KISS Notes The Cosmic Engine

keep it simple science TM

www.keepitsimplescience.com.au

Emmaus Catholic College SL#802440

1

but first, let’s revise...

Preliminary Physics Topic 4

THE COSMIC ENGINEWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. THE HISTORY OF OUR UNDERSTANDING OF THE UNIVERSE

2. HOW THE UNIVERSE BEGAN (THE "BIG BANG" THEORY)3. LIFE-CYCLES OF THE STARS

4. ENERGY FROM THE SUN, & ITS EFFECTS ON US

The Structure of the UniverseThe EARTH is a PLANET. The Earth and 7 other planets(plus dwarf planets, moons, asteroids, comets, etc) are inorbit around the Sun. The SUN and all these things in orbitaround it, make up our "SOLAR SYSTEM". Everythingstays in orbit around the Sun because of gravity.

The SUN is a STAR. Energy is being produced inside it, dueto NUCLEAR REACTIONS. The Sun is one of over 100billion stars that make up our GALAXY. Each star in thenight sky is another "Sun" within our galaxy, the "MILKYWAY". Our Sun and the other stars of the Milky Way areorbiting around the galaxy’s centre because of gravity.

Beyond our galaxy are billions of other galaxies. Thedistances involved are immense and unimaginable!

We have good reason to believe that the entire Universe isEXPANDING, with the space between galaxies increasing.

THE MILKY WAYGALAXY

OTHER GALAXIES

SSuunn

Mercury

Venus

Saturn

Jupiter

Mars

THE SOLAR SYSTEM

Asteroid Belt

Earth & Moon

Preliminary Physics Topic 4copyright © 2005-2007 keep it simple science


www.keepitsimplescience.com.auPreliminary Physics Topic 4copyright © 2005-2007 keep it simple science


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CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts andimportant facts. As you proceed through the topic, come back to this page regularly to see how each bit fits thewhole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

HistoricalSummary

•Aristotle•Aristarchus•Ptolemy•Copernicus•Brahe•Kepler•Galileo•Newton Discovery:

•Friedmann•Hubble

Einstein’sE=mc22

CosmicBackgroundRadiation

Stages in a Star’sLifeImpacts &

Effects

Propertiesof Radiation

Gammaγγ

Betaββ

Alphaαα

Supernovas,Pulsars &Black Holes

Geocentric&

HeliocentricModels

Evidence of the“Red-SShift”

Energy Sourcesin Stars

HowMatter

was FormedFormation

of Stars & Galaxies

Hertzsprung-RRussellDiagram

Brightness&

DistanceInverse Square

Law

Temperature&

Colourof Stars

Radiationfrom the

Sun

TThhee CCOOSSMMIICCEENNGGIINNEE

Historyof our

Understandingof the

Universe

How the Universe Began

Life Cyclesof theStars

Energyfrom the

Sun

Big BangTheory

Radioactivity




Different Models of the Universe

First, be aware that our understanding of galaxies and thetrue extent of the Universe was only discovered within thelast 100 years. Prior to that, any theory or model of theUniverse really only dealt with our Solar System. The starswere thought to be outside the Solar System, but relativelyclose to it.

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

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1. THE HISTORY OF OUR UNDERSTANDING OF THE UNIVERSE

Heliocentric Models correctly place the Sun at thecentre 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 is

spinning, so this idea is harder to accept on the basis ofcommon sense, even though it is correct.

Only the Moon truly orbits the Earth.

Geocentric Models incorrectly place the EARTH atthe centre ("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 revolves

around the Earth once per day. Also, we cannot feelthat the Earth is spinning, so this model makes

common sense, even though it is wrong!

Historical Summary

up until about 1700 AD

Aristotle ~330 BC Geocentric Theory

Thought that:

The Sun, Moon, planets & stars are carried on invisiblecrystal spheres rotating around the Earth.

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

Aristarchus ~240BC Heliocentric Theory

Thought that:

The Sun is in the centre with everything orbiting around it.The Earth must rotate on its axis, so it appears thateverything moves around us.

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

Claudius Ptolemy ~120AD Geocentric Model with "epicycles"

Based on the best (naked eye) measurements of the time,Ptolemy developed a model which could predict the motionof planets & the times of eclipses. Although we now knowit was wrong, it was a practical, working model used for1,400 years.

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

Ptolemy’s model was accepted for such a long time that itbecame part of the belief system of the Middle Ages, andwas even adopted as the official religious explanation ofthe Universe.

So, when new ideas and new discoveries emerged around1500 AD, they were seen as dangerous and heretical, andwere punishable by torture and death.

See “Further Explanations”at the end of this section

See “Further Explanations”at the end of this section

Earth MMoooonn

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Earth



4

Nicholas Copernicus 1473 - 1543 AD

Heliocentric Theory

As measurements improved, Ptolemy's model neededmore & more adjustments and epicycles to stay accuratein its description of the heavens. It got so complicatedthat Copernicus decided there must be a simplerexplanation. He decided that perhaps Aristarchus hadbeen correct after all, and the Sun was in the centre.

Copericus’s new model still relied on crystal spheres tocarry planets and stars in circular orbits, but it wasHeliocentric... Sun centred.

The accuracy of predicted motions remained much thesame as Ptolemy’s, but this model was much simpler inits explanations.

This model was NOT immediately accepted at the time.

Galileo Galilei 1564-1642Telescope Observations

Galileo was the first to use a TELESCOPE to view theheavens. His observations conflicted with the model ofPtolemy, and supported the Heliocentric idea ofCopernicus.

He observed that the planet Jupiter has moons orbitingaround it. (Only the Earth was supposed to have things goaround it!)

He saw that the planet Venus showed phases like the Moon.(This was only explainable if Venus orbited the Sun, notEarth!)



Tycho Brahe 1546-1601

Accurate Observations

Tycho 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 fit Brahe's extremely accurate data to theCopernicus model. Finally, he found it only fitted if theorbits were ellipses, not circles.

Eventually he proposed 3 "Laws of Planetary Motion" ,but could give no explanation of how or why the Earthand planets could orbit around the Sun.

The Heliocentric idea was still NOT accepted widely.

Sir Isaac Newton 1642-1727Mathematical Theory of Gravity

Newton’s Theory of Universal Gravitation provided theexplanation for things to be “in orbit”, and did away withthe clumsy “crystal spheres” of previous models.

From his equation for Gravity, Newton could proveKepler's Laws mathematically... this proved that theHeliocentric Model was correct.

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

The Significance of Telescopes in AstronomyAll of the models, until the time of Galileo, were limited bythe lack of the TELESCOPE.

Without telescopes, all measurements and observationswere made by naked-eye, and were of limited accuracy.

If telescopes had been available earlier, then PARALLAXmight have been observed in nearby stars, and greateraccuracy would have been possible in measuring planetarypositions and movements.

This would have led to rejection of the clumsy andcomplicated "epicycles" of Ptolemy and perhaps the correctHeliocentric model would have been accepted earlier.



