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keep it simple science TM

www.keepitsimplescience.com.au St Johns Park High School SL#802444

1

but first, let’s revise...

Preliminary Physics Topic 4

THE COSMIC ENGINE

What 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 Universe

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

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

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

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

THE MILKY WAY

GALAXY

OTHER GALAXIES

SSuunn Mercury Venus Saturn Jupiter Mars

THE SOLAR SYSTEM

Asteroid Belt

Earth & Moon

Preliminary Physics Topic 4

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Preliminary Physics Topic 4

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2

CONCEPT DIAGRAM (“Mind Map”) OF TOPIC

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

Historical Summary •Aristotle •Aristarchus •Ptolemy •Copernicus •Brahe •Kepler •Galileo •Newton Discovery: •Friedmann •Hubble Einstein’s E=mc22 Cosmic Background Radiation Stages in a Star’s Life Impacts & Effects Properties of Radiation Gamma

γγ

Beta

ββ

Alpha

αα

Supernovas, Pulsars & Black Holes Geocentric & Heliocentric Models Evidence of the “Red-SShift” Energy Sources in Stars How Matter was Formed Formation of Stars & Galaxies

Hertzsprung-RRussell Diagram Brightness & Distance Inverse Square Law Temperature & Colour of Stars Radiation from the Sun

TThhee

CCO

OSSM

MIICC

EEN

NGGIIN

NEE

History of our Understanding of the Universe How the Universe Began Life Cycles of the Stars Energy from the Sun Big Bang Theory Radioactivity

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Preliminary Physics Topic 4

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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 discovered within the last 100 years. Prior to that, any theory or model of the Universe really only dealt with our Solar System. The stars were thought to be outside the Solar System, but relatively close to it.

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

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1. THE HISTORY OF OUR UNDERSTANDING OF THE UNIVERSE

Heliocentric Models correctly place the Sun at the

centre of the Solar System. ("Helios" = Sun)

Heliocentric models require that the Earth rotates on its axis so that everything in the sky appears to go around us. However, we can't feel that the Earth is spinning, 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 incorrectly place the EARTH at

the 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 feel

that 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 invisible crystal 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 that everything moves around us.

This idea was not accepted because "parallax" could not be detected 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 motion of planets & the times of eclipses. Although we now know it was wrong, it was a practical, working model used for 1,400 years.

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

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

So, when new ideas and new discoveries emerged around 1500 AD, they were seen as dangerous and heretical, and were 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

SSuunn

PPllaanneettss FFiixxeedd SSttaarrss

M Moooonn SSuunn PPllaanneettss FFiixxeedd SSttaarrss Earth

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4

Nicholas Copernicus

1473 - 1543 AD

Heliocentric Theory

As measurements improved, Ptolemy's model needed more & more adjustments and epicycles to stay accurate in its description of the heavens. It got so complicated that Copernicus decided there must be a simpler explanation. He decided that perhaps Aristarchus had been correct after all, and the Sun was in the centre. Copericus’s new model still relied on crystal spheres to carry planets and stars in circular orbits, but it was Heliocentric... Sun centred.

The accuracy of predicted motions remained much the same as Ptolemy’s, but this model was much simpler in its explanations.

This model was NOT immediately accepted at the time.

Galileo Galilei

1564-1642

Telescope Observations

Galileo was the first to use a TELESCOPE to view the heavens. His observations conflicted with the model of Ptolemy, and supported the Heliocentric idea of Copernicus.

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

He saw that the planet Venus showed phases like the Moon. (This was only explainable if Venus orbited the Sun, not Earth!)

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Tycho Brahe

1546-1601

Accurate Observations

Tycho used the most advanced observatory of that time to gather outstandingly accurate data (accurate for naked eye measurement) of planetary movements. He favoured the geocentric model and hoped his observations would prove Copernicus wrong.

He jealously guarded his data from others, but when he died 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 the Copernicus model. Finally, he found it only fitted if the orbits were ellipses, not circles.

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

The Heliocentric idea was still NOT accepted widely.

Sir Isaac Newton

1642-1727

Mathematical Theory of Gravity

Newton’s Theory of Universal Gravitation provided the explanation for things to be “in orbit”, and did away with the clumsy “crystal spheres” of previous models. From his equation for Gravity, Newton could prove Kepler's Laws mathematically... this proved that the Heliocentric Model was correct.

Since the time of Newton, the Heliocentric model has been accepted as the scientifically correct description of the Universe, but it took another 200 years to discover

the full story of stars, galaxies and distances.

The Significance of Telescopes in Astronomy

All of the models, until the time of Galileo, were limited by the lack of the TELESCOPE.

