSequ

nce

Edge

ofagiant

star

ourSun.Diameter

1,300,000km

Dwarfstar

Onthescaleofthisdiagram,theEarthismicroscopic

Stages in the Life of a StarTheFutureEvolutionofourSun

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Nuclear FusionIf small atomic nuclei are slammed together hard enough, they may join together ("fusion") to form

one larger nucleus. When this occurs, the final nucleus is found to have slightly less massthan the original, separate nuclei a little bit of mass has "gone missing" . E = mc is at work.The missing mass has converted into energy. This is the process that powers a star.

When a star forms from the gravitational collapse of a cloud of gas(mostly hydrogen), the pressure and temperature in the core becomehigh enough to slam hydrogen nuclei together so that they undergofusion. Through a sequence of fusion reactions and other nuclearprocesses, 4 hydrogen nuclei (each is really just a proton) fuse toform one helium nucleus.

This sequence of reactions is called the Proton-Proton Chain,and is what produces the energy in a Main Sequence star like ourSun. In summary, the overall reaction is:

Energy Sources in a Main Sequence Star

4 Hydrogen Helium + Energy

4 1H14He2 + Energy

fusion

START WITH

4Hydrogennuclei (protons)

Energy

Energy

2protonsre-released

Energy

Helium-3 nuclei

FINAL PRODUCT=Helium-4 nucleus

heavy hydrogen(deuterium) nuclei

E

m

is

o

o

p

c

e

e

g

E

m

is

o

o

p

c

e

e

g

Reaction1

2moreprotons

Reaction

2

Reaction3

+ + + +

+

n

n

n

n

n

n

n+

+

+++

+

++

++

+

+ +


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Core Temperature and Star SizeA main sequence s tar like the Sun can "burn" steadily for bi ll ions of years. In the core of the Sun thetemperature is thought to be around 15 mill ion C. It would explode outwards like a huge atomic bomb

except that enormous gravitational forces hold it together.The size of any star is determined by the balance between gravity and the energy released by fusion.

Energy Source in Red Giant StarsMain Sequence stars "burn" hydrogen to helium forbill ions of years. The Sun is about 5 billion years old,and we think it will last another 5 billion years as a Main Sequence star .

Meanwhile, in the core, the amount of hydrogensteadily decreases and the amount of heliumincreases.

When the helium concentration reaches a certaincritical level, the amount of energy being producedin the core decreases rapidly. Without the outward

push of fusion energy, gravity takes over and thecore collapses inwards under its own weight. Thisgenerates immense heat from the conversion ofgravitation potential energy. Extra heat causes theouter layers above the core to expand outwards.the star may grow to thousands of times its originaldiameter.

When this happens in about 5 billion years, the Sunwill swell outwards beyond the Earth's current orbit,destroying the inner planets as it goes.


Meanwhile, down in the helium-rich core, thetemperature keeps increasing until it is hot enoughfor helium to begin fusing. Three helium nuclei, ifslammed together hard enough, will fuse to formcarbon and release even more energy.

Helium burning has begun.

3 Helium Carbon + energy3 4He2

12C6 + energy

Al though the star expands due to extra heat with in,conversely its outer layers become cooler and so its"peak" emitted wavelength is typically red light . Soit is much bigger, and is red: a RED GIANT.

Despite being cooler, its total luminosity increasesdue to its immense size. On the H-R diagram itmoves off the main sequence upwards to the right.

fusion

carbon

nucleus

energyrelease

h

u

m

n

e

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The Red Giant burns helium for a billion years or so,but gradually the fuel begins to run out and fusionslows down.

As its energy radiates away and the core cools,gravity now collapses the outer layers of the star.There may be a series of s tuttering explosions as fuelin a collapsing layer re-ignites fusion and blows partof the star away. Eventually, it shrinks down to the

size of a planet. Its density becomes immense(around 1,000kg per cm) and the atoms themselvesare compressed by gravity into " degenerate matter"...a fancy name for squashed atoms.

