Test 2 Notes - Keivan Stassun PDF

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Ch.14 – Our Star Why Does the Sun Shine?  What process creates energy in the sun? o The sun’s energy source  The first scientific hypotheses involved chemical reactions or gravitational collapse  failed  Development of nuclear theory led to the correct answer  The sun generates energy via nuclear fusion reactions  Hydrogen is converted into helium in the sun’s core  The mass lost in this conversion is transformed into energy  The amount of energy is given by: E=mc2  Given the sun’s mass, this will provide enough energy for the sun to shine for 10 billion years  Why does the Sun’s size remain stable? o Striking a balance  The sun began as a cloud of gas undergoing gravitational collapse  This caused the core of the sun to get hot and dense enough to start nuclear fusion reactions  Once began the fusion reactions generated energy which provided an outward pressure  This pressure perfectly balances the inward force of gravity o Deep inside the sun, the pressure is strongest where gravity is strongest o Near surface, the pressure is weakest where gravity is weakest  This balance is called gravitational equilibrium o It causes the sun’s size to remain stable  TEST Q: What can be used to directly measure the sun’s mass? o Venus-sun distance and the length of a venetian year  Orbital period of something small orbiting the bigger object we want to know Plunging to the center of the sun: an imaginary journey  Composition of the sun: hydrogen is 70%, and helium is 28%, the other parts are oxygen, carbon, and iron o We know this by identifying the absorption lines in the sun’s spectrum  These absorption lines are formed in the photosphere (sun’s atmosphere)  Layers of the sun (in order)

o Core: where the sun’s energy is generated o Interior zones: energy is transported from core to surface  Radiation zone  Convection zone o Photosphere: the “yellow” that we see; the light o Chromosphere: a thin layer above the photosphere where most of the sun’s UV light is emitted o Corona: the hot, ionized gas which surrounds the sun (emits mostly x-rays)  can be seen in visible light during an eclipse o solar wind: the stream of electrons, protons, and Helium ions which flow out from the sun  it extends out beyond Pluto The Cosmic Crucible  Why does fusion occur in the sun’s core? o Nuclear fusion- heavier nuclei are created by combining (fusing) lighter nuclei  All nuclei are positively charged o Electromagnetic force causes nuclei to repel each other  Nuclei must move fast enough to overcome this repulsion  This requires high temperatures and pressures (mankind can’t achieve yet) o When nuclei collide, the nuclear force binds them together  What is nuclear fission? o Nuclear fission- a reaction where lighter nuclei are created by splitting heavier nuclei (can be achieved by mankind)  Neutrons o Not stable! Do not exist for long! o Ve is a neutrino- a weakly interacting particle which has almost no mass and travels at nearly the speed of light  Proton-proton chain o Reaction where stars convert 4 hydrogen nuclei into 1 helium nucleus and energy is released o Why does the sun shine?  Mass of He – 99.3% of 4x mass of H  The missing 0.7% is converted into energy!! (E=mc2)  “observing” the solar interior o The sun’s interior is opaque- we can’t see directly into it with light o We can construct mathematical computer models of it, they are tested against the sun’s observable quantities o We can directly measure sound waves moving through the interior  We observe “earthquakes” in the photosphere by using Doppler Shifts

 Motion of sound waves can be checked against predictions The solar neutrino problem o Neutrinos are a fundamental prediction of the theory of nuclear fusion in the sun  The neutrinos from the sun have been detected, in the amount predicted! The sun-earth connection  Methods of energy transport o Radiation diffusion- A slow process that takes about 1 million years for energy to travel from the core to the surface o Convection zone  The bottom of the zone is heated… hot gas rises to the top  Cooler gas sinks to the bottom… just like when you boil a pot of water  Energy is brought to the surface via bulk motions of matter (convection)  Photospheric features o Sunspots- dark spot on the surface where the temperature is cooler  Occur in pairs; the pairs cluster into groups; and they rotate with the Sun  They come and go over an 11-year cycle o Granulation- the tops of convection cells seen “bubbling” on the solar surface o Coronal features  Prominences- gas trapped in the magnetic fields is heated and elevated through the chromosphere to the corona  Flares- when a magnetic loop breaks, it releases matter and energy into space o Solar wind- electrons, protons, He nuclei expelled by flares interact with earth’s magnetic field to cause o The aurorae- a strong solar wind can affect our technology by:  Interfering with communications  Knocking out power grids  Damage electronics in space vehicles Big picture ideas  How does the sun shine? o High temperatures and pressures in core lead to fusion (via proton-proton chain) of H  He o Conversion of mass to energy: E=mc2  Gravitational equilibrium o Balance between gravity and pressure o Over time, core shrinks, becomes hotter to maintain balance  How do we know? 

