8 sky watching charts! - teacher · pdf filebackground to chart the blanket of air that...

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8Charts!

Sky Watching Sky Watching Classroom hintsThese wall charts support teaching and learning about space and objects in our solar system.

Students will learn that:• theEarth’satmosphereisvitalforsustaininglife,andismade

upofanumberofdifferentlayers•theSunisastarthatprovidesalloftheheatandlightinour

solar system•theMoonistheEarth’sonlynaturalsatellite•starshavelifecyclesthatvarydependingontheirmass•constellationsarepatternsofstarsthatarevisiblefromEarth

fromdifferenthemispheresatdifferenttimesoftheyear•thereareeightplanetsinoursolarsystemthatvaryinsize

and composition•comets,asteroidsandmeteoroidsareclassifiedaccording

totheirsizeandthematerialstheyarecomposedof.

Usethechartstoprovideanoverviewofeachofthetopics.Youcouldreadthebackgroundinformationtostudentsandthenusethequestionstostimulateclassdiscussionandfurtherthinking about the topics.

The questions could also be allocated to pairs or groups ofstudentstoresearchanddevelopextendedresponses. These responses could include:

Also available:

MacmillanLibrarySeries:SkyWatching

Questions—lists questionsforyoutoaskstudents. The questions are designed to support andfacilitatethefollowingthinking skills:•Observing•Comprehending•Applying•Analysing•Reflecting•Evaluating.

Background to chart—givesabriefdescriptionofeachimageontheWallChart,plusanyrelevantinformation.

Key ideas—providesafewbriefstatementsthat sum up the key ideascoveredbytheimages.

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First published in 2011 byMACMILLAN EDUCATION AUSTRALIA PTY LTD15–19 Claremont Street, South Yarra, Vic 3141Visit our website at www.macmillan.com.auAssociated companies and representatives throughout the world

Copyright © Macmillan Education Australia 2011Macmillan Wall Charts: Sky WatchingISBN 978 1 4202 9123 0

Teaching notes by Virginia Joseph Publisher : Sharon Dalgleish Managing Editor : Bonnie WilsonEditors: Laura Jordan and Sarah Payne Editorial assistant: Haylie PretoriusProduction controller : Sunni CooperPhoto researcher: Kaitlin JordanPrinted in China

While every care has been taken to trace and acknowledge copyright, the publisher tenders apologies for any accidental infringement where copyright has proved untraceable. They would be pleased to come to a suitable arrangement with the rightful owner in each case.

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Spoken•role-playanddramatisations•speeches•debates•lectures•slideshows•discussions•explanations.

Written•factualreports•scientificreports•explanations•newspaperarticles•newspapereditorials,

advertising,letterstotheeditor.

Positional Only—For Dyeline guide only. Do not print PMS channel. Please hide PMS channel when printing. CMYK job only!

mesosphere (temperature decrease with height –15°C to –120°C)

mesopause

meteors (most burn up when they hit the mesosphere)

auroras ( 97–1000 km)

shuttle International Space Station ( 350 km)

thermosphere (temperature increase with height up to 2000°C)

Earth’s atmosphere

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de

(km

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India

Nepal

Mount Everest (8.5 km)

troposphere (temperature decrease with height 17°C to –51°C )

tropopause

ozone layer (concentrated ozone in lower stratosphere)

jet ( 10–12 km)

weather balloon ( 40 km)

stratosphere (temperature increase with height –60°C to –15°C)

stratopauserocket

thermopause

Hubble Space Telescope ( 600 km)

exosphere (edge of atmosphere thinning to a vacuum)

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85

600

Background to chart The blanket of air that surrounds the Earth is called its atmosphere. The Earth’s atmosphere is vital for sustaining life. It absorbs the energy from the Sun, recycles water and other chemicals, and works to provide a moderate climate. It also protects the Earth from high-energy radiation and the freezing vacuum of space.

