WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.648 Unit S P 7: Astronomy. 00:00:09.300 --> 00:00:16.003 Our Solar System contains the Sun, eight planets and their natural satellites, dwarf planets, asteroids and comets. 00:00:16.153 --> 00:00:20.521 The Sun is a star; the Moon is Earth’s natural satellite, not a planet. 00:00:23.200 --> 00:00:30.671 In increasing distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. 00:00:30.821 --> 00:00:35.003 Pluto is classified as a dwarf planet, not one of the eight planets. 00:00:35.153 --> 00:00:38.867 Read the inner column from top to bottom, then the outer column. 00:00:39.017 --> 00:00:41.812 Distances and sizes are not represented. 00:00:46.467 --> 00:00:49.027 Asteroids are mostly small rocky bodies. 00:00:49.177 --> 00:00:54.389 Comets contain ice and dust and often have long, oval-shaped (elliptical) orbits. 00:00:54.539 --> 00:01:01.887 Near the Sun, a comet develops a tail that points generally away from the Sun, not necessarily behind the direction it is moving. 00:01:04.567 --> 00:01:09.558 Mass is the amount of matter and stays the same when an object moves from Earth to the Moon. 00:01:09.708 --> 00:01:16.341 Weight equals mass times local gravitational field strength, so weight and G differ between celestial bodies. 00:01:19.000 --> 00:01:22.136 Early geocentric models placed Earth at the centre. 00:01:22.286 --> 00:01:32.386 Heliocentric models put the Sun at the centre of planetary motion; observations such as Venus’s phases and Jupiter’s moons helped challenge simple Earth-centred models. 00:01:35.067 --> 00:01:41.348 Improved telescopes, repeated observations and mathematical models changed scientific ideas. 00:01:41.498 --> 00:01:47.353 Accepting a new model depends on evidence and explanatory power, not only on who proposes it. 00:01:50.033 --> 00:01:53.572 Gravity provides the inward force that maintains an orbit. 00:01:53.722 --> 00:02:03.097 In a circular orbit, speed can be constant while velocity changes continuously because its direction changes; the body is accelerating towards the centre. 00:02:05.767 --> 00:02:10.003 On a circular-orbit diagram, draw gravity towards the central body. 00:02:10.153 --> 00:02:16.262 Draw velocity along a tangent: a line touching the circle at that point, at right angles to the radius. 00:02:16.412 --> 00:02:20.062 Gravity points inwards, not along the direction of motion. 00:02:20.212 --> 00:02:24.991 Speed may stay constant while direction changes, so velocity is changing. 00:02:29.667 --> 00:02:34.708 Planets orbit the Sun, moons orbit planets, and artificial satellites can orbit Earth. 00:02:34.858 --> 00:02:37.236 An orbit does not have to be circular. 00:02:37.386 --> 00:02:40.467 Comets often have more stretched-out elliptical orbits. 00:02:43.133 --> 00:02:52.326 For stable circular orbits around the same central body, a smaller orbital radius corresponds to a higher speed, and a larger radius to a lower speed. 00:02:52.476 --> 00:02:57.712 Changing the orbital speed generally requires a change in stable orbital radius. 00:03:00.367 --> 00:03:07.085 An orbiting satellite is continually falling under gravity while moving sideways fast enough to keep missing the surface. 00:03:07.235 --> 00:03:13.580 It does not need gravity to vanish; weightlessness inside an orbiting craft arises from shared free fall. 00:03:16.233 --> 00:03:23.901 For a circular orbit, average orbital speed equals two times pi times orbital radius divided by orbital period. 00:03:24.051 --> 00:03:31.229 Use radius from the central body's centre, not height above the surface, and convert the period to the required time unit. 00:03:33.900 --> 00:03:37.338 Stars form from a nebula, a cloud of gas and dust. 00:03:37.488 --> 00:03:43.075 Gravity draws matter together; temperature and pressure rise until fusion can sustain a star. 00:03:45.733 --> 00:03:53.188 During the main-sequence stage, outward pressure associated with energy from fusion balances inward gravitational attraction. 