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Goal: To understand special stars. Objectives: 1) To learn about Black holes 2) To learn about Neutron Stars 3) To understand Stars that erupt. 4) To understand Variable stars 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! • They also spin and have magnetic fields. • Pictured is the Crab Nebula – which supernovaed in 1054. 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. Energy has to come from somewhere. • Where does the energy the pulsars emit come from? • A) heat • B) nuclear fusion • C) gravity • D) Spin 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? White dwarfs in binaries • White dwarfs also can flare up in a binary system. • On a white dwarf though, the matter falling in stays as Hydrogen. • Eventually the temperatures on the surface go up and the density of Hydrogen gets high enough to fuse into Helium. • This creates a quick burning which is known as a nova (not to be confused with supernova). Black Holes in binaries • Just like with Neutron stars and white dwarfs, black holes will create an accretion disk. • However, you can see nothing from the actual accretion, so all you get to see is the accretion disk. • On the plus side, the accretion disk goes down to a few km in size at which point the gas has been heated quite a bit (infalling gas is slowed by frictional heating and interactions with the magnetic field). • The innermost parts will emit X-rays! Astro-mercial • But wait there’s more! • JETS! • Materials racing outward at close to the speed of light and going for up to millions of light Years! • (NGC 5532) Variable stars • Usually stars are held in equilibrium. • If they expand then they cool and that ends the expansion. • Their cores are stable. • The star is stable. • However, there is a region on the HR diagram where this is not the case – the instability strip. • In this region stars will pulsate – that is they will expand and contract. • This causes the star to get brighter and dimer. Variable types • RR Lyrae – lower mass stars after they undergo their Helium flash (the sun will do this someday). • RR Lyrae are Horizontal Branch stars. • Metal rich and Metal poor Cepheid variables (Type I and II). • These are the higher mass stars which pass back and forth through the instability strip. Observing RR Lyrae • RR Lyrae have periods of about 0.3 to 0.5 days. • A) Why do you think those periods are so short? • B) Why is this length of period a really bad thing when it comes to observing the star (hint, when can a good telescope look at stars?)? Absolute Magnitude • RR Lyrae have an average absolute magnitude of 0.75. • Why is that an advantage? • What is the disadvantage if you are looking at other galaxies? Why variable stars are important • Variable stars have a relationship between their period of pulsation and their absolute brightness. • The longer the period, the bigger the star is, and the brighter it is (sort of like a bigger bell has a larger period of vibration). • This allows us to measure distances (especially since these are very bright stars which can be seen a LONG distance away)! • In fact, the distance to Andromeda was first attempted to be estimated using Cepheid variables. Distance to Andromeda • Edwin Hubble tried to estimate the distance to Andromeda using Type II Cepheids (metal poor). • Type II Cepheids are in the globular clusters. • However, he made a slight mistake. Type I Cepheids (metal rich): Mv = -2.81 log(Period in days) -1.43 • Type I Cepheids (metal rich) • ones in the disk of our galaxy • have a pretty exact relationship between variability period and average absolute magnitude. • The brightness of Type Is is 4 times greater than Type IIs Distance misestimated • So, Hubble underestimated the distance to Andromeda by a factor of 50. Even today • We still know the distance to the Andromeda galaxy using the Type I (metal rich) Cepheids. Profile of a Cepheid Variable • Cepheids expand and contract. • As they do they change color (and temperature). • As they expand they cool and turn redder. • As they condense they get hotter and turn bluer. • When do you think they are brightest? Conclusion • There are some very special stars out there. • Many are in binary systems and do very weird things (and I have not covered the extremely rare ones such as Helium or Carbon stars). • Variable stars are quite simple to explain in general and can be used for very important distance calculations.