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P7

charlie croxford Inglês 70 flashcards

What is the life cycle of a star?

Flashcards do conjunto (70)

  1. 1What is a sidereal day?A sidereal day is the time take for the earth to spin once and this is about 23 hours and 56 minutes.
  2. 2What is a solar day?A solar day is the time taken for the sun to appear at the same position in the sky. Its 24 hours
  3. 3How long is the moons solar day?Around 25 hours
  4. 4Why do the stars we see change during the year?As we orbit the sun, the direction we face changes slightly each day. As a result we catch a different patch of the sky each night. An earth year is the same time it takes the earth to orbit the sun once, so on the same day each year you should be able to see the same stars in the night sky
  5. 5What are the phases of the moon?The moon only reflects light from the sun, only half is lit up. As the moon orbits the earth, we different amounts of the moon's dark and lit up surfaces. You see a 'full moon' when the whole of lit up side is facing the earth, and a 'new moon' when the dark half faces us. The rest of the phases are in between these two extremes
  6. 6What is an eclipse and state the two types?Eclipses happen when light from the sun is blocked. There is a lunar eclipse and a solar eclipse.
  7. 7What is a 'lunar eclipse'?As it orbits, the moon sometimes passes into the earths shadow. The earth blocks sunlight from the moon, so almost no light is reflected from the moon and it just seems to disappear. A total lunar eclipse is where no direct sunlight can reach the moon. More often, the moon isn't fully in the earths shadow so only part of it appears black - a partial lunar eclipse
  8. 8What is a 'solar eclipse'?The moon is just the right size and distance away that when it passes between the sun and the earth, it can block out the sun. This is called a solar eclipse. From some parts of the earth the sun is completely block - a total solar eclipse. For many places on earth only part of the sun will be blocked - a partial solar eclipse. For most places on earth the sun wont be blocked at all
  9. 9Why do eclipses not happen very often?The angle to of either the earth blocking the moon or the moon blocking the sun is so small that the chances of them lining up is very small.
  10. 10What are the two positions called where astronomers measure stars?THE POLE STAR: is a star that doesn't seem to move because its almost directly about the north pole.
    THE CELESTIAL EQUATOR: is an imaginary plane running across the sky, extending out from the earths equator
  11. 11What is retrograde motion?When a planet seems to change direction and go the other way for a bit, creating a loop of squiggle in its track before going back to normal
  12. 12Why does this happen?It happens because both the planets and earth are moving around the sun, so were seeing the motion of the planet relative to the earth
  13. 13What affects the speed of a wave?Either the density of the substance or medium its travelling through. So when a wave crosses a boundary between the substances (glass to air) it changes speed. This change in speed can cause the wave to change direction as well (refraction)
  14. 14How do converging or convex lenses work?They use refraction to focus light waves to form an image of an object. As a light ray hits the surface of the lens and passes from air to glass, it slows down. This causes light rays to bend towards the 'normal'. When it goes from 'glass to air' it bends away from the normal and speeds up.
  15. 15What can triangular prisms use refractions to form?A triangular prism can refract light to form a spectrum. Different wavelengths of light refract by different amounts. So white light disperses into its different colours as it enters a prism. In a rectangular has parallel boundaries so light enter and comes out together, however in a triangle it does not and is why the light disperses
  16. 16What is a converging lens?It gets fatter towards the middle and causes rays of light to come together to a focus.
  17. 17What is a focal point?The focal point of a lens is where ray initially parallel to the principle axis meet
  18. 18What is focal length?The focal length of a lens is just the distance between the middle of the lens and its focal point. It is more related to power, the more strongly it converges parallel rays of light, the shorter the focal length
  19. 19How do you calculate power (D)?Power (D) = 1 / Focal Length (m)
  20. 20How do you draw ray diagrams from a point source?1. Mark the focal point on the principle axis. Draw 3 parallel rays from the star - one to the centre of the lens, on towards the top and one towards the bottom. Only draw the rays going as far as the middle of the lens.