00:03:53.338 --> 00:03:57.229 This balance keeps the star approximately stable over a long period. 00:03:59.900 --> 00:04:05.179 When a Sun-like star uses up the hydrogen in its core, it expands into a red giant. 00:04:05.329 --> 00:04:09.764 It eventually loses its outer layers, leaving a hot, dense white dwarf. 00:04:09.914 --> 00:04:12.026 The white dwarf cools over time. 00:04:12.176 --> 00:04:17.497 The final remnant of a massive star depends on its mass; these are alternative paths. 00:04:22.167 --> 00:04:27.639 A star much more massive than the Sun can become a red supergiant and end in a supernova. 00:04:27.789 --> 00:04:32.766 The remnant may be a neutron star or, for sufficiently massive remnants, a black hole. 00:04:35.433 --> 00:04:45.128 Mass strongly affects a star’s evolution: more massive stars generally use their fuel faster and have shorter main-sequence lifetimes despite containing more fuel. 00:04:47.800 --> 00:04:51.656 Gravity and thermal pressure play different roles through the life cycle. 00:04:51.806 --> 00:04:57.905 A white dwarf is not simply a smaller ordinary main-sequence star burning hydrogen in the same way. 00:05:00.567 --> 00:05:07.947 When a wave source moves relative to an observer, the observed frequency and wavelength can change: this is the Doppler effect. 00:05:08.097 --> 00:05:12.164 A source moving away gives a longer wavelength and lower frequency. 00:05:12.314 --> 00:05:17.021 A source moving towards the observer gives a shorter wavelength and higher frequency. 00:05:19.700 --> 00:05:24.723 Spectral lines from many distant galaxies are shifted towards longer wavelengths. 00:05:24.873 --> 00:05:33.789 More distant galaxies generally show a greater red-shift, supporting expansion of space on large scales; not every nearby galaxy must be receding. 00:05:36.467 --> 00:05:41.149 The Big Bang model describes a Universe expanding from an early hot, dense state. 00:05:41.299 --> 00:05:46.706 It is not an explosion from one ordinary location into pre-existing empty space. 00:05:49.367 --> 00:05:58.464 The Steady State model also allowed expansion but proposed continual creation of matter to keep average density constant and no overall beginning. 00:05:58.614 --> 00:06:02.027 Red-shift alone therefore did not distinguish the two models. 00:06:04.700 --> 00:06:11.973 Cosmic microwave background radiation is detected across the sky and fits the cooled remnant of the hot early Universe. 00:06:12.123 --> 00:06:17.839 Together with red-shift and other evidence, it supports the Big Bang rather than the Steady State model. 00:06:20.500 --> 00:06:25.598 The Big Bang is the currently accepted model because the evidence supports it more strongly. 00:06:25.748 --> 00:06:33.244 Scientific models can be refined as new evidence appears; acceptance does not mean every question about the Universe is answered. 00:06:35.900 --> 00:06:45.434 Observations progressed from naked-eye records to optical telescopes, photography, electronic detectors, radio astronomy and space-based instruments. 00:06:45.584 --> 00:06:48.963 Different wavelengths reveal different processes and objects. 00:06:51.633 --> 00:06:59.866 Earth’s atmosphere absorbs some electromagnetic wavelengths, clouds obscure visible light, and atmospheric motion can blur images. 00:07:00.016 --> 00:07:06.511 Telescopes above the atmosphere avoid many of these limitations, although launch and maintenance are expensive. 00:07:06.661 --> 00:07:10.285 Different observing methods provide complementary evidence. 00:07:14.967 --> 00:07:20.629 Ground telescopes still provide valuable observations, including visible and radio windows. 00:07:20.779 --> 00:07:27.317 Choose an observing method for the wavelength, required resolution, practical constraints and scientific question. 00:07:29.967 --> 00:07:31.865 That completes Astronomy. 00:07:32.015 --> 00:07:35.361 Revisit the notes and test yourself on the revision website.