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Life story of a star Micro-world Macro-world Lecture 20 Life Cycle of Stars Recycling Supernovae produce - heavy elements - neutron stars - black holes Martin Rees - Our Cosmic Habitat Our favorite star: The Sun R๏ = 696,000 km (109 x Rearth) M= 2x1030kg ( 3x105 x Mearth) •Rotation period: 25 days(equator) 30 days (poles) Composition: 70% Hydrogen 28% Helium Stars have different colors Stars have different colors • B: blue – hottest • A: green – warm • C: red - cool Infer temperature of a star from the peak wavelength of its black body radiation Color, Brightness + Count them Sun May 2006April 2004 Belinda Wilkes Solar fusion processes + 1.4 MeV + 5.5 MeV + 12.9 MeV Neutrinos come directly from solar core Superkamiokande Sun as seen by a neutrino detector What happens when the Sun’s Hydrogen is all used up? Evolution of a Star Red Giant (Sun) Main Sequence Evolution • Core starts with same fraction of hydrogen as whole star • Fusion changes H He • Core gradually shrinks and Sun gets hotter and more luminous Evolution of the Sun • Fusion changes H He • Core depletes of H • Eventually there is not enough H to maintain energy generation in the core • Core starts to collapse The Sun will become a Red Giant The Sun 5 Billion years from now Earth The Sun Engulfs the Inner Planets Red Giant Phase • He core – No nuclear fusion – Gravitational contraction produces energy • H layer – Nuclear fusion • Envelope – Expands because of increased energy production – Cools because of increased surface area Helium fusion Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon Helium Flash • He core – Eventually the core gets hot enough to fuse Helium into Carbon. – This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. – We don’t see this flash because it’s buried inside the Sun. • H layer • Envelope Red Giant after Helium Ignition • He burning core – Fusion burns He into C, O • He rich core – No fusion • H burning shell – Fusion burns H into He • Envelope – Expands because of increased energy production What happens when the star’s core runs out of helium? – – – – The star explodes Carbon fusion begins The core starts cooling off Helium fuses in a shell around the core Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate Sun looses mass via winds • Creates a “planetary nebula” • Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen a “white dwarf star” Planetary nebula Planetary nebula Planetary nebula Hourglass nebula White dwarf • Star burns up rest of hydrogen • Nothing remains but degenerate core of Oxygen and Carbon • “White dwarf” cools • No energy from fusion, no energy from gravitational contraction • White dwarf slowly fades away… Time line for Sun’s evolution Brightest Star – Sirius A – (Sirius B is a white dwarf) Sirius Orion Constellation ( Nebula) Betelgeuse (Red Giant) Sirius B Comet HaleBop Betelgeuse is a red supergiant star about 600 light years distant 1. This is a Hubble Space Telescope image - the first direct picture of the surface of a star other than the Sun. 2. While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger. If placed at the center of our Solar System, it would extend past the orbit of Jupiter. 3. Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familiar constellation of Orion, the Hunter. 4. The name Betelgeuse is Arabic in origin. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova. In this historic image, a bright hotspot is revealed on the star's surface. The Sun Engulfs the Inner Planets The Sun becomes a White Dwarf Composition: Carbon & Oxygen What about M>1.4 M๏ stars? Nuclear burning continues past Helium 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells Fusion stops at Iron Fusion versus Fission Advanced reactions in stars make elements like Si, S, Ca, Fe Atomic collapse Supernova Explosion • Core pressure goes away because atoms collapse: electrons combine with protons, making neutrons and neutrinos • Neutrons collapse to the center, forming a neutron star Atomic Collapse Ordinary matter ~few grams/cm3 White Dwarfs ~1 ton/cm3 Neutron star ~108 ton/cm3 Core collapse • Iron core grows until it is too heavy to support itself • Atoms in the core collapse, density increases, normal iron nuclei are converted into neutrons with the emission of neutrinos • Core collapse stops, neutron star is formed • Rest of the star collapses in on the core, but bounces off the new neutron star (also pushed outwards by the neutrinos) Supernova explosion SN1987A Tarantula Nebula in LMC Neutrinos are detected Feb 23, 1987 Feb 22, 1987 Previously observed Supernova “Kepler’s Supernova” Oct 8, 1604 Chosun Silok Kepler’s Supernova today Light curve from Kepler’s Supernova Where do the elements in your body come from? • Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets • Supernova produce all of the heavier elements – Elements up to Iron can be produced by fusion – Elements heavier than Iron are produced by the neutrons and neutrinos interacting with nuclei during the supernova explosion How do high-mass stars make the elements necessary for life? http://en.wikipedia.org/wiki/Triple-alpha_process http://en.wikipedia.org/wiki/Neon_burning_process http://en.wikipedia.org/wiki/Silicon_burning_process Advanced Nuclear Burning • Core temperatures in stars with >8MSun allow fusion of elements as heavy as iron We We are are made made of of stardust! stardust May 2006April 2004 Belinda Wilkes What about M>8 M๏ stars? Gravity deforms space-time Light follows curved paths Gravity bends the path of light Curved Space • Einstein related gravity forces to space curvature. • Black holes deeply warp space. • Everything falls in, nothing can climb out. • How does this work? The Event Horizon • Event Horizon = black hole “surface” Object Mass Radius Earth 6 x 1024 kg 1 cm Jupiter 300 x Earth 3 m Sun 300,000 x Earth 3 km Mearth = 6x1024 kg Normal density R=6400km If the Earth was the density of a white dwarf If the Earth was the density of a neutron star R≈10km R≈2.5m If the Earth was Compressed into A Black Hole Rhoriz≈1cm A nonrotating black hole has only a “center” and a “surface” • The black hole is surrounded by an event horizon which is the sphere from which light cannot escape • The distance between the black hole and its event horizon is the Schwarzschild radius (RSch= 2GM/c2) • The center of the black hole is a point of infinite density and zero volume, called a singularity Black Holes • Light is bent by the gravity of a black hole. • The event horizon is the boundary inside which light is bent into the black hole. • Approaching the event horizon time slows down relative to distant observers. • Time stops at the event horizon. Binaries • Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1 “Seeing” Black Holes The First “First” Black Hole • Cygnus X-1 binary system • Most likely mass is 16 (+/- 5) Mo • Mass determined by Doppler shift measurements of optical lines Galaxy M84 core = “Super-massive”Black Hole? Gas, stars moving toward Gas, stars us Image of M84 Gas, stars moving away from us moving across Area STIS observes Spectrogram of gas and stars moving around the core Space Telescope Imaging Spectrograph spectrogram The core of Galaxy M84 contains a total mass = 300 million x M๏ in R<26 cyr! http://www.youtube.com/watch?v=dipFMJckZOM http://www.youtube.com/watch?v=dipFMJckZOM&feature=player_embedded