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Death of Stars Why Do Stars Leave the Main Sequence? • Running out of fuel Stage 8: Hydrogen Shell Burning • Cooler core imbalance between pressure and gravity core shrinks • hydrogen shell generates energy too fast outer layers heat up star expands • Luminosity increases • Duration ~ 100 million years • Size ~ several Suns Stage 9: The Red Giant Stage • Luminosity huge (~ 100 Suns) • Surface Temperature lower • Core Temperature higher • Size ~ 70 Suns (orbit of Mercury) Lifecycle • Lifecycle of a main sequence G star • Most time is spent on the main-sequence (normal star) The Helium Flash and Stage 10 • The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon • Initial explosion followed by steady (but rapid) fusion of helium into carbon • Lasts: 50 million years • Temperature: 200 million K (core) to 5000 K (surface) • Size ~ 10 the Sun Stage 11 • Helium burning continues • Carbon “ash” at the core forms, and the star becomes a Red Supergiant •Duration: 10 thousand years •Central Temperature: 250 million K •Size > orbit of Mars Deep Sky Objects: Globular Clusters • Classic example: Great Hercules Cluster (M13) • Spherical clusters • may contain millions of stars • Old stars • Great tool to study stellar life cycle Observing Stellar Evolution by studying Globular Cluster HR diagrams • Plot stars in globular clusters in Hertzsprung-Russel diagram • Different clusters have different age • Observe stellar evolution by looking at stars of same age but different mass • Deduce age of cluster by noticing which stars have left main sequence already Catching Stellar Evolution “red-handed” Main-sequence turnoff Type of Death depends on Mass • Light stars like the Sun end up as White Dwarfs • Massive stars (more than 8 solar masses) end up as Neutron Stars • Very massive stars (more than 25 solar masses) end up as Black Holes Reason for Death depends on Mass • Light stars blow out their outer layers to form a Planetary Nebula • The core of a massive star (more than 8 solar masses) collapses, triggering the explosion of a Supernova • Also the core of a very massive stars (more than 25 solar masses) collapses, triggering the explosion Supernova Light Stars: Stage 12 - A Planetary Nebula forms • Inner carbon core becomes “dead” – it is out of fuel • Some helium and carbon burning continues in outer shells • The outer envelope of the star becomes cool and opaque • solar radiation pushes it outward from the star Duration: 100,000 years Central Temperature: 300 106 K • A planetary nebula is formed Surface Temperature: 100,000 K Size: 0.1 Sun Deep Sky Objects: Planetary Nebulae • Classic Example: Ring nebula in Lyra (M57) • Remains of a dead, • exploded star • We see gas expanding in a sphere • In the middle is the dead star, a “White Dwarf” Stage 13: White Dwarf • Core radiates only by stored heat, not by nuclear reactions • core continues to cool and contract • Size ~ Earth • Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass! Stage 14: Black Dwarf • Impossible to see in a telescope • About the size of Earth • Temperature very low almost no radiation black! More Massive Stars (M > 8MSun) • The core contracts until its temperature is high enough to fuse carbon into oxygen • Elements consumed in core • new elements form while previous elements continue to burn in outer layers Evolution of More Massive Stars • At each stage the temperature increases reaction gets faster • Last stage: fusion of iron does not release energy, it absorbs energy cools the core “fire extinguisher” core collapses Region of instability: Variable Stars Supernovae – Death of massive Stars • As the core collapses, it overshoots and “bounces” • A shock wave travels through the star and blows off the outer layers, including the heavy elements – a supernova • A million times brighter than a nova!! • The actual explosion takes less than a second Supernova Observation Type I vs Type II Supernovae Type I – “Carbon Detonation” • Implosion of a white dwarf after it accretes a certain amount of matter, reaching about 1.4 solar masses • Very predictable; used as a standard candle – Estimate distances to galaxies where they occur Type II – “Core Collapse” • Implosion of a massive star • Expect one in our galaxy about every hundred years • Six in the last thousand years; none since 1604 SN1987A • Lightcurve What’s Left? • Type I supernova – Nothing left behind • Type II supernova – While the parent star is destroyed, a tiny ultracompressed remnant may remain – a neutron star – This happens if the mass of the parent star was above the Chandrasekhar limit More Massive Stars end up as Neutron Stars • The core cools and shrinks • Nuclei and electrons are crushed together • Protons combine with electrons to form neutrons • Ultimately the collapse is halted by neutron pressure, the core is composed of neutrons • Size ~ few km • Density ~ 1018 kg/m3; 1 cubic cm has a mass of 100 million kg! Manhattan Formation of the Elements • Light elements (hydrogen, helium) formed in Big Bang • Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae – Condense into new stars and planets – Elements heavier than iron form during supernovae explosions • Evidence: – Theory predicts the observed elemental abundance in the universe very well – Spectra of supernovae show the presence of unstable isotopes like Nickel-56 – Older globular clusters are deficient in heavy elements Neutron Stars (“Pulsars”) • First discovered by Jocelyn Bell (1967) – Little Green Men?!? Nope… • • • • Rapid pulses of radiation Periods: fraction of a second to several seconds Small, rapidly rotating objects Can’t be white dwarfs; must be neutron stars The “Lighthouse Effect” • Pulsars rotate very rapidly • Extremely strong magnetic fields guide the radiation • Results in “beams” of radiation, like in lighthouse Super-Massive Stars end up as Black Holes • If the mass of the star is sufficiently large (M > 25 MSun), even the neutron pressure cannot halt the collapse – in fact, no known force can stop it! • The star collapses to a very small size, with ultrahigh density • Nearby gravity becomes so strong that nothing – not even light – can escape! • The edge of the region from which nothing can escape is called the event horizon – Radius of the event horizon called the Schwarzschild Radius How might we “see” them? • Radiation of infalling matter Evidence for the existence of Black Holes • Fast Rotation of the Galactic Center only explainable by Black Hole • Other possible Black Hole Candidates: – Cygnus X-1 (X-ray source), LMC X-3 • Observational evidence very strong Black Holes – The Center of Galaxies? • IR picture of the center of the Milky Way Novae – “New Stars” • Actually an old star – a white dwarf – that suddenly flares up – Accreted hydrogen begins fusing • Usually lasts for a few months • May repeat (“recurrent novae”) Review: The life of Stars