Astronomy Section 4
Astronomy Section 4 ASTRO103
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Date Created: 05/04/16
Stellar Parameters I Lecutre Summary Apparent Magnitude o Intensity Absolute Magnitude o Luminosity Stellar Velocity Stellar Parameters Luminosity, Absolute Magnitude—How much energy does a star produce? Velocity—How fast does the star move through space? Temperature (Spectral Type) – How hot is the surface of the star? Radius—How big are stars? Mass—How much stuff do they contain? Distance—How far away are they? Intensity: What we measure (b) Power of the star distributed over a sphere with a radius equal to the star’s distance from us Inverse square Law What does the intensity tell us? When we measure the intensity, we can’t use that to determine th Luminosity—we don’t know if L and d are both large or both are small Apparent Magnitude A different way to write intensity Logarithmic in scale to accommodate the large range of values Larger numbers the less intense Magnitude 1 is 100 times brighter than magnitude 6 Magnitudes Magnitudes were first developed by the Greek Astronomer Hipparchus Absolute Magnitude Different way to write luminosity Logarithmic in scale It is the magnitude we would see IF the star were located 10 parsecs away Absolute Magnitudes range from + 19 for the intrinsically faintest stars to about =8 for the intrinsically brightest Distance Modulus If both apparent and absolute magnitude are now then you know how far away the star is Velocity of Stars Radial Component of Velocity Proper Motion The movement of a star perpendicularly to out line of sight Can only measure nearby stars Measured in units of arcseconds/years Vtis the Transverse Velocity in km/s D is the distance in parsecs U is the proper motion in “/years Space Velocity Stellar Parameters II Lecture Summary Stellar Temperature Balmer Thermometer Spectral Type HR Diagram Stellar Spectrum Balmer Thermometer Balmer absorption lines are produced by hydrogen atoms whose electrons are in the second energy level Cool stars Weak Balmer Lines Hot Stars Weak Balmer lines Medium Temperature Stars Strong Balmer Lines Beyond Balmer Lines Spectral Classification Original classification was done by Annie Cannon at Harvard Observatory ~1920 Examples of Spectral Types Spectral Class Chart Shapes of Spectral Lines Natural Width—the thin, but finite width owing to quantum uncertainty Rotational Broadening –spinning blue and redshifts the liens Doppler Broadening—Doppler shift from the thermal motion of the molecules Collisional Broadening—collisions spread out the lines, highly dependent on density UBV Filters Spectral shape on the cheap The V designation is the apparent brightness of an object through the V filter and is sometimes also written as m v Color Index Blackbody curve can be inferred from just 2 points Size of Stars (Hertzsprung-Russell Diagram) Lines of Equal Radius Conceptual HR Diagram Luminosity increases upward Temperature increases to the left Radius increases to the upper right Stellar Parameters III Lecture Summary Binary Stars o Visual Optical Double Stars o Astrometric o Spectroscopic Radial Velocity Curves o Spectrum o Eclipsing Binary Stars Light Curves Binary Stars Communicate stellar masses o Mass-Luminosity Relation Herschel—Binary Pioneer Optical Double: stars which appear close in the sky but are very far from each other Optical Double Stars Visual Binaries Binary stars are very common—out of the 59 nearest stars, 27 are members of systems containing more than one star Observing a VisuardBinary Kepler’s 3 law Barycenter/Center of Mass rd Combined mass given by Kepler’s 3 law Once the combined mass is known, astronomers can solve for the individual masses by noting the star’s distances from the center of mass Astrometric Binaries Spectroscopic Binaries Spectrum Binary Eclipsing Binaries Algol (the demon star) John Goodricke Correct interpretation of the nature of eclipsing binaries was first proposed by Goodricke in 1783. He studied Algol (beta Persei) and determined a period of 68 hours 50 minutes. Light Curves and Radii It is also possible to determine accurate values for stellar radii from eclipsing binary light curves. For example, the time between C (when the small star completely disappear behind the large one) and E (when the small star first reappears) times the orbital velocity gives the size of the large star Light Curves= Information Binary Stars and Masses An Approximate Relationship Stellar Parameters IV Lecture Summary Parallax Pulsating Variable Stars Main Sequence Fitting Spectroscopic Parallax Hipparcos Measured parallax to nearby stars Pulsating Variable Stars Stars that actually grow and shrink changing the heat and magnitude These stars are found in the Instability Strip The Pulsation Cycle RR Lyrae, Cepheid Variable Pulsating Variables are Standard Candles RR Lyrae Stars o All RR Lyraes have absolute magnitude of about M= 0.5 o Thus one observes m, assumes M= +0.5 and