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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
but first, let’s revise... The Structure of the Universe
Beyond our galaxy are billions of other galaxies. The distances involved are immense and unimaginable!
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.
We have good reason to believe that the entire Universe is EXPANDING, with the space between galaxies increasing.
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.
Mercury Earth & Moon
Jupiter
Sun
Venus As
te
OTHER GALAXIES
ro
id
Be
lt
Mars Saturn
THE SOLAR SYSTEM
THE MILKY WAY GALAXY
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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. 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.
Geocentric & Heliocentric Models
•Aristotle •Aristarchus •Ptolemy •Copernicus •Brahe •Kepler •Galileo •Newton
Evidence of the “Red-S Shift”
Einstein’s E=mc2 Cosmic Background Radiation How Matter was Formed
Discovery: •Friedmann •Hubble
Historical Summary
Formation of Stars & Galaxies
Big Bang Theory
How the Universe Began
History of our Understanding of the Universe
Temperature & Colour of Stars
The COSMIC ENGINE
Life Cycles of the Stars
Energy from the Sun
Radiation from the Sun
Brightness & Distance Inverse Square Law
Radioactivity
Hertzsprung-R Russell Diagram
Energy Sources in Stars
Alpha
α
Impacts & Effects
Properties of Radiation
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Beta
β
Gamma
γ
2
Stages in a Star’s Life
Supernovas, Pulsars & Black Holes
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1. THE HISTORY OF OUR UNDERSTANDING OF THE UNIVERSE Different Models of the Universe
Historical Summary
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.
up until about 1700 AD
Aristotle
Geocentric Theory
Thought that: The Sun, Moon, planets & stars are carried on invisible crystal spheres rotating around the Earth.
Over the centuries there have been TWO main models of the Universe competing for acceptance.
This basic concept was believed for about 2,000 years.
Geocentric Models incorrectly place the EARTH at the centre ("Geos" = Earth, “centric” = at the centre) Planets
~330 BC
Aristarchus
Fixed Stars
~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.
Sun
Earth
This idea was not accepted because "parallax" could not be detected at this time.
Moon
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!
See “Further Explanations” at the end of this section
Claudius Ptolemy ~120AD
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.
Heliocentric Models correctly place the Sun at the centre of the Solar System. ("Helios" = Sun)
The "epicycles" were needed to explain the "retrograde" motion of the planets.
Moon
Earth
Geocentric Model with "epicycles"
Fixed Stars Planets
See “Further Explanations” at the end of this section
Sun
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.
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.
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.
Only the Moon truly orbits the Earth.
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Nicholas Copernicus
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.
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.
He observed that the planet Jupiter has moons orbiting around it. (Only the Earth was supposed to have things go around it!)
Copericus’s new model still relied on crystal spheres to carry planets and stars in circular orbits, but it was Heliocentric... Sun centred.
He saw that the planet Venus showed phases like the Moon. (This was only explainable if Venus orbited the Sun, not Earth!)
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.
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.
Tycho Brahe
1546-1601
From his equation for Gravity, Newton could prove Kepler's Laws mathematically... this proved that the Heliocentric Model was correct.
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.
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.
He jealously guarded his data from others, but when he died it went to his student Kepler.
Johannes Kepler
The Significance of Telescopes in Astronomy All of the models, until the time of Galileo, were limited by the lack of the TELESCOPE.
1571-1630
Heliocentric Model, with elliptical orbits
Without telescopes, all measurements and observations were made by naked-eye, and were of limited accuracy.
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.
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.
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.
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.
The Heliocentric idea was still NOT accepted widely. Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
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Further Explanations The following may help your understanding. It is NOT a syllabus requirement to learn it.
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.
Parallax Parallax is the apparent movement of an object against a more distant background, when viewed from a different angle. 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.
Normal planetary “wanderings”
Your finger appears to move relative to the distant "landmark". This apparent movement is called "PARALLAX"
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. Earth
Retrograde motion
Fixed Stars
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.
line of observ
Planet
ation
Star being observed
Sun
Planet More distant stars
Earth, 6 months later
The position of the star should change against the background stars. Parallax!
