ASTR 100 Week 11 and 12 Notes Bob Berrington
ASTR 100 Week 11 and 12 Notes Bob Berrington ASTR 100
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This 9 page Class Notes was uploaded by Sarah Gardner on Sunday April 17, 2016. The Class Notes belongs to ASTR 100 at Ball State University taught by Dr. Bob Berrington in Fall 2016. Since its upload, it has received 14 views. For similar materials see Introduction to Astronomy: Solar System and Beyond in Astronomy at Ball State University.
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Date Created: 04/17/16
Week 11 & Week 12 ASTR 100 Properties of telescopes Light Gathering Power (LGP) o Proportional to collection area o Given by ratios of the squares of the radi o Example: how much LGP does an 8 inch telescope have over the human eye o 8 inch telescope gather is 816 times more light than the human eye Resolving power of telescopes (finer details) o Given by like a pixel size on the camera o The more pixels the better the resolving power o The smaller it is the better the resolution o Example: what is the angular resolution of the human eye? o If two objects are separated by less than 20 inches than the human eye will see one object magnification number of times image is enlarged o focal length equals distance between the lens or mirror and focus Altitude = angle above the Horizon o Zenith is at altitude of 90 degrees Azimuth = angle measured from North Cardinal Point toward East Cardinal Point o Time and location dependent Hertzsprungrussell diagram Giants and supergiant's o Cool temperature, high Luminosity cool temperature means low surface brightness F= σT 4 2 Surface area must be large L= 4πR F If F is low from T, then R must be large to make bright White dwarfs o High temperature, low luminosity 4 o High surface brightness F= σT o Low Luminosity L= 4πR F 2 o If F is high than R must be small (small surface area) o Known as a compact object o Dying star Hertzsprungrussel Diagram continued Luminosity classes o 1a bright supergiant o 1b supergiant o II bright giant o IV subgiants o V main sequence/dwarf o VI white dwarf Luminosity vs. Temperature o Plots of the diagram of Lumiosity versus Temperature o Spectral type Stars found in specific regions of the HR diagram o Main sequence = 90% of stars diagonally across the HR diagram o The sun is a G2 MS Helpful diagram of Star Luminosity (instead of me writing it down) Stellar classification Found to be a temperature sequence A type Stars Very blue = very hot 40000 degrees Kelvin M type Stars very red very cool 3000 degrees K f Further delineate into subclasses follow with a number 0 9 Represents the hottest nine Lines are strong for an AO star Measuring a star's temperature (Wein’s Law) measure of a star's maximum brightness gives temperature of a star Annie Jump Cannon Found a sequence in classification scheme Used Atomic absorption lines Cause of sequence was unknown Henry Norris Russell coined the mnemonic: “Oh be a fine girl kiss me” Cecilia Payne Gaposhkin Found a temperature sequence in Annie Jump Cannon Spectral classification study B Theoretical connector between Stellar Spectra and temperature Temperature sequence goes from high to low Line strength Temperature measures the average kinetic energy per atom Collisions between atoms can excite electrons in atoms The excited electrons will emit photons Temperature determines the typical energy and electron has in an atom Hotter stars have electrons with greater energy higher states in the atom Line strength continued Bomar thermometer Needs enough energy to excite electrons in n=2 and greater states Difficult to do for cool Stars Few electrons make it to the end state Line strength continued Other metals are similar to calcium Coolest Stars allows molecules to form helium Extremely difficult to excite seeing only and hottest stars with ionized helium Helium with one electron stripped like H but one negative electron held in orbit by two positive protons Very tightly bound seen only and hottest stars Line strength continued Other metals are similar to calcium o Coolest Stars aligned molecules to form o Helium extremely difficult to excite Seen only and hottest stars Ionized helium o Helium with one electron stripped o Like each but one electron held in orbit by two protons o Very tightly bound o Seen only in hottest stars Masses of stars How do we measure the mass of a star? Look at the gravitational influence Binary Stars Two stars in orbit about each other Gravity determines Mass Kepler's Third Law o o M 1 mass of an object 1 in solar units o M 2 mass of object 2 in solar units o P & a = years/AU’s Visual Binaries o Both stars seen in telescope o Able to follow orbit o Usually takes years o Can get massive both Stars o Orbit Center of mass Position in space where the system is mass behaves as if it were concentrated at a point can be called center of gravity