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Goal: To understand special stars. Objectives: 1) To learn about Neutron Stars 2) To learn about Pulsars 3) To understand Stars that erupt. Special stars – neutron stars • Neutron stars are stars that are about 1.4 times the mass of our sun and made entirely of neutrons. • These stars are only a few km in size. • They are essentially a giant atom! • Densities are HUGE (100 trillion times that of water)! • They also spin and have magnetic fields. Crab • Pictured is the Crab Nebula – which supernovaed in 1054. Special stars – Pulsars Special stars – Pulsars • Pulsars are neutron stars. • They spin very quickly (once per second to a thousand times per second). • The stars have strong magnetic fields, and only beam light from their pole (sort of like a lighthouse floodlight). • The pulses normally come in the radio. • However, they also emit a lot of X-rays. • The Crab for example spins 30 times per second. Other types of Pulsars • Hot spot – some emit x-rays from a hot spot that rotates around. • Accretion – this one is a combination of the other two. • Material tends to accrete more along the magnetic poles creating a magnetic hot spot. From NASA Again NASA Energy has to come from somewhere. • Where does the energy the pulsars emit come from? • A) heat • B) nuclear fusion • C) gravity • D) Spin Magnetars • Some neutron stars have a magnetic field 1000 times stronger than the others • Not sure how they form but may have to do with formation as they collapse • However they seem to have slower rotations (seconds) • Only seem to last 10,000 years or so • Hard to observe flare up only very randomly Neutron Stars in binary systems • Remember that most stars are in binary systems! • At the end of the life of the biggest star, sometimes the other stars get away because the dying star looses a lot of mass. • Sometimes they stay together. • Then, when the smaller star evolves… binary systems – Roche Lobes • As a star expands it has a looser and looser hold on its own materials (gravity decreases by the radius squared). • At some point a companion star will have more influence over the outermost parts of the star than the star itself does! • This is called the Roche Lobe. • If a red giant expands past its Roche Lobe, the companion star will accrete materials from it. But, what happens when you accrete matter onto a few km ball of mostly neutrons? • Well, at first the Hydrogen falls way down onto the surface. • This produces energy that helps to power the constant emission of X-rays by the neutron star. • Then, the H is fused into He and crushed onto the surface of the neutron star. • Soon you build up a layer of He (sort of like a layer of snow). He bomb • When the He layer is about 1 m thick, the Helium ignites! • As we saw with the Helium flash for a star, this is a tricky time. • The burning He heats the surface of the star – which speeds up the production of Helium! • The result is a spectacular explosion (although not as spectacular as a supernova) • This produces an X-ray burster! One other side effect • Another side effect of accreting matter is a change to the spin. • Will the spin get faster or slower? If all that was not strange enough • One seems to have a planets and maybe an asteroid belt • Formed from the debris field of its supernova Two binary neutron stars • Can eventually (do to gravity waves) merge together and supernova forming a black hole. Finally • There is one more thing to change spin • Sometimes there is a break in the crust that can make the star shrink down a little bit or change its state • This makes it spin faster usually. Conclusion • Neutron stars are very strange and interesting stars. • Some are in binary systems and do very weird things • Their very strong magnetic fields seem to be many of the causes of their strangeness.