Because it is small, its luminosity is very low.Residual heat causes the surface temperature toreach about 10,000C so the peak wavelength is

green, but it radiates the whole range of visiblewavelengths so that the star appears white:

it is a WHITE DWARF.

Over billions of years, the star cools and eventuallydies as a "brown dwarf" . In its death it moves downto the right and completely off the H-R diagram. Italso becomes virtually invisible and undetectable toEarth-bound astronomers.

Summary: Energy Sources in StarsMain Sequence: Proton-Proton Fusion

4 Hydrogen Helium + energy

Red Giants: Helium-Burning Fusion3 Helium Carbon + energy

White Dwarfs: Residual heat only. No energybeing produced once gravitational collapse iscomplete.

Energy Source in a White Dwarf

star

death

SUN

30,000 10,000 6,000 3,000blue green yellow red

TEMPERATURE(oC) &COLOUR

RedGiants

White

Dwarfs

MainSequence

TypicalLifeofaMainSequenceStar

Luminosity

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If a star forms a lot larger than normal (e.g. more than8 times the mass of the Sun) the compression and heatgenerated in the core causes more fusion reactions tooccur than just the basic hydrogen to helium reaction.

Larger nuclei are produced by a variety of fusionreactions; carbon, oxygen, silicon and other elementsas large as iron are formed in abundance. The star islarge, hot and luminous, so on the H-R diagram these

"Blue Supergiants" are near the top left of the grid.

Because they are so hot and dense in the core, BlueSupergiants burn their fuel very quickly and so have arelatively short life span.

L

um

i

n

t

y

Red Giants

White

Dwarfs

Main Sequence

BlueSupergiantStars

Supernova: what's the story?When the core runs out of fuel and fusion ceases,

gravity causes a collapse that is truly cataclysmic!The core collapses and shrinks rapidly, and whenthe outer layers fall in onto this dense core, theyrebound in a hugely energetic explosion...

a Supernova!

The Crab Nebula is the remains of a star whichexploded as a Supernova almost 1,000 years ago.

Photo courtesy of NASA & ESA

Temperature

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A "supernova" explosion has several in terest ingconsequences:-

The star briefly flares as bright as a million starscombined.

The explosion creates all the larger atoms (bynuclear reactions) and then sprays them outwards toform a dust cloud in space. Billions of years later, thiscloud may condense to form a new star, and the

heavier elements may collect to form planets likeEarth, rich in iron, silicon, oxygen and carbon, andperhaps capable of supporting life.

Our Solar System is 2ndgeneration . The Earth is richin iron, silicon, oxygen, etc.

and has heavy elements likelead, gold and uranium. Thesecan only have been made byfusion in a star which went

supernova.

The core of the explodingstar, collapsing under gravity

and further compressed by theexplosion, may become eithera "Neutron Star" and "Pulsar",or even (if the core was large

enough) a "Black Hole".

Aftermath of a SupernovaA Neutron Staris so dense that electrons get rammedinto the protons forming a single "nucleus" ofneutrons about 20km across. This far too small to beseen at cosmic distances, but we know they're outthere because they send us signals.

The neutron star rotates and emits high frequencyradiations in a tight beam. We detect "pulses" ofradiation as the beam sweeps past us. These"Pulsars" were discovered by early radio telescopes

and, for a while, thought to be possiblecommunications from ET's.

If the core of the exploding star exceeds a certainsize, the collapse inwards goes way beyond neutron

star stage. Matter col lapses intoitself forming a "singularity" with

a near-infinite density. The gravity

field becomes so strong that evena beam of light cannot escape the

singularity. Thus it cannot beseen and any light or matter

which goes near it willdisappear into it.

(Hence "Black Hole")

Within the black hole time stopsand all the laws of physics ceaseto operate. We think that our

galaxy (and probably most others)has one or more massive black

holes near the centre.

Artistsimpressionofablackholesuckingmatterfromanearby

star.Atomsaretornapartasthey

circleintotheeventhorizon.Twistedmagneticfields

cause2jetsofmattertobeejected.

