AQA GCSE Physics 3-4 Stars & Space GCSE Physics pages 266 to 275 April 10th 2010 AQA GCSE Specification STARS & GALAXIES 13.10 What is the life history of stars? Using skills, knowledge and understanding of how science works: • to explain how stars are able to maintain their energy output for millions of years • to explain why the early Universe contained only hydrogen but now contains a large variety of different elements. Skills, knowledge and understanding of how science works set in the context of: • Our Sun is one of the many millions of stars in the Milky Way galaxy. • The Universe is made up of at least a billion galaxies. • Stars form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets. • Gravitational forces balance radiation pressure to make a star stable. • A star goes through a life cycle (limited to the life cycle of stars of similar size to the Sun and stars much larger than the Sun). • Fusion processes in stars produce all naturally occurring elements. These elements may be distributed throughout the Universe by the explosion of a star (supernova) at the end of its life. The Milky Way The Milky Way is the name of our galaxy. From Earth we can see our galaxy edge-on. In a very dark sky it appears like a ‘cloud’ across the sky resembling a strip of spilt milk. A very dark sky is required to see the Milky Way this clearly Galaxies Galaxies consist of billions of stars bound together by the force of gravity. There are thought to be at least 200 billion galaxies in our Universe each containing on average 2 billion stars. The Andromeda Galaxy The Sun’s position in the Milky Way Types of galaxy Barred-Spiral – NGC 1300 Our galaxy is this type Spiral – The Whirlpool Galaxy Elliptical – M32 Irregular – The Small Magellanic Cloud Classifying galaxies Choose appropriate words to fill in the gaps below: Universe The ___________ is made up of billions of galaxies which billions consist of __________ of stars bound to each other by the force of ___________. gravity galaxy Sun is The name of our _________ is The Milky Way. The ______ located towards the outer edge of our galaxy. spiral The are different types of galaxy; ________, barred-spiral, barred-spiral elliptical and irregular. The Milky Way is a ____________ Andromeda galaxy. The _____________ Galaxy is the nearest spiral galaxy to the Milky Way. WORD SELECTION: Andromeda galaxy spiral Sun Universe gravity barred-spiral billions Galaxies Notes questions from pages 266 & 267 1. 2. 3. 4. 5. 6. 7. (a) What is a galaxy? (b) Name our galaxy. (c) How many galaxies are there? Copy and answer question (a) on page 266. Outline the history of the Universe to the present day. Explain the part played by gravity in the evolution of the Universe. Copy and answer questions (b) and (c) on page 267. Copy the ‘Key points’ table on page 267. Answer the summary questions on page 267. Galaxies ANSWERS In text questions: (a) When we use a powerful telescope to see a distant galaxy, we are seeing the galaxy as it was billions of years ago because the light from it has taken billions of years to reach us. (b) About 13 billion years. (c) They are both positively charged, so they repel each other. The force of repulsion is much greater than the force of gravity between them. Summary questions: 1. (a) Expanded, cooled. (b) Attracted. (c) Formed. 2. (a) (i) We could not send a probe far enough. (ii) Galaxies take millions of years to form; we couldn’t wait that long. (b) (i) Gravitational forces hold the stars together. (ii) The Universe has expanded leaving these vast spaces. Stars A star is a massive, luminous ball of gas that is held together by gravity. The Sun is a typical star that consists of about 75% Hydrogen, 24% Helium and 1% other elements such as carbon and oxygen. There are about 2 billion stars in our galaxy. The Pleiades Star Cluster Stars colours The colour of a star depends on its surface temperature. The Sun is an average temperature yellow star. RED = 3000°C ORANGE = 5000°C YELLOW = 6000°C WHITE = 8000°C BLUE = 10000°C and above Sirius, the brightest star in the night sky is a blue-white star The birth of the Sun Nebula Stars usually form inside a nebula. This is a cloud of mostly hydrogen along with smaller amounts of other elements ‘dust’. Many young stars are found inside the Orion nebula Protostar Due to gravitational attraction, the gas and dust clumps together. Gravitational potential energy is converted into heat energy. The gas starts to glow forming a protostar. Artist's conception of the birth of a star within a nebula Nuclear fusion When the temperature rises above about 10 million°C hydrogen nuclei join together to form Helium by the process of nuclear fusion. Energy is released. The star becomes stable when the radiation produced causes an outward pressure that prevents further gravitational collapse of the star. Nuclear fusion from hydrogen isotopes