    2. The middle ray doesnt get refracted, so just extend this ray past the focal point of the lens.
    3. Draw in the rest of the top and bottom rays, making them meet the middle ray above or below the focal point. The image is formed at this point - and its a real image, as the rays actually meet.
  21. 21How do you draw ray diagrams from an extended source?It almost the same as a point source.
    1. Treat two opposite edges of the object as point sources.
    2. As before, the parallel rays for each edge will meet in the line with the focal point of the lens.
    3. A real image is formed between the two points where the rays meet.
  22. 22How does a refracting telescope work?A refracting optical telescope is made up of two convex lenses with different powers - an objective lens and a more powerful eye lens. The objective lens collects the light from the object being observed and forms an image of it, the eyepiece magnifies this image so we can view it. The lenses are aligned to have the same principle axis
  23. 23How do you calculate magnification?Magnification = focal length of objective length / Focal length of eye lens
  24. 24How do more astronomical telescopes work?Most astronomical telescopes use a concave mirror instead of of convex. Concave mirrors are shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge. Concave mirrors are like a portion of a sphere. The centre of the mirrors surface is called the vertex. Halfway between the centre of curvature and the vertex is the focal point. Rays parallel to the mirrors axis reflect and meet at the focal point. By putting a lens near the focal point of the mirror to act as an eyepiece you can form a magnified image
  25. 25What is diffraction and how is it mainly affected?Diffraction is the spreading out of waves. The smaller the gap a waves pass through the bigger it is diffracted
  26. 26How do you detect faint sources?You need to use a huge objective lens, so that more light can be collected. The diameter of the objective lens is called the aperture. However making lenses bigger is difficult and expensive, whereas big mirrors are much easier to make accurately
  27. 27Why must aperture size be bigger than wavelength?Because all waves can diffract when they pass through a gap, radiation entering a telescope spreads out at the edges of the aperture, causing the image to blur. when the aperture is larger than wavelength the radiation passes through the aperture into your telescope with very little diffraction and you get a sharp image
  28. 28What can diffraction grating be used to make?A diffraction grating has very narrow slits, when white light passes through the gaps in a diffraction grating, the different wavelengths of coloured light are diffracted but by different amounts. This creates a spectrum of different coloured light. Astronomers can use these spectra to analyse the amount of light coming from stars.
  29. 29What is parallax?Parallax is an apparent change in position of an object against a distant background. It makes closer stars appear to move relative to distant ones over the course of a year.
  30. 30What is an arcsecond?Is the angle parallax is measured in.
    1 arcsecond =1" = (1 / 36000)°
  31. 31How do we measure this distance?In parsecs.
    1 parsec = about 3 light years
    A parsec is thr distance to a star with a parallax angle of 1 arcsecond.
  32. 32How do you calculate distance?Distance (pc) = [1 / angle (arcsec)]°
  33. 33What depends on the distance to a star?The luminosity of a star depends on its size and temperature. The bigger and hotter it is, the more energy it gives out, so the brighter it is. As you move away from a star, it looks dimmer - because the energy reaching you gets less. So the observed intensity of the light of a star as seen on earth depends of its luminosity and how far away it is from earth.
  34. 34What is a cepheid variable?It is a group of stars that pulsate, they get dimmer and brighter over a period of several days.
  35. 35What does a cepheid variable star's pulse depend on?How quickly they pulse is directly linked to their luminosity. They greater the luminosity, the longer the time between the pulses. So two cepheid variables might have the same observed brightness but one has a longer pulse period so you know that it has a higher luminosity. So the star with the longer pulse period must be further away. Astronomers work out the distance to a cepheid variable by comparing the luminosity and the observed brightness of the star.