5

Further ExplanationsThe following may help your understanding. Itis NOT a syllabus requirement to learn it.

ParallaxParallax is the apparent movement of an objectagainst a more distant background, when viewedfrom a different angle.

Opponents of any Heliocentric modelthroughout history could argue (correctly) that ifEarth was orbiting the Sun, then the stars shouldshow some parallax movements relative to otherstars, when viewed from one part of our orbitcompared to another.

This parallax motion could not be detected bynaked eye observations, even with the mostaccurate instruments invented right up until the17th century, so heliocentric theories tended tobe rejected.

In fact, nearby stars DO show parallaxmovement, but you need a telescope to detect it,because even the nearest stars are billions ofkilometres away.

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 planetnight after night, it seems to move slowlyeastward compared to the background of stars.However, sometimes the planet moves westwardfor a while. This was called "retrograde"(backwards) motion.

To explain it, Ptolemy proposed that the planetswere carried on smaller crystal spheres (theepicycles) which rotated on the rim of the mainspheres ("deferents") surrounding the Earth.

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

The real explanation for retrograde motion is thatwe view the moving planets from a movingEarth. At certain parts of our orbit, we"overtake" other planets and so they appear tomove "backwards" for a while. Retrogrademotion is easily explained by a Heliocentricmodel, with the Earth and other planets allorbiting the Sun.



A Simple Example of Parallax:

Hold up one finger and view it with one eye against adistant tree or post. Hold the finger still whileswitching to view it with your other eye.

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

This apparent movement is called "PARALLAX"

Normal planetary “wanderings”

Retrograde motionFixed Stars

SSuunn

Earth

Earth, 6 months later

line of observation

Star beingobserved

Moredistantstars

The position of the starshould change against thebackground stars. Parallax!

Earth Deferent

Epicycle

Planet

Planet

Each planet’s main orbit is arotating glass sphere, called

the “deferent”.It revolves around the Earth.

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Worksheet 1Fill in the Blanks.Check your answers at the back.

A a)................................................. model of theUniverse places the Earth at the centre, witheverything revolving around us. The other mainmodel is called b)......................................................,which places the c)................................... at thecentre.

Aristotle proposed a d)..............................................model. This basic concept was accepted foralmost e)............................ years.

f).................................................. was the firstperson to propose a g)............................................model. His idea was not accepted because“parallax” could not be observed in the stars,which were thought to be quite close to theEarth.

Claudius h)........................................ developed amathematically accurate model which couldpredict i)......................................... and the motionsof the planets. His model wasj)....................................... and imagined all the“heavenly bodies” to be carried around thek)............................................ by crystal spheres. Hehad to add smaller spheres, calledl)............................................, in order to explain them)........................................................ motion of theplanets. This model was accurate (for the time)and so was accepted for about 1400 years.

Nicholas n)................................................ was thefirst in (relatively) modern times to propose ao)................................................ model.

Tycho Brahe’s contribution was the gathering ofp)...................................................................................He hoped it would prove Copernicus to beq)...................................

Brahe’s student r).......................................................got access to the data after Brahe died, and usedit to develop a Heliocentric model in which theplanetary orbits were s).......................................instead of circles.

Galileo was the first to make observations with at).................................. He saw that the planetJupiter has u)...................................................................................................... and that Venuswent through v)........................................ like theMoon. These observations conflicted with thew)....................................................... model, andsupported the x).......................................................model.

It was Sir y).............................................................who finally proved that thez)...................................................... model iscorrect. His mathematical theory ofaa)........................................... explained how thingscould be in ab)............................. without needingcrystal spheres. More importantly, he could provemathematically that ac)............................................’sLaws of Planetary Motion were in agreementwith gravity.

All the models developed before the time ofGalileo were limited by the available technology.Without ad)...................................., all observationswere by ae)................................................... and oflimited af).................................................... Forexample, it is impossible to measure anyag)............................................ even in nearby stars,without a telescope. Since ag)...................................could not be observed, it was logical to accept theah).......................................................... models ofAristotle and Ptolemy.

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COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

Outline of the “Big Bang” Theory

• The universe began approx. 13-15 billion years ago.

• At the beginning, all the space, matter and energy of theuniverse was concentrated in a "primeval atom" or"singularity".i.e. in one tiny point of incredible density and temperature.

• This exploded outwards in all directions, becoming coolerand less dense as it expanded very rapidly.

• This expansion is still occurring today. Galaxies aremoving further apart as the space between them expands.Within a galaxy, gravity attracts matter and holds stars andplanets together in their orbits around each other, so thereis no apparent expansion noticeable in the "local" area ofspace.

This theory seems strange and unbelievable whendescribed in simple outline, so why is it accepted as beingcorrect? Simple! ...because the theory explains manyobserved facts about the universe:-

Facts that the “Big Bang” Explains

• We believe that the Universe is expanding. The mainevidence is the "Red-Shift" of the spectral lines of distantgalaxies. Expansion is due to the original explosion.

• The "Cosmic Background Radiation". It was discoveredin 1965 that the entire Universe seems to be filled withmicrowave radiation coming from every direction. This isexplained as being the "afterglow" of radiation from soonafter the Big Bang explosion.

• The observed chemical composition of the universe(almost entirely Hydrogen and Helium) agrees withtheoretical predictions of what should have happenedduring the first seconds of the Big Bang.

Discovery of the Expanding UniverseIn 1922, the Russian Alexander Friedmann predicted thatthe universe was expanding.

His prediction arose from working on the equations ofEinstein's "General Theory of Relativity". This was a braveprediction at the time, since other galaxies beyond ours hadnot been discovered, and there was no known evidence ofexpansion.

During the 1920's new, bigger telescopes led to thediscovery of other distant galaxies. The American, EdwinHubble, analysed the spectral lines from distant galaxiesand discovered the "cosmological red-shift".

What is the "RED-SHIFT"?The "Red-Shift" is when the lines in a galaxy's lightspectrum have “stretched” to longer wavelengths (i.e. nearer to the red end of the visible light spectrum).

This is due to the Doppler Effect:The waves emitted by a stationary object spread out evenlyin all directions, with the same wavelength.

However, when the object is moving, the waves in front get“bunched up” and their wavelength is shortened. Thewaves behind get “stretched” and the wavelength islengthened.

The Red-Shift in the light from distant galaxies seems tobe caused by them moving away from us as the universeexpands. The wavelength of light gets longer (redder). Ifthey were approaching, we would see a “blue shift” in thelight.

7


2. HOW THE UNIVERSE BEGAN




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

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

Explanation of the “Red Shift”

Waves spreading outevenly from astationary object

In Front,wavelengthshortened

Light Bluer

Behind,wavelengthlengthened

Light redder

Light Waves Spreading Out From a Moving Galaxy



8

How the Matter of the Universe was FormedIn 1915, Albert Einstein had deduced his famous equation

E = energy, m = massc = the speed of light

= 3x108 ms-1

The equation predicts that matter and energy are equivalentand inter-changeable.

