Discover the Universe Week 13
Discover the Universe Week 13 AST 1002
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This 7 page Class Notes was uploaded by Jocelyn on Tuesday December 1, 2015. The Class Notes belongs to AST 1002 at University of Florida taught by Reyes, Francisco J in Summer 2015. Since its upload, it has received 41 views. For similar materials see Discover the Universe in Science at University of Florida.
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Date Created: 12/01/15
Tuesday November 17 2015 Discover the Universe Week 13 The spectral analysis provides information to determine the temperature of the star or the spectral classification We didn39t need to know the distance Next we also made use of the HR Diagram If we know the temperature or the luminosity class then we can deduce the luminosity The Distance Ladder We get distances to nearby planets from radar ranging If we know the distance we can apply Kepler s 3rd law Obtaining Stellar Masses Binary stars are classified as visual spectroscopic and eclipsing Visual Sirius is the brightest star in the sky Sirius A has a companion Sirius B called white dwarf P2 a3m1 m2 The plane of the orbit for Sirius is not face on incline 46 degrees Spectroscopic Many binary stars are too far away and too close to each other to be resolved but they can be discovered from periodic spectral line shifts The shift of the spectral lines is caused by eh Doppler Effect Eclipsing We identify eclipsing binary stars by observing the light curve of the star a plot of the apparent brightness of the star as function of time Why is the mass of the star so important Together with the initial composition mass defines the entire life cycle and all other properties of the star A star with more mass means More gravity More pressure in the core Higher core temperatures Tuesday November 17 2015 Faster nuclear reaction rates Fast production of energy Higher luminosity Shorter lifetime Life time directly proportional to fuel availablehow fast fuel is burned Big stars live shorter lives and burn their fuel fast During the early stages of a star formation the objects are called protostars The internal temperature is not high enough to produce fusion Nuclear reactions slowly covert H to He in the core That is called core hydrogen burning Burning here means fusion In the Sun s core the conversion of H to He will take 10 billion years its Main Sequence lifetime What happens when the core Hydrogen is used up Nuclear reactions stop Core pressure decreases Core contracts and gets hotter heating overlapping layers 4Hgt 1He burning moves from the core to a hot wheel surrounding the core What happens to the core as it continues to contract and get hotter Helium begins to fuse into Carbon at 100 million K Triple Alpha 3He gt C Structure of stars in evolutionary stages Hydrogen burning core Main sequence of a star Hydrogen burning shell Helium core no thermonuclear reactions Red Giant Star The increased shell burning causes the outer layers to expand and cool again The star moves up the asymptotic giant branch Thursday November 19 2015 During this phase helium to carbon burning creates a carbon core which starts to contract an heat up Then He burning moves to a shell With H burning in an outer shell Next in the core for a Sunlike star nothing will happen This is because solar mass stars cannot squeeze and heat the core enough to ignite carbon It will need to reach a temperature of 600 million K With more production of energy in the core the carbon core continues to contract and heat Shell He burning grows more intense Surface layers pulsate and are finally ejected Planetary nebulae have nothing to do with plants It is the last stage of the evolution of the star with the mass of the Sun They emit liner addition hotlow pressure gas but in size they are much smaller than the emission nebulae They are important sources of heavy elements C N and O which contain interstellar clouds and will go into the next generation of stars White dwarfs have about the Sun s mass The white dwarf is an object about the size of the Earth 001 solar radii 12000 km The collapse is topped due to electron degeneracy pressure Density 1 million gcm3 Very low luminosities What will eventually happen to a white dwarf Mets cooler and fainter at the same radius usually takes billions of years What happens to higher mass stars Gravity squeezes and heats the core The temperature needs to increase enough to be able to ignite Carbon around 600 million K Then it will continue fusing oxygen then neon as each fuel gets exhausted in the core its burning moves to a shell Thursday November 19 2015 Concentric fusion shell form an onion skin structure The formation of an Iron Core is the last stage THIS IS BECAUSE creating elements heavier than Iron requires energy The mass is too large and the electron degeneracy pressure cannot stop the collapse Protons and electrons are crushed together in the collapsing core making neutrons Eventually the neutrons are so close together they touch They generate the neutron degeneracy pressure Densities up to 100 trillion gcm3 Crab Nebula Supernova Remnant Located in the Taurus constellation 6500 ly away 1000 years old The central object is a rotating neutron star with a strong magnetic field The object is called a pulsar The crab pulsar first detected as a radio wavelength has a rotation period of 33 milliseconds What happens to the core Composition of the core Neutrons Mass 13 Msun Radius 10 km 6 miles Density 10A17 kgm3 The core rotates fast 1 cm3 weighs as much as Mt Everest Why does the core neutron star rotate fast The angular momentum L is conserved L before L after gt va mVR Thursday November 19 2015 Magnetic field of neutron stars The first pulsar was discovered by Jocelyn Bell in England who measured this radio signal from an unresolved source In 1974 Anthony Huewish won the Nobel Prize in Physics for explaining the pulsar A pulsar model Strong magnetic fields lead to hot spots at the magnetic poles Accelerated particles at hot spots emit beamed radiation If the rotation axis is not aligned with the magnetic field axis then the searchlight rotates lf Earth is in the path of the rotating beam for every rotation a pulse is detected lf Mass lt 14 it will collapse into a white dwarf Electrons run out of room to move around Electrons prevent further collapse Protons and neutrons still free to move mourned stronger gravity more compact Size of white dwarf 12000 km diameter lf Mass gt 14 lt 3 solar masses it will collapse into a neutron star Neutrons run out of room to move around If Mass gt 3 solar masses it will collapse into a Black Hole Black Holes are thought to be the endpoints of stars that exceed 2530 solar masses on the main sequence They are concentrations of mass where gravity is so strong that nothing can escape In terms of theory of relativity the mass of the black hole distort and curve the spacetime and creates an extremely deep gravitational well The mass will collapse into what is called a SINGULARITY The radius and density of the resulting object cannot be determined The state of the matter inside cannot be described Thursday November 19 2015 Einstein s General Theory of Relativity Masses curve the space around them Eventually the escape velocity would exceed the speed of light The trajectory of alight beam will be so distorted The beam of light will remain inside the gravity well Nothing could get out including ightgt that s a black hole The critical radius at which the escape velocity equals the speed of light is called the Schwarzschild Radius This is considered the radius of a black hole because there is no solid surface at that radius The sphere around the black hole at the Schwarzschild Radius is called the event horizon because no event inside that sphere can ever be seen heard or known What would happen to the orbit of Earth if the Sun suddenly became a Black Hole Would Earth get sucked in NO The Earth will be at the same distance at one AU from solar mass Black Hole it will feel the same gravitational attraction from the Sun at 1 AU The gravitational force depends on the product of the masses and is inversely related to the distance squared Example A supermassive black hole in the center of the Milky Way The mass is about 43 million soar masses How can we determine the mass Newton version of Kepler s 3rd Law Mbha3P2 P Orbital Period of a star orbiting the black hole a radius of the orbit Mass of Black Hole Mbh Thursday November 19 2015 Chapter 14 The Milky Way Galaxy A galaxy is a collection of stellar and interstellar matter star fas dust white dwarfs black holes held together by gravity Only three object outside of the Milky Way Galaxy are visible to the Naked Eye the Andromeda Galaxy and two Magellanic Clouds The band of diffuse light that stretches across the sky is what we see when looking along the plane of the Galaxy The dark regions are regions of higher concentrations of dust We cannot see the star behind the dust The Andromeda Galaxy 25 million y away Three Main Parts of a Spiral Galaxy Galactic disk Galactic bulge Galactic halo
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