CH12 - Astronomy 1F03 2010/11 Fall Term 2010/11...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 12 The Evolution of Stars Last chapter… Last Stars form from collapsed gas clouds Stars At some point fusion reactions begin in the core of the star the Stars on the main sequence steadily burn hydrogen to helium via fusion in the core the Star birth to Main-Sequence Main Life (For a star with (For mass equal to the Sun) to What keeps a star together? What Stars are dense and feel their own selffeel gravity Gravity attempts to make the star collapse collapse High gas pressure can oppose gravity can Pressure vs. Gravity Pressure High temperatures create high pressures pressures As long as there is a source of heat the star can support itself with ordinary gas pressure from very hot gas pressure Hydrostatic Hydrostatic Equilibrium For a stable equilibrium: For stable If you squash it, it must If get hotter/higher pressure faster than gravity gets stronger gravity This causes it to bounce This back to the equilibrium position position This can’t go on… This Stars burn for a long time on the Main Sequence the Eventually the hydrogen in the core is used up the The nuclear burning is the source of the heat than maintains the high central pressure pressure What happens next? Core-Hydrogen Burning In the Sun The end of core Hydrogen burning Hydrogen The core loses pressure and collapses and This moves the star away from the main sequence sequence Stellar Evolution: Stellar High Mass vs. Low Mass Stars Stars have a huge range of masses: Stars 0.08 up to over 100 times the Sun How dramatic the end of core hydrogen burning is depends on the mass of the star star Stellar Evolution: Stellar High Mass vs. Low Mass Stars Low-Mass Evolution: Low Evolution is slow Evolution At each stage, the star changes dramatically dramatically High-Mass Evolution High Evolution is rapid Evolution Stages blur together Low Mass Stars: Low Evolution of Sun-like Stars The Helium core collapses and gets hotter hotter This heat slows its collapse its Low Mass Stars: Low Evolution of Sun-like Stars There is a shell of burning Hydrogen There burning around the hotter Helium core The hot core makes the hydrogen shell burn faster burn The star slowly gets brighter The The hot, high pressure shell makes the outer part of the star expand outer Red Giant Branch Red The core shrinks and gets hotter still hotter The hot hydrogen burning shell eventually puffs up the outer envelope of the star to 100 times the initial radius: A Giant The outer envelope is cooler: Red cooler: Red Giant Phase Red A star’s surface only has to be hot star surface enough to radiate the inner heat away enough For large Red Giant stars, a cool red surface radiates just fine surface RRED GIANT = 100 RSUN TRED GIANT = ½ TSUN Luminosity = 1002 x 1/16 = 600 LSUN Luminosity L = 4π R 2 STAR σT 4 STAR Origins Public Talk Origins Prof. Dimitar Sasselov Dimitar HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS LECTURE INFORMATION On Completing the Copernican Revolution On What is the nature of life and is there life on other planets? It took us 450 years from the realization that Earth is a planet to took discovering Earths orbiting other stars. We are just beginning to understand the diversity of planets like our own and their origins. Now astronomy and biology could combine to tell us something new about our place in the universe. 2/ 11/2010 Chester New Hall, Rm104 8:00pm This can’t go on… This The Helium core can’t contract forever The For Sun-llike or larger stars the core ike For eventually gets to 100 million degrees eventually This is hot enough to fuse Helium into Carbon Carbon Helium Core Burning Helium The Helium in the core ignites in a “Helium Flash” The star now has a heat source at the core again the It contracts back to an intermediate size size Core Helium Burning Helium Core Helium burning occurs on the Horizontal Branch Branch Core Helium Helium burning is faster burning The second Red Giant Phase The Horizontal Branch stars accumulate a Carbon core accumulate The Carbon core contracts and heats up just as before and Helium and Hydrogen now both burn in shells both Red Supergiants Supergiants The core is hotter so the second red giant is larger and more luminous luminous The Star may get larger than Earth’s larger orbit! orbit! The end of fusion… The The Carbon core never gets hot enough to burn using fusion to BUT… It can’t just keep contracting BUT just forever forever Degenerate dwarfs Degenerate There is second source of pressure due to quantum mechanical effects to Electrons will not occupy the same volume: Pauli Exclusion principle Pauli At 1 million g/cc (109 kg/m3) this At kg/m this degenerate electron