The Earth’s atmosphere is divided into five distinct layers, based on changes in thermal characteristics, chemical composition, movement and density. Each layer is separated by a pause. These layers do not have a constant thickness.

The troposphere extends about 20 kilometres above the Earth’s surface. Almost all weather occurs in this region. All life on Earth is found in the troposphere, as this is the densest part of the atmosphere. The density of the gases decreases with height, so the air becomes thinner as altitude increases. The temperature also decreases with height, from about 17 degrees Celsius to about –51 degrees Celsius. The tropopause separates the troposphere from the next layer. Together the troposphere and the tropopause make up the lower atmosphere.

The stratosphere extends from the tropopause to about 50 kilometres above the Earth’s surface. The temperature increases with height due to the absorption of solar ultraviolet radiation, from about –60 degrees Celsius to about –15 degrees Celsius. Most ozone is found in the

stratosphere. The ozone layer absorbs and scatters ultraviolet radiation. The stratopause separates the stratosphere from the next layer.

The mesosphere extends from the stratopause to about 85 kilometres above the Earth’s surface. The temperature decreases with height, from about –15 degrees Celsius to –20 degrees Celsius. While the gases in the mesosphere become thinner as the altitude increases, they are thick enough to slow down meteors as they hurtle through the atmosphere. The mesopause separates the mesosphere from the next layer. The stratosphere, stratopause, mesosphere and mesopause make up the middle atmosphere.

The thermosphere extends from the mesopause to over 600 kilometres above the Earth’s surface. The gases in the thermosphere are even thinner than in the mesosphere. As the molecules absorb energy from the Sun, the temperature can reach 2000 degrees Celsius. The thermopause separates the thermosphere from the next layer.

The exosphere is the outermost layer of the Earth’s atmosphere. It extends from the thermopause until it merges with space. The thermosphere, thermopause and exosphere make up the upper atmosphere.

Questions Observing • Howmanylayersarethereinthe

Earth’s atmosphere?• Whatisbetweeneachlayerof the

Earth’s atmosphere?

COmprehending

• Wherewouldaweatherballoonneed to be positioned? Why do you think that?

• Isiteasiertobreatheatthebottomor the top of the troposphere? Why do you think that?

Applying

• Whatwouldyouexpectthetemperature outside a jet aircraft to be like as it reaches higher altitude?

• Howwouldmountainclimbersneed to protect themselves from atmospheric conditions?

AnAlysing

• Mostmeteorsburnupinthemesosphere. What does this tell you about the gases of the thermosphere?

• Insomelayersof theatmospherethe temperature increases with altitude. Why does this happen?

refleCting

• Howwouldyoufeelif oneof thelayers of the Earth’s atmosphere was badly damaged?

evAluAting

• Whatistheimportanceof theozone layer?

• Whatissuesshouldbeconsideredwhen manned shuttles are sent through the Earth’s atmosphere into space?

SKY WATCHING Chart 1

Key ideas • Theblanketofairsurrounding

theEarthiscalleditsatmosphere.•TheEarth’satmosphereprotects

andsustainslifeontheplanet. AcknowledgementsEarth’s Atmosphere: Cliff Watt.

Earth’s atmosphere

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dark, cooler temperatures

corona

diameter: 1.4 million kilometres

prominence

light, hottest temperatures

The Sun

Background to chart The Sun is the star at the centre of our solar system. It is the largest object in the solar system, occupying over 98 percent of its total mass. The Sun has a diameter of 1.4 million kilometres, while Earth’s diameter is only 12 756 kilometres. The energy produced through fusion in the Sun’s core produces all of the heat and light in our solar system.

The Sun is mostly hydrogen and helium, but it is neither a solid nor a gas; it is plasma. Plasma is a gas whose temperature is so high that it is sensitive to magnetism. Plasma is unstable and gaseous near the surface but becomes denser towards the centre of the Sun.