  36. 36What is the Curtis-Shapley debate?In the 1920s there was a debate about the size and structure of the universe, led by two famous astronomers. Through telescopes, people had seen some faint, fuzzy objects that they called nebulae. Some of these objects looked spiral shaped but some were just blobs. Shapley and Curtis argued about what these nebulae were and where they were
  37. 37What was Shapley's arugment?Shapley believed the universe was just one gigantic galaxy about 100000 parsecs across. He reckoned our sun and solar system were far from the centre of the galaxy. He believed that nebulae were huge clouds of gas and dust. These clouds were relatively nearby and actually part of the milky way
  38. 38What was Curtis' argument?Curtis thought the universe was made up of many galaxies. He thought our galaxy was smaller than Shapley suggested, about 10000 pc across with the sun at or nearly near the centre. The spiral nebulae were other very distant galaxies, completely separate from the milky way.
  39. 39So who was right?They both were. Shapley was right about the solar system is far from the centre of our galaxy, but Curtis was right about that there were many galaxies in the universe. Curtis was also right about the spiral nebulae - Hubble used cepheid variable stars to show that they're really far away.
  40. 40How did Hubble help solve this big debate?He helped solve this big debate with his observations of the andromeda nebula. Using images taken using the largest telescope at the time. he found that this spiral-shaped fuzzy blob contained many stars, some of which were cepheid variables. Hubble calculated the distance to the andromeda nebula by working out distance to the cepheid variables within it, using the relationship between their brightness and pulse frequency.
  41. 41How do we know that distant galaxies are moving away from us?Because of redshift. By looking out the amount of redshift, you can calculate the recession velocity of the galaxy. The greater the redshift the greater the speed of recession. This suggests that the whole universe is expanding from a single point, which could be explained from an initial explosion (big bang theory).
  42. 42What is Hubble's law?Speed of recession (km/s) = Hubble's constant x distance
  43. 43What is the value of Hubble's constant?70 km/s per Mpc
  44. 44What is the kinetic theory?Kinetic theory says that gases consist of very small particles. Which they do - oxygen consists of oxygen molecules. These particles are constantly moving in completely random directions. They constantly collide with each other and with the walls of their container. When they collide, they bounce off each other.
  45. 45What is absolute zero (0)?Absolute zero is 0 kelvin (-273°C). This is the coldest thing anything can ever get. This is because atoms have as little kinetic energy as its possible to get. Absolute 0 is the start of the Kelvin scale of temperature. To convert °C to kelvin just add 273 and -273 to convert the other way round.
  46. 46How is kinetic energy proportional to temperature?Anything that's moving has kinetic energy. If you increase the temperature of a gas, you give its particles more energy, so as you heat up a gas the average speed of the particles increases. If you double the absolute temperature (in kelvins) you double the average kinetic energy.
  47. 47What happens to pressure when volume is decreased?Gas particles have some mass, so when they collide with something, they exert a force. In a sealed container, gas particles smash against the container, creating an outward pressure. If you put the same amount of gas in a bigger container, the pressure will decrease, cause there will be fewer collisions. When volume is decreased particles get more squashed up and pressure increases.
  48. 48What is the equation for working out constant pressure?Pressure x volume = constant
  49. 49How do you work out the constant volume?Pressure / temperature (K) = constant
  50. 50How do you calculate the constant pressure?Volume / temperature (K) = constant
  51. 51When and how did scientists discover fusion?In the early 20th century. Einstein realised that mass could be converted to energy. It was suggested that hydrogen is turned into helium inside the sun, and that when this happens some mass gets lost (maybe powering the sun)?
  52. 52How do you start fusion?Two nuclei can fuse to create a larger nucleus. Nuclei can only fuse like this is they are brought close together. For that you need lots of energy (high pressures and temperatures)
  53. 53How does nuclear fusion convert mass into energy?E=mc^2
  54. 54How do you balance nuclear equations?1. Two hydrogen nuclei combine to make a larger hydrogen nucleus
    2. A small hydrogen nucleus and a larger hydrogen nucleus combine to make a helium nucleus
    3. Two helium nuclei combine to make a larger helium nucleus.
  55. 55What is a positron?Its a bit like a positive electron, and conserves charge
  56. 56What is the absorption spectra?At high temperatures, electrons become excited and jump into higher energy levels by absorbing radiation. Because there are only certain energy levels an electron can occupy, electrons absorb a particular frequency of radiation to get to a higher energy level. You can see this if a continuous spectrum of visible light shines through a gas - the electrons in the gas atoms absorb certain frequencies of the light, making gaps in the otherwise continuous spectrum (dark lines)
  57. 57What is the emission spectra?Electrons are unstable in the higher energy levels so they tend to fall from a higher to lower levels, losing energy by emitting radiation of a particular frequency. This gives a series of bright lines formed by the emitted frequencies.