Without telescopes, all measurements and observations were made by naked-eye, and were of limited accuracy. If telescopes had been available earlier, then PARALLAX might have been observed in nearby stars, and greater accuracy would have been possible in measuring planetary positions and movements.

This would have led to rejection of the clumsy and complicated "epicycles" of Ptolemy and perhaps the correct Heliocentric model would have been accepted earlier.

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Preliminary Physics Topic 4

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5

Further Explanations

The following may help your understanding. It

is NOT a syllabus requirement to learn it.

Parallax

Parallax is the apparent movement of an object

against a more distant background, when viewed

from a different angle.

Opponents of

any Heliocentric model

throughout history could argue (correctly) that if

Earth was orbiting the Sun, then the stars should

show some parallax movements relative to other

stars, when viewed from one part of our orbit

compared to another.

This parallax motion could not be detected by

naked eye observations, even with the most

accurate instruments invented right up until the

17th century, so heliocentric theories tended to

be rejected.

In fact, nearby stars DO show parallax

movement, but you need a telescope to detect it,

because even the nearest stars are billions of

kilometres away.

Retrograde Motion & Epicycles

Epicycles were a device invented by Ptolemy to

explain the "retrograde" motion of the planets.

Firstly you must know that, while the stars always

appear in exactly the same relative positions every

night, the planets do not.

("Planet" means

"wanderer" in Greek.) If you observe a planet

night after 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 it, Ptolemy proposed that the planets

were carried on smaller crystal spheres (the

epicycles) which rotated on the rim of the main

spheres ("deferents") surrounding the Earth.

This "wheels-on-wheels" idea was able to explain

retrograde motion adequately, if rather clumsily.

The real explanation for retrograde motion is that

we view the moving planets from a moving

Earth.

At certain parts of our orbit, we

"overtake" other planets and so they appear to

move "backwards" for a while.

Retrograde

motion is easily explained by a Heliocentric

model, with the Earth and other planets all

orbiting the Sun.

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A Simple Example of Parallax:

Hold up one finger and view it with 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 to the distant "landmark".

This apparent movement is called "PARALLAX"

Normal planetary “wanderings”

Retrograde motion Fixed Stars SSuunn Earth Earth, 6 months later line of observation Star being observed More distant stars The position of the star

should change against the background stars. Parallax!

Earth Deferent

Epicycle Planet

Planet

Each planet’s main orbit is a rotating glass sphere, called

the “deferent”. It revolves around the Earth.

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tthhee ““eeppiiccyyccllee””,, w

whhiicchh rroottaatteess oonn tthhee ddeeffeerreenntt..

(6)

Worksheet 1

Fill in the Blanks.

Check your answers at the back.

A a)... model of the

Universe places the Earth at the centre, with

everything revolving around us. The other main

model is called b)...,

which places the c)... at the

centre.

Aristotle proposed a d)...

model. This basic concept was accepted for

almost e)... years.

f)... was the first

person 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 the

Earth.

Claudius h)... developed a

mathematically accurate model which could

predict i)... and the motions

of

the planets.

His model was

j)... and imagined all the

“heavenly bodies” to be carried around the

k)... by crystal spheres. He

had to add smaller spheres,

called

l)..., in order to explain the

m)... motion of the

planets. This model was accurate (for the time)

and so was accepted for about 1400 years.

Nicholas n)... was the

first in (relatively) modern times to propose a

o)... model.

Tycho Brahe’s contribution was the gathering of

p)...

He hoped it would prove Copernicus to be

q)...

Brahe’s student r)...

got access to the data after Brahe died, and used

it to develop a Heliocentric model in which the

planetary orbits were s)...

instead of circles.

Galileo was the first to make observations with a

t)... He saw that the planet

Jupiter has u)...

... and that Venus

went through v)... like the

Moon. These observations conflicted with the

w)... model, and

supported the x)...

model.

It was Sir y)...

who finally proved that the

z)...

model is

correct.

His mathematical theory of

aa)... explained how things

could be in ab)... without needing

crystal spheres. More importantly, he could prove

mathematically that ac)...’s

Laws of Planetary Motion were in agreement

with gravity.

All the models developed before the time of

Galileo were limited by the available technology.

Without ad)..., all observations

were by ae)... and of

limited af)...

For

example, it is impossible to measure any

ag)... even in nearby stars,

without a telescope. Since ag)...

could not be observed, it was logical to accept the

ah)... models of

Aristotle and Ptolemy.

6

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Preliminary Physics Topic 4

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COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

(7)

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 the universe 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 cooler and less dense as it expanded very rapidly.