The Sun looks like a huge ball of fire, but it actually has a number of layers. The innermost quarter of the Sun’s radius is its core. Beyond this are the radiative zone and the convective zone. The Sun’s visible layer is the photosphere, which is about 5800 degrees kelvin. The photosphere has a mottled appearance due to turbulent eruptions of energy at the surface. Above the photosphere is a thin layer called the chromosphere. Above this is a region of hot plasma called the corona, which is about two million degrees kelvin. It extends millions of kilometres into space, but is only visible during a solar eclipse.

On the front of this chart is an Extreme Ultraviolet Imaging Telescope (EIT) image of the Sun. The Sun’s surface varies in temperature. The darker areas in the image indicate cooler

temperatures, while the light areas show the hottest temperatures.

The Sun has a magnetic field which forms sunspots and active regions. At 3800 degrees kelvin, sunspots are ‘cool’ regions, which look dark in comparison to the surrounding regions. There are no sunspots on the EIT image on this chart. The Sun’s magnetic field sometimes changes explosively, spitting out clouds of plasma and energetic particles into space.

The Sun, like the rest of the solar system, is about 4.6 billion years old. It has enough nuclear fuel to remain much as it is for another five billion years. Then, it will grow to become a type of star called a red giant.

On this EIT image is a huge, handle-shaped prominence on the Sun. Prominences are giant clouds of relatively cool, dense plasma suspended in the Sun’s hot, thin corona. They can take the shape of huge loops or arches. At times prominences can erupt, escaping the Sun’s atmosphere.

Questions Observing • HowlargeistheSun?• Whatdoesasolarprominencelook

like?

COmprehending

• WhatistheSunmadeupof ?• WhatdoestheSunprovidetothe

restof thesolarsystem?

Applying

• HowfarthroughitslifeistheSun?• Whatisthehottestpartof theSun?

AnAlysing

• Whyaresunspotsconsideredtobe‘cool’regions?

• ApproximatelyhowbigistheSun’score?

refleCting

• Whatdoyouthinkwouldhappenif wedidn’thaveaSun?

• ShouldscientistscontinuetostudytheSun?Whydoyousaythat?

evAluAting

• WhydoyouthinktheSunisimportant to the rest of the solar systemandtoEarth?

• Whatdoyouthinkwouldbethebest method to use in studying the Sun?


Key ideas • TheSunisthestaratthe

centreofoursolarsystem.• TheSunisneitherasolidnor

agas;itisplasma.• TheSuniscomposedofa

numberoflayers.

AcknowledgementsSOHO/ExtremeUltravioletImagingTelescope(EIT)consortium–SOHOisaprojectof internationalcooperationbetweenESAandNASA/courtesyof SpaceImages.

The Sun

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Mare Frigoris

Mare Imbrium

Aristarchus

Copernicus

Oceanus Procellarum

Kepler

Mare Humorum

Mare Nubium

TychoMare Nectaris

Mare Foecunditatis

Langrenus

Dionysius

Mare Tranquillitatis

Proclus

Mare Crisium

Mare Serenitatis

Menelaus

Plato

The Moon

Background to chart The Moon is the Earth’s only natural satellite. The Moon has no atmosphere. As a result, the sky on the Moon always appears dark and it is completely silent.

The Moon also has nothing to protect it from the impact of asteroids, comets and meteorites. Its surface is scarred by millions of impact craters. With no erosion to wear away the craters, they remain unchanged until the surface is hit by something else. The Moon’s craters vary in size and some are many hundreds of kilometres in diameter. The biggest intact lunar crater is Clavius, with a diameter of 160 kilometres.

Some well-known craters on the Moon include Aristarchus, Copernicus, Kepler, Tycho, Plato, Menelaus, Proclus, Dionysius and Langrenus.

Dark, lightly cratered maria cover about 16 percent of the lunar surface and are mostly located on the nearside of the Moon. Maria means ‘seas’, but maria on the Moon are dry plains that have never held water. They are called maria because Galileo, the first person to study the night sky using a telescope, thought the dark, smooth areas on the Moon were seas. The first moon landing occurred in Mare Tranquillitatis (the Sea of Tranquillity), on 20 July 1969.