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.
Earth The planet is carried on a smaller sphere, the “epicycle”, which rotates on the deferent.
Deferent
Each planet’s main orbit is a rotating glass sphere, called the “deferent”. It revolves around 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.
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.
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Epicycle
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Worksheet 1 Fill in the Blanks. Check your answers at the back. 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.
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.
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.
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.
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.
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.
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.
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)...................................
COMPLETED WORKSHEETS BECOME SECTION SUMMARIES
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2. HOW THE UNIVERSE BEGAN Discovery of the Expanding Universe
Outline of the “Big Bang” Theory
In 1922, the Russian Alexander Friedmann predicted that the universe was expanding.
• The universe began approx. 13-15 billion years ago.
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.
• 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.
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".
Note: You must NOT think of this as if the matter exploded outwards into the space surrounding it.
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).
The explosion and expansion was of space itself. Before the explosion there was no space or time.
This is due to the Doppler Effect: The waves emitted by a stationary object spread out evenly in all directions, with the same wavelength.
• 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.
Waves spreading out evenly from a stationary object
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:-
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.
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.
Light Waves Spreading Out From a Moving Galaxy
Explanation of the “Red Shift” Behind, wavelength lengthened
• 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.
Light redder
Light Bluer
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.
• 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.
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In Front, wavelength shortened
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How the Matter of the Universe was Formed
Formation of Stars and Galaxies
In 1915, Albert Einstein had deduced his famous equation
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.
E=mc²
E = energy, m = mass c = the speed of light = 3x108 ms-1
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.
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
Eventually, the particles became cool enough (and slow enough) for Overall expansion continues gravity to have an but “clumps” of matter effect. collapse due to gravity
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.
If the so galaxies form 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".
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.
Note: we know this is true because the Cosmic Background Radiation (the afterglow of the Big Bang fireball) shows distinct patterns of unequal distribution.
After further cooling, some protons & neutrons were able to combine into simple atomic nuclei.
H
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.
He
The atoms formed were nearly all hydrogen, with a small amount of helium and a trace of lithium
Roughly 13 billion years later, here we are… • on a planet, in a solar system, orbiting a star.
Li
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.
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• 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. 8
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Worksheet 2 Fill in the blank spaces Einstein’s famous equation, E= w).......................... predicts that x)............................... and ....................................... are equivalent and interchangeable. For example, in a nuclear reaction a small amount of y)............................... will “disappear” because it has been converted into a large amount of z)..........................................
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)..................................................
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 ..........................................
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.
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 .........................................
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)...................................................................................
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.
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 “bunchedup” 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)........................................... Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
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3. LIFE-CYCLES OF THE STARS Temperature and Colour of Stars
Relationship Between Temperature & Dominant Wavelength of Radiation from a Hot Object
With light waves, wavelength (and frequency) determines colour.
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.
Shorter wavelengths are toward the BLUE end of the spectrum. Longer wavelengths are towards the RED end of the spectrum.
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.
For stars, this means there is a relationship between their TEMPERATURE and their COLOUR. Relatively cool stars (surface temp 3,000ºC or less) emit radiation which peaks at longer wavelengths in the infrared and red light part of the spectrum.
The graph shows the relationship.
Amount of Energy Radiated
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 object
“peak” wavelength shorter
Very hot stars (30,000ºC and more) have a peak emission at the shorter wavelengths of blue light.
hot object
“peak” wavelength
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.
warm object “peak” wavelength longer
shorter
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.
longer Wavelength of Radiation
At (relatively) low temperature, there is less energy being emitted, and the peak wavelength is longer.
white light is a mixture of wavelengths
At higher temperatures, there is more energy emitted and the “peak” wavelength gets shorter.
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.
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.
Red Orange Yellow Green Blue Violet
A spectroscope is simply a more sophisticated version of the prism, and allows the intensity of each wavelength to be measured.