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

This theory seems strange and unbelievable when described in simple outline, so why is it accepted as being correct? Simple! ...because the theory explains many observed facts about the

universe:-Facts that the “Big Bang” Explains

• We believe that the Universe is expanding. The main evidence is the "Red-Shift" of the spectral lines of distant galaxies. Expansion is due to the original explosion.

• The "Cosmic Background Radiation". It was discovered in 1965 that the entire Universe seems to be filled with microwave radiation coming from every direction. This is explained as being the "afterglow" of radiation from soon after the Big Bang explosion.

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

Discovery of the Expanding Universe

In 1922, the Russian Alexander Friedmann predicted that the universe was expanding.

His prediction arose from working on the equations of Einstein's "General Theory of Relativity". This was a brave prediction at the time, since other galaxies beyond ours had not been discovered, and there was no known evidence of expansion.

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

What is the "RED-SHIFT"?

The "Red-Shift" is when the lines in a galaxy's light spectrum 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 evenly in all directions, with the same wavelength.

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

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

7

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2. HOW THE UNIVERSE BEGAN

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Preliminary Physics Topic 4

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Note: You must NOT think of this as if the matter exploded 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 out evenly from a stationary object In Front, wavelength shortened Light Bluer Behind, wavelength lengthened Light redder

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8

How the Matter of the Universe was Formed

In 1915, Albert Einstein had deduced his famous equation

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

= 3x108ms-1

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

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

For example, during a nuclear explosion a small amount of matter "disappears". It has been converted into the energy of the explosion. In the Sun, as in all stars, 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 and mass of the universe was formed from this energy, according to Einstein's equation. Obviously it must have taken large amounts 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 was too hot for matter to form, or rather, any matter that formed was instantly torn apart again.

As the fireball expanded, however, it cooled rapidly until particles of matter (protons, electrons & neutrons) were "condensed" from the energy according to E=mc². After further cooling, some protons & neutrons were able to combine into simple atomic nuclei.

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

Formation of Stars and Galaxies

As the early universe (now made up of large amounts of atoms) continued to expand, it also cooled further. At this time the entire universe may be pictured as a single, hot cloud of gas, still expanding as space itself grows.

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

Eventually, the particles became cool enough (and slow enough) for gravity to have an effect. If the atoms in the cloud had been perfectly evenly distributed, then gravitational attractions

would have been equal in every direction and cancelled out. However, it seems that random fluctuations within the cloud had caused a degree of "lumpiness".

Gravity was able to attract the matter within each "lump" of gas and cause it to collapse in on itself. Eventually, each separate "lump" of matter became a galaxy. Further "accretion" of "lumps" within each galaxy led to the formation of stars. Later, the debris of exploded stars, containing heavier elements, accreted to form solar systems like 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 from

each other as space itself continues to expand.

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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 matter

collapse due to gravity

The atoms formed were nearly all hydrogen, with a small amount of helium

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Worksheet 2

Fill in the blank spaces

The accepted theory for the origin of the

Universe 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 single

point, or d)...

• This e)...

outwards,

eventually forming the universe we see today,

which is still f)...

The Big Bang Theory is accepted because it

explains:

• the g)... of light from

distant galaxies

• the h)...

radiation

• the observed i)... composition

of

the universe, which is about 99%

j)... and ...

atoms.

The idea of an expanding universe was first

proposed by Alexander k)...

in 1922. This was based on his analysis of the

equations of Einstein’s “General Theory of

l)...”.

It was Edwin m)... who actually

discovered evidence of expansion. He analysed

the n)... lines of light from

distant galaxies and found they were

o)...

This p)...-Shift” is thought to be due to

the q)... Effect... the

phenomenon in which the

r)... of waves being emitted

by a s)... object get

“bunched-up”

in front of

the object,

and

t)... behind. If a galaxy is

moving fast enough, its light emitted in front of it

will appear to be u)... than

normal, while light behind it will appear

v)...

Einstein’s famous equation, E= w)...

predicts that x)...

and

... are equivalent and

inter-changeable. For example, in a nuclear reaction a

small amount of y)... will

“disappear” because it has been converted into a

large amount of z)...

In the early stages of the Big Bang, we believe the

opposite occurred. Initially, the entire universe

was composed of aa)...

As the fire-ball expanded and cooled, some of

the aa)... converted into

ab)..., in the form of the

sub-atomic particles ac)...,

... and ...

After futher expansion and cooling some of

these particles were able to combine to form

ad)... of

the elements

ae)... and ...,

with a trace of af)...