o Neutrinos  fusion! o Sunquakes details of solar interior o Model of sun accurately predicts what we observe

Ch.15 – Surveying the Stars Snapshot of the heavens  Studying the life cycle of stars o A star can live for millions to billions of years o All stars were not born at the same time- seeing them at different stages in life  We can piece together the story of how stars evolve with time  Classification of stars o Stars were originally classified based on their brightness and location in sky  But, told us little about a star’s physical nature o Astronomers have developed a more appropriate classification system based on:  A star’s luminosity  A star’s surface temperature o These properties depend on a star’s mass and its stage in life  Measuring them allows us to reconstruct stellar life cycles  Mass is the most important property Stellar luminosity  Luminosity- the total amount of power radiated by a star into space  Apparent brightness- refers to the amount of a star’s light which reaches us per unit area o The farther away a star is, the fainter it appears to us o How much fainter it gets obeys an inverse square law o Its apparent brightness decreases as the distances increases

o Depends on 2 things:  How much light is emitting: luminosity (watts)  How far away it is: distance (meters)  Apparent brightness = L / 4d2  Measuring distance to stars o Parallax- apparent wobble of a star due to earth’s orbiting of the sun o 1 parsec = 3.26 light years  d=1/P Stellar surface temperature  Colors of stars o The color tells us the temperature: bluer = hotter  Spectral type classification system o Historically: OBAFGKM (Oh Be A Fine Guy, Kiss Me!) o From hotter  cooler  Spectral types: absorption lines in a star’s spectrum arising from different elements o Not determined by a star’s composition o Determined by a star’s surface temperature Stellar Masses  Masses of stars o MASS is the single most important property of any star o It determines:  Its luminosity  Color / spectral type (surface temperature) o Can only be measured directly by observing the effect of gravity from another object o This is most easily done for 2 stars which orbit one another… a binary star!  Binary Stars o Optical doubles- two unrelated stars which are in the same area of the sky o Visual binaries- a binary which is spatially resolved (two stars are seen) o Spectroscopic binaries- a binary which is spatially unresolved; only one star is seen; the existence of the second star is inferred from the Doppler shift of spectral lines o Eclipsing binaries- a binary whose orbital plane lies along our line of sight, thus causing “dips” in the light we see o Binary stars  The stars orbit each other via gravity  Newton’s version of Kepler’s law: p2= 4pi2/G(m1+m2)  Measure orbital period of the binary (P) and distance between the stars (a), then calculate the masses of the stars (m1+m2)  The Hertzsprung-Russell (HR) Diagram

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o Very useful diagram for understanding stars o Plot 2 major properties of stars:  Surface temperature (x) vs. Luminosity (y)  strong relationship with mass also  Stars tend to group into certain areas  Knowing the mass of a star, helps find luminosity and surface temp. o 90% of all stars lie on the main sequence Stellar Luminosity o How can two stars have the same temperature, but vastly different luminosities?  The stars have different sizes! o The luminosity of a star depends on 2 things:  Surface temperature  Surface area (radius)  L = T44R2 o The largest stars are in the upper right corner of the HR diagram (supergiants) o We use binary stars to measure directly the masses of stars of every type. This leads to be the: mass-luminosity relation (for main sequence stars only) o As one moves to the upper left of the main sequence:  Stars become more massive, much more luminous, and fewer in number Lifetime on the Main Sequence o O & B stars burn fuel like a bus o M stars burn fuel like a compact car o Our sun will last 1010 years on the Main Sequence o Every M star that was ever created is still on the main sequence! The Instability Strip o There is a region on the HR diagram where all stars within it are variable- these stars pulsate o Cepehid variables  Henrietta Leavitt studied behavior of Cepheid variable stars in the Magellenic clouds  same distance  The brightness of the stars varied in a regular pattern  Distance indicator: F-G bright giants whose pulsation periods get longer with increasing luminosity Clusters Open clusters o 100’s of stars between 106-109 years old o Irregular shapes o Gas or nebulosity is sometimes seen Globular clusters o 105 stars

o 8-15 billion years old o Spherical shape o No gas  Cluster HR Diagrams Indicate Age o All stars in the cluster were born at about the same time o The position of the hottest, brightest star on a cluster’s main sequence is called the main sequence turnoff point o All stars above the turnoff have used up their H fuel and are dead Big Picture Ideas  All stars are made primarily of H and He  Differences in stars are due to mass and age  The HR diagram is one of the most important tools for understanding stars  Stars spend most of their lives as main sequence stars, fusing H into He in their cores. As they evolve, their positions in the HR diagram change  HR diagrams of star clusters allow us to determine their ages