Itisthesejetsandtheirradiationwhichallowusto findablack hole

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Life of Stars Student Name .................................1.a) What is meant by nuclear fusion?

b) Write a word equation to summarise the fusion reaction that releases most ofthe energy in a main sequence star.

c) Where does the energy come from?

2. Write a word equation for the main fusion reaction in Red Giant star.

3. How is the temperature of a star related to its size?

4. Sketch a H-R graph showing the positionsof the Main Sequence, Red Giants,White Dwarfs and Blue Supergiants.

5.a) Name 2 possible stars which can result from a supernova.

b) Circle one of your answers to (a) which may also be a Pulsar.c) Outline the evidence that our Solar System formed from supernova debris.


4 ENERGY FROM THE SUN

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4. ENERGY FROM THE SUN& ITS EFFECTS ON US

Energy From the NucleusThere are basically 3 different ways that energy can be released from the nuclei of atoms .

1. Nuclear FusionWhen 2 small nuclei are slammed together

hard enough they join and become one.A small amount of mass goes missing ...

it has converted to energy according to

E = mc2.

This is the process which powers the stars.

p

p

p

p

p

n

n

p

Hydrogennuclei.(protons)

Helium

nucleus

Fusion

Energy

released

2. Nuclear FissionUnder certain condi tions, a very large nucleus

(e.g. uranium or plutonium) can break apart intosmaller nuclei. Once again, a small amount

of matter disappears ...

E = mc2

is at work again!

This is the process occurring in a nuclear reactorused to generate electrici ty in many countries.

It is also the energy source inan atomic bomb.

PhotooftheatombombexplosionontheJapanese

cityofNagasaki,1945.

Another Nuclear ProcessNext Slide

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Radiation Causes Ionisation

All 3 radioactive radiat ions can causeionisation... they can cause electrons to beknocked out of their orbit around an atom,turning the atom into an ion.

This is why radiation isdangerous to living things.Ionisation of atoms in a livingcell can disrupt membranes, cause genetic

mutations or alter the cells DNA sothat it becomes cancerous.

The massiveALPHA part iclehas the highest

ionisationability, BETA is

much lessionising and

GAMMA lessagain.

Effects of Electric & Magnetic Fields

Alpha and Beta radiat ions are part ic les and both carryelectric charges. Alpha is positive (+ve), Beta negative(-ve).

This means that both Alpha and Beta can be deflected by anelectric field and by a magnetic field. The deflection of alphacompared to beta will be opposite in both cases.

Note that Gamma rays are NOT deflected by either field, becausethey have no electric charge.

Alpha,BetaorGammaradiation

ElectricFieldbetweenchargedplates

Alpha(+ve)smalldeflectionduetolargemass

Gamma.(nocharge)nodeflection

Beta(-ve)largerdeflectionduetosmallmass.

DeflectionofRadiationsbyElectricField

MagneticField(intopage)

betweenmag.poles

Alpha(+ve)smalldeflection

Gamma.(nocharge)nodeflection

Beta(-ve)largerdeflection

DeflectionofRadiationsbyaMagneticField

Atombecomesionised

Electronknockedoutoforbit

+

-

Properties of Alpha, Beta & Gamma Radiation

Radiati

on

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Penetrating Ability

Alpha, Beta and Gamma radiation are qui te different in their abi lty to penetrate throughdifferent substances. You may have done Practical Work on this.

FIRST-HAND INVESTIGATION,that you may have done to test the penetration

of radiation through different materials.

GeigerTube.Detects

radiationbytheionisation

itcauses.

Alpha,BetaorGammasource.Eachtestedseparately.

Different

materialsplacedhere (e.g.paper,lead,aluminium)toseewhatcan

blocktheradiation.

Datasentto

electronic counting

devicetomeasuretheradiationlevels

What You Might Have DiscoveredALPHA part ic les have low penetrating abili ty .They are so likely to collide and interact with atoms in theirpath, that they usally do not penetrate far. A few centimetres inair is as far as theyll get, and a piece of paper will stop 99% ofthem.

BETA particles penetrate further than alpha.They are less likely to interact, and so penetrate further, butrarely go more than 10-20cm in air and most can be stoppedby thin metal sheets such as aluminium foil.