Because the c² term in the equation is a very large number,it follows that a very small amount of matter is equivalentto a large amount of energy

For example, during a nuclear explosion a small amount ofmatter "disappears". It has been converted into the energyof the explosion. In the Sun, as in all stars, energy isconstantly being released from the conversion of matter toenergy.

The reverse happened during the Big Bang. Originallythere was only energy. The matter and mass of theuniverse was formed from this energy, according toEinstein's equation. Obviously it must have taken largeamounts of energy to form each tiny particle of matter.

In the first split second of the Big Bang explosion, all the"substance" of the universe was radiation energy. It wastoo hot for matter to form, or rather, any matter thatformed was instantly torn apart again.

As the fireball expanded, however, it cooled rapidly untilparticles of matter (protons, electrons & neutrons) were"condensed" from the energy according to E=mc².

After further cooling, some protons & neutrons were ableto combine into simple atomic nuclei.

After approximately 300,000 years it became cool enoughfor electrons to combine with nuclei to form atoms of(mainly) hydrogen and helium, with a trace of lithium.

Formation of Stars and GalaxiesAs the early universe (now made up of large amounts ofatoms) continued to expand, it also cooled further. At thistime the entire universe may be pictured as a single, hotcloud of gas, still expanding as space itself grows.

Expansion of a gas causes it to cool, so the temperature ofthe fireball must have fallen as the cloud expanded. Sincetemperature is really a measure of the Kinetic Energy(i.e. speed) of the particles, it follows that the KE of theatoms must have dropped too.

Eventually, the particlesbecame coolenough (andslowenough)forgravityto haveaneffect.

If theatoms inthe cloudhad beenperfectly evenlydistributed, thengravitational attractionswould have been equal in every direction and cancelledout. However, it seems that random fluctuations withinthe cloud had caused a degree of "lumpiness".

Gravity was able to attract the matter within each "lump" ofgas and cause it to collapse in on itself. Eventually, eachseparate "lump" of matter became a galaxy. Further"accretion" of "lumps" within each galaxy led to theformation of stars. Later, the debris of exploded stars,containing heavier elements, accreted to form solar systemslike ours.

Roughly 13 billion years later, here we are…• on a planet, in a solar system, orbiting a star.

• our star is one of billions, orbiting around our galaxy.

• our galaxy is one of billions, all flying apart fromeach other as space itself continues to expand.



E=mc²

H He

Li

Note: we know this is true because the Cosmic Background Radiation

(the afterglow of the Big Bang fireball) shows distinct patterns of unequal distribution.

Overall expansion continues

so galaxies form

but “clumps” of mattercollapse due to gravity

The atoms formed were nearly all hydrogen,

with a small amount of heliumand a trace of lithium

Worksheet 2Fill in the blank spaces

The accepted theory for the origin of theUniverse is the a).........................................................Theory. According to this theory:

• The universe began about b)..................................years ago.• At the beginning, all the c)...................................and ........................ was concentrated in a singlepoint, or d).............................................• This e).................................... outwards,eventually forming the universe we see today,which is still f)..................................................

The Big Bang Theory is accepted because itexplains:• the g)............................................... of light fromdistant galaxies• the h).........................................................................radiation• the observed i)................................... compositionof the universe, which is about 99%j).......................................... and ............................atoms.

The idea of an expanding universe was firstproposed by Alexander k).........................................in 1922. This was based on his analysis of theequations of Einstein’s “General Theory ofl)..........................................”.

It was Edwin m)............................ who actuallydiscovered evidence of expansion. He analysedthe n).................................. lines of light fromdistant galaxies and found they wereo)...................................................................................

This p).....................-Shift” is thought to be due tothe q)................................................. Effect... thephenomenon in which ther).......................................... of waves being emittedby a s)................................... object get “bunched-up” in front of the object, andt)............................................. behind. If a galaxy ismoving fast enough, its light emitted in front of itwill appear to be u)....................................... thannormal, while light behind it will appearv)...........................................

Einstein’s famous equation, E= w)..........................predicts that x)............................... and....................................... are equivalent and inter-changeable. For example, in a nuclear reaction asmall amount of y)............................... will“disappear” because it has been converted into alarge amount of z)..........................................

In the early stages of the Big Bang, we believe theopposite occurred. Initially, the entire universewas composed of aa)..............................................As the fire-ball expanded and cooled, some ofthe aa)................................. converted intoab)........................................, in the form of thesub-atomic particles ac)...................................,.................................... and ..........................................

After futher expansion and cooling some ofthese particles were able to combine to formad)...................................... of the elementsae)............................... and ...................................,with a trace of af).................................

As the universe continued to expand, it alsoag)..............................., which means that theatoms lost some of their ah)..........................energy. Eventually, they lost enough K.E. for theforce of ai).......................................... to cause localconcentrations of matter to “clump” together.Each “clump” was caused to collapse in on itself,eventually forming aj)................................ and.........................................

So, although the universe as a whole isak)........................................., at the local levelal)............................................ is able to hold mattertogether in galaxies containing stars and solarsystems.

9









Relationship Between Temperature & Dominant Wavelength of

Radiation from a Hot ObjectTo understand the life of a star, you first need to knowsome basics about the radiation of energy (e.g. light) froma hot object such as a star.

Any hot object will radiate energy (typically infra-red heatand light) from its surface. The hotter it gets, the moreenergy will be radiated. This energy will be radiated at avariety of wavelengths, but for any given temperature thereis a particular "peak" wavelength that dominates theemitted energy.

The graph shows the relationship.

At (relatively) low temperature, there is less energy beingemitted, and the peak wavelength is longer.

At higher temperatures, there is more energy emitted andthe “peak” wavelength gets shorter.

Temperature and Colour of StarsWith light waves, wavelength (and frequency) determinescolour.

Shorter wavelengths are toward the BLUE end of thespectrum. Longer wavelengths are towards the RED endof the spectrum.

Relatively cool stars (surface temp 3,000ºC or less) emitradiation which peaks at longer wavelengths in the infra-red and red light part of the spectrum.

COOL STARS ARE RED

Hotter stars (our Sun's surface temp is about 6,000ºC) alsoemit a lot of infra-red and the whole range of visible light,but the peak is yellow light rather than red. (shorterwavelength)

Very hot stars (30,000ºC and more) have a peak emission atthe shorter wavelengths of blue light.

HOT STARS ARE BLUE

Some bright stars can be seen to be reddish or blue-ish tothe naked eye, but generally the "peak" colour of a star canonly be determined by using a Spectroscope to analyse thewavelengths of light gathered via a telescope.

The spectrum of light from a star gives us a lot ofinformation, but the "peak" wavelength (i.e. the dominantcolour) tells astronomers the star's surface temperature.This turns out to be vitally connected to the star's life andultimate death.