Basically, everything we know about stars comes from studying the radiation they emit! Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
different wavelengths spread out to form a spectrum
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Brightness and Distance: the Inverse Square Law
One way to understand this is explained in the diagram. Light spreading out from a star
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.
nce
a dist
x
Obviously, how bright a star appears depends on how luminous it is, AND how far away it is.
dista
nce
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.
“d”
”
“2d
Square with sides twice as long.
Square Area
2x
x2
If you start with the mathematical relationship:
I.d2 = constant,
this means that no matter how far you are from a star the product (brightness x distance squared) has the same value.
IAdA2 = k
Therefore, at position “A”,
or
Same amount of light falls on 4 times the area
Luminous Star
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.
Iα 1 d2
Area = 4x2
I.d2 = constant
and at position “B”,
IBdB2 = k
therefore,
IAdA2 = IBdB2
The “α” symbol means “proportional to” TRY THE WORKSHEET at the end of this section.
This relationship was previously studied in an earlier topic (Revise Topic 1 “The World Communicates”)
The Hertzsprung-Russell Diagram
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.
To an astronomer, the Sun is a pretty average “Main Sequence” star, classified “G3” on the H-R diagram. Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
hot, bright, blue stars
-5
Hertzsprung and Russel found that when they graphed luminosity against surface temperature like this, the vast majority of stars plotted in this shaded zone.
(Absolute Magnitudes) +10 +5 0
This zone is now called the “MAIN SEQUENCE”
our Sun cool, dull, red stars
+15
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 = IBdB2
Luminosity increasing
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.
-10
Now we put together the Colour-Temperature relationship, and the Brightness-Distance relationship:
Spectral Classes Colours Temp. (oC)
O Blue 30,000+
B
A White 10,000
F
G
K
M
Yellow
Red
5,000
2,500
Note: Temp scale decreases to the right
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. 11
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Stages in the Life of a Star
The future evolution of our Sun
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
“M a in
Luminosity
"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.
Seq
uen c
SUN
Wh ite Dw arfs
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.
30,000 blue
e”
10,000 6,000 green yellow TEMPERATURE (oC) & COLOUR
3,000 red
So, what causes a star to change from one type to another during its life?
The SUN as it is now
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.
this dot shows the size of a White
The edge of a RED GIANT
Dwarf
.
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.
+
Emission of particles & energy
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.
+
+n “heavy hydrogen” (deuterium) nuclei
Energy
n
+
Reaction 2
+ +
To keep it simple... (K.I.S.S.)...
4 Hydrogen 4 1H1
2 more protons
+
Reaction 3
fusion
2 protons re-released
Helium + Energy 4He + energy 2
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+
Reaction 1
+n
This sequence of reactions is called the Proton-Proton Chain, and is what produces the energy in a Main Sequence star like our Sun.
+
+
n
+ +
+ n
Emission of particles & energy
START WITH 4 Hydrogen nuclei (protons)
Energy Sources in a Main Sequence Star
+ n Energy
n + +
Helium-3 nuclei
+ n
Energy
FINAL PRODUCT = Helium-4 4 nucleus
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Energy Source in a White Dwarf
Core Temperature and Star Size
The Red Giant “burns” helium for a billion years or so, but gradually the fuel runs out and fusion stops.
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.
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".
The size of any star is determined by the balance between gravity and the energy released by fusion.
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.
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.
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.
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.
Typical Life of a Main Sequence Star Red Giants
Luminosity
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.
3 helium nuclei
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.
fusion
Seq
Wh ite Dw arfs 30,000 blue
uen c
e”
SUN
10,000 6,000 green yellow TEMPERATURE (oC) & COLOUR
3,000 red
carbon nucleus
star death
Summary: Energy Sources in Stars
energy release
Main Sequence: Proton-proton fusion reactions. 4 Hydrogen Helium + energy
“Helium burning” has begun. 3 Helium Carbon + energy 12C 3 4He2 6 + 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.
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.
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. Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
“M a in
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Supernova: what's the story?
This "supernova" explosion has several interesting consequences:-
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.
• 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.
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.
