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Chapters 11 & 12

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by: Jasmine Knight

Chapters 11 & 12 ASTR 101 001

Jasmine Knight
GPA 3.0

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Introductory Astronomy 1
Jimmy Irwin
Study Guide
50 ?




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1 review
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"Why didn't I know about this earlier? This notetaker is awesome, notes were really good and really detailed. Next time I really need help, I know where to turn!"
Terrell Bergstrom

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This 9 page Study Guide was uploaded by Jasmine Knight on Wednesday November 4, 2015. The Study Guide belongs to ASTR 101 001 at University of Alabama - Tuscaloosa taught by Jimmy Irwin in Fall 2015. Since its upload, it has received 94 views. For similar materials see Introductory Astronomy 1 in Astronomy at University of Alabama - Tuscaloosa.


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Date Created: 11/04/15
Chapter 11 Our Star • Surface Temp: 5800 Kelvin • Core Temp: 15 Million Kelvin • Composition: 71% hydrogen, 28% Helium, 1% other • The Sun’s energy source was long considered a major mystery? • Is it on fire...NO! Total chemical energy/ energy rate per year ~ 10,000 years - even if the Sun were one big lump of coal or wood it would only provide energy for 10,000 years • Is it contracting.. NO! Gravitational potential energy/ energy rate per year = 20 million years • Is it powered by Nuclear Energy? YES! Nuclear Mass-Energy/energy rate per year • Converting a fraction of the Sun’s mass to energy via fusion can power it for 10 billion years Gravitational Potential Energy • In space, an object or gas cloud has more gravitational energy when it is spread out than when it contracts • A contracting cloud converts gravitational potential energy to thermal energy Fission • Big nucleus splits into smaller nuclei + energy (Nuclear power plants) Fusion • Small nuclei stick together to make a bigger nucleus + energy (sun, stars) Fusion Within The Sun • Fusion required very high temperatures (and pressures) to occur at the center of the Sun • How does the Sun get hot? • Must understand the role of gravity within the Sun • Gravity: inward pull of outer layers of the Sun toward the center, thus heating the gas at the center of the Sun • The weight of upper layers compresses lower layers • The upper layers of gas compress and heat up the lower layers of gas → when central temperature reaches at least 10 million K, fusion turns on • High gas pressure at center of Sun supports the outer layers • Gas at center of Sun has high density and temperature How does Fusion work? • If temperature is too low (protons moving at too low velocities), positively- charged protons will repel each other (both are positive charges) • If temperature reaches the crucial barrier of 10 million K, particles move fast enough to overcome repulsion and stick together • If 4 1^H nuclei (protons) stick together, two protons are changed into neutrons → 4^He nucleus + energy • The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus • Proton-Proton Chain: 1^H fusing into 4^He - Net result • IN: 4 protons (1^H nuclei) → OUT: 4^He nucleus, 2 gamma rays → energy, 2 positrons (anti-electrons), 2 neutrinos • Four individual protons weight 0.7% more that one 4^He nucleus - this “missing mass” has been converted into energy (the two gamma rays) via E=mc^2 • The Sun balances gravitational pressure with gas pressure • Inward pull of gravity is counterbalanced by the outward force of gas pressure (gas heated by gamma-rays created by fusion trying to escape) • “Hydrostatic or Pressure Equilibrium” • Sun neither collapses nor blows apart How can we really tell what is going on inside the core of the Sun? • Neutrinos (nearly-massless neutral particles) created during fusion fly directly through the Sun • Observations of solar neutrons can tell us what’s happening in core Solar Neutrino Detection • Neutrinos are notoriously difficult to detect- perhaps one in a trillion neutrinos passing through the large detector interacts with matter • Would take a layer of lead 1 light year thick to stop an average neutrino • Very small fraction of neutrinos can be detected on specially made neutrino detectors- the predicted number of neutrinos are detected What is the Structure of the Sun? • Solar Wind: Flow of ions, charged particles (mostly protons and electrons) from the Sun’s surface • Corona: Outermost layer of solar atmosphere, Tenuous, low density ~ 1 million K, hot gas emits