Ch.16 & 17 - Stellar Birth and Star Stuff Stellar Evolution  Stars are born, grow up, mature, and die  A star’s mass determines what life path it will take o Low mass (.08-2M) o Intermediate mass (2-8M) o High mass (8M+)  If born with < 0.08M the star is stillborn  brown dwarf  The life of any star can be described as a battle between two forces: o Gravity vs. pressure (gravitational equilibrium)  Gravity always wants to collapse the star  Pressure holds up the star  The star’s life is determined by what provides this pressure. We examined the conditions under which gravity can overcome pressure in

interstellar gas, causing fragments of a molecular cloud to contract into protostars, and we saw that gravity's advantage over pressure continues until fusion begins in a star's core. Once hydrogen fusion begins, the energy it generates balances the energy the star radiates into space. With energy balanced, the star's internal pressure stabilizes and halts the crush of gravity. The star is then in a state of equilibrium much like our Sun, with thermal pressure balancing gravity and fusion energy from the core balancing the flow of radiative energy from the star's surface. Stays in this state for millions of years but then exhausts hydrogen, so fusion ceases in the core, and gravity winds over pressure.  the mass of a main-sequence star determines both its luminosity and its lifetime because it determines the core temperature and fusion rate at which the star can remain in gravitational equilibrium Stellar Womb  Stars are born deep in molecular clouds o So cold that H2 can exist  A cold cloud can fragment o Gravity causes cloud fragments to collapse o These regions (cores) become more dense and compact o Heat up as they collapse Life as a Low Mass Star  Life on the main sequences o Stay on MS until they use up their fuel (H) o Massive stars have more fuel, but they are also more luminous, so they use it up faster  Leaving the main sequences o When H used up in core, the core begins to collapse  H shell heats up and H fusion begins there  Increased pressure causes outer layers of the star to expand  The star is now in the subgiant phase of its life  Red giants o The He collapses until it heats to 108 K  He fusion begins (He  C)  The star, called a red giant, is once again stable  Planetary Nebulae o When the red giant exhausts its He fuel  The C core collapses  Low mass and intermediate mass stars don’t have enough gravitational energy to cause carbon to fuse  The outer envelope of the star is gently blown away  planetary nebula o No other recitation after this o The collapsing carbon core becomes a white dwarf

When nuclear fusion stops, core begins to shrink, but the outer layers begin to expand. So, begins to grow in size and luminosity to become a red giant Life as a High Mass Star  Massive star evolution on the HR diagram o When H is exhausted in the star’s core, it leaves the main sequence o The star moves toward the upper right corner of the HR diagram  It becomes a red supergiant  Super giants o What happens to high mass stars when they exhaust their He fuel?  Have enough gravitational energy to heat up to 6x10 8K  C fuses into O o C is exhausted, core collapses until O fuses o The cycle repeats itself: O  Ne  Mg  Si  Fe (can’t go farther than iron)  The Iron (Fe) Problem o The supergiant has an Fe core which collapse and heats  Fe cannot fuse  Fe cannot fuse into another element without creating mass; this requires energy, and it does not produce energy  So, the Fe core continues to collapse...  Supernova o The force of gravity continues to collapse the core  Extremely high temperatures  Atoms in core are smashed apart  Electrons are smashed into protons  neutrons o The neutron core collapses until it is a tightly packed ball of neutrons  This happens in seconds  The rest of the star collapses and bounces off of the neutron core, exploding into space (supernova!) (most energetic thing that happens in universe after big bang)  Neutron star is left behind o The amount of energy released is so great, that fusion can crate most of the elements heavier than Fe o Expelled into space! Summary of differences between high and low mass stars o Compared to low mass stars, high mass stars:  Live much shorter lives  Die as supernova; low mass stars die as planetary nebula  Can fuse elements up to Fe; low mass stars only fuse up to C  In supernova, elements heavier than Fe created 