10


3. LIFE-CYCLES OF THE STARS

shorter longer

Wavelength of Radiation

very hotobject

hotobject“peak”

wavelength

“peak” wavelengthlonger

“peak”wavelengthshorter

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f Ene

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Radi

ated

warmobject

For stars, this means there is arelationship between their

TEMPERATURE and their COLOUR.

You are familiar with the way that a prism can break “white”light up into the “colours ofthe rainbow” by refracting each wavelength so that they separate.

A spectroscope is simply a more sophisticatedversion of the prism, and allows the intensity of eachwavelength to be measured.

Measuring the “peak” wavelength of the spectrum of lightfrom a star allows astronomers to determine the star’ssurface temperature.

There are also fine dark lines present in the spectrum whichreveal the chemical composition of the star.

Basically, everything we know about stars comes fromstudying the radiation they emit!

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11

Brightness and Distance:the Inverse Square Law

Definitions:"Luminosity"= amount of light energy being emitted froma glowing object such as a star."Brightness" or “Intensity”= amount of light beingreceived when you look at it from a distance.

Obviously, how bright a star appears depends on howluminous it is, AND how far away it is.

Example: a really luminous star (i.e. emitting a lot of light)could look quite dull (low brightness) if viewed from ahuge distance. A less luminous star could appear verybright if viewed from close up.

Mathematically, the relationship is that the apparentbrightness or intensity (I) is inversely proportional to theSQUARE of the distance (d²) from which it is viewed.

This relationship was previously studied in an earlier topic(Revise Topic 1 “The World Communicates”)

I αα 1 or I.d2 = constantd2

The “α” symbol means “proportional to”

One way to understand this is explained in the diagram.

If you start with the mathematical relationship:I.d2 = constant,

this means that no matter how far you are from a star theproduct (brightness x distance squared) has the same value.

Therefore, at position “A”, IAdA2 = k

and at position “B”, IBdB2 = k

therefore, IAdA2 = IBdB

2

TRY THE WORKSHEET at the end of this section.

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The Hertzsprung-Russell DiagramNow we put together the Colour-Temperature relationship,and the Brightness-Distance relationship:

The Hertzsprung-Russell (H-R) diagram is a graphical plotof the Luminosity of stars against Temperature.It is named after the 2 astronomers who independentlydiscovered the relationship.

Hertzsprung and Russel foundthat when they graphed

luminosity against surfacetemperature like this, the vastmajority of stars plotted in this

shaded zone.

cool, dull,red stars

hot, bright,blue stars

To an astronomer, the Sun is a prettyaverage “Main Sequence” star, classified“G3” on the H-R diagram.

LLuummiinnoouussSSttaarr

xdistance “d”

distance “2d”

2xSquare

Area x2

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Same amountof light fallson 4 timesthe area

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our Sun

This zone is now called the“MAIN SEQUENCE”

To calculate a star's luminosity, astronomersmust measure the apparent brightness as

seen from Earth, and measure (or estimate)the star's distance from us. The luminositycan then be calculated using IAdA

2 = IBdB2

Luminosity is often expressed on anumerical scale of "magnitudes" as shownon the graph. Our Sun has a magnitude

of +3 on this scale.The temperature scale is often described by "spectral class".

This uses letters to classify stars according to the peakwavelength, and colour, being emitted. For example, our

star (the Sun) is classified as spectral class "G". Thistranslates to a peak wavelength of yellow light and a

surface temperature about 5,700°C.

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12

Stages in the Life of a StarNot all stars fit into the "main sequence", however. Somestars have luminosity-temperature combinations that placethem 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 a seriesof changes during their life. Most stars spend most oftheir life on the Main Sequence, but later they will rapidlychange to become Red Giants, and end their life as a WhiteDwarf. The H-R diagram shows what our Sun is likely todo in the future, while below is a rough guide to the relativesizes of these star types. So, what causes a star to change from one type to

another during its life?

To answer that, you must understand where theenergy of a star comes from,

and that different types of star (at different phases of their life)

are powered by different energy sources.

SUN

The future evolution of our Sun

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

TTEEMMPPEERRAATTUURREE ((oCC)) && CCOOLLOOUURR

TheSUN

as it is now

this dot showsthe size of a WhiteDwarf .

The edge ofa RED GIANT

Energy Sources in a Main Sequence StarWhen a star forms from the gravitational collapse of acloud of gas (mostly hydrogen), the pressure andtemperature in the core become high enough to slamhydrogen nuclei together so that they undergo fusion.Through a sequence of fusion reactions and other nuclearprocesses, 4 hydrogen nuclei (each is really just a proton)fuse to form one helium nucleus.

This sequence of reactions is called the Proton-ProtonChain, and is what produces the energy in a Main Sequencestar like our Sun.

To keep it simple... (K.I.S.S.)...

4 Hydrogen Helium + Energy4 1H1

4He2 + energy

fusion

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 mass than the original, separate nuclei…a little bit of mass has "gone missing". E = mc² is at work.

The missing mass has converted into energy.This is the process that powers a star.

SSTTAARRTT WWIITTHH44 HHyyddrrooggeenn nnuucclleeii ((pprroottoonnss))

Energy

Energy

2 protonsre-released

Energy

Helium-3 nuclei

FFIINNAALL PPRROODDUUCCTT == HHeelliiuumm-44 nnuucclleeuuss

“heavy hydrogen”(deuterium) nuclei

Emis

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2 more protons

RReeaaccttiioonn 22

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+ + + +

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n

nn

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++

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“Main SSeeqquuenncce””



13

Energy Source in a White DwarfThe Red Giant “burns” helium for a billion years or so, butgradually the fuel runs out and fusion stops.

As its energy radiates away and the core cools, gravity nowcollapses the outer layers of the star and it shrinks rapidlydown to the size of a planet. Its density becomes immense(around 1,000kg per cm³) and the atoms themselves arecompressed by gravity into "degenerate matter".

Because it is small, its luminosity is very low. Residual heatcauses the surface temperature to reach about 10,000°C sothe “peak” wavelength is green, but it radiates the wholerange of visible wavelengths so that the star appears white:it is a WHITE DWARF.

Over billions of years, the star cools and eventually dies asa "brown dwarf". In its death it moves down to the rightand completely off the H-R diagram. It also becomesvirtually invisible and undetectable to Earth-boundastronomers.



Core Temperature and Star SizeA main sequence star like the Sun can "burn" steadily forbillions of years. In the core of the Sun the temperatureis thought to be around 15 million °C. It would explodeoutwards like a huge atomic bomb except that enormousgravitational forces hold it together.

The size of any star is determined by the balancebetween gravity and the energy released by fusion.

Energy Source in a Red Giant StarMain Sequence stars "burn" hydrogen to helium forbillions of years. For example, the Sun is about 5 billionyears old, and we think it will last another 5 billion yearsor so.

Meanwhile, in the core, the amount of hydrogen steadilydecreases and the amount of helium increases.

