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Stellar evolution 2015


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Stellar evolution 2015

  1. 1. ASTROPHYSICS Stellar characteristics and stellar evolution
  2. 2. Black body radiation  A black body is a perfect emitter. A good model for a black body is a filament light bulb: the light bulb emits in a very large region of the electromagnetic spectrum.  There is a clear relationship between the temperature of an object and the wavelength for which the emission is maximum. That relationship is known as Wien’s law: Km2.9x10T constantT 3- max max    
  3. 3. Commons wikipedia Wien Displacement law By analysing a star’s spectrum, we can know in what wavelength the star emits more energy. The Sun emits more energy at λ=500 nm. According to Wien’s law, the temperature at the Sun’s surface is inversely proportional to the maximum wavelength. So: K5800 500x10 2.9x102.9x10 T 9- -3 max -3  
  4. 4. Black body radiation and Wien Law
  5. 5. Star’s Colour and Temperature Blackbody Applet
  6. 6. Black body radiation Apart from temperature, a radiation spectrum can also give information about luminosity. The area under a black body radiation curve is equal to the total energy emitted per second per unit of area of the black body. Stefan showed that this area was proportional to the fourth power of the absolute temperature of the body. The total power emitted by a black body is its luminosity. According to the Stefan-Boltzmann law, a body of surface area A and absolute temperature T has a luminosity given by: 4 σATL  where, σ = 5.67x108 W m-2 K-4 , A = 4πr2
  7. 7. Phet simulation
  8. 8. Why is this important? The spectrum of stars is similar to the spectrum emitted by a black body. We can therefore use Wien Law to find the temperature of a star from its spectrum. If we know its temperature and its luminosity then its radius can be found from Stephan-Boltzmann law.
  9. 9. Real spectra are more complicated than this (remember emission and absorption lines?) Blackbody Spectrum Emission and Absorption Lines
  10. 10. Example 1 The apparent brightness of our Sun is 1,393 Wm-2. This can be determined using light sensors on Earth. We know that the Earth is 1AU from the Sun. The Sun has an approximate black body spectrum with most of the energy radiated at a wavelength of 5.0 X 10-7 m. This is done using a spectrometer on Earth.
  11. 11. Example 1  Use the above information to find out the  1. Luminosity of the Sun  2. Surface temperature of the Sun  3. Radius of the Sun  USE YOUR DATA BOOKLET!
  12. 12. Atomic Spectra The spectrum of atomic hydrogen was discussed and accounted for using the Bohr model of the atom. Remember that the electron shells of a given atom can absorb a specific frequency of energy. E = hf Lets look at Hydrogen as an example.
  13. 13.
  14. 14. Atomic Spectra An electron transition downwards leads to an emission of a specific frequency of light. This produces an emission spectrum if observed through a spectrometer.
  15. 15. Atomic Spectra Another good example of line emission spectra is the burning of sodium. The gaseous sodium’s electrons produce two distinct spectral lines in the yellow region of the E-M spectrum.
  16. 16. Atomic Spectra A particular gas, like Hydrogen can also ABSORB specific frequencies of light. This removes particular frequencies from a continuous spectrum. This is called an ABSORPTION SPECTRUM.
  17. 17. Atomic Spectra In all cases the absorption and the emission spectra will match perfectly.
  18. 18. Atomic Spectra The spectrum seen from a star is due to the presence of a particular chemical element in the outer atmosphere of the star. The sun produces absorption lines of Hydrogen, iron, calcium and sodium.
  19. 19. Atomic Spectra The absorption spectrum also tells us the outer temperature of the sun’s surface. For every element there is a temperature range which will produce strong absorption lines.
  20. 20. Atomic Spectra Examples would be…  Hydrogen absorption lines occur at temperatures of 4000 to 12 000 K.  Helium lines require temperatures of between 15 000 and 30 000 K in order to get their electrons to absorb energy.
  21. 21. Atomic Spectra
  22. 22. Atomic Spectra Different atoms are sensitive to different temperatures. It is possible to determine a star’s temperature by the absorption spectra that the star is producing.
  23. 23. Atomic Spectra The chemical composition of stars due to their line absorption spectra are found to be remarkably similar. The average composition of stars is 74% Hydrogen, 25% Helium and only 1% other elements.
  24. 24. Atomic Spectra In summary, line absorption spectra tell us more about a star’s temperature rather than its chemical composition (as most stars have the same composition).