As the universe continued to expand, it also

ag)..., which means that the

atoms lost some of their ah)...

energy. Eventually, they lost enough K.E. for the

force of ai)... to cause local

concentrations 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 is

ak)..., at the local level

al)... is able to hold matter

together in galaxies containing stars and solar

systems.

9

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Preliminary Physics Topic 4

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Relationship Between Temperature

& Dominant Wavelength of

Radiation from a Hot Object

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

Any hot object will radiate energy (typically infra-red heat and light) from its surface. The hotter it gets, the more energy will be radiated. This energy will 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 is less energy being emitted, and the peak wavelength is longer.

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

Temperature and Colour of Stars

With light waves, wavelength (and frequency) determines colour.

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

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

COOL STARS ARE RED

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

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

HOT STARS ARE BLUE

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

The spectrum of light from a star gives us a lot of information, 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 and ultimate death.

10

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3. LIFE-CYCLES OF THE STARS

shorter longer Wavelength of Radiation very hot object hot object “peak” wavelength “peak” wavelength longer “peak” wavelength shorter Am ou nt o f E ne rg y Ra di at ed warm object

For stars, this means there is a

relationship between their

TEMPERATURE and their COLOUR.

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

A spectroscope is simply a more sophisticated version of the prism, and allows the intensity of each wavelength to be measured.

Measuring the “peak” wavelength of the spectrum of light from a star allows astronomers to determine the star’s surface temperature.

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

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

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wavelengths wavelengthsdifferent spread out to form a spectrum

(11)

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11

Brightness and Distance:

the Inverse Square Law

Definitions:

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

"Brightness" or “Intensity”= 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 really luminous star (i.e. emitting a lot of light)

could look quite dull (low brightness) if viewed from a huge distance. A less luminous star could appear very bright if viewed from close up.

Mathematically, the relationship is that the apparent brightness or intensity (I) is inversely proportional to the SQUARE 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.d

2

= constant

d

2

The “α” symbol means “proportional to”

One way to understand this is explained in the diagram.

If you start with the mathematical relationship:

I.d

2

= constant,

this means that no matter how far you are from a star the product (brightness x distance squared) has the same value. Therefore, at position “A”,

I

A

d

A2

= k

and at position “B”,

I

B

d

B2

= k

therefore,

I

A

d

A2

= I

B

d

B2

TRY THE WORKSHEET at the end of this section. LLiigghhtt sspprreeaaddiinngg oouutt ffrroomm aa ssttaarr

The Hertzsprung-Russell Diagram

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

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

Hertzsprung and Russel found that when they graphed luminosity against surface temperature like this, the vast majority of stars plotted in this

shaded zone.

cool, dull, red stars hot, bright,

blue stars

To an astronomer, the Sun is a pretty

average “Main Sequence” star, classified

“G3” on the H-R diagram.

LLuummiinnoouuss SSttaarr x distance “d” distance “2d” 2x Square Area x2 Square with sides twice as long. Area = 4x2 Same amount of light falls on 4 times the area SSppeeccttrraall O B A F G K M CCllaasssseess

CCoolloouurrss Blue White Yellow Red TTeemmpp.. 30,000+ 10,000 5,000 2,500 (oC) Lu m in os ity in cr ea si ng (Absolute Magnitudes) +15 +10 +5 0 -5 -10 our Sun

This zone is now called the “MAIN SEQUENCE”

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

the star's distance from us. The luminosity can then be calculated using IAdA2 = I

BdB2

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 to classify 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 a peak wavelength of yellow light and a

surface temperature about 5,700°C.

N

(12)

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12

Stages in the Life of a Star

Not all stars fit into the "main sequence", however. Some stars have luminosity-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 a series of changes during their life. Most stars spend most of their life on the Main Sequence, but later they will rapidly change 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, while below is a rough guide to the relative

sizes 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 the energy 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 3,000 blue green yellow red

TTEEMMPPEERRAATTUURREE ((oCC)) && CCOOLLOOUURR

The SUN as it is

now

this dot shows the size of a White Dwarf . The edge of

a RED GIANT

Energy Sources in a Main Sequence Star

When a star forms from the gravitational collapse of a cloud of gas (mostly hydrogen), the pressure and temperature in the core become high enough to slam hydrogen nuclei together so that they undergo fusion. Through a sequence of fusion reactions and other nuclear processes, 4 hydrogen nuclei (each is really just a proton) fuse to form 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 our Sun.