Example is Sirius Types of binary Stars Spectroscopic Binary o Uses Doppler shift o Only one star visible o Lines shift as a function of time o Give the line of sight of velocity o Can see one set of Shifting lines o Sb1 o Can see two sets of Shifting lines o SB2 Spectroscopic binary star Much like Kepler’s Second Law (planets sweep equal orbits in equal time, in this case stars) Types of binary Stars Eclipsing binary Stars o See only one star o 1 star eclipses the other (Earth is in nearly the same orbital) o Measure period from light curve Brightness of a star as a function of time During eclipse see only one of two stars Binary Stars tell us how a massive systems are To get independent masses of components of the system that has to be eclipsing binary and spectroscopic. Mass and luminosity About a hundred fifty Stars give reliable masses o We get Spectral types from these ones o O type Stars (main sequence) Massive, 55100 solar units As bright as 1 million suns (1x10 L) Hot, about 50,000 degrees K o M type Stars (main sequence) Least massive, about 0.1 solar units Least luminous (1x10 L), about as bright as our sun Very cool, 3000 degrees K Formation and Structure of Stars Interstellar medium (ISM) o Composition o Mostly hydrogen 75% by mass, 92% by particle number o Helium 25% of mass, 0.07% by particle number Some trace elements like C, O, N, Ca, Na o Other heavier elements Roughly 1% is dust o Cigarette smoke sized particles o Silicate carbon mixed o 150 meters distance between dust particles average ISM Not evenly distributed Seen in cool clouds with high density o 10 100 particles per cubic centimeter Hot tenuous gas o Seen as luminous red clouds Nebulae Luminous clouds o Usually red o Hydrogen Alpha emission line (example = rosette nebula) o Light from excited He, H, N, etc Some blue o Reflected/scattered light o Same reason why sunset is red Associated with O & B type Stars o Needs energy to ionize hydrogen o Heats cloud to 10,000 degrees K Trapezium o 4 stars that illuminate Orion Nebula o Spectral types equal B3 B1 B1 and O3 o Found in other galaxies Example: m64 Galaxy ISM Cont. o Dark Cloud o Regions where no stars appear in the sky o Cloud is nonluminous Opaque Made of dust particles Very cool gas with typical temperature of 1030 degrees K Most likely regions where star formation occurs o Often called molecular clouds o Contains dust grains and complex molecules Horsehead nebula is an example Interstellar reddening o Same phenomenon causes sunsets to be red o Blue light is preferentially scattered by dust in the cloud o Red light makes it through, therefore color is red Dark clouds continued o Gravitationally stable o Outward gas pressure equals inward gravitational force o Need something to initiate Cloud collapse Shock wave o High Velocity gas need flow velocity gas o Causes gas compression and Cloud collapse o Examples: Supernova explosions, spiral arms, and galaxy collisions Galaxy collision star formation o Formation of protostars o Internal pressure of cloud not able to Halt gravitational collapse o Complex molecules allows energy to escape o Gravitational collapse as quick relatively Few million years which is 1% lifetime of a star Depends on mass of star generated O type stars collapse fastest b/c they're massive M type stars collapse slowest because they're the least massive Ideal gas law Describes behavior of gas Describes relationship between pressure, temperature, and volume If you're given these you can figure it out Cloud collapse Large molecular hydrogen Cloud collapses o Angular momentum problem (ice skater effect) o Majority of matter falls into core where star forms Angular momentum causes some matter to fall into a disc around central core Clouds heat up converting gravitational potential energy into kinetic energy of atoms o Ideal gas law says that if he goes down and T goes up then P must go up Presence of dust critical to Cloud collapse o Allows cloud to cool which allows collapse o Emission from dust and molecules in light are transparent to Cloud Core of cloud heats up o Reacts few million degrees Kelvin as Cloud collapses o Nuclear fusion happens at a few million degrees Kelvin o Begins conversion of hydrogen into helium protonproton chain source of power for majority 90% of stars lifetime Solar nebula theory o Angular momentum of cloud around Center of the Galaxy causes Cloud to flatten as it collapses o Central region attracts most of matter o Location of sun = hottest region o Disk forms planets o Inner regions heated by sun o Outer regions remain cool o Only materials with high condensation temperature for planetesimals o Silicate, iron, nickel, etc o Built in size like raindrops in clouds o Terrestrial planets o Typical composition like sun without hydrogen and helium o Formed in high temperature region of solar nebula
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