uses the distance modulus to get distance Cepheids o Obeys a Period-Luminosity Relation o One observes the period, looks up M in the chart to the right, observes m, and then applies the distance modulus Main Sequence Fitting Plot observe HR diagram of a cluster of stars (in a “cluster” the stars are all approximately the same distance) Assume the stars will obey the same HR relationship as other stars Adjust distance to “fit” the cluster onto the main sequence Spectroscopic Parallax Ia—Bright Supergiant Ib—Supergiant II—Bright Giant III—Giant IV—Sub Giant V—Main-sequence star Spectroscopic Parallax 1. Determine the spectral class of the star from the types of spectral lines present 2. Determine the luminosity class of the star from the thickness of the spectral lines 3. Plot the point on the HR Diagram at the intersection of the spectral type and luminosity class 4. Read off M on the HR diagram for the star 5. Observe m 6. Use the distance modulus m-M to determine the distance to the star Stellar Evolution I Lecture Summary Star Formation o Evaporating Gaseous Globules Protostars Main Sequence o Brown Dwarfs o Red Dwarfs o Sun-like Stars Post-Main Sequence Stages o Red Giants, Yellow Giants, Planetary Nebula Star Formation Eagle Nebula Protostars Protostars Moving to the Main Sequence Masses of Forming Stars Quick Approximation of Star Lifetime Brown Dwarfs Never become main sequence stars M < 0.08M Red Dwarfs Fully Convective Stars Probably the most common type 0.08M<M<0.4M Solar Mass Stars Won’t fuse Carbon 0.4M < M < 3.0 M Evolving to become a Red Giant Stellar Evolution Law #1 o Whenever the core of a star runs out of fuel, the star energy production region moves outward (toward the surface) cuasing the star to expand into a red giant Helium Ash Core The Sun Will Eventually Get Big To the Horizontal Branch Stellar Evolution Law #2 o Whenever a new energy source is found in the core, the star will move back down toward the main sequence Shedding Gas Planetary Nebula Planetary Nebula of a Binary System Interstellar Medium Lecture Summary Nebulae o Dark o Emission o Reflection Evidence for ISM o Interstellar extinction, reddening, spectral lines Components of the ISM o HI Clouds, HII regions, Giant Molecular Clouds, Coronal Gas Nebulae Singular is Nebula Form the Greek word for cloud Obvious evidence for the ISM Dark Nebulae Emission Nebulae Reflection Nebulae Complex Nebula Some nebulae show characteristics of the various “types” of nebulae The Cocoon Nebulae shown has elements of all three types Interstellar Medium Subtle evidence for its existence Interstellar Extinction Interstellar Reddening Interstellar Absorption Composition of the ISM 99% Gas o ~75% H, ~25% He, traces of other stuf 1% Dust o Similar to cigarette smoke particles Electron Spin HII Cloud Dark Molecular Clouds Map of Nearby ISM Interstellar Probe Sending a Probe Deep into Space Stellar Evolution II Lecture Summary Evolution of Massive Stars Supernova o Type II—Exploding Massive Stars Cluster Evidence o Algol Paradox Evolution of Massive Stars Fusion Chart Gum Nebula Why do Astronomers Believe in Stellar Evolution? Clusters—All Stars o Formed at the same age o Formed from the same cloud of gas and thus have the same composition o Are in the same location—same distance same stellar extinction, reddening o But have different masses h and chi Persei—Very young clusters (10 million years) Hyades Cluster—600 million years Globular Cluster 47 Tucanae ~ 11 billion years old Roche Lobes Algol Paradox—Blue Stragglers Compact Objects Lecture Summary White Dwarfs Neutron Stars Black Holes White Dwarfs M< 1.44 M—Chandrasekhar’s Limit Radius about the size of the earth High Density—1,000,000 g/cm 3 Temperature between 6,000K and 30,000K, most around 10,000K Ball of Carbon and Oxygen Nuclei with unbound electrons mixed in Supported by electron pressure—degenerate material (pressure doesn’t depend upon temperature) White Dwarfs as Companion Stars Nova Supernova Characterisitics Type I o Exploding white dwarf o Lack H spectral lines o Peak around M ~-1V o Initial rapid decline, slower end time decline Type II o Exploding massive star o Strong H spectral lines o Peak around M ~ -V7 o Pause for about 100 days before rapid decline in brightness Neutron Stars 1.2 M < M < 2.5? M Radius~ a city (10 km) Extremely High density o 10 14g/cm 3 Ball of neutrons held together by gravity o e + p N + v Supported by neutron pressure Immense Magnetic Fields Neutron Stars Spin Rapidly Pulsars Pulsars=Neutron Stars Lighthouse Model Glitches X-Rays from Crab Pulsar Black Holes M > ~3M Zero Radius—a singularity Black holes have no hair o Density, temperature, and composition are not meaningful. All structure is destroyed It is called a black hole because its gravity is strong enough to keep even light from escaping General Relativity Mass bends spacetime Spacetime tells matter and light where to go Black Holes
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