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

4 Hydrogen

Helium + Energy

4

1

H

1 4

He

2

+ energy

fusion

Nuclear Fusion

If 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 WWIITTHH 44 HHyyddrrooggeenn nnuucclleeii ((pprroottoonnss))

Energy Energy 2 protons re-released Energy Helium-3 nuclei

FFIINNAALL PPRROODDUUCCTT == HHeelliiuumm-44 nnuucclleeuuss “heavy hydrogen”

(deuterium) nuclei

Emission of particles & energy Emission of particles & energy

RReeaaccttiioonn 11 2 more protons RReeaaccttiioonn 22 RReeaaccttiioonn 33 + + + + + n n n n n n n n + + + + + + + + + + + + + Lu m in os ity RReedd GGiiaannttss Whitee Dwaarfs “Main SSeeqquuenncce””

(13)

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Energy Source in a White Dwarf

The Red Giant “burns” helium for a billion years or so, but gradually the fuel runs out and fusion stops.

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

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

Over billions of years, the star cools and eventually dies as a "brown dwarf". In its death it moves down to the right and completely off the H-R diagram. It also becomes virtually invisible and undetectable to Earth-bound astronomers.

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Core Temperature and Star Size

A main sequence star like the Sun can "burn" steadily for billions of years. In the core of the Sun the temperature is thought to be around 15 million °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 a Red Giant Star

Main Sequence stars "burn" hydrogen to helium for billions of years. For example, the Sun is about 5 billion years old, and we think it will last another 5 billion years or so.

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

When the helium concentration reaches a certain critical level, the amount of energy being produced in the core decreases rapidly. Without the outward push of fusion energy, gravity takes over and the core collapses inwards under its own weight. This generates immense heat (by conversion of gravitation potential energy) which causes the outer layers above the core to expand outwards…. the star may grow to thousands of times its original diameter.

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

Meanwhile, down in the helium-rich core, the temperature keeps increasing until it is hot enough for helium to begin fusing. Three helium nuclei, if slammed together hard enough, will fuse to form carbon and release even more energy.

“Helium burning” has begun.

3 Helium Carbon + energy

3 4He

2 12C6 + energy

Although 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 is much bigger, and is red: a RED GIANT.

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

fusion carbonnucleus

energy release

3 helium nuclei

Summary: Energy Sources in Stars

Main Sequence: Proton-proton fusion reactions.

4 Hydrogen Helium + energy

Red Giants: Heat energy from gravitational collapse of

core, followed by “Helium burning” fusion: 3 Helium Carbon + energy

White Dwarfs: Residual heat only. No energy being

produced once gravitational collapse is complete.

star death SUN

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

TTEEMMPPEERRAATTUURREE ((oCC)) && CCOOLLOOUURR

Lu m in os ity RReedd GGiiaannttss Whitee Dwaarfs “Main SSeeqquuenncce””

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Supernova: what's the story?

If a star forms much larger than normal (e.g. more than 8 times the mass of the Sun) the compression and heat generated in the core causes more fusion reactions to occur than 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 iron are formed in abundance.

The star is large, 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, they burn their fuel very quickly and so have a relatively short life span.

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 when the outer layers fall in onto this dense core, they rebound in a hugely energetic explosion...

a Supernova!

This "supernova" explosion has several interesting

consequences:-• The star briefly flares as bright as a million stars combined.

• The explosion creates all the larger atoms (by nuclear reactions) and then sprays them outwards to form a dust cloud in space. Billions of years later, this cloud may condense to form a new star, and the heavier elements may collect 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 in iron, silicon, oxygen, etc. and has heavy elements like lead, gold and uranium. These can only have been made by fusion in a star which went supernova.

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

A Neutron Star is so dense that electrons get rammed into the protons forming a single "nucleus" of neutrons about 20km across. This far too small to be seen at cosmic distances, but we know they're out

there:-The neutron star rotates and emits high frequency radiations in a tight beam. We detect "pulses" of radiation as the beam sweeps past us. These "Pulsars" were discovered 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, the collapse inwards goes way beyond neutron star stage. Matter collapses into itself forming a "singularity" with a density approaching infinity. The gravity field becomes so strong that even a beam of light cannot escape the singularity. Thus it cannot be seen and any light or matter which goes near it will disappear into it.

(Hence "Black Hole") Within the black hole time stops and all the laws 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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Lu m in os ity RReedd GGiiaannttss Whitee Dwaarfs “MainSSeeqquuenn cce””

BBlluuee SSuuppeerrggiiaanntt SSttaarrss

Photo © Laurence Diver laurence.diver@gmail.com

The Energy Sources and Life Cycles of Stars can be studied further in the HSC Option Topic

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The hotter an object is, the a)... radiation it will emit. As well as the amount of radiation, the b)... of the radiation changes with temperature too. The higher the temperature, the c)... the wavelength of the “peak” radiation emitted. This means that for stars, the cooler stars are d)... coloured, while very hottest stars are e)... coloured.