Properties of Alpha, Beta & Gamma Radiation

GAMMA rays are highly penetrating.They are like X-rays, only more so. Gamma cantravel many metres through air and other substances. To absorb gamma rays,several centimetres of lead or a metre of concrete are a good start.

Alpha

Beta

Gamma

Paper Aluminium Lead

foil

A ti it 5

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Nuclear Reactions Student Name .................................

1. Compare (similarities) and contrast (differences) nuclear fission & fusion.

2. List the 3 common emissions of radioactivity.

For each one, state exactly what is emitted and write a symbol for it.

3. How are the 3 radioactive radiations affected by electric and magnetic fields?

4. Imagine you had a small amount of an unknown radioactive substance and ageiger counter. Describe a simple experiment using common, at-handsubstances which could allow you to identify the type(s) of radiation beingemitted.Describe the results expected for each type of radiation.


R di ti F th S

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Every second from the corona, trillions of charged

particles (electrons and ionised atoms, especially ionisedhydrogen = protons) with enough energy to escape theSuns gravity, stream outwards into space. They exertenough force to push comet tails outwards, and affect theorbits of the smaller members of the Solar System such asasteroids.

This Solar Wind would be very dangerous to life, but the

Earths magnetic field deflects, traps and channels theparticles, so very few get through to the surface.

EMRWith a surface temperature around 5,700oC, mostof the EMR from the Sun is at the wavelengthscorresponding to visible light (with the peakbeing yellow) and infra-red (heat).

Some radiation is also at the longer wavelengthsof radio and microwaves, but most of this isabsorbed by the Earths atmosphere.

A small fract ion of the Suns EMR is at shorterwavelengths corresponding to ultra-violet (UV)rays. These could be very dangerous, butfortunately the ozone layer in the upperatmosphere absorbs most of the UV.

The Solar WindThe Suns corona is an atmosphere of hot gasextending millions of kilometres into space. It isonly visible during a solar eclipse when thebrighter face of the Sun is blotted out by theMoon.

solarwind

solarwinddeflectedbymagneticfield

Earth

Radiation From the SunThe Sun emits huge amounts of energy every second. Some is electromagnetic radiation

(EMR), but i t also gives out streams of high energy particles... the Solar Wind .

magneticfielddistortedbysolarwind

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The flow of charged particles that make up thesolar wind is not a constant stream. It fluctuates

with changes in the Suns magnetic field, whichscientists monitor by studying the sunspots.

Galileo was the first to see sunspots with histelescope... dark spots on the Suns bright

surface.

We now know that sunspots appear darkbecause they are regions that are cooler (only

4,500oC). They are associated with regionswhere the Suns magnetic field is st ronger, and

this causes more partic les to be ejected in thesolar wind.

AND, the Suns magnetic field undergoes cyclicchanges over an 11 year period. Every 11 yearsthere are more sunspots and more intensity inthe solar wind, sometimes to the extent that it

can affect our power suppl ies andcommunications.

More evidence against Ptolemys geocentric

model: Sunspots were obvious blemisheson one of the heavenly bodies which were

believed to be perfect!

Earth

Earths

magnetic

field

Theparticlesspirallingdownintothepolesalsocausethebeautifulauroradisplaysof

theNorthernLights&

SouthernLights.

Sometimes,theSolarWind

penetratesthemagneticfield

Sunspots & the Solar Wind

When sunspot activity peaks, our magnetic field can beoverwhelmed by the solar wind. Charged particlespenetrate the field and are sent into spiralling pathstowards the Earths poles. Intense pulses of EMR at radiofrequencies can result, which can cause static ,

jamming our communications, especially satel l ite

telephone links which use radio and microwaves.

Extreme pulses can causes surges in electric power linesand damage electronic equipment. In one event some 25years ago, the EMR pulse set off a surge in the power gridof the eastern USA & Canada which was so severe thatthe entire system shut down. Millions of people were leftwithout power for several days in mid-winter!

Spirallingchargedparticlesproduce

EMRpulses.

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