deuterium (H2) and tritium (H3) The birth of the solar system About 99.9% of the original gas and dust formed the Sun. The remaining 0.1% formed the planets and other bodies of the solar system. The future of the Sun Main sequence The Sun is about half way through a 10 billion year period in its life cycle called ‘main sequence’ During this time hydrogen in the core of the Sun is converted into Helium by the process of nuclear fusion. The Sun will gradually become hotter over time so that in about two billion years time life will no longer be possible on Earth. Red Giant In about 5 billion years time the hydrogen in the Sun’s core will run out. Without outward radiation pressure the core will collapse under gravity and become even hotter. Eventually the temperature will be high enough to cause the fusion of helium into heavier elements such as carbon and oxygen. The now greater outward radiation pressure will cause the Sun to expand into a Red Giant. When the Sun becomes a Red Giant it will be nearly as big as the Earth’s orbit about the Sun. Planetary Nebula and White Dwarf After only a few million years the Helium will also run out in the Sun’s core. A final collapse of the core occurs to form a very hot dense object about the size of the Earth called a white dwarf. The rest of the Sun is blown away to form a planetary nebula (from which a new star might form). The white dwarf will gradually cool over billions of years to form a black dwarf. The Ring Nebula Low mass stars The Sun is an average star. There are many cooler stars of lower mass called red dwarfs. These are very faint and can only be seen through telescopes. The nearest star to the Sun is a red dwarf called Proxima Centauri. It is just over 4 light years away. High mass stars Most of the stars we can see in the sky are more massive than the Sun. Compared with the Sun they: - are larger - are brighter - are bluer when main sequence - pass through their life cycles more quickly - sometimes end their lives differently All the main stars in the constellation of Orion are more massive than the Sun. Red supergiants Higher mass stars form larger red giants. The star Betelgeuse (top left in Orion) is larger than the orbit of Mars about the Sun. Supergiants will also cause elements such as carbon and oxygen to undergo nuclear fusion to form even heavier elements such as silicon and iron. The internal ‘onion’ structure of a red supergiant star Supernovae When a red supergiant star causes iron in its core to undergo nuclear fusion energy is absorbed causing a great implosion. This rebounds and causes a massive explosion that can for a few days outshine a whole galaxy. This is called a supernova. Supernovae are very rare. They can be seen in the daylight sky. The last observed supernovae in our galaxy took place in 1604. The Crab Nebula was formed after a supernova observed in 1054 Neutron stars The core left over from a supergiant star can be so massive that gravity causes electrons and protons to combine to form neutrons. This is a neutron star. A neutron star is only about 10km in diameter and is extremely dense. A teaspoon full of neutron star has a mass of about two billion tonnes. Some neutron stars, called pulsars, emit regular radio signals. X-ray image of the neutron star inside the Crab Nebula Black holes The most massive stars collapse to form black holes. The gravity caused by black holes is so strong that nothing can escape, including light. Black holes can only be observed from the affect they have on surrounding objects such as a companion star. The life cycle of the Sun NOTE: Due to its relatively low mass the Sun will not become a red supergiant, supernova, neutron star or black hole. Star evolution summary protostar main sequence star CORE MASS red supergiant > 1.4 ʘ supernova red giant nebula MASS < 0.23 ʘ CORE MASS planetary nebula brown dwarf (failed star) < 1.4 ʘ MASS 0.23 to 4 ʘ MASS > 0.05 ʘ MASS < 0.05 ʘ MASS >4ʘ ʘ = Sun white dwarf <3ʘ >3ʘ neutron star black hole Choose appropriate words to fill in the gaps below: hydrogen Stars are made mostly from __________ and generate their energy by nuclear _________. Stars are formed in nebulae fusion when ________ gravity causes gas and dust to clump together. expand to form a Towards the end of its life, the Sun will _________ red giant after which most of its material will be blown away nebula leaving behind a small white _____ dwarf star. as a planetary ______ massive stars than the Sun may undergo a supernova More ________ neutron explosion and become _________ stars or black holes. WORD SELECTION: expand neutron gravity fusion massive hydrogen nebula dwarf The life history of a star Notes questions from pages 268 & 269 1. 2. 3. 4. 5. 6. 