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.
Blue Supergiant Stars
Luminosity
“M a in
Red Giants
Seq
uen c
• 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".
e”
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:-
Wh ite Dw arfs
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.
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...
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")
a Supernova!
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.
Photo © Laurence Diver
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The Energy Sources and Life Cycles of Stars can be studied further in the HSC Option Topic “Astrophysics”
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Worksheet 3 Part A Fill in the blanks. Check answers at the back. 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.
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)..................
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)................................................................................................
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.
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)............................................................................”
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.
COMPLETED WORKSHEETS BECOME SECTION SUMMARIES
Part B Practice Problems Inverse Square Law 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? Check your answers at the back.
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. Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
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Worksheet 3 (continued) Part C Inverse Square Law More Difficult Problems
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 IA=20, dA= 10, dC=80, solve equation to find IC )
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".
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?
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)
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?
These problems require the use of IAdA2 = IBdB2
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 108LY) 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?
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:
IAdA2 = IBdB2 IA x 102 = 22.5 x 62 IA = (22.5 x 36) / 100 = 8.10 units
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?
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 IA=20, IB=5, dA=10, solve equation to find dB)
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?
Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION, APPROPRIATE PRECISION and UNITS Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
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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.
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 = mc2 is at work again!
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:
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”.
BETA RADIATION (β) also involves emission of a particle... this time an electron, ejected at high speed.
-
ALPHA RADIATION (α) 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.
n
Symbol often used:
2
-1 1
e-
(γ)
GAMMA RADIATION involves the emission of a high frequency wave of the electromagnetic (EMR) type.
+ n +
For that reason it is often given the symbol 4
0
He Gamma rays often accompany Alpha or Beta emission.
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The Properties of Alpha, Beta & Gamma
Effects of Electric & Magnetic Fields Alpha and Beta radiations are particles and both carry electric charges... Alpha is positive (+ve), Beta negative(-ve).
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 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.
Electron knocked out of orbit
Deflection of Radiations by Electric Field
Alpha, Beta or Gamma radiation
Alpha (+ve) small deflection due to large mass
-
Gamma. (no charge) no deflection
+ Electric Field between charged plates
Atom becomes ionized
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.
Beta (-ve) larger deflection due to small mass.
Note that Gamma rays are NOT deflected by either field, because they have no electric charge. Deflection of Radiations by Magnetic Field
The massive ALPHA particle has the highest ionization ability, BETA is much less ionizing and GAMMA less again.
Alpha (+ve) small deflection Gamma. (no charge) no deflection Magnetic Field (into page) between mag. poles
Beta (-ve) larger deflection
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 You may have done Practical Work in class to investigate this. will stop 99% of them.
Penetrating Ability Alpha, Beta and Gamma radiation are quite different in their abilty to penetrate through different substances.
• 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.
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.
Alpha
Data sent to electronic counting device to measure the radiation levels
Beta Gamma Paper
Lead
• 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.
Different materials placed here (e.g. paper, lead, aluminium) to see what can block the radiation.
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Aluminium foil
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Radiation From the Sun
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”.
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”. 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).
Galileo was the first to see sunspots with his telescope... dark spots on the Sun’s bright surface. More evidence against Ptolemy’s geocentric model: Sunspots were obvious “blemishes” on one of the “heavenly bodies” which were believed to be perfect!
Some radiation is also at the longer wavelengths of radio and microwaves, but most of this is absorbed by the Earth’s atmosphere.
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.
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.
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.
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.
Sometimes, the Solar Wind penetrates the magnetic field
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.
Earth
Spiralling charged particles produce EMR “pulses”.
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.
The particles spiralling down into the poles also cause the beautiful “aurora” displays of the “Northern Lights” & “Southern Lights”.
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.
solar wind
Earth
Earth’s magnetic field
magnetic field distorted by solar wind
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!
solar wind deflected by magnetic field
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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.
Worksheet 4 Fill in the blanks. the back.
Check your answers at
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)....................
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.
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
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.
• 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)....................................................
“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.
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.