When the helium concentration reaches a certain criticallevel, the amount of energy being produced in the coredecreases rapidly. Without the outward push of fusionenergy, gravity takes over and the core collapses inwardsunder its own weight. This generates immense heat (byconversion of gravitation potential energy) which causesthe outer 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 Sun willswell 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 enough forhelium to begin fusing. Three helium nuclei, if slammedtogether hard enough, will fuse to form carbon andrelease even more energy.

“Helium burning” has begun.3 Helium Carbon + energy3 4He2

12C6 + energyAlthough the star expands due to extra heat within,conversely its outer layers become cooler and so its"peak" emitted wavelength is typically red light. So it ismuch bigger, and is red: a RED GIANT.

Despite being cooler, its total luminosity increases due toits immense size. On the H-R diagram it moves off themain sequence upwards to the right.

fusioncarbonnucleus

energy release3 he

lium

nuc

lei

Summary: Energy Sources in StarsMain Sequence: Proton-proton fusion reactions.

4 Hydrogen Helium + energy

Red Giants: Heat energy from gravitational collapse ofcore, followed by “Helium burning” fusion:

3 Helium Carbon + energy

White Dwarfs: Residual heat only. No energy beingproduced once gravitational collapse is complete.

stardeath

SUN

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

TTEEMMPPEERRAATTUURREE ((oCC)) && CCOOLLOOUURR

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Typical Life of a Main Sequence Star



14

Supernova: what's the story?

If a star forms much larger than normal (e.g. more than 8times the mass of the Sun) the compression and heatgenerated in the core causes more fusion reactions to occurthan just the basic hydrogen to helium reaction.

Larger nuclei are produced by a variety of fusion reactions;carbon, oxygen, silicon and other elements as large as ironare formed in abundance.The star is large, hot and luminous, so on the H-R diagramthese "Blue Supergiants" are near the top left of the grid.

Because they are so hot and dense in the core, they burntheir fuel very quickly and so have a relatively short lifespan.

When the core runs out of fuel and fusion ceases, gravitycauses a collapse that is truly cataclysmic! The corecollapses and shrinks rapidly, and when the outer layers fallin onto this dense core, they rebound in a hugely energeticexplosion...

a Supernova!

This "supernova" explosion has several interestingconsequences:-

• The star briefly flares as bright as a million starscombined.

• The explosion creates all the larger atoms (by nuclearreactions) and then sprays them outwards to form a dustcloud in space. Billions of years later, this cloud maycondense to form a new star, and the heavier elements maycollect to form planets like Earth, rich in iron, silicon,oxygen and carbon, and perhaps capable of supporting life.

Our Solar System is “2nd generation”. The Earth is rich iniron, silicon, oxygen, etc. and has heavy elements like lead,gold and uranium. These can only have been made byfusion in a star which went supernova.

• The core of the exploding star, collapsing under gravityand further compressed by the explosion, may becomeeither a "neutron star" and "pulsar", or even (if the corewas large enough) a "black hole".

A Neutron Star is so dense that electrons get rammed intothe protons forming a single "nucleus" of neutrons about20km across. This far too small to be seen at cosmicdistances, but we know they're out there:-

The neutron star rotates and emits high frequencyradiations in a tight beam. We detect "pulses" of radiationas the beam sweeps past us. These "Pulsars" werediscovered by early radio telescopes and, for a while,thought to be possible communications from ET's.

If the core of the exploding star exceeds a certain size, thecollapse inwards goes way beyond neutron star stage.Matter collapses into itself forming a "singularity" with adensity approaching infinity. The gravity field becomes sostrong that even a beam of light cannot escape thesingularity. Thus it cannot be seen and any light or matterwhich goes near it will disappear into it.

(Hence "Black Hole")

Within the black hole time stops and all thelaws of physics cease to operate.

We think that our galaxy (and probably most others)

has one or more massive black holes near the centre.



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BBlluuee SSuuppeerrggiiaanntt SSttaarrss

Photo © Laurence [email protected]

The Energy Sources and Life Cycles of Stars

can be studied further in the HSC Option Topic

“Astrophysics”

The hotter an object is, the a).................................... radiationit will emit. As well as the amount of radiation, theb)................................ of the radiation changes withtemperature too. The higher the temperature, thec)............................................. the wavelength of the “peak”radiation emitted. This means that for stars, the cooler starsare d).............................. coloured, while very hottest stars aree)................................. coloured.

Luminosity refers to the amount of f)...........................energy being g).................................. from a star, whilebrightness refers to the amount beingh).................................... by an observer some distance away.There is a relationship between the observed brightness (orintensity) of light and the distance from the source. This isthat the brightness (intensity) is proportional toi)................................................................................................

Two astonomers, j).............................. and ................................independently discovered that when thek)..................................... of a star is plotted graphicallyagainst its l).............................................., most stars are foundto lie in a narrow band of points known as the“m)............................................................................”

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It is thought that most stars spend most of their life asm)........................................................................... stars. Theirenergy source is a series of n)................................ reactionscalled the o)........................... - .................................. Chain, inwhich p).................................. nuclei (protons) fuse intoq)............................ nuclei. During the reaction, a smallamount of r).......................... is converted intos)................................... according to E= t)..................

After billions of years, the star’s core is depleted inu)....................... and rich in v)....................................... TheProton-Proton Chain cannot sustain the energy output, sothe core begins to collapse. This can result in“v)............................... -burning” fusion starting, in whichv)............................. fuses to form w)...........................Meanwhile, the outer layers of the star expand outwardsand the star becomes enormous. Its luminosityx)....................................., but the surface temperature isrelatively low, so the dominant colour is y)......................... Ithas become a z)................................................. star.

After a billion years or so, the z)....................................... staruses up all its fuel. Without internal energy, it rapidlyshrinks down to become a aa)........................................ star.This produces a lot of heat from conversion ofab).................................. potential energy, but fusion reactshave virtually ceased. It continues radiating its residualenergy, gradually cooling as it dies.

Worksheet 3 Part AFill in the blanks. Check answers at the back.

Part B Practice Problems Inverse Square Law

Note:Many problems involving the brightness-distancerelationship do not need the full calculationtreatment. They can be solved using the inversesquare idea as a ratio.

The basic idea is this:

• If distance is doubled, brightness willDECREASE by 2² (ie decrease by a factor of 4)to ¼ of original.

• If distance is tripled, brightness will decrease bya factor of 3² (ie 9 times) to one-ninth of original.

• If distance is HALVED (decreased by a factorof 2) then brightness must INCREASE by 2² = 4 times brighter.

• If you went 10 times closer, brightness mustincrease by 10² i.e. 100 times brighter.

1.By what factor would the apparent brightness ofa star change when viewed from a point 5 timesfurther away?2.When viewed from Earth, a star has a brightnessof 10 units. Where would you have to be for it'sbrightness to be 40 units?3.At distance D, a star's brightness is 32 units.What would the brightness be when viewed fromdistance 4D?4.At distance "d" from a star, its brightness is 8units. What would be its brightness at distanced/5 ?5.Two stars have the same apparent brightnesswhen viewed from Earth. However, star "X" isknown to be 3 times further away than star "Y".How do their luminosities compare?