7. Copy Figure 2 on page 269. Outline the life history of a star like our Sun. Your account should include what is meant by (a) protostar, (b) red-giant, and (c) white dwarf. Explain the additional stages undergone by the most massive stars. Your account should include what is meant by (a) supernova, (b) neutron star, and (c) black hole. (a) How does a star produce energy? (b) Explain why the Sun is neither expanding or contracting at the present time. Copy and answer questions (a), (b), (c) and (d) on pages 268 and 269. Copy the ‘Key points’ table on page 269. Answer the summary questions on page 269. The life history of a star ANSWERS In text questions: (a) The potential energy of gas and dust decreases when it gathers and is transformed into heat energy. (b) The outward pressure of radiation from its core stops it collapsing. (c) Gravity. (d) Gravity. Summary questions: 1. (a) B, A, C, D. (b) (i) A (ii) It will fade out and go cold. 2. (a) (i) Expand, collapse. (i) Explode, collapse. (b) (i) The neutron star must have sufficient mass. (ii) The gravitational field is so strong that nothing can escape from it. The formation of elements Hydrogen and helium Hydrogen and some helium was formed at the time of the Big Bang. Helium is also formed by nuclear fusion in main sequence stars like the Sun. Lighter elements Elements such as carbon, oxygen and silicon are formed by nuclear fusion in red-giant stars. The heaviest element formed in red-giants is iron. The internal ‘onion’ structure of a red supergiant star Heavier elements All elements heavier than iron are thought to have been formed during supernovae explosions. The fact that such elements exist on Earth is evidence that our Sun and the entire solar system has been formed out of the supernova explosion of an earlier star. Elements such as copper, gold and uranium were formed in supernovae explosions Choose appropriate words to fill in the gaps below: The lightest and most common element in the Universe is hydrogen helium ___________. Hydrogen and some __________ were formed from the Big Bang. fusion Most elements have been formed by nuclear _________ in the lighter cores of stars. Helium and the __________ elements such as carbon _________ are formed by stars like the Sun. iron Elements up to ______ are formed in the core of red supergiant stars. The heaviest elements are formed in supernovae explosions. ___________ WORD SELECTION: carbon hydrogen fusion helium lighter iron supernovae How the chemical elements formed Notes questions from pages 270 & 271 1. 2. 3. 4. 5. 6. Explain the two different processes by which (a) lighter and (b) heavier elements were formed. Copy and answer questions (a) and (b) on pages 270 and 271. Outline the ways of trying to discover the presence of extra-terrestial life. Copy and answer question (c) on page 271. Copy the ‘Key points’ table on page 271. Answer the summary questions on page 271. How the chemical elements formed ANSWERS In text questions: (a) In a supernova explosion. (b) Its half-life is very short compared with the age of the Sun. Any plutonium formed when the Sun formed would have decayed long ago. (c) Carbon atoms are in all the molecules that make up living objects. Summary questions: 1. (a) Hydrogen. (b) Uranium. (c) Helium, iron. (d) Hydrogen. 2. (a) Stars, supernova. (b) Supernova, galaxy. (c) Stars, supernova. The light year A light year is the distance travelled by light in one year. Light travels at 300 000 000 metres per second = 300 000 kilometres per second = 18 million kilometres per minute = 1.08 billion km per hour = 26 billion km per day = 9.5 trillion km per year! Question Calculate the distance to the nearest star to the Sun, Proxima Centauri, in kilometres and how long it would take to reach it travelling at 100 km/h (60 mph) if this star is 4.2 light-years away. Distance = 40 trillion kilometres (4.0 x 1012 km) It would take about 45 million years to reach this star. Universal issues Notes questions from pages 272 & 273 1. Answer questions (a), (b) and (c) on page 272. Universal issues ANSWERS (a) 2 km (b) 9000 km (c) over 30 million km How Science Works ANSWERS a) b) c) d) Myths: how the Earth was created by Phan Ku. Observations: that the Sun, moon, planets and stars move across the sky. The data concerning the size of the Moon. Ptolomy’s Earthcentred model required the Moon to speed up and slow down and hence therefore to change its size as seen from Earth. The Moon, when measured, did not show these changes in apparent size. Example of hypothesis: Anaxagoras hypothesised that the Sun and the Moon were made of rocks. e) Ptolomy’s Earth-centred theory of the Universe. f) Copernicus’ theory of the universe because it is supported by much evidence, but theories are not completely proven in all instances and therefore always open to being disproved. This is more obviously shown by Bruno. Planets are being discovered around stars, but there is no evidence of life outside of the Earth. g) Anaxagoras and Bruno were examples of political influences on science.