pressure comes into play and holds up the core into Degenerate Matter Degenerate Degenerate pressure is completely unimportant for ordinary matter: unimportant The electrons are too far apart to notice The each other each Degenerate matter is so dense a single ml (e.g. A grape) weighs 1,000 kg (1 tonne) tonne The core containing most of the star is less than the size of the Earth What now? What The core is dead The The envelope continues to expand to The outer envelope drifts off into space drifts The core is exposed as a small, hot white dwarf star dwarf Planetary Nebula Planetary When the envelope is ejected the white dwarf illuminates the gas dwarf Called a planetary nebula planetary Misnomer: 18th Century astronomers Misnomer: Century thought the rounded nebulae might be young solar systems young Planetary Nebulae Planetary Cat’s Eye Planetary Nebula Simple Picture White Dwarfs White White Dwarfs are initially hot White They start very blue and slowly become redder as they cool redder Small radius: Low luminosity Small Degenerate pressure holds the white dwarf up against gravity dwarf Life of a G type star Life Timescales HR Diagram HR Where stars spend their time … Where Low mass stars only spend a long time on the main sequence or as dwarfs sequence Everything else is “fast” and therefore rare in and space space Giant stars are seen because they are very bright so we sample rare stars from far away stars Finding White Dwarfs Finding Faint but hot and blue A llong ong observation will reveal white dwarfs white Picked out Picked by blue colour colour White dwarfs in M4 Globular cluster Novae: White Dwarf binaries Novae: Many stars are binaries Many What if one star dies first to become a white dwarf… dwarf Close Binaries Close When the second star goes red giant the white dwarf can “steal” gas from gas the envelope the Hot material falls onto the white dwarf Hot Occasionally it explosively burns (using fusion) to produce a nova fusion) Nova Nova Nova: “New” (star) (Actually an old star 10,000 times brighter than usual for a month or so) for Many novae repeat periodically periodically Mass ejected from a Nova from Ring of matter Ring ejected during a nova in 1901 seen in 1951 seen Another nova Another (1992), +12 months, +19 months (HST) months High Mass Stars: High Stars heavier than the Sun High mass stars have more weigh to hold up against gravity hold More weight requires more pressure More High mass stars tend to be hotter in the centre centre This makes transitions to high core temperatures smoother temperatures Big stars, big losses… Big Large stars have strong winds and can Large even be unstable, shedding lots of mass even (This may be why there is a maximum star mass) Eta Carina 100 MSUN 5x106 as bright! as High Mass Stars High High Mass stars undergo less dramatic luminosity changes as burning changes changes Below around 8 solar masses the result is still a Carbon white dwarf Carbon High Mass Stars High At more than 8 solar masses the star will get hot enough to burn Carbon burn Massive stars transition to burning new elements smoothly smoothly “Layers, Like an Onion…” The core of a high mass star steadily becomes hotter to fuse heavier and heavier elements heavier The core is surrounded by many shells burning the lighter elements lighter The problem with Iron… The Fusion generates energy because the elements you get out weigh less than what went in what e.g. 1 Carbon weighs less than 3 e.g. Heliums Heliums E = m c2 Einstein’s famous formula calculates the Einstein famous energy you get out energy The Problem with Iron The Protons Neutrons Iron Iron “lightest” The total number of protons and neutrons always stays the same in nuclear reactions nuclear Iron weighs less than any other combination of protons and neutrons neutrons The End of Energy production The An Iron core can’t produce energy An Fusion reactions still occur but these use up energy (to make heavier elements) elements) The star loses its pressure support very rapidly… rapidly Core collapse Core The Massive star collapses down to very high densities Gravity takes it beyond degenerate beyond degenerate 10 kg/m Electron pressure (1010 kg/m3) Electron Electrons and protons are converted into neutrons releasing neutrinos into Degenerate Neutron pressure can hold can hold up the Star (1015 kg/m3) up Rebound Rebound The core overshoots the point of support by degenerate Neutron pressure (to 1018 kg/m3) pressure It rebounds back to (1015 kg/m3) It The shock throws off the entire outer part of the star in a massive explosion part Supernova! Supernova! The star gives of vast amounts of light in just a few weeks in Equal to the output of the Sun over its entire Main Sequence lifetime entire