Coursework Booklet 1

Transcription

1 Level 3 Applied Science UNIT 16: Astronomy and Space Science PHYSICS SECTION Coursework Booklet 1 1 P a g e

2 Astronomy and space science Learning aim A Understand the fundamental aspects of the solar system Pass criteria 1. State the main features of the Solar System. The structure position of the Sun, planets and their moons. Forces gravity, centripetal force. Orbital characteristics what orbits what? Rotation what is special about Earth s and the Moon s rotation? Atmospheric composition what gases are found in Earth s atmosphere? 2. Details about Earth, Moon and Sun. Day and night (D) Moon phases (D) Solar and lunar eclipses (D) Tidal effects (D) 3. Details about other Solar System objects. Known planets (8 of them). Prominent moons. Asteroids. Comets. Meteors. Kuiper Belt. Oort Cloud. 4. Life cycle of a star. What happens to the different stars depending on their initial mass (flowchart and D). 5. Cosmological theories of the present day with descriptions of supporting evidence for each. Big Bang theory in terms of redshift. Olbers paradox. 2 P a g e

3 Merit criteria 6. Describe the main features of the Solar System. Planetary axes of rotation. Composition of planets, moons, asteroids, comets, meteors, planetary ring system labels for Saturn (D), and Van Allen radiation belts (D). Surface features of chosen moons (The Moon and Europa) and planets (Mars and Jupiter). 7. Describe star types with reference to the Hertzsprung-Russel (HR) diagram. What do the HR diagram show? Examples Alpha Centauri and Betelgeuse. Relationship between temperature and mass looking at the HR diagram. Determining spectral classes of stars. Description of what happens to stars with i) a similar mass to the Sun, and ii) a much larger mass than the Sun in terms of their life cycles. 8. Explain how we measure astronomical distances. Trigonometric parallax and the limitations which arise with this measurement. The use of Cepheid Variables which are known as standard candles. What eclipsing binaries are and how we can use them to measure distances. 9. Describe features of the Universe. Red and Blue shift. Hubble s Law. Fate of the Universe based on density. Olbers paradox. 3 P a g e

4 Distinction criteria 10. Description of the forces which allow the Sun to remain in equilibrium. Description of hydrostatic equilibrium and what forces it balances out. Why is there a force due to gas pressure? Why does this change when the star has fused all its hydrogen, and what effect does this have on the Solar System? 11. Nuclear fusion. Description of nuclear fusion. Magnetic forces and features on Sun s surface. 12. Description of the Sun. Composition. The layered structure (D). Identifying the gases involved in the structure. 4 P a g e

5 1. Features and characteristics of the Sun 1.1 Structure of the Sun Figure 1: the layered structure of the Sun. INNER Layers Temperature What happens there? Core 15 million degrees Celsius Nuclear fusion occurs 2 hydrogen nuclei are fused into a helium nucleus. Photons are also produced. Radiative zone 2 million degrees Celsius Emits radiation. Convective zone 2 million degrees Celsius Photons produced in the core make their way to the surface by convection. OUTER layers Temperature What happens there? Photosphere 5,500 degrees Celsius Emits visible light so this is the part of the Sun that we see. Sunspots arise on the photosphere. Chromosphere 7,700 degrees Celsius Hydrogen emits light that gives off a reddish colour. Corona 500,000 degrees Celsius This can only be seen during a solar eclipse. Table 1: details of the layered structure of the Sun. The entirety of the Sun is gaseous due to the extreme temperatures. Task: you need to know what gases are present in the Sun: 5 P a g e

6 1.2 Nuclear fusion H + 1 H 2He Releases energy! Figure 2: fusion of hydrogen into helium. Nuclear fusion occurs in the core of the Sun where the temperatures exceed 15 million degrees Celsius. This immense heat and pressure are required to bring 2 hydrogen nuclei together and fuse them together forming a helium nucleus. Photons (packets of light energy) are also produced. Task: draw a diagram and write what is happening at each stage of a proton-proton chain below: 6 P a g e