Some of the more easily recognised seas of the Moon include Mare Frigoris (the Sea of Cold), Mare Imbrium (the Sea of Showers), Oceanus Procellarum (the Ocean of Storms), Mare Humorum (the Sea of Moisture), Mare Nubium (the Sea of Clouds), Mare Serenitatis (the Sea of Serenity), Mare Crisium (the Sea of Crises), Mare Tranquillitatis (the Sea of Tranquillity), Mare Foecunditatis (the Sea of Fecundity) and Mare Nectaris (the Sea of Nectar).

The relatively bright, heavily cratered highlands are called terrae. The craters and basins in the highlands are formed by meteorite impact.

The Moon is about one-quarter of the size of the Earth and its diameter is 3476 kilometres. The Moon’s gravity is one-sixth of that on Earth, and its temperatures are more extreme, ranging from about 110 degrees Celsius during the day, to –150 degrees Celsius at night.


Key ideas • TheMoonistheEarth’sonlynatural

satellite.• TheMoonhasnoatmosphere.• TheMooniscoveredinthousandsof

craters.

AcknowledgementsShutterstock/Brad Thompson.

The MoonQuestions Observing

• Whatdoesthesurfaceof theMoonlooklike?• WhydoestheMoon’sskyalwayslookdark?

COmprehending

• WhyaretheresomanyimpactcratersontheMoon’ssurface?• Whyaretheseascalledmariaeventhoughtheycontainno

water?

Applying

• WhydoyouthinktheMoonexperiencessuchanextremerangeof temperatures?

• WhydoyouthinkthefirstApollomissionchosetolandinoneof themaria?

AnAlysing

• WhyistheMoon’ssurfacesodifferentfromtheEarth’ssurface?

• WouldanobjectbelighterorheavierontheMoonthanontheEarth?Whydoyousaythat?

refleCting

• Themostrecentmoonlandingoccurredin1972.Whydoyouthinktherehavebeennonesincethen?

• Howwouldyoufeelif younolongersawtheMooninthenightsky?

evAluAting

• WhatinsightsdoyouthinkcanbegainedbystudyingtheMoon?

• WhatfactorswouldneedtobeaddressedbeforepeoplecouldliveontheMoon?

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Life cycle of a star

massive star

nebula

solar-type star

low mass star cycle

massive star cycle

red giant

red giant

black hole

supernova

white dwarf

neutron star

nebula

black dwarf

Background to chart All stars are born in huge clouds of dust and gas called nebulae. Gravity pulls the dust and gas together until an enormous amount has been collected. When the temperature at the centre of the ball reaches about 15 million degrees kelvin (as a result of all the gas and dust colliding under the great pressure of the surrounding material), nuclear fusion begins and the ball of gas and dust starts to glow. A star is born.

New stars vary in size and colour, from blue to red and from less than half to over 20 times the Sun’s size. The greater the amount of matter in a nebula, the greater the mass of the star, and the brighter and hotter it will be. The star will become either a massive star (five or more times the mass of the Sun) or a solar-type star (0.4 to 3.4 times the mass of the Sun).

After millions to billions of years, a massive star or solar-type star begins to run out of its main fuel, hydrogen, and the core becomes unstable and contracts. The outer shell of the star starts to expand, cooling and glowing red. The star becomes a red giant.

When a massive star reaches the red giant phase, the core starts to shrink, and a series of different elements begin to fuse in the core. When the core becomes essentially just iron, fusion stops and the iron atoms start to absorb energy. The core temperature rises to over 10 billion degrees kelvin and the energy is released in a powerful

explosion called a supernova, propelling material from the star’s outer layers to form new nebulae. The core of the star becomes a dense ball of neutrons.