optical and X-ray light • Chromosphere: Middle layer of solar atmosphere ~10,000-100,000 K, emits UV light • Photosphere: visible surface on Sun, 5800 K • Convection Zone: Energy transported upward by rising hot gas bubbles, hot bubbles rise → cool off → sink → get reheated → rise, same mechanism that moves rock in mantles of terrestrial planets (but much, much hotter and involving gas) Granulation • Convection (rising hot gas, falling cooler gas) takes energy to surface • Hotter gas rises (lighter patches), cooler gas sinks (darker patches) A Closer Look: Granulation • 35-minute time lapse image of surface of Sun • Convection brings hotter parcels of gas up to the surface of the Sun, where the gas cools off and sinks back down - leads to mottled appearance of Sun Radiation Zone • Energy transported upward by photons • Photons continuously bumped and re-directed by atoms comprising Sun Core • Energy generated by nuclear fusion, 15 million K • Pressure, 10^11 atmospheres - keeps Sun from collapsing (10% by mass) • Fusion only happens in core (not in other layers) Chapter 12 • Luminosity: amount of power an object radiates (energy per second, such as a Watt), Intrinsic property of an object • Apparent Brightness: amount of light that reaches an observer (energy per second), depends on observer’s location relative to light Luminosity Versus Brightness • Observer 1 is one meter away while observer 2 is five meters away • Both observers must agree the luminosity of the bulb is 100 watts • The observers measure a different apparent brightness for the bulb • Amount of light passing through each sphere is the same • Area of sphere = 4pi(radius)^2 • Divide luminosity by area to get brightness • Light intensity needs to spread out over a larger area as it propagates out → star looks dimmer the farther one is from it • Think of equal amounts of paint needed to paint walls of different sizes, the larger wall gets a thinner layer of paint • The relationship between apparent brightness and luminosity depends on distance, Brightness = luminosity/ 4pi(distance)^2 • We can determine a star’s luminosity if we can measure its distance and brightness • Luminosity = 4pi(distance)^2 x (brightness) • This makes finding the distances to astronomical objects crucial So how far away are these stars? Are they luminous and far away or dim and nearby? • Parallax: apparent positions of nearest stars shift as Earth orbits Sun • Relies on simple geometry - no assumptions needed • Just need to know Earth-Sun distance (AU) • Parallax angle depends on distance: the closer the object, the greater the angular shift • Measure angle → get distance Parallax and Distance • We measure angles in arcseconds • Parallax currently only accurate for distances <1600 light years (GAIA spacecraft will extend that to tens of thousands of light years - launched in December 2013 • The angular resolution of the human eye is 1’=60’’. Cannot measure parallax with naked eye for objects farther than 0.05 light years (nearest star is 4.4 light years) • Most luminous stars 1 million Lsun • Least luminous stars 0.0001Lsun • Hottest Stars: 50,000 K • Coolest Stars: 3000 K • Sun’s surface is 5800 K • Surface temperature is not core temperature Properties of Thermal Radiation • Hotter objects emit more light per unit area at all frequencies • Hotter objects emit photons with a higher average energy/shorter peak wavelength (Wien’s Law) • Blue stars are hottest • Red stars are coolest • Color of star can tell us roughly its temperature Remembering Spectral Types • Hottest: O B A F G K M Coolest • Oh, Be A Fine Guy, Kiss Me Spectral Subtypes • Each spectral class is broken into 10 subclasses: B1, B2, B3 - the lower the number the hotter the star • Each star also has a luminosity class • I - Supergiant • II - Giant/Supergiant • III - Giant • IV - Subgiant • V - main sequence stars (hydrogen burning) • Most massive stars: 100 Msun • Least massive stars: 0.08 Msun How do we measure mass? • We measure mass using gravity • Direct mass measurements are possible only for stars in binary star systems Types of Binary Star Systems • Visual • Eclipsing • Spectroscopic Visual Binary • We can directly observe the orbital motions of these stars (need stars to be well- separated • Analogous to direct method of finding extrasolar planets (where you can actually see the planet) Eclipsing Binary • We can measure periodic eclipses when one star passes in front of another star • We don’t actually see two stars, we just measure dips in light from one apparent star • Analogous to transit method for finding extrasolar planets Spectroscopic Binary • We determine