7 1.3 Hydrostatic equilibrium Hydrostatic equilibrium occurs when the force due to pressure exactly balances the force due to gravity. Figure 3: illustration of hydrostatic equilibrium. As nuclear fusion occurs, the gas pressure within the star increases. The force due to this gas pressure pushes outwards trying to make the star expand. The gravitational force common to all matter is an inwards force which keeps the star together and in a spherical shape. Both forces are massive but balance each other out equal and opposite, and this is known as hydrostatic equilibrium. 1.4 Surface features of the Sun Magnetic field The Sun s magnetic field is responsible for the space weather we view on Earth (the Northern lights), the interplanetary magnetic field through which spacecraft must follow to journey around the solar system, etc. The Sun is made of plasma (super-hot mix of electrons and ions) which naturally create magnetic fields when they move. Figure 4: the magnetic fields of the Sun. The magnetic fields follow an 11-year cycle of solar minima and maxima. During solar minima, solar explosions are infrequent; whereas, during solar maxima, they occur a lot more frequently. It is these bursts of solar energy (the solar wind) that cause the Aurora Borealis on Earth (see Van Allen Radiation belts). 7 P a g e

8 1.4.2 Sunspots The photosphere is the layer of the Sun that we observe. Sun spots are darker, cooler areas on the photosphere layer of the Sun. They are around 2000 K cooler than the photosphere and can be up to 50,000 kilometres in diameter. They occur over a region of intense magnetic activity and they are the site at which solar ejections (coronal mass ejections or solar flares) are erupted from. Figure 5: Sunspots on the Sun s surface. 8 P a g e

9 1.5 Life cycle of a star YOU MUST HAVE A HAND DRAWN SKETCH OF THIS IN YOUR COURSEWORK. Figure 6: the life cycle of a star depending on mass. Stars with a similar mass to the Sun follow the top cycle. Stars with larger masses than the Sun follow the bottom cycle. The Sun is currently at the Main Sequence (average star) stage. When a significant portion of the hydrogen is fused into helium, the Sun s internal core will collapse and heat up until it is hot enough to fuse helium into larger atoms. At this point, the outward force due to pressure overcomes the inward force due to gravity and the Sun begins to swell into a Red Giant. During this, the Sun will engulf at least Mercury, Venus and Earth but this is not due to happen for over 5 billion years. The Sun will spend a few thousand to 1 billion years as a red giant before the helium in the core runs out and fusion stops. At this point, the star begins to shrink again until a helium shell is formed. This helium the ignites and the outer layers of the Sun will be blown off forming a planetary nebula. Finally, the core will collapse in on itself and the Sun will become a white dwarf. 9 P a g e

10 Task: read this information below and summarise it in the flow chart on the next page: A star's life cycle is determined by its mass. The larger its mass, the shorter its life cycle. A star's mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust from which it was born. Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes as a protostar. Eventually the temperature reaches 15,000,000 degrees and nuclear fusion occurs in the cloud's core. The cloud begins to glow brightly, contracts a little, and becomes stable. It is now a main sequence star and will remain in this stage, shining for millions to billions of years to come. This is the stage our Sun is at right now. As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take from there. For low-mass stars, after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed from the outer layer. The core remains as a white dwarf and eventually cools to become a black dwarf. High-mass stars are also born in nebulae and evolve and live in the Main Sequence. However, their life cycles start to differ after the red giant phase. When the core contains essentially just iron, fusion in the core ceases. This is because iron is the most compact and stable of all the elements so it taken much energy to break up compared with any nucleus in any other element. Creating heavier elements than iron requires an input of energy rather than the release of energy. At this point, the star begins the final stage of the gravitational collapse. The core reaches a temperature of over 100 billion degrees as the iron atoms are crushed together and the star explodes into a supernova. If the remnant explosion is times the mass of our Sun after the explosion, it does something very different. The force of gravity overcomes the nuclear forces which keep protons and neutrons from combining. The core is thus swallowed by its own gravity. It has become a black hole which readily attracts any matter and energy that comes near it. You may also wish to use this website: 10 P a g e

11 Condense the information above and summarise what happens at each stage. Stellar nebulae Average star Massive star Red Giant Red Supergiant Planetary nebula Supernova White dwarf Neutron star Black hole 11 P a g e