Two different things can happen at this stage. If the core remains intact after the supernova, it becomes a neutron star. Neutron stars spin rapidly, giving off radio waves. Alternatively, if the original star was very massive, the core will be swallowed by its own gravity, becoming a black hole, attracting and swallowing up any matter and energy that comes near it.

When a solar-type star reaches the red giant phase, its outer layers continue to expand, the core contracts inward, helium atoms in the core fuse together to form carbon and the core is stabilised. The star begins to shed its outer layers, continuing to cool and shrink until it becomes a white dwarf. Once all of the energy is gone, it no longer emits light and it becomes a black dwarf.


Key ideas • Allstarsareborninnebulaeandastar’slife

cycledependsontheamountofmaterialcollectedduringitsformation.

• Somestarsbecomesupernovas,thenblackholesorneutronstars.

•Somestarsbecomewhitedwarfs,thenblackdwarfs.

AcknowledgementsBased on the ‘Life Cycle of Stars’ poster produced by NASA’s Imagine the Universe!, http://imagine.gsfc.nasa.gov/

Life cycle of a starQuestions Observing

• Wherearestarsborn?• Whichstarsbecomeredgiants?

COmprehending • Whatarenebulaemadeof ?• Whydoesawhitedwarf eventuallybecome

ablackdwarf ?

Applying

• Whatmightyoufindinablackhole?• WilltheSunbecomeasupernova?Whydoyousay

that?

AnAlysing

• Whatdetermineshowbrightandhotastarwillbe?•Whichstarwouldhavethelongestlifecycle:astar

the size of the Sun or a star 10 times greater than the Sun?

refleCting

•Whatdoyouthinkwouldhappenif therewerenostarsinouruniverse?

•Howwouldyoufeelif therewasablackholeontheedgeof oursolarsystem?

evAluAting

•Whatchemicalelementismostimportanttooursolarsystem?Whydoyousaythat?

•WhatwillbetheeffectonthesolarsystemwhentheSunmovesintothenextphaseof itslifecycle?

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AlkaidMizar

Megrez

Dubhe

Alioth

Phecda

Merak

Ursa Major (The Great Bear)

Crux (The Southern Cross)

Orion (The Great Hunter)

Betelgeuse

Bellatrix

MintakaAlnilamAlnitak

RigelSaiph

Alpha Crucis

Gamma Crucis

Beta Crucis(or Mimosa)

Delta Crucis

Epsilon Crucis

Constellations

Background to chart People have always used the stars to help them with navigation, guide them in the planting and harvesting of their crops, and to support their myths and legends. The ancient Greeks were the first to describe the constellations in a formal way. In 150 AD, the Greek scientist Ptolemy published a book summarising Greek astronomical knowledge. This included a catalogue of 1022 stars, arranged into 48 constellations.

In 1930, Eugène Delporte listed the 88 modern constellations on behalf of the International Astronomical Union.

Different constellations are visible in the northern and southern hemispheres, and they change with the seasons. The movement of the Earth around the Sun changes the position of the constellations over the course of the year.

The constellation Ursa Major (the Great Bear) is the third largest constellation in the sky. The seven stars which make up the rump and tail of the Great Bear are commonly called the Big Dipper. The Big Dipper is not a constellation itself, but an asterism, which is a distinctive group of stars. Although the whole of Ursa Major is difficult to see without very dark skies, the Big Dipper is one of the most recognisable patterns in the northern hemisphere.

The Southern Cross, or Crux, is the smallest of all the 88 constellations, but it is made up of some of the brightest stars in the sky. Alpha Crucis is the thirteenth

brightest star visible in both the northern and southern hemispheres, and Beta Crucis is the nineteenth brightest star in the sky. The best times to look at the Southern Cross are the early evenings of April, May, June and July.