Leave behind a neutron star (or possibly black hole)  Low mass stars leave behind a white dwarf behind  Are far less numerous o Have the same beginning 

*know process, sequence, what events brought what elements, and why iron is end of road* Low-Mass Stellar Evolution Summary  halt of core collapse  planetary nebula formation  stellar death  white dwarf High-Mass Stellar Evolution Summary  ignition of carbon core  simplified core fusion cycle  central core has been fused into iron o star has layered onion structure representing the previous stages of fusion it has undergone  stellar death  supernova explosion with neutron star or black hole in the middle of the expansion/explosion 17.4 Mass Exchange in Binary Stars  two stars in a binary system can be close enough to transfer mass from one to the other o gaining or losing mass will change the life path of a star Chapters 16-17: Big Picture Ideas  the tug of war between gravity and pressure determines how stars evolve from birth in a cloud of gas and dust to their death  low mass stars (like our Sun) live long lives and die with the ejection of a planetary nebula, leaving behind a white dwarf  high mass stars live fast and die young, exploding dramatically as supernovae, enriching the galaxy with new heavy elements  all elements heavier than H and He were forged in the cores of stars o when they die, low mass stars expel Carbon, massive stars expel all heavier elements in supernova explosions Chapter 18 - The Bizarre Stellar Degeneracy Pressure  quantum Mechanics: two particles cannot occupy the same space with the same energy  for very dense solids, the electrons cannot be in their ground states, they become very energetic – approaching the speed of light  this provides a pressure to hold up the star that no longer depends on temperature  this holds up dead stars: white dwarf stars and neutron stars

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if gravity is too strong for degeneracy pressure, gravity crushed the star into a black hole

Degenerate Core Leftover  after planetary nebula, the carbon core collapses  gravity is finally stopped by electron degeneracy pressure  the stellar corpse at the center of a planetary nebula is a white dwarf White Dwarf  they are in gravitational equilibrium… o gravity vs electron degeneracy pressure  they generate no new energy  as they slowly radiate their heat into space, they become cooler and fainter  they are very dense; 0.5 – 1.4 M packed into a sphere the size of the Earth  degenerate matter obeys different laws of physics  the more mass the star has, the smaller the star becomes! o increased gravity makes the star denser o greater density increases degeneracy pressure to balance gravity Limit on White Dwarfs  Chandrasekhar formulated the laws of degenerate matter o won Nobel Prize  theory predicts that gravity will overcome the pressure of electron degeneracy if a white dwarf has a mass greater than 1.4M (Chandrasekhar Limit) o energetic electrons, which cause this pressure, reach the speed of light White Dwarfs in Binary Star Systems  if a white dwarf is in a close binary: o matter from its companion can be accreted onto the white dwarf o the matter forms a disk around the white dwarf o friction in the accretion disk heats it  it emits visible, UV, and even X-ray light o if matter falls onto the white dwarf, H fusion begins  the white dwarf temporarily becomes brighter Novae  They typically become 100-1000x brighter got a few days, then fade  Accretion disk: is a rotating disk of gas orbiting a star  The hydrogen buildup on the surface of the white dwarf can ignite into an explosive fusion reaction that blows off a shell of gas  Temporary, but reoccur.  Because so little mass if blown off during a nova, the explosion does not disrupt the binary system

Ignition of the infalling hydrogen can recur again with periods ranging from months to thousands of years White Dwarf Supernovae  If accretion rings the mass of a white dwarf above the Chandrasekhar limit, electron degeneracy can no longer support the star o The white dwarf collapses  The collapse raises the core temperature and runaway carbon fusion begins, which ultimately leads to an explosion of the star  Such an exploding white dwarf is called a white dwarf supernova  Always contains the same luminosity (about 10 billion Suns) o Good distance indicators o More luminous than Cepheid variable stars o Can be used to measure out to greater distances  There are two types of supernovae: o White dwarf- no hydrogen seen in spectrum o Massive star- prominent hydrogen in spectrum  Chart o Time 0 = max brightness of supernova 

Neutron Stars  They are the leftover cores from supernova explosions  If the core < 3 M, it will stop collapsing and be held up by neutron degeneracy pressure  Neutron stars are very dense  They rotate very rapidly: period = 0.03 to 4 seconds  Their magnetic fields are 10 1...