12 1.6 Hertzsprung-Russel Diagram The HR diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, colour, temperature and evolutionary stage. The graph has 5 main regions. Once they are in a stage, they DO NOT progress up that stage. Figure 7: the Hertzsprung-Russel diagram. (Giant stars are represented by larger circles.) The main sequence contains your average-sized stars for which the luminosity is related directly to temperature. Main sequence stars tend to be in the beginning or middle of their lives, when the pressure outwards from nuclear fusion still balances out the inward force of gravity they are said to be hydrostatic equilibrium. Red Giants and Super giants are older small/medium sized stars which have bloated, and are therefore more luminous despite not being very hot. White dwarfs have shed their outer layers and are super-hot, but not very luminous because they are small. Blue giants are hot and massive but do not live for very long. 12 P a g e

13 1.7 Spectral classes Spectral Class Intrinsic Color Temperature (K) Solar masses Prominent Absorption Lines O Blue Over 25, He+, O++, N++, Si++, He, H B Blue 11,000 25, He, H, O+, C+, N+, Si+ A Blue-white 7,500 11, H(strongest), Ca+, Mg+, Fe+ F White , H(weaker), Ca+, ionized metals G Yellow-white 5,000-6, H(weaker), Ca+, ionized & neutral metal K Orange 3,500-5,000 M Red Under 3, Ca+(strongest), neutral metals strong, H(weak) Strong neutral atoms, TiO Table 2: showing the spectral classes of the different types of stars compared with their temperature and masses. The Sun has an outer layer temperature of around 5,500 K meaning its spectral class is G. As it is closer to G than K it is said to be G2. TYPE Ia Ib II III IV V VI Star Very luminous supergiants Less luminous supergiants Luminous giants Giants Subgiants Main sequence stars (dwarf stars) Subdwarf Table 3: showing the classification for different types of stars. The Sun is a main sequence star meaning it is a type V (five) star. The overall spectral class of the Sun is G2V. 13 P a g e

14 Task: Complete the table below using the tables 2 and 3 to help you. Type of star (from HR Name of star Temperature Spectral class diagram) Main sequence The Sun 5,500 K Red giant Betelgeuse 3,500 K White dwarf Sirius 9,940 K Table 2 shows that the mass of the star is proportional to the temperature, i.e. the hotter the star, the more massive it is this does not mean bigger. For example, from table 1 it can be seen that white dwarves can be 2-4 time more massive compared to main sequence stars and they are twice as hot. 2. Features, characteristics and relationship factors of the Earth and Moon 2.1 Structure of the Earth Figure 8: the structure of the Earth. Layer Composition Inner core Solid iron and nickel Outer core Liquid iron and nickel Mantle Has the properties of a solid, but can flow very slowly Crust Relatively thin and rocky Table 4: composition of each layer of the Earth s structure. 14 P a g e

15 Quick question: can you remember which gases make up our atmosphere? (Extension: what about their respective amounts?) Figure 9: pie chart showing the relative amounts of each gas in Earth s atmosphere. 2.2 Rotations and orbital characteristics Day and night The Earth is tilted by It is this tilt (or axis) that the Earth rotates around. Solar day time taken for the Sun to reach the same point in the sky 24 hours. Sidereal day - time taken for the Earth to make one complete rotation about its axis 23 hours 56 minutes and 4 seconds. Figure 10: Earth s tilted axis. 15 P a g e

16 A solar day is represented by numbers 1 and 3. A sidereal day is represented by numbers 1 and 2. Figure 11: illustration of solar and sidereal days. One complete rotation about the axis gives us one full day. During this time, Earth experiences day and night: Figure 12: illustration of day and night on Earth. 16 P a g e

17 2.2.2 Orbits The moon orbits the Earth in a solar month which is 28 days long. The Earth orbits the Sun in a solar year which is days long. The rest of the planets in our Solar System all orbit the Sun but take different amounts of time to do so. Figure 13: Earth and Moon orbits. The time it takes Earth to orbit the Sun once is known as the orbital period and is represented by the letter T. The distance from the Earth to the Sun is represented by the letter a this is known as the semi-major axis. This varies as the Earth goes round the Sun in an elliptical orbit. The relationship which links these together is: T 2 a 3 This relationship describes the orbits of all the planets in the Solar System i.e. due to the elliptical orbit, when the planet is closest to the Sun (a is smaller), it will travel further in one month compared with when the planet is further away (a is larger). Figure 14: Kepler s 3 rd law. The Sun s gravitational force and the centripetal force keeps the planets in these curved orbits. 17 P a g e

