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Stellar Remnants Stellar Evolution • The end of a star’s life as with its birth depends on its mass. • Solar mass stars generally will evolve smoothly to a white dwarf. • Larger masses result in several endings: – Supernova, Black hole, Neutron star White Dwarfs • Hot compact stars • Mass of the Sun, size of the Earth • Core of carbon and oxygen • Very compact, no fuel supply • Increasing mass causes them to shrink. Degenerate Matter • White dwarfs are very dense objects. • They are made of degenerated matter. • It is so dense that a beach ball size lump would weigh as much as an ocean liner. ( 1 ton per cm3) The End of a High-Mass Star A high-mass star can continue to fuse elements in its core right up to iron (after which the fusion reaction is energetically unfavored). As heavier elements are fused, the reactions go faster and the stage is over more quickly. A 20-solar-mass star will burn carbon for about 10,000 years, but its iron core lasts less than a day. The End of a High-Mass Star The neutrinos escape; the neutrons are compressed together until the whole star has the density of an atomic nucleus, about 1015 kg/m3. The collapse is still going on; it compresses the neutrons further until they recoil in an enormous explosion as a supernova. Supernovae A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star. There are two different types of supernovae, both equally common: Type I, which is a carbon-detonation supernova; Type II, which is the death of a high-mass star just described. Supernovae Carbon-detonation supernova: white dwarf that has accumulated too much mass from binary companion If the white dwarf’s mass exceeds 1.4 solar masses, electron degeneracy can no longer keep the core from collapsing. Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion. The Formation of the Elements The elements that can be formed through successive alpha-particle fusion are more abundant than those created by other fusion reactions: The Formation of the Elements The last nucleus in the alpha-particle chain is nickel-56, which is unstable and quickly decays to cobalt-56 and then to iron-56. Iron-56 is the most stable nucleus, so it neither fuses nor decays. However, within the cores of the most massive stars, neutron capture can create heavier elements, all the way up to bismuth-209. The heaviest elements are made during the first few seconds of a supernova explosion. The Cycle of Stellar Evolution Star formation is cyclical: stars form, evolve, and die. In dying, they send heavy elements into the interstellar medium. These elements then become parts of new stars. And so it goes. Chandrasekhar Limit • Extra mass increases the gravity and compresses it. • The Chandrasekhar limit implies that a star greater than 1.4 solar mass cannot evolve smoothly to become a white dwarf. • Too much mass causes white dwarf to collapse. Binary System • One star maybe a white dwarf • Mass can be transferred between them. • An accretion disk spirals toward the compact. • Some binary systems produce a nova explosion if the gas layer reaches ignition temperature of hydrogen. • Novae may repeat the process if it doesn’t accumulate too much mass. • If the mass is over the Chandrasekhar –Limit, it will explode. Neutron Star • In very massive stars, the core is iron. • When the iron core collapses, the result is a supernova explosion. • The collapsing core is squeezed past degenerate matter. • A neutron star is formed. Supernovae • There are two types of supernovae. • Type I luminosity declines rapidly at first, then slowly as time passes. • Type II maintains brightness for up to 100 days and then declines in luminosity. • A supernova leaves behind a nebula. • This is called a supernova remnant. Neutron Stars Neutron stars, although they have 1–3 solar masses, are so dense that they are very small. This image shows a 1-solar-mass neutron star, about 10 km in diameter, compared to Manhattan. Pulsars • 1967 Jocelyn Bell noticed odd radio signal. • Discovered the first pulsar, a pulsating star. • These objects seem to be associated with super nova remnants. (SNR) • It is believed that a pulsar is a rapidly rotating neutron star. • Instead of pulsing, a pulsar is a rapidly spinning neutron star. • From the period of the pulse, it had to be extremely dense but associated with a supernova explosion. • Spin of a neutron star and its magnetic field generates powerful electric fields. • Emission created by accelerating charges called synchrotron radiation (low energy radiation at radio wavelengths.) Pulsars But why would a neutron star flash on and off? This figure illustrates the lighthouse effect responsible: Strong jets of matter are emitted at the magnetic poles, as that is where they can escape. If the rotation axis is not the same as the magnetic axis, the two beams will sweep out circular paths. If the Earth lies in one of those paths, we will see the star blinking on and off. Black Holes The mass of a neutron star cannot exceed about 3 solar masses. If a core remnant is more massive than that, nothing will stop its collapse, and it will become smaller and smaller and denser and denser. Eventually the gravitational force is so intense that even light cannot escape. The remnant has become a black hole. Black Hole • Core remnant mass that is greater than 3 solar masses. • A Black hole would result from a star’s core complete collapse. • Degenerate neutrons con hold up core against its own gravity. Event Horizon • Is the boundary of a black hole. • The distance at which the escape velocity equals the speed of light for the size of the black hole. • Space is so curved that any light emitted is bent back to the point mass. • The size of the event horizon is called the Scharzschild radius. Black Holes The radius at which the escape speed from the black hole equals the speed of light is called the Schwarzschild radius. The Earth’s Schwarzschild radius is about a centimeter; the Sun’s is about 3 km. Once the black hole has collapsed, the Schwarzschild radius takes on another meaning – it is the event horizon. Nothing within the event horizon can escape the black hole. Distorted Space-Time • As you approach a black hole, time slows down. • This happens because the gravity field has distorted space-time. • Einstein’s theory of relativity helps explain this. Detecting Black Holes • Event horizon is only several miles across. • Black hole and visible star will orbit around the center of mass between them. • Guestamate the mass of visible star, then using Kepler’s 3rd Law, calculate the mass of the other object. • If mass is too large for neutron star or white dwarf, most likely a black hole. • Black holes advertise their presence with X-rays. • Gas material will form an accretion disk. • Particles rub against each other. • This causes heat that is hot enough to emit X-Rays. • Gravitational Waves: – If black hole orbits a companion, it motion generates a gravitational wave. – Hawking Radiation: – Black star emits blackbody radiation. – Using Wien’s Law you can calculate the temperature of the black hole. (6 x 10-8K)