End of high mass stars Nova Supernova Type I Type II Stellar nucleo-synthesis Stellar Recycling Nova (means new): is a star that suddenly increases greatly in brightness, then slowly fades back to its normal appearance over a period of months or few weeks. Once a decade, on average, we observe a `new' star in the heavens. Credit: http://csep10.phys.utk.edu/astr162/lect/binaries/accreting.html Comparing before and after images of that region of the sky demonstrates that novae are old stars that dramatically increase in brightness, such as Nova Herculis shown above. The light curves for a nova look like the following. or brightness The change is brightness is typical a factor of 10 6 (whereas a supernova is 10 8, a different object all together). There are many reasons why a star might increase in brightness in a sudden and explosive-like manner; the collision of two stars, core changes, unstable pulsations. However, novae are often recurrent, meaning that after 50 to 100 years the nova will go off again. This means that whatever causes the brightness changes must be cyclic (i.e. it doesn't destroy the star). The best explanation for novae is surface fusion on a white dwarf. By definition, white dwarfs no longer have any hydrogen to burn in a fusion reaction. They have used all there hydrogen at earlier phases of their life cycle. However, a white dwarf in a binary system can `steal' extra hydrogen from its companion by tidal stripping. 1
A Binary system with a normal main sequence star and an old white dwarf will look like the following: Eventually the main sequence star will evolve to become a red giant star. As the red giant star continues to expand it will exceed its Roche limit and hydrogen gas will stream across to the white dwarf, spiraling inward to form an accretion disk. Hydrogen gas will build up on the surface of the white dwarf where the surface gravity is extremely high. After a few decades, the pressure and density of the hydrogen outer shell will reach the point where fusion can begin and the shell explodes in a burst of energy. accretion disk ( So hot that emits in visible as well as in ultraviolet and x-ray) After the shell is fused, the process starts over again, thus explaining why we see recurrent novae. 2
Credit: http://csep10.phys.utk.edu/astr162/lect/binaries/accreting.html O & B Stars Form Fast Live Fast, Burn Hot Leave Main-Sequence Fast Die Young, Die Explosively q massive stars have the same types of internal changes as low mass stars same types of compositional changes occur But q massive stars evolve more rapidly extra mass causes extra gravity extra gravity causes extra core heating extra core heating causes faster nuclear burning more luminous, so they use fuel more rapidly leave main sequence fast Dividing line between high and low mass occurs at 8 M This dividing line really refers to sun. the mass of the star at the time the carbon core forms. Star Mass Time on MS 1 M sun 10 billion yrs 5 M sun (B-type) 100 million yrs 10 M sun (O-type) 20 million yrs For all stars > 2.5 M sun Helium burning starts smoothly without any helium flash. In Brief: Stars with Mass < 0.08 M sun failed stars brown dwarf (nuclear burning never starts) Stars with 0.08 < Mass < 0.25 M sun burn through Helium blow off their envelope and core becomes He white dwarf Stars with 0.25 < Mass < 4 M sun burn through Carbon blow off their envelope and core becomes a C-O white dwarf Stars with 4 < Mass < 8 M sun (intermediate mass stars) burn through Carbon blow off their envelope with massive stellar winds and core becomes a C-O or possibly an O-Ne-Mg white Dwarf Stars with Mass > 8 M sun (high-mass stars) burn Carbon, Neon, Oxygen & Silicon build up a heavy Iron core & burning shells. Final stage occurs when the Iron core begins to catastrophically collapse supernova (boooom.) 3
In Brief: Stars with Mass < 0.08 M sun failed stars brown dwarf (nuclear burning never starts) Stars with 0.08 < Mass < 0.25 M sun burn through Helium blow off their envelope and core becomes He white dwarf Stars with 0.25 < Mass < 4 M sun burn through Carbon blow off their envelope and core becomes a C-O white dwarf Stars with 4 < Mass < 8 M sun (intermediate mass stars) burn through Carbon blow off their envelope with massive stellar winds and core becomes a C-O or possibly an O-Ne-Mg white Dwarf Stars with Mass > 8 M sun (high-mass stars) burn Carbon, Neon, Oxygen & Silicon build up a heavy Iron core & burning shells. Final stage occurs when the Iron core begins to catastrophically collapse supernova (boooom.) High Mass Stars: M > 8 M sun They burn through a succession of nuclear fusion fuels and form heavier elements. All elements other than H and He are produced from stars. The material in you was formed by a star! The process of building up heavy elements from light ones is called nucleosynthesis. Evolution is very fast e.g. for star: M > 20 M sun * Hydrogen burning: 10 Myr * Helium burning: 1 Myr * Carbon burning: 1000 years * Neon burning: ~10 years * Oxygen burning: ~1 year * Silicon burning: ~1 day Composition of high Mass Stars: M > 8 M sun Credit: prof. W. Pogge (OSU) 4
Energy is released when elements lighter than iron are fused together. Likewise energy is released when element heavier than iron are split apart (fission). Conversely, to fuse heavier elements or split light elements requires extra energy. Iron is the most tightly bound nucleus. This means that moving towards iron releases energy. It is like a ball rolling to the lowest point. The most stable element is Iron ( 26 Fe 56 ). Need energy to split up Fe or to add to Fe. For elements lighter than iron: Fusion releases energy For elements heavier than iron: Fission releases energy The universe is slowly turning to iron! Credit:Terry Herter, Cornell University In stars iron plays a role of fire extinguisher central fire ceases equilibrium goes away forever T = several billions K Gravity overpowers and star implodes core starts collapsing temperature reaches T >10 Billion K & density ~10 12 kg/m 3 Two Energy consuming processes kick in: Iron nuclei breaks into lighter nuclei e.g. He, p & n. This process is called p + e n + neutrinos. (called ) The neutrinos escape & carry away energy. Both processes rob the energy of core cooling the star reducing the pressure accelerating its collapse. Core is collapses fast and reaches a point where Now the opponent of gravity is there Before the halt, gravity overshoots takes only one sec and then 5