18 2.2.3 Eclipses Figure 15: illustration of lunar and solar eclipses. Lunar eclipse only happen when the moon is full. A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra. Solar eclipse occurs when the Moon gets in between Earth and the Sun, and the moon casts a shadow over Earth. 18 P a g e

19 2.2.4 Moon phases Task: move the ping pong ball around you and draw what you see every 45. The whole cycle (of the moon orbiting the Earth) takes 28 days so some months we see 2 full moons. This is called a lunar cycle. When the Sun is effectively behind the Moon, the side of the Moon that faces the Earth looks like a shadow this is called a new moon. When the side of the Moon that faces the Earth is fully illuminated by the Sun, this is called a full moon. 19 P a g e

20 The Moon is tidally locked so that we only ever see one side of the Moon. It takes 28 days for the Moon to make one complete rotation about its axis which is also the length of a lunar month. Because we only ever see one side of the moon the other side is known as the dark side of the moon. Figure 17: the two sides of the Moon. Figure 16: the tidal locking of the Moon. The side of the moon that faces us is lightly cratered and has mares which are similar to the Grand Canyon but on the moon. Water sculpted the Grand Canyon so it is believed that water once sculpted these sections of the Moon before it was stripped of its atmosphere. The other side is heavily cratered as it has been hit by space debris. 20 P a g e

21 2.2.5 Tidal effects on Earth The Moon causes the tides to go in and out. We experience 2 high tides in 24 hours with the one closest to the Moon having the highest tide. Figure 18: high and low tide on Earth this happens twice daily. The Moon is 391 times closer to the Earth than the Sun is, and this results in the Moon s gravitational force being 175 times larger on the Earth when compared with the Sun. The gravitational force from the Moon is not strong enough to shape rock but is strong enough to shape the water oceans. Figure 19: The Moon s effect on the tides on Earth. 21 P a g e

22 2.2.6 Van Allen Radiation Belts The Van Allen belts are a collection of charged particles, gathered in place by Earth s magnetic field. The Earth s magnetic field is similar to that of a bar magnet. Figure 20: The Van Allen radiation belts. The Northern lights (Aurora Borealis) are formed when the Sun s solar wind (electrically charged particles) collides with the Earth s magnetic field causing it to stretch and eventually, these charged particles travel into the Earth s atmosphere down the poles where the magnetic field is weakest. Figure 22: the Aurora Borealis (or Northern lights). Figure 21: the effect of the Sun s solar wind on the Earth s magnetic field. 22 P a g e

23 2.3 Forces involved in the structure of the Solar System All objects are attracted to each other by the force of gravity. Every object will have its own gravitational force, but it only becomes noticeable when there is a massive object like a planet, star or moon. The Sun s gravitational force is responsible for keeping the planets in their orbits. These orbits are all elliptical. As this force is pulling the objects towards the centre, it is known as a centripetal force. The Planets also have strong gravitational forces, with Jupiter s being the strongest as it is the most massive. Objects (like comets and asteroids) which are flying around the Solar System can be captured by the gravitational force of the planets and brought into their orbits this is how Moon s come to be. Figure 23: the structure of the Solar System. The planets all follow elliptical orbits around the Sun as it is the Sun s gravitational field strength that keeps these planets in these orbits. 23 P a g e

24 3. Features and characteristics of the inner and outer planets 3.1 Inner and outer planets Can you remember the planets in the Solar System, in order? M V E M J S U N Planet Position Composition and atmospheric conditions Mass Distance Ring system Surface features Axis of rotation M V E M J S U N These planets all orbit the Sun in elliptical orbits which are fixed due to the gravitational force of the Sun acting on each planet individually. YOU WILL NEED TO FIND DIAGRAMS OF EACH OF THE PLANETS. 24 P a g e