The constellation Orion (the Great Hunter) is another easily identified pattern of stars in the northern hemisphere. It is visible in the night sky from December to April. In Orion’s shoulder is Betelgeuse, one of the rare supergiants at almost 1000 million kilometres in diameter. Its bright red colour contrasts with Rigel, the blue supergiant in Orion’s foot. Although it is just one-tenth the size of Betelgeuse, Rigel is still almost 100 times the size of the Sun. In the northern hemisphere, the three bright stars that form Orion’s Belt, Alnitak, Alnilam and Mintaka, are easily visible on the southern horizon on winter evenings.

Questions Observing • WhichthreestarsformOrion’sBelt?•HowmanystarsmakeuptheGreat

Bear?

COmprehending •Canyouseethesameconstellations

allyearround?Whydoyousaythat?•Inthepast,whywereconstellations

soimportantforsailors?

Applying

•WouldyoubeabletoseetheSouthern Cross in the northern hemisphere?Whydoyousaythat?

•Howdidfarmersusetheconstellationstohelpthem?

AnAlysing

•InwhichseasonsisiteasiertoviewtheSouthernCross?

•IsitpossibletoseeOrioninsummer?Whydoyousaythat?

•Whataresomeproblemsyoumightexperience if you had to navigate acrosstheseausingthestars?

refleCting

•Wouldyourathersailacrosstheseausing the stars to navigate or using GPS?Whydoyousaythat?

•Howwouldyoufeelif alltheconstellationsdisappeared?

evAluAting

•Whyisitusefultohavean‘official’listof constellations?

•Doyouthinkthislistwilleverchange?Whydoyousaythat?


Key ideas • Constellationsarethegroups

ofstarsthatformpatternsinthenightsky.

• Thereare88officialconstellations.

• Differentconstellationsarevisibleinthenorthernandsouthernhemispheres.

ACknOwledgeMentSIllustrations:Cliff Watt.

Constellations

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Neptune

Mercury

Venus

Earth

Moon

Mars

Jupiter

Saturn

Uranus

The Planets

Background to chart Our solar system consists of eight planets. The planets closest to the Sun—Mercury, Venus, Earth and Mars—are called the terrestrial planets because of their rocky surfaces. The gas giants or Jovian planets—Jupiter, Saturn, Uranus and Neptune—are composed almost entirely of hydrogen and helium.

Mercury is the smallest planet, with a diameter of 4879 kilometres. Mercury has very little atmosphere to stop meteorite impacts, and its temperatures vary from intense heat during the day to freezing at night. Mercury’s orbit is 88 Earth days.

Venus’ temperatures are hot enough to melt lead as its thick, toxic atmosphere traps heat. Below its clouds are volcanoes and misshapen mountains. Venus has a diameter of 12 104 kilometres and an orbit of 225 Earth days.

Earth is the only planet known to support life. It has liquid water, weather and seasons. The Earth’s atmosphere, composed largely of nitrogen and some oxygen, protects the planet’s surface. The Earth has a diameter of 12 756 kilometres and an orbit of 365.25 days.

Mars has seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is very thin. Most of its surface is covered in red dust. Mars has a diameter of 6794 kilometres and an orbit of 687 Earth days.

Jupiter is a giant ball of liquid and gas, with little or no solid surface. It is the

largest planet in our solar system, with a diameter of 142 984 kilometres and a greater mass than all the other planets combined. Jupiter has an orbit of almost 12 Earth years.

Saturn, a ball of hydrogen and helium with a diameter of 120 536 kilometres, is known for its rings, which are made up of ice particles that travel around the planet. Saturn has a solid inner core, but its surface is made up of a thick layer of cloud. It takes 29.5 Earth years for Saturn to orbit the Sun.

Uranus has a blue tint, due to methane in its atmosphere. Uranus has a diameter of 51 118 kilometres and an orbit of 84 Earth years.

Neptune is mainly composed of hydrogen, helium, water and silicates. Neptune has a diameter of 49 528 kilometres and an orbit of almost 165 Earth years.

The picture on the front of this chart is a montage of several images and does not reflect the actual size of the planets. The images were taken by different spacecrafts. The inner planets are roughly to scale to each other, and the outer planets are roughly to scale to each other.