Check your answers at the back.

Worksheet 3 (continued)

Part CInverse Square Law More Difficult Problems

Note on Units of Measurement:The "brightness" or intensity of light can bemeasured in a variety of units such as watts persquare metre (Wm-2).However, to keep this worksheet as simple aspossible, brightness values are expressed as just"units".

In keeping with the astronomical context,distances are in "Light Years (LY)"… the distancethat light can travel in one year. (1LY is about 10billion billion kilometres)

These problems require the use ofIAdA

2 = IBdB2

Example problem:When viewed from a distance of 6.00 light years,a star has a brightness of 22.5 units. How brightwill it appear from a distance of 10.0 light years?

Solution:IAdA

2 = IBdB2

IA x 102 = 22.5 x 62

IA = (22.5 x 36) / 100= 8.10 units

Try These:6.When viewed from planet A, a star's apparentbrightness is 20 units. When viewed from planetB the same star has an apparent brightness ofonly 5 units.. If planet A is 10 light years fromthe star, how far is planet B from the star? (hint: let IA=20, IB=5, dA=10, solve equation tofind dB)

7.The same star as in Q6 is viewed from planet Cwhich is 80 light years from the star. How brightwill it appear to be? (hint: let IA=20, dA= 10, dC=80, solve equationto find IC )

8.When viewed from 3.25 light years away, a star'sbrightness is 5.77 units. How bright will it bewhen viewed from 1.40 light years?

9.A star has a measured brightness of 15 unitswhen viewed from a distance of 5.5 light years.How far from the star does an observer need tobe for the apparent brightness to be 6.2 units?

10.The "Andromeda Nebula" is a faint cloud-likeobject just visible to the naked eye. With a goodtelescope, it turns out to be a whole galaxy about200 million LY (2.0 x 108LY) away. Its brightnessas seen from Earth is only 0.0045 (4.5 x 10-3)units. What would it's brightness be if you couldapproach to only 1 million LY from it?

11.The apparent brightness of a star is “I” units.You now move to a point half the originaldistance away. While your spaceship wastravelling, the star’s luminosity increased by afactor of 3. In terms of “I”, what is thebrightness of the star at your new position?

12.Two stars “A” and “B” are 12.0 LY apart. Fromthe exact mid-point between them the brightnessof star “A” is 9,000 units and star “B” is 1,000units. Staying on the line between them, wheremust you move to so that the 2 stars have thesame brightness?

16





Remember that for full marksin calculations, you need to show

FORMULA, NUMERICAL SUBSTITUTION,APPROPRIATE PRECISION and UNITS



www.keepitsimplescience.com.au17


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:

Nuclear Fusionis when 2 small nuclei are slammed together

hard enough so that 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.

RadioactivitySome atoms have an unstable nucleus and canspontaneously re-adjust themselves to a more

stable form.When they do so, excess energy and matter is

emitted in any of 3 different ways:

Nuclear Fissionis the opposite of fusion. Under certain conditions,a very large nucleus (e.g. uranium or plutonium) can

break apart forming 2 smaller nuclei and oftenseveral individual neutrons. Once again, if the

masses before and after are compared it seems asmall amount of matter has “disappeared”...

E = mc2 is at work again!

This is the process occurring in a nuclear reactorused to generate electricity in many countries. It is

also the energy source in an “atomic bomb”.

ALPHA RADIATION ((αα))is a particle ejected from a nucleus which is

simply too big.The alpha particle is made up of

2 protons and 2 neutronsand is the same as the

nucleus of a helium atom.

For that reason it is often given the symbol

GAMMA RADIATION ((γγ))involves the emission of a

high frequency waveof the electromagnetic (EMR) type.

Gamma rays often accompany Alpha orBeta emission.

BETA RADIATION ((ββ)) also involves emission of a particle...

this time an electron,ejected at high speed.

Symbol often used:

n ++

n

24 He

-

-110 e-




18

The Properties of Alpha, Beta & Gamma

Radiation Causes IonizationAll 3 radioactive radiations can cause ionization...i.e. can cause electrons to be knocked out of their orbitaround an atom, turning the atom into an ion.

This is why radiation is dangerous to living things.Ionization of atoms in a living cell can disrupt membranes,cause genetic mutations or alter the cell’s DNA so that itbecomes cancerous.

The massive ALPHA particle has the highest ionizationability, BETA is much less ionizing and GAMMA lessagain.

Effects of Electric & Magnetic FieldsAlpha and Beta radiations are particles 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 either type of field.

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

Alpha, Beta or Gammaradiation

Electric Field between charged plates

Alpha (+ve)small deflection due tolarge mass

Gamma. (no charge)no deflection

Beta (-ve)larger deflection due to small mass.

Deflection of Radiations by Electric Field

Magnetic Field (into page)

between mag. poles

Alpha (+ve)small deflection

Gamma. (no charge)no deflection

Beta (-ve)larger deflection

Deflection of Radiations by Magnetic Field

Penetrating AbilityAlpha, Beta and Gamma radiation are quite different intheir abilty to penetrate through different substances.

FIRST-HAND INVESTIGATION, that you mayhave done in class to test the penetration of radiationthrough different materials.

Geiger Tube.Detectsradiation bythe ionization itcauses.

Alpha, Beta or Gamma source.All 3 tested separately.

Different materialsplaced here (e.g. paper, lead,aluminium) to seewhat can block theradiation.

Data sent toelectronic countingdevice to measurethe radiation levels

What You Might Have Discovered & Explanations• ALPHA particles have low penetrating ability.They are so likely to collide and interact with atoms in theirpath, that they usally do not penetrate far. A fewcentimetres in air is as far as they’ll get, and a piece of paperwill stop 99% of them.

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

• GAMMA rays are highly penetrating.They are like X-rays, only more so. Gamma can travel manymetres through air and other substances. To absorb gammarays, several centimetres of lead or a metre of concrete area good start.

You may have done Practical Work in class to investigate this.

Alpha

Beta

Gamma

Paper Aluminium Leadfoil

Atom becomes ionized

Electron knockedout of orbit

+

-



19

Radiation From the SunThe Sun emits huge amounts of energy every second.Some is electromagnetic radiation (EMR), but it also givesout streams of high energy particles... the “Solar Wind”.

The Solar WindThe Sun’s corona is an “atmosphere” of hot gas extendingmillions of kilometres into space. It is only visible duringa solar eclipse when the brighter face of the Sun is blottedout by the Moon.

Every second from the corona, trillions of chargedparticles (electrons and ionized atoms, especially ionizedhydrogen = protons) with enough energy to escape theSun’s 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 theEarth’s magnetic field deflects, traps and channels theparticles, so very few get through to the surface.