Luminosity refers to the amount of f)... energy being g)... from a star, while brightness refers to the amount being h)... by an observer some distance away. There is a relationship between the observed brightness (or intensity) of light and the distance from the source. This is that the brightness (intensity) is proportional to i)... Two astonomers, j)... and ... independently discovered that when the k)... of a star is plotted graphically against its l)..., most stars are found to 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 as m)... stars. Their energy source is a series of n)... reactions called the o)... - ... Chain, in which p)... nuclei (protons) fuse into q)... nuclei. During the reaction, a small amount of r)... is converted into s)... according to E= t)... After billions of years, the star’s core is depleted in u)... and rich in v)... The Proton-Proton Chain cannot sustain the energy output, so the core begins to collapse. This can result in “v)... -burning” fusion starting, in which v)... fuses to form w)... Meanwhile, the outer layers of the star expand outwards and the star becomes enormous. Its luminosity x)..., but the surface temperature is relatively low, so the dominant colour is y)... It has become a z)... star.

After a billion years or so, the z)... star uses up all its fuel. Without internal energy, it rapidly shrinks down to become a aa)... star. This produces a lot of heat from conversion of ab)... potential energy, but fusion reacts have virtually ceased. It continues radiating its residual energy, gradually cooling as it dies.

Worksheet 3 Part A

Fill in the blanks. Check answers at the back.

Part B Practice Problems

Inverse Square Law

Note:

Many problems involving the brightness-distance

relationship do not need the full calculation

treatment. They can be solved using the inverse

square idea as a ratio.

The basic idea is this:

• If distance is doubled, brightness will

DECREASE by 2² (ie decrease by a factor of 4)

to ¼ of original.

• If distance is tripled, brightness will decrease by

a factor of 3² (ie 9 times) to one-ninth of original.

• If distance is HALVED (decreased by a factor

of 2) then brightness must INCREASE by

2² = 4 times brighter.

• If you went 10 times closer, brightness must

increase by 10² i.e. 100 times brighter.

1.

By what factor would the apparent brightness of

a star change when viewed from a point 5 times

further away?

2.

When viewed from Earth, a star has a brightness

of 10 units. Where would you have to be for it's

brightness to be 40 units?

3.

At distance D, a star's brightness is 32 units.

What would the brightness be when viewed from

distance 4D?

4.

At distance "d" from a star, its brightness is 8

units. What would be its brightness at distance

d/5 ?

5.

Two stars have the same apparent brightness

when viewed from Earth. However, star "X" is

known to be 3 times further away than star "Y".

How do their luminosities compare?

(16)

Worksheet 3 (continued)

Part C

Inverse Square Law

More Difficult Problems

Note on Units of Measurement:

The "brightness" or intensity of light can be

measured in a variety of units such as watts per

square metre (Wm

-2

).

However, to keep this worksheet as simple as

possible, brightness values are expressed as just

"units".

In keeping with the astronomical context,

distances are in "Light Years (LY)"… the distance

that light can travel in one year. (1LY is about 10

billion billion kilometres)

These problems require the use of

I

A

d

A2

= I

B

d

B2

Example problem:

When viewed from a distance of 6.00 light years,

a star has a brightness of 22.5 units. How bright

will it appear from a distance of 10.0 light years?

Solution:

I

A

d

A2

= I

B

d

B2

I

A

x 10

2

= 22.5 x 6

2

I

A

= (22.5 x 36) / 100

= 8.10 units

Try These:

6.

When viewed from planet A, a star's apparent

brightness is 20 units. When viewed from planet

B the same star has an apparent brightness of

only 5 units.. If planet A is 10 light years from

the star, how far is planet B from the star?

(hint: let I

A

=20, I

B

=5, d

A

=10, solve equation to

find d

B

)

7.

The same star as in Q6 is viewed from planet C

which is 80 light years from the star. How bright

will it appear to be?

(hint: let I

A

=20, d

A

= 10, d

C

=80, solve equation

to find I

C

)

8.

When viewed from 3.25 light years away, a star's

brightness is 5.77 units. How bright will it be

when viewed from 1.40 light years?

9.

A star has a measured brightness of 15 units

when viewed from a distance of 5.5 light years.

How far from the star does an observer need to

be for the apparent brightness to be 6.2 units?

10.