The COSMIC ENGINE
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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.
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.
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.
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
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.
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.
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
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.
Q R
S
10. The vertical scale on this graph measures: A. Luminosity B. Colour C. Surface Temperature D. Diameter
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. Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
P
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
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18. (cont) Which line of this table correctly identifies the type of radiation, and the electric charge on the TOP plate?
12. Our Sun is expected to "evolve" and undergo changes in the future. Which of the following shows the expected changes as our aging Sun moves around the H-R diagram? A. P>Q>R B. Q>P>S C. R>Q>S D. P>S>R
RADIATION A. B. C. D.
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.
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.
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.
20. The "Ozone Layer" of the Earth is very effective in blocking which radiation from the Sun? A. Radio waves B. Ultra-violet waves C. Solar Wind D. Light waves
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.
Part B
21. (2 marks) Contrast a geocentric model of the universe with a heliocentric model. 22. (3 marks) Discuss how the historical development of models of the universe was limited by the technology available. In your answer, refer to one specific model, naming the person responsible for it.
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 type of radiation was passed between electrically charged plates in a vacuum. Later, the experiment was repeated with a thin piece of paper in the path of the radiation.
23. (5 marks) Place the following men in chronological order, and state whether each supported a heliocentric or geocentric model of the universe. Kepler, Ptolemy, Aristotle, Copernicus, Aristarchus
Detector screen
24. (3 marks) About 1930, Edwin Hubble used a new, large telescope to discover the "Red-Shift". a) What is the "red-shift"? b) Explain how the red-shift gives evidence of an expanding universe. c) How is expansion of the universe explained by the "BigBang" theory?
radiation radiation detected here
Later,
radiation
Electrically charged plates
Paper inserted here
NO RADIATION DETECTED
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Longer Response Questions.
Mark values given are suggestions only, and give you an idea of how detailed an answer is appropriate.
16. Which combination of radiations would show LEAST total 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.
Electrically charged plates
Beta Gamma Alpha Beta
CHARGE ON TOP PLATE +ve -ve +ve -ve
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29. (2 mks) Write a simple equation to describe the reaction that produces energy in a typical main sequence star.
25. (5 marks) At the instant of the “Big Bang” all the substance of the universe is thought to have been in the form of radiant energy. 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 mainly formed in the early universe? c) The "Big Bang" is all about expansion. How then did matter get together to form galaxies and stars? Explain.
30. (5 marks) Complete the table to describe the properties of alpha, beta and gamma radiation. RADIATION TYPE
Alpha Beta Gamma
26. (2 marks) This graph shows the distribution of radiation emitted by an object at 5,000oC.
WHAT IT IS
(a) electron (d)
IONIZATION ABILITY
high (c) v. low
PENETRATION ABILITY
(b) medium (e)
Amount of Energy
31. (7 marks) Using a "geiger counter" a scientist detected a source of radioactivity coming from a mineral sample. When placed 2 mm in air from the sample the counter registered 1,687 ionization events in 1 minute. Next she placed a piece of aluminum foil in the 2 mm space between sample and Geiger tube. The counter registered 802 events/min.
Wavelength of Radiation
On the graph sketch clearly the curve you would expect if the object was at 2,000oC.
When the aluminium foil was replaced with a piece of paper the count was 1,538 ionizations/min.
27. (3 marks) If a star has an apparent brightness of 37.5 units when viewed from a distance of 22 LY, at what distance would an observer see its brightness to be 16.3 units?
With a piece of lead foil in the gap, the result was 366 ionizations/min. From these results she was able to deduce what type(s) of radio-active emissions were coming from the sample.
28. (7 marks) Use the graph provided to sketch the Hertzsprung-Russell diagram, showing clearly:
a) State what type(s) of radiation(s) were coming from the sample. b) Explain your reasoning. c) Account for the result obtained with a piece of paper shielding 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?
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.
32. (3 marks) Radiation from the Sun includes high energy charged particles, radio waves and ultra violet waves. Each of these is partially or totally absorbed, blocked or deflected before it reaches the Earth's surface. By what? State specifically what EACH of these radiations is blocked or deflected by.