25 3.2 Moons of the Solar System Planet Number of Moons Names of Moons Image Surface features Mercury 0 Venus 0 Earth 1 The Moon Maria (seas) Craters Highlands Mars 2 Jupiter 69 known moons, 4 main moons. Ganymede Half is old and cratered Half is young and lighter Europa Water-ice crust Striated cracks and streaks Callisto Most heavily cratered object in the Solar System Io Highly volcanic Saturn 18 known moons, 1 main moon. Titan Lakes of liquid methane which erode crater impacts Volcanic Uranus 27. Titania Neptune P a g e

26 3.3 Planetary ring systems All the outer planets have rings some are more prominent than others. YOU NEED TO LABEL SATURN S RINGS you might want to research another diagram. Jupiter Saturn Uranus Neptune Figure 24: the ring systems of the outer solar system planets. 26 P a g e

27 4. Features and characteristics of other solar system objects Asteroids Asteroids are rocky, airless worlds that orbit our Sun but are too small to be called planets. They form a belt between Mars and Jupiter. Kuiper belt Disc-shaped region of icy bodies including dwarf planets such as Pluto and comets beyond the orbit of Neptune. The Kuiper belt contains asteroids, comets and dwarf planets. Meteors Comets or asteroids passing through the Earth s atmospheres these are known as shooting stars. These are short period comets. Comets Icy body that releases gas or dust. They are often referred to as dirty snowballs. They have tails which are formed when the sunlight reflects off the dust they release. Oort cloud Spherical shell of icy objects which exist in the outermost reaches of the solar system. It houses long-period comets. 4.1 Asteroids Found in the asteroid belt which is located between Mars and Jupiter. You need to include images of each and write about features of large asteroids such as size, composition, location, and a BRIEF description of its mission. NEAR Shoemaker to Eros: Rosetta/ Philae to Comet 67P: 27 P a g e

28 4.2 Comets You need to include an image of each and BRIEFLY write about: 1) Short period comets (generally less than 200 years orbit): Halley s comet: Shoemaker-Levy 9 comet: 2) Long period comets (generally more than 200 years orbit): Hale-Bopp comet: Haykutake comet: Kohoutek comet: Meteors Meteors are meteroids, comets or asteroids which have entered the Earth s atmosphere. When they burn up in the atmosphere we observe meteor showers of shooting stars. At different points during the year we can observe meteor showers on Earth: Perseids (August), Orionids (October) and Geminids (December). Meteors have 3 main classifications stony, stony-iron and iron. 28 P a g e

29 5 Measuring astronomical distances 5.1 Trigonometric parallax We can find out how far stars are away using trigonometric parallax. Look at the diagram below the left hand side shows the Earth orbiting the Sun. As the Earth reaches point A on the orbit, we can mark where a particular star appears in the night sky. As the Earth reaches point B, we can mark where that same star lies on the night sky. It appears to be in a different place. Figure 25: the use of trigonometric parallax to measure distances to stars. Where these two lines cross over gives the location of the star. To find the distance to the star we need to use the equation d = 1. The parallax angle, p, is found by halving the angle we measure, and is measured in minutes of arc p and seconds of arc. This is because the angles are tiny and degrees are too large. (Think of it like measuring the width of a cell in metres it would be impossible so we would measure it in micrometres first and then convert it afterwards. The conversion from degrees to minutes of arc to seconds of arc is shown below: Seconds of arc Minutes of arc Degrees SECONDS OF ARC CAN ALSO BE REFERRED TO AS ARCSECONDS. MINUTES OF ARC CAN ALSO BE REFERRED TO AS ARCMINUTES. 29 P a g e

30 Questions: 1) How many seconds of arc are there in 1 minute of arc? 2) How many seconds of arc are there in 1 degree? 3) How many seconds of arc are there in 1 minute of arc? 4) How many seconds of arc are there in 1 degree? Limitations of parallax Parallax is a great method of measuring distances; however, it does have its limitations because the angles that we are measuring are so tiny, so the uncertainties in the values are very high. Any stars which are further that 100 Parsecs away give an angle which is too small for Earth based telescopes to measure. Also, atmospheric turbulence can affect the readings so again, the angle may not be completely accurate. 5.2 Cepheid variables Cepheid variables are stars which pulsate in luminosity (brightness). The period (in days) of these pulses is regular and can be found by noting when the star is at its brightest, and then noting when it reaches that point later on (see diagram below brightness variation of Cephei). Magnitude tells you its brightness/luminosity. Figure 26: the pulsation of Cepheid variable stars in luminosity over a 2-week period. 30 P a g e