Questions Observing • Howmanyplanetsareinoursolar

system?•WhichplanetsareclosesttotheSun?

COmprehending •Whichtwogasgiantsarealmost

identical in size?•WhydoesUranusappearblue?

Applying

•AbouthowmuchbiggerisJupiterthan Earth?

•Whatfeaturedoallof theterrestrialplanets share?

AnAlysing

•HowareEarthandVenussimilar?•HowareEarthandVenusdifferent?

refleCting

•Whatwouldhappenif amannedspacecraftvisitedNeptune?Whydoyou think that?

•Whatdoyouthinkwouldhappenif other planets had the same atmosphereasEarth?Whydoyouthink that?

evAluAting

•Whatmajorfactorswouldneedtobe addressed before people could live on Mars?

•Williteverbepossibletocaptureall of the planets in a single photographicimage?Whydoyouthink that?


Key ideas • Thereareeightplanetsinour

solarsystem.•Theterrestrialplanetsare

Mercury,Venus,EarthandMars.•TheJovianplanetsareJupiter,

Saturn,UranusandNeptune.Theyarealsocalledthegasgiants. AckNowlEdgEMENtS

Dreamstime/Iofoto.

The Planets

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Comet Hale-Bopp

coronawhite, hottest temperatures

Near-Earth Asteroid 433 Eros

Comets

Asteroids

Background to chart A comet is an icy body that orbits the Sun. It consists of a solid nucleus surrounded by a cloudy atmosphere called a coma and one or two tails. The nucleus of a comet is a ball of ice and rocky dust particles. When a comet’s orbit brings it close to the Sun, it heats up. Some of the ice on the surface of the nucleus is vaporised and the coma is formed. The Sun’s radiation and the solar wind push dust and gas particles away from the coma in the form of a tail or tails. Comet tails always point away from the Sun and they can stretch for millions of kilometres. Some comets become visible as they pass close to the Sun because the gas and dust in their comas and tails reflect sunlight.

On 23 July 1995, Alan Hale and Thomas Bopp both identified an unusually bright comet outside of Jupiter’s orbit. Its intense brightness was thought to be due to its exceptionally large size. Most comets have a nucleus of about 1.6 to 3.2 kilometres across; Hale-Bopp’s was estimated to be 40 kilometres across. Comet Hale-Bopp holds the record for the longest period of naked-eye visibility, at 19 months. It will not appear again for another 2400 years. Another famous comet is Halley’s Comet, which becomes visible to the naked eye about every 76 years.

Asteroids are rocky, airless worlds that orbit the Sun, often called ‘minor planets’. Most are located in the asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-Earth objects.

Near-Earth Asteroid 433 Eros was discovered on 13 August 1898 by Gustav Witt in Berlin, and independently observed on the same date by Auguste HP Charlois in France. Eros has a potato-shaped body 33 kilometres by 13 kilometres by 13 kilometres in size. Eros’ orbit crosses Mars’ path but doesn’t intersect that of Earth. There is no air and no evidence of water on Eros and its gravity is very weak. During the day, its temperature is about 100 degrees Celsius, but at night this plunges to –150 degrees Celsius.

Although comets and asteroids are both tiny bodies that orbit the Sun, their composition is different. Asteroids are mostly rock with some ice, while comets are mostly ice with some rock.

Questions Observing • Whatdoesacometlooklike?•HowwouldyoudescribeNear-Earth

Asteroid433Eros?

COmprehending •Whatarethedifferentpartsof a

comet?Namethem.•Whyareasteroidsreferredtoas

‘minorplanets’?

Applying

•Docometsalwayshavetails?Whydoyousaythat?

•WhyaresomecometseasiertoseeastheyapproachtheSun?

AnAlysing

•Whatisthedifferencebetweenacometandanasteroid?

•Whatdocometsandasteroidshaveincommon?

refleCting

•Howwouldseeingacometmakeyoufeel?Whydoyousaythat?