  25. 25. Stars can be arranged into categories based on the features in their spectra… This is called “Spectral Classification” 1. by the “strength” (depth) of the absorption lines in their spectra 2. by their color as determined by their blackbody curve 3. by their temperature and luminosity How do we categorise stars? A few options:
  26. 26. First attempts to classify stars used the strength of their absorption lines… Williamina Fleming They also used the strength of the Harvard “computers”! Stars were labeled “A, B, C…” in order of increasing strength of Hydrogen lines.
  27. 27. OBAFGKM(LT)! Later, these categories were reordered according to temperature/color… Annie Jump Cannon
  28. 28. OBAFGKM - Mnemonics Only Boring Astronomers Find Gratification in Knowing Mnemonics! O Be A Fine Girl Kiss Me
  29. 29. Eventually, the connection was made between the observables and the theory. Observable: • Strength of Hydrogen Absorption Lines • Blackbody Curve (Color) Theoretical: • Using observables to determine things we can’t measure: Temperature and Luminosity Cecilia Payne
  30. 30. The Spectral Sequence Class Spectrum Color Temperature O ionized and neutral helium, weakened hydrogen bluish 31,000-49,000 K B neutral helium, stronger hydrogen blue-white 10,000-31,000 K A strong hydrogen, ionized metals white 7400-10,000 K F weaker hydrogen, ionized metals yellowish white 6000-7400 K G still weaker hydrogen, ionized and neutral metals yellowish 5300-6000 K K weak hydrogen, neutral metals orange 3900-5300 K M little or no hydrogen, neutral metals, molecules reddish 2200-3900 K L no hydrogen, metallic hydrides, alkalai metals red-infrared 1200-2200 K T methane bands infrared under 1200 K
  31. 31. “If a picture is worth a 1000 words, a spectrum is worth 1000 pictures.”  Spectra tell us about the physics of the star and how those physics affect the atoms in it
  32. 32. The Hertzsprung-Russell diagram This diagram shows a correlation between the luminosity of a star and its temperature. The scale on the axes is not linear as the temperature varies from 3000 to 25000 K whereas the luminosity varies from 10-4 to 106, 10 orders of magnitude.
  33. 33. H-R diagram  The stars are not randomly distributed on the diagram.  There are 3 features that emerge from the H-R diagram:  Most stars fall on a strip extending diagonally across the diagram from top left to bottom right. This is called the MAIN SEQUENCE.  Some large stars, reddish in colour occupy the top right – these are red giants (large, cool stars).  The bottom left is a region of small stars known as white dwarfs (small and hot)
  34. 34. H-R Diagram
  35. 35. H-R Diagram  H-R Diagram animation
  36. 36. What is the role of interpretation in the sciences?  Go to and participate in real science now!
  37. 37. Types of Stars  Red Giants Very large, cool stars with a reddish appearance. All main sequence stars evolve into a red giant. In red giants there are nuclear reactions involving the fusion of helium into heavier elements.
  38. 38. Red Giants  The fuel is expended much faster than in stars like our sun.  Within a red giant is a core still increasing in temperature.  When the temperature rises to 100 million degrees Kelvin helium fusion takes place.
  39. 39. Red Giants  There are now two layers of energy production;  the hydrogen burning shell,  the helium-burning core.  This process eventually yields a carbon and oxygen core,that may eventually produce an iron core,in the most massive stars.
  40. 40. Red Giants  The fusion process stops with iron;  Iron represents the most stable form, in which protons and neutrons can exist.  Once the Iron core is formed energy production comes to an end.  The pressure forcing the star to expand no longer is present, gravity takes over.
  41. 41. Red Giants  Within seconds, iron core collapses with such a force,not even the space within the orbital structure of the atom is preserved.  The layers within the iron core fall into the centre,at different rates,an outward shock wave is produced.
  42. 42. Red Giants  This shock wave is capable of driving off most of the mass of the star.  For a star of size 10 solar masses;  85% of the mass is lost,  the star goes supernova.
  43. 43. Types of Stars  White dwarfs A red giant at the end stage of its evolution will throw off mass and leave behind a very small size (the size of the Earth), very dense star in which no nuclear reactions take place. It is very hot but its small size gives it a very small luminosity. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense. A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.
  44. 44. Types of Stars  Neutron stars A neutron star is formed from the collapsed remnant of a massive star (usually supergiant stars – very big red stars). Models predict that neutron stars consist mostly of neutrons, hence the name. Such stars are very hot. A neutron star is one of the few possible conclusions of stellar evolution. The first direct observation of a neutron star in visible light. The neutron star being RX J185635-3754.
  45. 45. Types of Stars  Supernovae A supernova is a stellar explosion that creates an extremely luminous object. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant. Crab Nebula
  46. 46. Types of Stars  Supernovae A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun would emit over 10 billion years. Supernova remnant N 63A lies within a clumpy region of gas and dust in the Large Magellanic Cloud. NASA image.