31 They are sometimes known as standard candles because they pulsate in brightness as follow the inverse square law meaning their brightness quarters when the distance from them doubles, the brightness ninths when the distance trebles: Figure 27: showing the inverse square law relationship of star light at increasing distances. If you know the source strength, or absolute luminosity, of an astronomical object then you can calculate the distance from the observed luminosity using the inverse square law. 5.3 Eclipsing binaries Binary stars are stars which orbit a common centre of mass (orbit the same centre), have a look at this link: 6 Expanding Universe 6.1 Redshift and blue shift As a galaxy accelerates away from us, the wavelength of light emitted by the galaxy is stretched. Due to the wavelength being stretched, the light from the galaxy appears red (see both diagrams below). This is known as RED SHIFT and only happens when galaxies accelerate AWAY from us. If a galaxy moves towards us (Andromeda galaxy), the wavelength of light it emits is squished. Due to this, the light from the galaxy appears blue (see both diagrams below). This is known as BLUE SHIFT and only happens when galaxies accelerate TOWARDS us. 31 P a g e

32 Figure 28: visible light wavelengths. Figure 29: red and blue shift of light. 6.2 Hubble s Law Edwin Hubble uncovered evidence that the Universe is expanding. He compared redshift velocities (speeds of galaxies moving away from us), with his estimated distances to those galaxies from Earth and found that: Hubble s Law The further away a galaxy is from another point in space, the faster it appears to recede, and therefore, the Universe is expanding. Figure 30: graph confirming Hubble s Law. The equation for Hubble s Law is: v = H 0 d Where v is the velocity of recession, d is the distance to the galaxy, and H 0 is Hubble s constant. Hubble s constant can be found by finding the gradient of a velocity-distance graph: H 0 = v d Finally, by taking the reciprocal of Hubble s constant, you can find the age of the Universe! Age of Universe = 1 H 0 32 P a g e

33 This then has to be converted into years. The current age of the Universe has been estimated to be 13.7 billion years old!!! If the Universe is expanding, it must mean that if we look back in time, the Universe must have been shrinking to a point this point is known as the Big Bang Theory! 6.3 Possible fate of the Universe based on density You need to include a BRIEF description on these possible fates Cosmological theories of the present day 7.1 The Big Bang Theory Redshift provides evidence that the Universe is expanding as we go forward in time. But what about if we go back in time? If it expands going forward, then going back it must shrink. As the Universe shrinks, everything in it gets closer together until it reaches a point in space this point is thought to have been where the Big Bang originated. Scientists have gathered a lot of evidence and information about the Universe. They have used their observations to develop a theory called the Big Bang. The theory states that about 13.7 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today. The fact that galaxies are moving away from us, and the further away a galaxy, the faster it is moving is evidence for the Big Bang. Scientists have also detected cosmic microwave background radiation (CMBR) which is thought to be the electromagnetic radiation left over from an early stage of the Universe. Figure 31: The Cosmic Microwave Background Radiation believed to have been left over from the Big Bang. 33 P a g e

34 7.2 Olbers paradox A scientific model describes the Universe to be infinitely old. If this was true then eventually, we would see a star in any direction, even if it is far away. If the Universe was infinitely old then the light that has travelled from very distant stars (40 billion light years away for example), will have already reached us, and this would mean that the night sky should look uniformly (evenly) bright which is does not. For this model to work, it must also describe a static Universe which means it is not expanding and therefore light from galaxies would not be red or blue shifted. The fact that the night sky is not as bright as the Sun is called Olbers paradox: 1) There is too much dust to see the distant stars. 2) The Universe only has a finite (opposite of infinite) number of stars. 3) The position of stars in not even stars could hind behind one another. 4) The Universe is expanding. 5) The Universe is young meaning distant light has not reached us yet. Figure 32: infinite universe paradox. Task: use the web address below to make a BRIEF comment about all these explanations and if they are true or false P a g e