•Whatdoyouthinkwouldhappenif youdiscoveredanewcomet?

evAluAting

•WhatfactorsmakeErosuninhabitable?

•WhatmakesHale-Boppauniquecomet?


Key ideas • Cometsandasteroidsboth

orbittheSun.•Cometsandasteroidsare

differentincomposition.•Mostasteroidsarelocatedin

theasteroidbeltbetweentheorbitsofMarsandJupiter.

AcknowledgementsComet Hale-Bopp: Shutterstock/Artshots; Near-Earth Asteroid 433 Eros: NASA/Johns Hopkins University Applied Physics Laboratory. Courtesy SpaceImages.

Cometsandasteroids

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S

Largest known meteorite, Namibia

Meteor shower

Aerial view of meteorite crater, Arizona

Meteorites

Background to chart The pieces of rock and iron that travel through space which are smaller than asteroids are called meteoroids. A meteor is a streak of light that can be seen when a meteoroid enters the Earth’s atmosphere. These are sometimes called shooting stars. Millions of meteoroids enter the Earth’s atmosphere every day, but most burn up in the mesosphere. Sometimes, meteor clusters seem to fill the sky with a shower of sparks. These are known as meteor showers. In a meteor shower, a number of meteors seem to radiate from one point in the night sky. This is the result of streams of meteoroids entering the Earth’s atmosphere in close succession.

Meteoroids that do reach the Earth are called meteorites. Most are quite small, usually about the size of a pebble, but some can be very large. The largest known meteorite, at about 60 tonnes, is the Hoba Meteorite in Namibia which was discovered in 1920. It is estimated to be between 200 million and 400 million years old and is thought to have landed on the Earth some 80 000 years ago. The Hoba Meteorite is the largest naturally-occurring piece of iron on the Earth’s surface. It is composed of approximately 82.4 percent iron, 16.4 percent nickel, 0.76 percent cobalt and 0.04 percent phosphorus with trace amounts of other elements.

When large bodies such as meteorites, asteroids and comets strike a planet, an impact crater or impact basin is created. Impact craters are bowl-shaped depressions that can measure up to about 25 kilometres in diameter. One of the most famous, the Meteor Crater in Arizona, is about 1275 metres across and 175 metres deep. It was formed nearly 50 000 years ago when an iron meteorite weighing 300 000 tonnes struck the Earth.


Key ideas • Smallpiecesofrockandironthattravel

throughspacearecalledmeteoroids.•Clustersoftinymeteoroidsthatburnup

intheEarth’satmosphereareknownasmeteorshowers.

•Ameteoriteisameteorthatreachesaplanet’ssurfacewithoutburningup.

AcknowledgementsMeteorite Shower: Shutterstock /Shalygin; Meteorite Crater, Arizona: Shutterstock /Robert Spriggs; Largest known meteorite, Namibia: Shutterstock /Pichugin Dmitry.

MeteoritesQuestions Observing

• Comparedtothesizeof aperson,howbigistheHobaMeteorite?

•WhatdoesameteorlooklikewhenitenterstheEarth’satmosphere?

COmprehending

•Whatisameteorshower?•Whathappenswhenalargemeteoroidhitsaplanet?

Applying

•Whatisthedifferencebetweenameteor,ameteoroidandameteorite?

•Whyisameteorsometimescalleda‘shootingstar’?

AnAlysing

•Whatlayerof theatmosphereprotectsEarthfrommeteoroids?

•Whatwouldhappenif itdidn’t?

refleCting

•Whatdoyouthinkwouldhappenif ameteoritethesizeof the Hoba Meteorite were heading towards the Earth today?

•Howwouldyoufeelif yousawameteorshowerinyourlocalarea? Why do you say that?

evAluAting

•Whymightitbeusefultostudythefeaturesandcompositionof the Hoba Meteorite?

•WhatparallelscanyoudrawbetweentheHobaMeteoriteand the Meteor Crater in Arizona?

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