  47. 47. Types of Stars  Pulsars Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Periods of rotation vary from a few milliseconds to seconds. Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission beams.
  48. 48. Types of Stars  Black Holes A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.
  49. 49. Types of Stars  Cepheid variables Cepheid variables are stars of variable luminosity. The luminosity increases sharply and falls of gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star. A Cepheid is usually a giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of Cepheid stars range from 103 to 104 times that of the Sun. Cepheid Variable Sim Real Data used by Astronomers
  50. 50. Types of Stars  Binary stars A binary star is a stellar system consisting of two stars orbiting around their centre of mass. For each star, the other is its companion star. A large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries. Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left).
  51. 51. Binary stars There are three types of binary stars  Visual binaries – these appear as two separate stars when viewed through a telescope and consist of two stars orbiting about common centre. The common rotation period is given by the formula: )( 4 21 32 2 MMG d T    where d is the distance between the stars. Because the rotation period can be measured directly, the sum of the masses can be determined as well as the individual masses. This is useful as it allows us to determine the mass of singles stars just by knowing their luminosities.
  52. 52. Binary stars  Eclipsing binaries – some binaries are two far to be resolved visually as two separate stars (at big distances two stars may seem to be one). But if the plane of the orbit of the two stars is suitably oriented relative to that of the Earth, the light of one of the stars in the binary may be blocked by the other, resulting in an eclipse of the star, which may be total or partial Eclipsing Binary Simulation
  53. 53. Binary stars  Spectroscopic binaries – this system is detected by analysing the light from one or both of its members and observing that there is a periodic Doppler shifting of the lines in the spectrum.
  54. 54. Binary stars A blue shift is expected as the star approaches the Earth and a red shift as it moves away from the Earth in its orbit around its companion. If λ0 is the wavelength of a spectral line and λ the wavelength received on earth, the shift, z, is defined as: 0 0    z If the speed of the source is small compared with the speed of light, it can be shown that: c v z  The speed is proportional to the shift
  55. 55. TOK  Stars are a long way away. How can we claim we know anything about them?  Do stars die?  Are we stardust?
  56. 56. H-R diagram  The stars are not randomly distributed on the diagram.  There are 3 features that emerge from the H-R diagram:  Most stars fall on a strip extending diagonally across the diagram from top left to bottom right. This is called the MAIN SEQUENCE.  Some large stars, reddish in colour occupy the top right – these are red giants (large, cool stars).  The bottom left is a region of small stars known as white dwarfs (small and hot)
  57. 57. Star Formation  Protostar  High temperature leads to ionisation of elements.  E-M energy is emitted.  The star can have considerable Luminosity.  5000 times the surface area and 100 times as Luminous as our Sun.
  58. 58. Star Formation  Protostar-  Temperature continues to increase…  Electrons stripped from the atoms in the core.  A plasma is formed.
  59. 59. Star Formation  Main Sequence Star-  Nuclear Fusion starts up.  Temperatures now high enough to fuse Hydrogen into Helium.  Gravitational contraction will now stop as the Fusion process will offset the contraction.  “Hydrostatic Equilibrium”
  60. 60. You are not Required to Memorize these Reactions
  61. 61. Main Sequence Stars  Where a star lands on the Main Sequence depends on its mass.  A star will stay at this place on the main sequence for its lifetime as a Main Sequence Star. QuickTime™ and a Photo - JPEG decompressor are needed to see this picture.
  62. 62. Main Sequence Stars  The life of a Main Sequence star is determined by its mass.  High Mass = Short Lifespan.  Sun (1M) - 10 Billion Years.  15 M - 10,000 Years. QuickTime™ and a Photo - JPEG decompressor are needed to see this picture.
  63. 63. Main Sequence Stars  Once hydrostatic equilibrium is reached… the star will remain stable for as long as it is a Main Sequence Star (thousands to up to billions of years).
  64. 64. The Future of our Sun  There is enough Hydrogen in our Sun’s core to produce fusion for another 5 billion years.  Eventually the core will be mostly Helium.  Hydrogen Fusion will then begin outside the core.
  65. 65. The Future of our Sun  No fusion happening in the core to counteract the gravitational contraction.  The core will begin to collapse.  This raises the cores temperature.  Sun will start to expand.  Surface temperature will drop.  Sun enters RED GIANT STAGE.
  66. 66. The Future of our Sun  Luminosity increases due to massive size.  Out to the orbit of Venus.  Core temperature now so high that Helium can be fused.  Higher temperature required to fuse Helium as it has two positive charges.