NOTICE ANY ERRORS? Our material is carefully proof-read but we’re only human If you notice any errors, please let us know Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
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Answer Section
Worksheet 3 Part B Inverse Square Law problems 1. Decrease to 1/25 as bright. 2. To be 4X brighter, must get closer by factor of 2. i.e. half the distance. 3. 1/16 as bright = 2units 4. at 1/5 distance will be 25X brighter. 8x25=200units. 5. “X” is 3x further away, so its luminosity must be 9 times “Y”’s.
Worksheet 1 a) Geocentric b) Heliocentric c) Sun d) geocentric e) 2,000 f) Aristarchus g) heliocentric h) Ptolemy i) eclipses j) geocentric k) Earth l) epicycles m) retrograde n) Copernicus o) heliocentric p) very accurate data on the movement of planets q) wrong r) Kepler s) ellipses t) telescope u) moons in orbit around it v) phases w) Ptolemy x) Copernicus y) Isaac Newton z) heliocentric aa) Gravity ab) orbit ac) Kepler’s ad) telescopes ae) naked eye af) accuracy ag) parallax movement ah) geocentric
Part C More Difficult Problems 6. IAdA2 = IBdB2 20 x 102 = 5 x dB2 dB2 = (20 x 102) / 5 dB2 = 400 dB = 20 LY 7. IAdA2 = Icdc2 20 x 102 =Ic x 802 Ic = (20 x 102) / 802 = 0.31 units 8. IAdA2 = IBdB2 5.77 x 3.252 =IB x 1.402 IB= (5.77 x 3.252) / 1.402 = 31.1 units 9. IAdA2 = IBdB2 15 x 5.52 = 6.2 x dB2 dB2 = (15 x 5.52) / 6.2 = 73.185... dB = 8.6 LY 10. IAdA2 = IBdB2 4.5x10-3 x (2x108)2 = IB x (1x106)2 IB = (4.5x10-3 x (2x108)2) / (1x106)2 = 180 units
Worksheet 2 a) Big Bang b) 13-15 billion c) matter & energy d) singularity e) expanded (exploded) f) expanding g) “red shift” h) Cosmic Background i) chemical j) Hydrogen & Helium k) Friedmann l) Relativity m) Hubble n) spectral o) shifted to longer wavelengths p) Red q) Doppler r) wavelength s) moving t) stretched out u) bluer v) redder w) mc2 x) mass & energy y) mass z) energy aa) (radiant) energy ab) matter ac) protons, electrons & neutrons ad) atoms ae) hydrogen & helium af) lithium ag) cooled ah) kinetic ai) gravity aj) stars & galaxies ak) expanding al) gravity
11. At half the original distance, the star will be 4 times brighter, AND if it has also increased luminosity by 3 times, then total brightness change is 12 times. ie Answer = 12x “I”. 12. Since the observer is viewing each star from the same distance, it follows that star “A” must be 9 times more luminous than “B”. To see the stars showing the same brightness you need to be 3 times further from “A” than from ”B”. (3 times further away reduces brightness to 1/9. Compare Q5) Therefore need to be at the point 9LY from “A” and 3LY from “B”.
Worksheet 3 Part A a) more c) shorter e) blue g) emitted i) the inverse of the square of j) Hertzsprung & Russell l) temperature n) nuclear p) hydrogen r) mass t) mc2 v) helium x) increases z) Red Giant ab) gravitational
b) wavelength d) red f) light h) received the distance k) luminosity m) Main Sequence o) Proton-Proton q) helium s) energy u) hydrogen w) carbon y) red aa) White Dwarf
Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
Worksheet 4 a) fusion c) mass e) large g) mass into energy i) nucleus k) stable m) α o) helium q) electron s) γ u) electrons w) beta y) membranes aa) cancerous ac) paper ae) metal (aluminium) foil
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b) join together d) energy f) uranium / plutonium h) mc2 j) radioactivity l) Alpha n) 2 protons + 2 neutrons p) β r) Gamma t) high v) alpha x) gamma z) mutations ab) alpha ad) medium af) gamma
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Worksheet 4 (cont...)