At the start of Iron Core collapse, the core properties are: Radius ~ km (~R earth ) Density ~ kg/m 3 A second later, the properties are: Radius ~ km Density ~ kg/m 3 Collapse Speed ~ c! Energetic shock waves sweep through the star q At shock breakout: Star brightens to ~10 Billion L sun in minutes. Can outshine an entire galaxy of stars! q Outer envelope is blasted off: accelerated to a few x 10,000 km/sec gas expands & cools off q Only the core remains behind. q After its initial brilliance, the Supernova fades out after a few months. q This is called. Credit: http://zebu.uoregon.edu/~js/ast122/lectures/lec15.html Nova and supernova both represent sudden enhancement in the brightness of a star for few weeks or months and eventually the star becomes dim again. In the sky they may look alike but both are quite different processes. Differences: v A supernova is more than a million times brighter than a nova. v In a matter of few months it radiates away as much energy as Sun will radiate in its entire 10 10 yrs of life period. v A star can become supernova only once but it may become nova many times. v According to observations supernovae are divided into two classes. 6
Type I q q (rise in luminosity followed by steady, gradual decline) Type II q q (luminosity remains at same level for a few months after peak, before decline) Type II: Core-collapse supernova is the same as the death of the high mass star as described in previous slides. Type I: (come back to the white dwarfs, descendent of low mass stars!) A) > In binary system when white dwarf goes through nova cycles some material it burns out or some expels away but some material it keeps collecting, hence mass of WD increases slowly If the mass of the WD exceeds the the limit of 1.4 solar mass (called Chandrasekhar mass limit) electron degeneracy pressure cannot any more work against gravity WD immediately starts collapsing/contracting Temperature increases rapidly Carbon fusion begins everywhere throughout WD simultaneously Star explodes as carbon-detonation supernova. B) Two white dwarf in a binary system may collide and merge to form a massive, unstable star. End result is again carbon-detonation supernova. v 1054 AD: "Guest Star" in Taurus Observed by Chinese astronomers Visible in daylight for 23 days Visible at night for ~6 months Left behind the Crab Nebula v 1572: Tycho Brahe's Supernova v 1604: Johannes Kepler's Supernova Important supernovae that were influential at the beginnings of modern astronomy. v 6000-9000 BC: Vela supernova Observed by the Sumerians; appears in legends about the God Ea. Credit: Prof. W. Pogge (OSU) Crab Supernova Remnant: remnant of the supernova observed by Chinese astronomers in 1054 A.D. This is type II supernova expanding into space at several 1000 Km/sec. 7
Nearest naked-eye visible supernova seen since 1604. Explosion occurred on February 23, 1987: 15 M sun Blue Supergiant Star named SK-69 o 202 exploded in the Large Magellanic Cloud. (satellite galaxy of the Milky Way ~50,000 pc away). Particle experiments on Earth recorded a pulse of neutrinos arriving just before the burst of light from shock breakout. Astronomers have continued to follow its development over the last 15 years. Vela Supernova Remnant: expansion velocities imply that its central star exploded around 9000 B.C. SN1987a has provided us with a great wealth of information about supernova physics, and help to largely experimentally confirm the basic predictions of the core-bounce picture (although with good data, many details still remain murky). After Before The material in you was formed by the stars! There are about 115 naturally occurring elements in the Universe. Stellar evolution theory successfully explanations the origin of all these elements. The process of building up heavy elements from light ones is called nucleosynthesis: Fusion in the very early Universe, immediately after the Big Bang produced hydrogen, helium, lithium, beryllium and boron, the first 5 elements in the Periodic Table. Other elements, from carbon to iron, were formed by fusion reactions in the cores of stars. 8
He Capture: Elements heavier than iron are not produced in stars, so what is their origin? The construction of elements heavier than iron involves neutron capture. a nuclei can capture or fuse with a neutron because the neutron is electrically neutral and, therefore, not repulsed like the proton. there exist numerous free neutrons in the stars as the byproduct of many reactions. each neutron capture by heavy nuclei produces an isotope, some are stable, some are unstable. unstable isotopes will decay by emitting a positron and a neutrino to make a new element. E.g. Fe 56 + n Fe 57 (stable isotope) Fe 57 + n Fe 58 (stable isotope) Fe 58 + n Fe 59 (unstable isotope) In about a month Iron-59 radioactively decays into cobalt-59. Cobalt-59 captures a neutron to form Cobalt-60 and decays to nickel-60 and so on. Another example: Cadmium decays to form Indium Neutron capture can happen by two methods: 1) s-process (s means slow) The s-process works as long as the decay time for unstable isotopes is longer than the capture time. Up to the element bismuth (atomic number 83), the s-process works, but above this point the more massive nuclei that can be built from bismuth are unstable. 2) r-process (r means rapid) In this process the capture of neutrons happens in such a dense environment that the unstable isotopes do not have time to decay. The high density of neutrons needed is only found during a supernova explosion and, thus, all the heavy elements in the Universe (radium, uranium and plutonium) are produced this way. The supernova explosion also has the side benefit of propelling the new created elements into space to seed molecular clouds which will form new stars and solar systems. Ref: http://zebu.uoregon.edu/~js/ast122/lectures/ 9
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