Sunspots & the Solar WindThe flow of charged particles that make up the solar windis not a constant stream. It fluctuates with changes in theSun’s magnetic field, which scientists monitor by studyingthe “sunspots”.

Galileo was the first to see sunspots with his telescope...dark spots on the Sun’s bright surface.

We now know that sunspots appear dark because they areregions that are cooler (only 4,500oC). They are associatedwith regions where the Sun’s magnetic field is stronger, andthis causes more particles to be ejected in the solar wind.

AND, the Sun’s magnetic field undergoes cyclic changesover an 11 year period. Every 11 years there are moresunspots and more intensity in the solar wind, sometimesto the extent that it can affect our power supplies andcommunications.

When sunspot activity peaks, our magnetic field can beoverwhelmed by the solar wind. Charged particlespenetrate the field and are sent into spiralling paths towardsthe Earth’s poles. Intense pulses of EMR at radiofrequencies can result, which can cause “static”, jammingour communications, especially satellite telephone linkswhich 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 that theentire system shut down. Millions of people were leftwithout power for several days in mid-winter!



EMRWith a surface temperature around 5,700oC, most of

the EMR from the Sun is at the wavelengthscorresponding to visible light (with the “peak” being

yellow) and infra-red (heat).

Some radiation is also at the longer wavelengths ofradio and microwaves, but most of this is absorbed by

the Earth’s atmosphere.

A small fraction of the Sun’s EMR is at shorterwavelengths corresponding to ultra-violet (UV) rays.These could be very dangerous, but fortunately the

“ozone layer” in the upper atmosphere absorbs mostof the UV.

magnetic fielddistorted bysolar wind

solar wind

solar wind deflectedby magnetic field

More evidence against Ptolemy’s geocentric model:Sunspots were obvious “blemishes” on one of the“heavenly bodies” which were believed to be perfect!

Earth

Earth

Earth’s magnetic field

The particlesspiralling down intothe poles also cause

the beautiful“aurora” displays of

the “Northern Lights”& “Southern Lights”.

Sometimes, theSolar Wind

penetrates themagnetic field

Spiralling chargedparticles produce

EMR “pulses”.

Worksheet 4

Fill in the blanks. Check your answers atthe back.

Nuclear a)........................................... occurs when2 small nuclei are slammed together so hard thatthey b)....................................... In the process, asmall amount of c)............................ is convertedinto d).................................... Fission is when ae)....................... nucleus (such as f).........................)splits into fragments. Once again, there is aconversion of g).............................. into............................... according to E= h)....................

Another way that energy is released from anatomic i)................................. is known asj)............................................ This occurs becausesome nuclei are unstable, and can spontaneouslyre-adjust themselves to a morek)................................... form by emitting particlesand/or EMR. The 3 forms of radioactiveradiation are:

• l).........................................................,symbol = m)...........................

This involves emission of a particle consisting ofn)...................................................................................This is equivalent to the nucleus of ao).................................... atom

• Beta radiation, symbol = p)...........................This involves the emission of anq)......................................

• r)............ ......................., symbol = s)....................This is the emission of a t).............................frequency EMR wave.

All 3 types of radioactive emissions can causeionization, by knocking u)...................................out of their orbits in an atom.v)............................................... radiation has thehighest ionization ability, thenw)................................... less so, andx).............................. least of all. It is this ionizationwhich makes radioactivity dangerous to life: livingcells can be killed because theiry)........................................ are disrupted, or theirDNA can be damaged, resulting in geneticz)...................................... or the cell becomingaa)....................................................

Each radiation is different in its penetratingability:ab)..................................... is least penetrating, andmost can be stopped by a sheet ofac)..............................................Beta has ad)....................................... penetration. Itcan usually be stopped by a thin sheet ofae)...............................................The most penetrating radiation isaf)......................................... which can penetratemany metres of air and needs ag)...........................or ............................................. to stop it.

Each radiation is also affected differently byah).............................. or ............................................fields. Because ai)........................ and........................are particles carrying aj)......................................,both will be ak).......................................... by a field.They will deflect in al)..................................directions because alpha carries aam)............................... charge, while beta isan).................................. Also, ao)...............................will deflect through a greater angle thanap).................................. because aq)...............................................................................................................ar)............................................. radiation is NOTaffected by either type of field.

The Sun emits a range of EM waves, some ofwhich could be dangerous to life on Earth.Luckily, most of the dangerousas)................................ waves are absorbed by theat)......................... layer in the upper atmosphere.As well as EMR, the Sun emits streams ofau)................................................... known as theav).................................................................................This could be very dangerous too, but very littlegets to the Earth’s surface because of the Earth’saw)......................................................whichax)........................................... most of it.

“Sunspots” are darker spots on the Sun’s surfacewhich are ay)..............................................(cooler/hotter) and are areas where the Sun’saz).................................................... is more intense.The presence of sunspots results in the solarwind becoming more ba)...........................................Sunspot activity goes up and down in a cycle overbb)................. years. When sunspots are at amaximum, the solar wind can overwhelm theEarth’s bc)............................................... When thishappens, charged particles can give off bursts ofbd)............................................. which can interferwith be)............................................. and damagebf).................................................. equipment. Inextreme cases, disruption has occurred tobg)................................. supplies.

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21

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorizing the OUTLINE of a topic

helps them learn and remember the concepts and important facts.Practise on this blank version.

TThhee CCOOSSMMIICCEENNGGIINNEE

Practice QuestionsThese are not intended to be "HSC style" questions, but tochallenge your basic knowledge and understanding of thetopic, and remind you of what you NEED to know at theK.I.S.S. principle level.

When you have confidently mastered this level, it is stronglyrecommended you work on questions from past exampapers.

Part A Multiple Choice1.The astronomer who supported a heliocentric model of theuniverse was:-A. AristotleB. PtolemyC. CopernicusD. Tycho Brahe

2.The use of "epicycles" in the geocentric model of theuniverse was to explain:-A. the retrograde motion of the planets.B. the lack of observable parallax motion of stars.C. the elliptical shape of planetary orbits.D. the occurrence of eclipses and how to predict them.

3.One reason why early heliocentric theories of the universewere rejected was:-A. heliocentric models cannot explain retrograde motion of

the planets.B. heliocentric models predict parallax movement of stars,

and none could be seen.C. geocentric models were a simpler way to explain the

motion of the planets.D. geocentric models could be proven correct, once the

telescope was invented.

4.Which of the following is the correct sequence of scientificevents?A. Einstein's theories, then Hubble's observations, which

prompted Friedmann's prediction.B. Hubble's observations, followed by Friedmann's

prediction, led to Einstein's theory.C. Einstein's theory led to Friedmann's prediction, which

was confirmed by Hubble's observations.D. Friedmann's prediction was confirmed by Hubble's

observations, which led Einstein to his theory.

5.Einstein's famous equation, E=mc², means:-A. a small amount of energy is equivalent to a large amount

of mass.B. an expanding universe must cool down.C. the speed of light is constant and cannot be exceeded.D. a small amount of matter can be made from a large

quantity of energy.