  58. 58How do astronomers use spectra to work out what stars are made of?By comparing known spectra to the ones emitted by the stars they are able to see what it is made of
  59. 59How do stars form?Stars are born in a cloud of dust and gas. Gravity cause the denser regions of the cloud to contract very slowly into clumps. When these clumps get dense enough, the cloud breaks up into protostars. Protostars continue to collapse under gravity, reducing in volume. This makes the particles more squashed up, increasing pressure and temperature. Eventually the temperature at the centre of a protostar reaches a few million °C, and hydrogen nuclei start to fuse together to form helium. This releases an enormous amount of energy and creates enough outward pressure to stop the gravitational collapse. The star has now reached the MAIN SEQUENCE stage. It stays like that, relatively unchanging, while it fuses hydrogen into helium in the core.
  60. 60Where does fusion happen?A star is made up of a core surrounded by different layers. The closer to the centre of the star, the hotter it will be.
    THE CORE: Most of the fusion in a star takes place in the centre. The pressure from the weight of the rest of the star makes the core hotter and denser than the rest of the star. So, the nuclei in the core are close enough to fuse together
    SURFACE: The outer region of the star, from where energy is radiated into space. Energy released from fusion in the core is transported by photons of radiation and convection currents to the surface of the star. This is the part of the sun we see.
  61. 61When does a main sequence star not being main sequence?A star stops being in the main sequence when it runs out of hydrogen in the core. It then swells up to become a red giant of supergiant star. In the process of swelling up, the stars photosphere cools down
  62. 62What is the life cycle of a star?What is the life cycle of a star?
  63. 63Explain the Hertzsprung-Russell diagram.If you plot luminosity against temperature, you don't just get a random collection of stars. Different types of stars group together in distinct areas. This is called the Hertzsprung-Russell diagram. The different areas show the main stages of a star's life cycle, the main sequence, reg giants and supergiants, and white dwarfs. The reason you can see these areas is because stars exist in these stable stages of their life cycle for long periods. You don't see unstable phases like supernovas on the diagram because they happen too quickly!
  64. 64What are the advantages of using computer controlled telescopes in space?1. Light is not refracted from the atmosphere
    2. No light pollution
  65. 65Search for extraterrestrial activity?So far no evidence however scientists believe that it is probably likely that life does exist on other planets
  66. 66How does the atmosphere mess up measurements?Because the atmosphere only lets some types of radiation through and it blocks all the others. Visible light is effected the worst. It gets refracted and ends up very blurry.
  67. 67Why are space telescopes better than those on earth?No refraction. No light pollution.
    HOWEVER
    expensive and hard to position
  68. 68Why are there uncertainties in space programmes?They are really expensive - $135 billion in today's money.
    They have to balance this with health services and coping with natural disasters.
    Space programmes are linked with other countries so cut backs can have a knock on effect.
  69. 69What are the 4 reasons in which decide where observatory locations are chosen?Optical - they are put in remote areas with little light pollution
    Height - they want as little atmosphere before the observatory and telescope as possible
    Dry - they want it in a dry location so water does not refract it
    Clouds - they want it in a clear area with little to none clouds so the view is not blocked
  70. 70What are 4 other factors which need to be taken into account?COST - very expensive
    ACCESS - if they are in remote areas, accessibility has to be made/built
    ENVIRONMENT - have to be careful not to damage surrounding environment
    SOCIAL - they need facilities like water, electricity and accommodation which is expensive, however creates jobs.

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FlashCards sobre P7, criado por charlie croxford em 20-06-2014.
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