The "Andromeda Nebula" is a faint cloud-like

object just visible to the naked eye. With a good

telescope, it turns out to be a whole galaxy about

200 million LY (2.0 x 10

8

LY) away. Its brightness

as seen from Earth is only 0.0045 (4.5 x 10

-3

)

units. What would it's brightness be if you could

approach 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 original

distance away. While your spaceship was

travelling, the star’s luminosity increased by a

factor of 3.

In terms of “I”, what is the

brightness of the star at your new position?

12.

Two stars “A” and “B” are 12.0 LY apart. From

the exact mid-point between them the brightness

of star “A” is 9,000 units and star “B” is 1,000

units. Staying on the line between them, where

must you move to so that the 2 stars have the

same brightness?

16

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(17)

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4. ENERGY FROM THE SUN & ITS EFFECTS ON US

Energy From the Nucleus

There are basically 3 different ways that energy can be released from the nuclei of atoms:

Nuclear Fusion

is 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.

Radioactivity

Some atoms have an unstable nucleus and can spontaneously 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 Fission

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

break apart forming 2 smaller nuclei and often several individual neutrons. Once again, if the masses before and after are compared it seems a

small amount of matter has “disappeared”...

E = mc2is at work again!

This is the process occurring in a nuclear reactor used 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 neutrons

and 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 wave of the electromagnetic (EMR) type.

Gamma rays often accompany Alpha or Beta emission.

BETA RADIATION

((ββ))

also involves emission of a particle... this time an electron, ejected at high speed.

Symbol often used:

n

+

+

n

2 4

He

--11 0

e

(18)

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The Properties of Alpha, Beta & Gamma

Radiation Causes Ionization

All 3 radioactive radiations can cause ionization...

i.e. can cause electrons to be knocked out of their orbit around 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 it becomes cancerous.

The massive ALPHA particle has the highest ionization ability, BETA is much less ionizing and GAMMA less again.

Effects of Electric & Magnetic Fields

Alpha and Beta radiations are particles and both carry electric charges...

Alpha is positive (+ve), Beta negative(-ve). This means that both Alpha and Beta can be deflected by an electric field and by a magnetic field. The deflection of alpha compared 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 Gamma radiation

Electric Field between charged plates

Alpha (+ve)

small deflection due to large 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 Ability

Alpha, Beta and Gamma radiation are quite different in their abilty to penetrate through different substances.

FIRST-HAND INVESTIGATION, that you may

have done in class to test the penetration of radiation through different materials.

Geiger Tube. Detects radiation by the ionization it causes.

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

Different materials placed here (e.g. paper, lead, aluminium) to see what can block the radiation. Data sent to electronic counting device to measure the 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 their path, that they usally do not penetrate far. A few centimetres in air is as far as they’ll get, and a piece of paper will stop 99% of them.

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

• GAMMA rays are highly penetrating.

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

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

Alpha Beta Gamma

Paper Aluminium Lead foil

Atom becomes ionized Electron knocked out of orbit

+

(19)

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Radiation From the Sun

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

The Solar Wind

The Sun’s corona is an “atmosphere” of hot gas extending millions of kilometres into space. It is only visible during a solar eclipse when the brighter face of the Sun is blotted out by the Moon.

Every second from the corona, trillions of charged particles (electrons and ionized atoms, especially ionized hydrogen = protons) with enough energy to escape the Sun’s gravity, stream outwards into space. They exert enough force to push comet tails outwards, and affect the orbits of the smaller members of the Solar System such as asteroids.

This “Solar Wind” would be very dangerous to life, but the Earth’s magnetic field deflects, traps and channels the particles, so very few get through to the surface.

Sunspots & the Solar Wind

The flow of charged particles that make up the solar wind is not a constant stream. It fluctuates with changes in the Sun’s magnetic field, which scientists monitor by studying the “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 are regions that are cooler (only 4,500oC). They are associated with regions where the Sun’s magnetic field is stronger, and this causes more particles to be ejected in the solar wind. AND, the Sun’s magnetic field undergoes cyclic changes over an 11 year period. Every 11 years there are more sunspots and more intensity in the solar wind, sometimes to the extent that it can affect our power supplies and communications.

When sunspot activity peaks, our magnetic field can be overwhelmed by the solar wind. Charged particles penetrate the field and are sent into spiralling paths towards the Earth’s poles. Intense pulses of EMR at radio frequencies can result, which can cause “static”, jamming our communications, especially satellite telephone links which use radio and microwaves.

Extreme pulses can causes surges in electric power lines and damage electronic equipment. In one event some 25 years ago, the EMR pulse set off a surge in the power grid of the eastern USA & Canada which was so severe that the entire system shut down. Millions of people were left without power for several days in mid-winter!