6.Observational evidence supporting the idea of anexpanding universe comes mainly from:-A. the red-shift of spectral lines.B. the "Big-Bang" theory.C. the equations in Einstein's General Theory of Relativity.D. observed motions of stars moving apart in the galaxy.

7.The characteristic of the early universe which allowedgalaxies to form was:-A. its chemical composition being mostly hydrogen and

helium.B. the cosmic background radiation forming.C. "lumpiness" or uneven distribution of matter.D. gravity acting equally in all directions.

8.Which of the following statements about radiation from ahot object is correct?A. Hotter objects emit redder light at a longer wavelength.B. The cooler the object the shorter the wavelength of the

"peak" emission.C. The "peak" emission from a very hot star would be

infra-red.D. The hotter the object the shorter and "bluer" the "peak"

emission.

9.A light is viewed from 1 metre distance, and again from a 5metre distance. At 5 metres, its apparent brightness wouldbe:A. 1/5B. 5 timesC. 1/25D. 25 times

This is a simplifiedversion of theHetzsprung-Russell (H-R) diagram. Itis used forquestions 10-12

10.The vertical scale on this graph measures:A. LuminosityB. ColourC. Surface TemperatureD. Diameter

11.At which position would a star classified as a "white dwarf"be located on the H-R diagram?A. P B. Q C. R D. S

22





S

QP

R

12. Our Sun is expected to "evolve" and undergo changesin the future. Which of the following shows the expectedchanges as our aging Sun moves around the H-R diagram?A. P>Q>RB. Q>P>SC. R>Q>SD. P>S>R

13. A "Red Giant" star is characterized by having:A. high luminosity and low temperature.B. low luminosity and low temperature.C. low luminosity and high temperature.D. high luminosity and high temperature.

14. The "proton-proton" fusion reaction:A. produces hydrogen from helium in supergiant stars.B. heats a star up so it expands to become a Red Giant.C. produces large atomic nuclei during a supernova

explosion.D. produces helium from hydrogen in a Main Sequence star.

15. The main energy source in a White Dwarf star is from:A. "helium burning" fusion reactions.B. degenerate matter reactions.C. residual heat following gravitational collapse.D. nuclear fission of large nuclei.

16. Which combination of radiations would show LEASTtotal deflection if passed through a strong magnetic field?A. Alpha and beta.B. Alpha and gamma.C. Beta and gamma.D. Alpha, beta and gamma.

17. Gamma radiation exhibits properties of:A. low penetration and low ionization.B. low penetration and high ionization.C. high penetration and low ionization.D. high penetration and high ionization.

18. The diagrams show an experiment in which a single typeof radiation was passed between electrically charged platesin a vacuum. Later, the experiment was repeated with a thinpiece of paper in the path of the radiation.

18. (cont)Which line of this table correctly identifies the type ofradiation, and the electric charge on the TOP plate?

RADIATION CHARGE ON TOPPLATE

A. Beta +veB. Gamma -veC. Alpha +veD. Beta -ve

19. The "Solar Wind" is best described as:A. electromagnetic radiation emitted by the Sun.B. a stream of air blowing from the Sun.C. the outer "atmosphere" or corona of the Earth.D. stream of charged particles ejected from Sun.

20. The "Ozone Layer" of the Earth is very effective inblocking which radiation from the Sun?A. Radio wavesB. Ultra-violet wavesC. Solar WindD. Light waves

Part B Longer Response Questions.Mark values given are suggestions only, and give you an ideaof how detailed an answer is appropriate.

21. (2 marks) Contrast a geocentric model of the universe with aheliocentric model.

22. (3 marks) Discuss how the historical development of models of theuniverse was limited by the technology available. In youranswer, refer to one specific model, naming the personresponsible for it.

23. (5 marks) Place the following men in chronological order, and statewhether each supported a heliocentric or geocentric modelof the universe.Kepler, Ptolemy, Aristotle, Copernicus, Aristarchus

24. (3 marks) About 1930, Edwin Hubble used a new, large telescope todiscover the "Red-Shift".a) What is the "red-shift"?b) Explain how the red-shift gives evidence of anexpanding universe.c) How is expansion of the universe explained by the "Big-Bang" theory?

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radiation

Detector screen

radiationdetected here

Electricallycharged plates

radiation

Electricallycharged plates

LLaatteerr,

NO RADIATIONDETECTED

Paper inserted here

25. (5 marks)At the instant of the “Big Bang” all the substance of theuniverse is thought to have been in the form of radiantenergy.a) Where did all the matter (e.g. atoms) come from?Explain with reference to Einstein's equation.b) Which 2 types of atoms are thought to have been mainlyformed in the early universe?c) The "Big Bang" is all about expansion. How then didmatter get together to form galaxies and stars? Explain.

26. (2 marks) This graph shows the distribution of radiation emitted byan object at 5,000oC.

On the graph sketch clearly the curve you would expect ifthe object was at 2,000oC.

27. (3 marks) If a star has an apparent brightness of 37.5 units whenviewed from a distance of 22 LY, at what distance would anobserver see its brightness to be 16.3 units?

28. (7 marks) Use the graph provided to sketch the Hertzsprung-Russelldiagram, showing clearly:

a) what property is measured on each axis.b) the approximate positions of main sequence, red giant,white dwarf and blue supergiant stars. (label clearly)c) the approximate position of our Sun.

29. (2 mks) Write a simple equation to describe the reaction thatproduces energy in a typical main sequence star.

30. (5 marks) Complete the table to describe the properties of alpha, betaand gamma radiation.

RADIATION WHAT IT IS IONIZATION PENETRATIONTYPE ABILITY ABILITY

Alpha (a) high (b)Beta electron (c) mediumGamma (d) v. low (e)

31. (7 marks) Using a "geiger counter" a scientist detected a source ofradioactivity coming from a mineral sample. When placed 2mm in air from the sample the counter registered 1,687ionization events in 1 minute.

Next she placed a piece of aluminum foil in the 2 mmspace between sample and Geiger tube. The counterregistered 802 events/min.

When the aluminium foil was replaced with a piece ofpaper the count was 1,538 ionizations/min.

With a piece of lead foil in the gap, the result was 366ionizations/min.

From these results she was able to deduce what type(s) ofradio-active emissions were coming from the sample.

a) State what type(s) of radiation(s) were coming from thesample.b) Explain your reasoning.c) Account for the result obtained with a piece of papershielding the sample.d) Account for the result obtained with the lead foil.e) Why was it important to keep the Geiger tube always 2 mm from the sample?

32. (3 marks) Radiation from the Sun includes high energy chargedparticles, radio waves and ultra violet waves. Each of theseis partially or totally absorbed, blocked or deflected beforeit reaches the Earth's surface.By what?State specifically what EACH of these radiations is blockedor deflected by.

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Wavelength of Radiation

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KISS Notes The Cosmic Engine

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