WC14 - Recap The Life of Stars Recap The Life of Stars What...

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Unformatted text preview: Recap The Life of Stars Recap The Life of Stars What is the key phenomenon in Sun’s ages 2­5 ? What is a Hertzsprung­Russell diagram ? What is the relation between stars masses and their luminosities and length of life ? What is the range of star masses for which the star life is similar to our Sun’s ? What is considered a very large star ? Define the conditions for the formation of supernovas/pulsars. Discuss the types of supernovas. What is a type Ia supernova ? What is Crab nebula ? Explain the origin of the pulsar’s radio pulses. Define a close binary system. Black Holes Black Holes (from Chapter 7) The Life of a Very Large Star Black Holes Einstein’s General Relativity White Holes Beyond Neutron Stars Beyond Neutron Stars In 1939 Oppenheimer and Volkoff demonstrated that for a neutron star with 3 solar masses the neutrons will not resist the pressure and no structure will oppose the gravitational collapse. The idea of the black hole is actually much older: in the 18th century Laplace speculated that if light would feel gravitation, a very large star would be invisible. Black holes correspond to infinitely distorted space­time, which are predicted by Einstein’s General Relativity Visualizing Black Holes Visualizing Black Holes 250:1 Red giant 100:1 700:1 3:1 White Neutron Sun star today dwarf Near a star Black hole Far from stars Black hole space space time singularity Event horizon Einstein’s Special Relativity Einstein’s Special Relativity Einstein’s Special Relativity (1905): “the speed of light is the same in all constantly moving frames”. Einstein used the space­time correlation to demonstrate that space and time form a 4 dimensional continuum. Mind experiments show that time and space are correlated. Einstein’s General Relativity Einstein’s General Relativity Einstein General Relativity (1916). Matter creates the curvature of the space­time continuum around stars and a 1919 experiment demonstrate it. Einstein did not have the math skills to express his ideas. Through his friend Marcel Grossman he found the 60­year old metric tensor work of Bernhard Riemann. He liked so much his equations that he used to compare them with Greek architecture (i.e. beautiful and serene). The Physics of the Black Hole (I) The Physics of the Black Hole (I) A black hole is defined as a volume of space­time from where nothing can be emitted to the outside world. The symmetric model (Schwarzchild, Penrose): the radius of the sphere (or the event horizon) is proportional to the collapsed mass (for a solar mass it would be 3 km, for Earth less than 1cm !). The asymmetric rotational model (Kerr, Newman): the ergosphere, outside the event horizon, has space dragged round by the black hole. More important, the singularity is of a ring shape and the black hole is shaped like a donut. Extremely small objects can go through its center to other point in space­time without being broken into smaller pieces. The Physics of the Black Hole (II) The Physics of the Black Hole (II) Huge tidal effects as one approaches a black hole is its main characteristic (they exist even near a dwarf star). However, if the mass of the black hole is huge the tidal effects at the event horizon are negligible (no signs for spaceships that cross the event horizon). Time dilation is another consequence of a black hole. A remote observation ofan astronaut falling into the hole will see him hovering for ever on the event horizon (as the time intervals become infinite). The X­Ray of the Universe (I) The X­Ray of the Universe (I) The X rays are associated with huge temperatures and an extremely dynamic plasma, such the plasma sucked in by a black hole. X­rays satellites: 1970 “Uhuru”, 1972 “Copernicus”, 1975 “SAS­3”, etc. Uhuru identified the first candidate for a black hole: Cygnus X­1 in the Swan constellation. That source was in the vicinity of the supergiant star HDE 226868. The X­Ray of the Universe (II) The X­Ray of the Universe (II) X rays can also come from pulsars. In the case of a black hole the X emission is irregular, with increases in intensity being due to larger chunks of matter being swallowed by the black hole. The life of a X source is extremely long as the rate of mass transfer is extremely small (by comparison to the mass of the regular star). Pairs of stars orbiting one around each other are known from the search for neutron stars (pulsars). In those cases the matter from a regular star is sucked through the magnetic poles of the neutron star, which become hot and emit pulses of X rays of frequency related to its rotation (ex.Centaurus X­3). However, for old pulsars (with weak magnetic fields) the emission of X rays is in bursts, when enough material reaches the surface of the neutron star. White Holes (I) White Holes (I) Black holes do not emit anything except: Hawking proposed in 1975 that the intense gravitational field surrounding a very small black hole can create real pairs of particles, which, because of the strong tidal effects, will be separated. gravitons (in the case of a non­symmetric rotating black hole) the matter scattered before the equilibrium. The net effect is that gravitational energy is used to emit energetic particles making the black hole “warm”. This process accelerates exponentially as particles emission causes the black hole to decrease in size and the increased distortion of space­time increases the tidal effects. In the end the black hole explodes (becoming a white hole). White Holes (II) White Holes (II) The white holes process requires a very small black hole, much smaller than the conventional stellar black holes. Hawking proposed that mini­black holes were created during the Big Bang and might produce particles (cosmic rays) today. According to Hawking, advanced civilizations might be able to create mini black holes, where one could dump all the garbage and then use them for energy production. For a regular black hole to start this process its temperature has to be about the same as the outside temperature (theory predicts 1065 years). Galactic Black Holes Galactic Black Holes Black Holes can be formed if there is enough pressure to push matter into itself Big Bang was a place where this kind of pressure occurred and galactic astronomy relies on black holes during the Big Bang to act as “seeds” around which some galaxies were formed. White holes correspond to very small such implosions Centers of spiral galaxies (like our galaxy) correspond to larger implosions ...
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