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EMR

With a surface temperature around 5,700oC, most of the EMR from the Sun is at the wavelengths corresponding to visible light (with the “peak” being

yellow) and infra-red (heat).

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

the Earth’s atmosphere.

A small fraction of the Sun’s EMR is at shorter wavelengths corresponding to ultra-violet (UV) rays.

These could be very dangerous, but fortunately the “ozone layer” in the upper atmosphere absorbs most

of the UV.

magnetic field distorted by solar wind

solar wind

solar wind deflected by 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 particles spiralling down into the poles also cause

the beautiful “aurora” displays of the “Northern Lights”

& “Southern Lights”. Sometimes, the Solar Wind penetrates the magnetic field Spiralling charged particles produce EMR “pulses”.

(20)

Worksheet 4

Fill in the blanks.

Check your answers at

the back.

Nuclear a)... occurs when

2 small nuclei are slammed together so hard that

they b)... In the process, a

small amount of c)... is converted

into d)... Fission is when a

e)... nucleus (such as f)...)

splits into fragments. Once again, there is a

conversion of

g)...

into

... according to E= h)...

Another way that energy is released from an

atomic i)... is known as

j)... This occurs because

some nuclei are unstable, and can spontaneously

re-adjust themselves to a more

k)... form by emitting particles

and/or EMR. The 3 forms of radioactive

radiation are:

• l)...,

symbol = m)...

This involves emission of a particle consisting of

n)...

This is equivalent to the nucleus of a

o)... atom

• Beta radiation, symbol = p)...

This involves the emission of

an

q)...

• r)... ..., symbol = s)...

This is the emission of a t)...

frequency EMR wave.

All 3 types of radioactive emissions can cause

ionization, by knocking u)...

out of

their orbits in an atom.

v)... radiation has the

highest ionization ability,

then

w)...

less so,

and

x)... least of all. It is this ionization

which makes radioactivity dangerous to life: living

cells can be killed because their

y)... are disrupted, or their

DNA can be damaged, resulting in genetic

z)... or the cell becoming

aa)...

Each radiation is different in its penetrating

ability:

ab)... is least penetrating, and

most can be stopped by a sheet of

ac)...

Beta has ad)... penetration. It

can usually be stopped by a thin sheet of

ae)...

The most penetrating radiation is

af)... which can penetrate

many metres of air and needs ag)...

or ... to stop it.

Each radiation is also affected differently by

ah)... 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 a

am)... charge, while beta is

an)... Also, ao)...

will deflect through a greater angle than

ap)... because aq)...

...

ar)... radiation is NOT

affected by either type of field.

The Sun emits a range of EM waves, some of

which could be dangerous to life on Earth.

Luckily,

most of

the dangerous

as)... waves are absorbed by the

at)... layer in the upper atmosphere.

As well as EMR, the Sun emits streams of

au)... known as the

av)...

This could be very dangerous too, but very little

gets to the Earth’s surface because of the Earth’s

aw)...which

ax)... most of it.

“Sunspots” are darker spots on the Sun’s surface

which are ay)...

(cooler/hotter) and are areas where the Sun’s

az)... is more intense.

The presence of sunspots results in the solar

wind becoming more ba)...

Sunspot activity goes up and down in a cycle over

bb)... years. When sunspots are at a

maximum, the solar wind can overwhelm the

Earth’s bc)... When this

happens, charged particles can give off bursts of

bd)... which can interfer

with be)... and damage

bf)... equipment. In

extreme cases, disruption has occurred to

bg)... supplies.

20

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COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC

Some 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

CCO

OSSM

MIICC

(22)

Practice Questions

These are not intended to be "HSC style" questions, but to challenge your basic knowledge and understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level.

When you have confidently mastered this level, it is strongly recommended you work on questions from past exam papers.

Part A Multiple Choice 1.

The astronomer who supported a heliocentric model of the universe was:-A. Aristotle B. Ptolemy C. Copernicus D. Tycho Brahe 2.

The use of "epicycles" in the geocentric model of the universe 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 universe were 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 scientific events?

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 an expanding 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 allowed galaxies 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 a hot 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 5 metre distance. At 5 metres, its apparent brightness would be: A. 1/5 B. 5 times C. 1/25 D. 25 times This is a simplified version of the Hetzsprung-Russell (H-R) diagram. It is used for questions 10-12 10.

The vertical scale on this graph measures: A. Luminosity

B. Colour

C. Surface Temperature D. 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

St Johns Park High School SL#802444

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Preliminary Physics Topic 4

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References

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