the orbit by measuring Doppler shifts of emission lines in the spectrum of one star (or both). • Analogous to Doppler method for finding extrasolar planets What is a Hertzsprung-Russell diagram? • An H-R diagram plots the temperature and luminosity of stars • Location of star on H-R diagram tells us about its size, temperature, luminosity, and nature • Most stars (90%) fall somewhere on the main sequence of the H-R diagram • Stars occupy a rather narrow path in luminosity-temperature space. • Do not find hot, low-luminosity or cool, high-luminosity stars on the main sequence • Main sequence is a mass sequence • Massive stars are blue, hot, and luminous (upper left) • Low mass stars are red, cool and dim, (lower right) • Sun like stars are somewhere in between • Stars on the main sequence are happily converting hydrogen to helium in their cores → stable (for now) • Stars not on the main sequence (giants, white dwarfs, etc) are not happily converting hydrogen to helium in their cores Why are more massive stars more luminous? • Core pressure and temperature of a higher-mass star need to be higher in order to balance gravity • Higher core temperature boost fusion rate, leading to larger luminosity • O stars have the shortest lifetimes • M stars have the longest lifetimes Sizes of Stars • Very, very few star sizes measured directly (too far away and/or are too small) even with modern instrumentation • Most sizes deduced from modeling of stellar emission Indirect Determination of Star sizes • Eclipsing binaries help us determine the sizes of stars • If velocity of secondary star is known, the time is takes the light curve to decline to the minimum is related to size of secondary • Time spent in minimum is related to primary star size • Lunar occultations of background stars also tell us about the sizes of stars • The time it takes a star’s light to extinguish as it passes behind the Moon can be measures • If we know the distance to the star we can determine its radius this way Main Sequence Star Summary • High mass: High luminosity,short lived, large radius, Blue (high temp) • Low mass: Low luminosity, long lived, small radius, Red (low temp) • Initial mass determines nearly every property of a star • Stars not on the main sequence are not happily converting hydrogen to helium • Thermal Radiation: hotter objects emit more light per unit area • Stars with lower T and higher L than main sequence stars must have larger radii: giants and supergiants, these stars are on the main sequence • Stars with higher T and lower L than main sequence stars must have smaller radii: white dwarfs, these stars used to be main sequence and then giant stars, but are now at the end of their stellar lifetimes Off the Main Sequence • Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence • All stars become larger and redder (initially) after exhausting their core hydrogen: giants and supergiants • Most stars eventually end up small and white after fusion has ceased: white dwarfs Stellar Properties Review • Luminosity: from brightness and distance • Temperature: from color and spectral type • Mass: from period (p) and average separation (a) of binary-star orbit Star Clusters • Open Cluster: a few thousand (or less) loosely packed young stars - will not remain together for very long • Globular Cluster: up to a million or more old (12-13 billion years) stars in a dense ball bound together by gravity Star Clusters and Stellar Lives • Our knowledge of the life stories comes from comparing mathematical models of stars with observations • Star clusters are particularly useful because they contain stars of different mass that were born about the same time and at the same distance • All stars have same distance, same age, so we can study differences in mass of stars How do we measure the age of a star cluster? • The hottest, most massive stars on the main sequence age first and leave the main sequence first • They are followed by successively lower mass stars • Place where main sequence of a cluster ends: turn off point • younger clusters have bluer turnoff points on the H-R diagram • Massive blue stars die first, followed by white, yellow, orange and red stars - that is first O, then B, A, F, G, K, M stars in order • Main sequence turnoff point of a cluster tells us its age • Older clusters have a turnoff point at a lower luminosity/temperature/ mass than younger clusters • To determine accurate ages, we compare models of stellar evolution to the cluster data • Detailed modeling of the oldest globular cluster reveals that they are 13 billion years old


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