25. a) Matter formed from energy according to Einstein’s E=mc2. b) Hydrogen and helium c) Although the general universe expanded, the distribution of matter was not even. There was a “lumpiness” where matter was more concentrated in parts, and more spread out in others. In the more concentrated parts, gravity was able to attract matter and cause it to collapse together to form galaxies and stars.
ag) lead or concrete ah) electric or magnetic ai) alpha & beta aj) electric charge ak) deflected al) opposite am) positive an) negative ao) beta ap) alpha aq) alpha particles are much more massive ar) Gamma as) U.V. at) ozone au) particles av) Solar Wind aw) magnetic field ax) deflects ay) cooler az) magnetic field ba) intense bb) 11 bc) magnetic field bd) EMR be) communications bf) electronic bg) power (electricity)
Amount of Energy
26. Your sketch must show the 2000oC curve to be lower down, and with the peak further to the right (longer wavelegth).
Practice Questions Part A Multiple Choice 1. C 2. A 3. B 4. C
5. D 6. A 7. C 8. D
9. C 10. A 11. D 12. C
13. A 14. D 15. C 16. B
17. C 18. C 19. D 20. B
Part B
In some cases there may be more than one correct answer. The following “model” answers are correct but not necessarily perfect.
27.
IAdA2 = IBdB2 37.5 x 222 = 16.3 x dB2 dB2 = (37.5 x 222 ) / 16.3 = 1,113.49... dB = 33.4 LY
28.
LUMINOSITY
blue supergiants
22. The development of models of the universe up until the 17th century was limited by the lack of telescopes to observe and measure planets and stars. For example, about 200BC Aristarchus proposed a heliocentric universe. His idea was rejected because no-one could observe parallax in any stars. (It should be observable if the Earth orbits the Sun)
Ma
White dwarfs
Red giants in
se
qu
en
ce
TEMPERATURE
29. 4 Hydrogen or, even better answer 4 1H1
In fact it IS observable with a telescope, but without this technology Aristarchus’s theory could not be developed.
Helium + energy He2 + energy
4
30. a) helium nucleus (2 protons + 2 neutrons) ( 4He2 ) b) low c) low d) Short wavelegth EMR (e) v. high
Aristotle (Geocentric) Aristarchus (Heliocentric) Ptolemy (Geo..) Copernicus (Helio...) Kepler (Helio...)
31. a) Beta and Gamma b) Inserting paper had very little effect, so alpha not present. Aluminium foil stopped about half the radiation. Suggests beta. Some radiation penetrated even lead... suggests gamma. c) Paper stopped a small amount of beta particles, but nearly all the radiation went straight through it. d) The lead stopped all the beta and about half of the gamma. e) This is to keep the experiment “fair”. The various results could not be compared with each other if the distance was changed. This “variable” in the experiment needs to be controlled.
24. a) The “red shift” is the “stretching” of spectral lines from distant galaxies. The wavelengths have become longer, moving toward the red end of the spectrum. b) The stretching of the spectral lines is thought to be due to the Doppler Effect. For this to be so, it means the galaxies must be rushing away from us at high speed. This is evidence that the space between the galaxies is expanding.
32. High energy charged particles (solar wind) are deflected by the Earth’s Magnetic field. Most radio waves are absorbed by the Earth’s atmosphere. Most UV is absorbed by the “ozone layer” in the upper atmosphere.
c) Big Bang theory proposes a huge explosion at the beginning of time. The current expansion of the universe is simply the continuation of that outward explosion.
Preliminary Physics Topic 4 copyright © 2005-2007 keep it simple science
2,000oC
Wavelength of Radiation
21. Geocentric models place the Earth at the centre, with Sun, Moon, planets, etc in orbit around Earth. Heliocentric models place the Sun at the centre, with Earth and planets in orbit around Sun.
23.
5,000oC
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