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The Deaths of Massive Stars The Deaths of Massive Stars • Massive stars live spectacular lives and destroy themselves in violent explosions. • Because of their high mass, they can fuse heavier elements since they can achieve higher pressures and temperatures in their cores. Nuclear Fusion in Massive Stars • At higher temperatures than carbon fusion, nuclei of oxygen, neon, and magnesium fuse to make silicon and sulfur. • At even higher temperatures, silicon can fuse to make iron. • Iron is the last element to form by fusion. Nuclear Fusion in Massive Stars The heavy elements are used up, and fusion goes very quickly in massive stars. Hydrogen fusion can last 7 million years in a 25-solar-mass star. The same star will fuse its oxygen in 6 months and its silicon in just one day. Supernova Explosions of Massive Stars • Silicon fusion produces iron— the most tightly bound of all atomic nuclei. Supernova Explosions of Massive Stars • As a star develops an iron core, energy production declines, and the core contracts. –Nuclear reactions involving iron begin. –However, they remove energy from the core—causing it to contract even further. –Once this process starts, the core of the star collapses inward in less than a tenth of a second. Types of Supernovae Type I supernovae are formed by the collapse and explosion of a white dwarf star. Their light curves exhibit sharp maxima and then die away gradually. They from from Population II stars in elliptical galaxies. Type II supernovae are produced by the collapse and explosion of a massive star. They die away more sharply than the Type I (about 15 days), but then their magnitude plateaus until about 100 days past their explosion. They form from Population I stars in spiral galaxies. Observations of Supernovae • In AD 1054, Chinese astronomers saw a ‘guest star’ appear in the constellation known in the Western tradition as Taurus the Bull. – The star quickly became so bright that it was visible in the daytime. – After a month, it slowly faded, taking almost two years to vanish from sight. Observations of Supernovae • When modern astronomers turned their telescopes to the location of the guest star, they found a what is now known as the Crab Nebula. Observations of Supernovae The Recycling of Stardust Neutron Stars • A supernova will produce one of two star remnants: A neutron star or a black hole. • A neutron star contains a little over 1 solar mass compressed to a radius of about 10 km. Nuclear arithmatic • A proton has a mass of 1 and a charge of +1. • An electron has a mass of 0 and a charge of -1. • A neutron has a mass of 1 and a charge of 0. 1 proton+ 1 electron = 1 neutron + lots of energy! (annihilation) Formation of Neutron Stars • The collapse would force protons to combine with electrons and become neutrons. Neutron Stars • The matter is so dense that a single teaspoon would weigh ten billion tons on Earth. • Predictions show that the stars have a 1km surface crust that consists of iron 10,000 times more dense and stiff than on Earth. The Make-up of a Neutron Star Formation of Neutron Stars • The rapidly - spinning star creates collimated beams of radiation visible in X-ray and radio wavelengths. • If these beams sweep over Earth, we see them as a series of regular pulses: a pulsating star, or pulsar. Click on image at left to play an animation. Discovery of Pulsars In November 1967, Jocelyn Bell found a new, regular pattern in data from a radio telescope. Originally she and her team suspected they had made the first detection of alien life, and named it LGM 1. What they really found was the first pulsar. Magnetars • A magnetar is a neutron star with a magnetic field so strong that it slows the star’s rotation and causes quakes. • These quakes cause small bursts of gamma radiation. Quark Stars • Quark Stars are smaller and more dense than neutron stars. They can get as small as 11 km across. • The physics of a quark star is unknown. Gravity may not be needed to hold it together. • The matter of a quark star is strange- really “strange”! A Real Quarker! • This is RX J1858-3754, one of the first quark stars studied by the Chandra X-Ray Observatory. It is in the constellation of Centaurus. Hypernovae • Hypernovae are much more luminous and 100X more powerful than supernovae • Source of gamma-ray bursts • Hypernovae can be thought of as “failed supernovae”, because most of the star does not explode off into space Formation of Black Holes • When the most massive stars collapse, gravity wins. • If the contracting core of a star becomes small enough, the escape velocity in the region around it is so large that no light can escape. It is a black hole. Black Holes • A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. • It can be identified by either how much space it takes up or by the mass. Black Holes • It is a common misconception to think of black holes as giant vacuum cleaners that will suck up everything in the universe. A black hole is just a dead star with a massive gravitational field. At a reasonably large distance, its gravity is no greater than that of a normal object of similar mass. If the Sun became a black hole, the planets’ orbits would not change at all. Black Holes • The event horizon is the boundary between the isolated volume of space-time and the rest of the universe. The Search for Black Holes • The areas around black holes (their accretion disks) can be detected using X-ray and radio telescopes. • Black holes can also be detected if a star is seen to orbiting a non-visible point. What Happens After A Black Hole? • One fanciful idea is that a black hole is attached to something called a white hole that is in another universe. • It does the exact opposite of a black hole, it spits things out. • But, a white hole has never been discovered.