  67. 67. The Future of our Sun  Helium fused into Carbon and Oxygen.  This is where our carbon comes from.  Once the Helium is used up in the core, the star collapses due to the high mass core even more… this raises the temperature even higher!  Burning of Hydrogen in the outer layers causes further expansion.
  68. 68. The Future of our Sun  At this point the Sun’s surface will reach out to the Earth’s Orbit!  10,000 times as luminous as today.  The sun will start to shed layers… these layers are called planetary nebula.
  69. 69. The Future of our Sun  The core will now be exposed.  The core is called a White Dwarf.  About the size of the Earth.  Very hot, very small.  Not luminous.  Will eventually cool into a cold, “dead” star called a brown dwarf.
  70. 70. Types of Stars (review)  Cepheid variables Cepheid variables are stars of variable luminosity. The luminosity increases sharply and falls of gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star. A Cepheid is usually a giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of Cepheid stars range from 103 to 104 times that of the Sun.
  71. 71. Cepheid variables The relationship between a Cepheid variable's luminosity and variability period is quite precise, and has been used as a standard candle (astronomical object that has a known luminosity) for almost a century. This connection was discovered in 1912 by Henrietta Swan Leavitt. She measured the brightness of hundreds of Cepheid variables and discovered a distinct period- luminosity relationship.
  72. 72. A three-day period Cepheid has a luminosity of about 800 times that of the Sun. A thirty-day period Cepheid is 10,000 times as bright as the Sun. The scale has been calibrated using nearby Cepheid stars, for which the distance was already known. This high luminosity, and the precision with which their distance can be estimated, makes Cepheid stars the ideal standard candle to measure the distance of clusters and external galaxies. Cepheid variables
  73. 73. Using cepheids
  74. 74. Cepheid variables
  75. 75. Mass – Luminosity Relationship  There is a relationship between the luminosity of a star and its mass  L = M3.5  Where L is luminosity, M is mass in solar units and applies to all main sequence stars  The power (3.5) can be any value between 3 and 4 as it is itself mass dependant.
  76. 76. The Chandrasekhar Limit  If the initial solar mass of a star is greater than 4 solar masses… then the star will become a…  Supergiant Star and eventually collapse to a…  Neutron Star or a Black Hole.
  77. 77. The Chandrasekhar Limit  A star greater than 4 solar masses (1 solar mass is the mass of our Sun)… will collapse to a core…  With a mass of greater than 1.4 Solar Masses.  A neutron star or a black hole  If the core is less than 1.4 solar masses the star will become a White Dwarf.
  78. 78. The Chandrasekhar Limit  This 1.4 solar mass of the core boundary is called the Chandrasekhar limit.  Famous Indian astronomer.  Less than 1.4 solar mass for the core… electron degeneracy prevents further collapse.  Electrons cannot be packed together any further.
  79. 79. THE OPPENHEIMER-VOLKOFF LIMIT  is the equivalent to Chandrasekhar but applies to neutron stars.
  80. 80. The Supergiants  Stars with masses greater than 4 solar masses initially will evolve into a Supergiant star…  Eventually collapse to a core greater than 1.4 solar masses.  Become a Neutron Star or a Black Hole.
  81. 81. The Supergiants  The star will undergo the same process as a Red Giant…  Fusion of Helium in the inner core.  Fusion of Hydrogen in the outer layers.  Expansion.  Cooling at the surface of the star.  Subsequent fusion of Oxygen and Carbon in the core.
  82. 82. The Supergiants  The difference is that the star is massive enough to continue to…  Collapse after the Oxygen and Carbon fusion.  Further gravitational collapse leads to further temperature rises.  Capable of beginning the fusion of silicon.
  83. 83. The Supergiants
  84. 84. The Supergiants  The fusion of silicon makes iron.  Supergiants are the brightest stars visible due to their enormous size.  Betelgeuse in the constellation Orion.
  85. 85. The Supergiants  Iron cannot undergo fusion due to its very high coulombic repulsion (26 protons).  It would need astronomical temperatures.  The star has reached a critical state.
  86. 86. The Supergiants  The star will once again begin to collapse into the core.  But no more fusion will take place to counteract the gravitational collapse.  Incredibly high temperatures lead to the combining of electrons and protons.
  87. 87. The Supergiants  Neutrons and neutrinos are formed in large quantities.  High energy neutrinos form an outward pressure wave.  This wave hurtles outward.
  88. 88. The Supergiants  This shock wave rips the outer layers off of the star.  The inner core is now exposed.  Huge amount of radiation floods into space.