Supernova 1987A SNO saw 13 SNO neutrinos from this! neutrinos Stellar Evolution in a nutshell: Stellar Keeping a star steady Something has to hold up stars Hot gas can provide support with pressure support Nuclear fusion in the core it can keep the star stable star Stellar Evolution in a nutshell: Stellar Core burning When the core stops burning it starts to compress and heat up compress Options: 1) The core starts burning something new new 2) The star dies 2) Stellar Evolution in a nutshell: Star Death Star The final fate of a star depends on how massive it is massive Low Mass Star: Low White Dwarf + Planetary Nebula High Mass Star: High Neutron Star or Black Hole + Supernova The fate of the stars … Stellar Evolution in Stellar Star Clusters Star clusters are collections of stars formed together formed Same composition (elements) Same Same time Same Star clusters are a laboratory for studying stellar evolution for different star masses star Evolving Star Cluster Evolving High Mass stars reach the main sequence first sequence Starting with the most massive, stars run out of core hydrogen fuel and leave the main sequence sequence The brightest stars still there mark the main sequence turnoff turnoff Evolving Star Cluster Evolving Stars off the main sequence evolve Stars to become: to Red giants and supergiants supergiants Horizontal Branch stars Horizontal Dead stars: Dead White Dwarfs, Neutron Stars and White Black Holes Black Old Cluster HR Diagram Old You can estimate the age of star clusters from the main sequence turnoff main Old clusters like Globular Cluster M80 can be as old as 12 billion years old Ordinary Ordinary Pressure Pressure Normally pressure is supplied by motions of particles motions The hotter they are, the higher the pressure pressure This depends on having a source of heat heat Degenerate Pressure Degenerate Quantum mechanics restricts particles like electrons or neutrons from getting too close together too This is called Degenerate Pressure This This effect can prevent very dense things collapsing things Degenerate Pressure Degenerate Case I: White Dwarfs Degenerate electons hold up a white dwarf at electons hold 1 tonne per cubic cm tonne The star is compacted down to the size of a planet smaller than Earth planet It’s not enough for a heavy star… It A white dwarf must weigh less than 1.4 Suns to successfully resist gravity to Degenerate Pressure: Degenerate Case II: Neutron Stars Degenerate Neutron pressure and related effects can hold up a Neutron Star at up to 1 billion tonnes per cubic cm tonnes Neutrons stars are compacted down to the size of a city! down Neutrons stars must weigh less than approximately 2-3 Suns to than Suns successfully resist gravity successfully Nova vs. Supernova Supernova A Nova is up to around 1000 times brighter than the Sun brighter A Supernovae is 1,000,000,000 times 1,000,000,000 brighter than the Sun Varieties of Supernovae? Varieties Astronomers saw two distinct patterns for how supernovae fade over time time Supernova Light Curves Type I and Type II Type Supernovae Type I Supernovae fade quickly Type There is almost no hydrogen seen in spectra Type II Supernovae have a light curve with a “plateau” The is a lot of hydrogen seen in the spectra The Need some theory here… Explanation: Explanation: Type II Supernova Type II match what we’d expect from expect a big star undergoing core collapse: big Lots of hydrogen on the outside Lots A flat light curve consistent with a big envelope swept up by a shockwave (tested using simulations) simulations) 3d shockwave simulation Explanation: Explanation: Type I Supernova Is there another way to get a supernova? Is In a close binary system you can add mass to a White Dwarf from a low mass star White 1.4 Solar masses is the maximum mass for a white dwarf white Above this white dwarf collapses, heats up and explosively detonates all iits Carbon at all ts once once Supernova Comparison Supernova Type I and Type II are unrelated but co-iincidentally release similar amounts co ncidentally of energy of Type I rely on special circumstances for a low mass star Type II apply to all high mass stars Type Supernova Comparison Supernova Supernova Remnants Supernova Crab Nebula 1054 AD Vela Supernova Remnant 9000 BC Supernova Side-effects: Supernova Recycling Initially the universe was 99.9% Hydrogen and Helium Hydrogen Supernovae eject new Heavy elements Heavy elements into space that form new stars, planets and ultimately life and people and New generations of stars contain more heavy elements each time (still mostly Hydrogen and Helium) Hydrogen Stellar Recycling Stellar “We are all We stardust” stardust -- Carl Sagan ...
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