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1-10-06 Hubble above atmosphere because atmosphere interferes with telescopes Pointed Hubble above dipper in black for 30 days Hubble Deep Field: in that tiny piece of black, there were 3000 galaxies, proportionally there are 50 billion over the entire sky Ian Shipsey Course webpage: www.physics.purdue.edu/academic_programs/courses/astr264 The Cosmic Landscape-Our Place in the Universe 1.1 A Modern View of the Universe Planet-solar system-galaxy-local group-local super cluster Stars-a large, glowing ball of gas that generate heat and light through nuclear fusion Planet- a moderately large object that orbits a star; it shines by reflected light. Planets may be rockey, ice, or gaseous in composition. Moon (or satalite)- an object that orbits a planet note that terms Moons and Satallite are used interchangeably, spacecrafts are satalites, but not moons Asteroid- a relatively small and rocky object that orbits a start. They are smaller than planets and often called minor planets. Comet- a relatively small and icy object that orbits a star Solar (star) system- a star and all t he material that orbits it, including its planets and moons. Solar systems, planets, moons, and asteroids and comets are all in it Nebula-an interstellar cloud of gas and/or dust Galaxy- a great island of stars in space, held together by gravity and orbiting a common center Universe-the sum total of all matter and energy; that is, everything within it How did we come to be? 1-12-06 guy who made up the big bang name was from BBC Radio off the top of his head universe’s energy remains the same Speed of light: finite speed, 300,000km/s Destination-Light Travel Time Moon-1 second Sun-8 minutes Sirius-8 years Andromeda-2.5 million years We see objects as they were in the past Light-year: the distance light can travel in one year, 10 trillion km, 6 trillion miles 1 light-year=(speed of light)x(1year) =(300,000 km/s)x(365x24x60x60) =9.4 trillion km, 6 trillion miles light-year is a unit of distance, not time 14 billions years is widely accepted as good current estimate for age of universe 15 billion year old galaxies we couldn’t see since light hasn’t gotten here yet What’s our physical place in the universe? -earth is part of the solar system which is in the milky way, which is a member of the local group of galaxies in the local supercluster How did we come to be? -matter in our bodies came from big bang, which produced hydrogen and helium -all other elements were constructed from H and He in start and then recycled into new solar systems How can we know that the universe was like in the past? -when we look to great distances we are seeing events that happened long ago because light travels at a finite speed Can we see the whole universe? -no, observable portion is about 14 billion years in radius because universe is about 14 billion years old 1.2 Scale of the Universe Reduce the size of the solar system by 10 bllion, sun is size of a grapefruit: Earth is the size of a ball point on a pen, 15m away Alpha Centauri is 2500 miles away 1-17-06 in a 1-10billion scale, the entire galaxy is the distance from the earth to the sun how long to count the stars in our galaxy at 1 per second? A few thousand years Milky way is 1 of 100billion galaxies, 10tothe11th stars/galaxy x 10tothe11th galaxies=10tothe22 stars, or as many as grains of (dry) sand on all of earth’s galaxies 1 AU=distance of earth to the sun The Cosmic Calendar: a scale on which we compress he history of the universe into 1 year Jan 1: big bang: 0:00Hrs 1 month: 1.2 billion years Earth is about 4.6 billion years, September 3rd Dec 17th, multi-cell Dec 26th, Dinosaurs, gone by Dec 30th Dec 31st, 11:58pm, Humans, Earth rotates around its axis once every day North Pole: 0 km/hr North: 1275 km/hr Equator: 1650 km/hr South: 1275 km/hr Earth orbits the sun, 1 AU=150 million, KM With Earth’s axis tilted by 23.5 degrees, pointing to polaris Roatating in the same direction it orbits, counter-clockwise as viewer from about he north pole Our sun moves relative to other galaxies, speeds relatively 70,000 km/h, stars are so far away we can’t see their movement, our sun orbits galaxy every 230 million years Most of the mily way’s mass is in its halo. It’s dark matter, we can’t see it and don’t really know what it is Galaxies moving: Galaxies are like rasins in bread as they move. As the bread rises, galaxies are further apart, this is how we know universe is expanding, further away they are, faster they’re moving Stars move randomly relative to one another, but all rotate once about every 230 million years 1-18-06 The Science of Astronomy Scientific thinking is based on everyday ideas of observation and trial and error experiments How did astro observations benefit ancient societies? Keeping track of time and seasons -for practicul purposes, including agriculture -for religious and ceremonial purposes aid to navagation Why does modern science trace roots to greeks? Greeks were the first people to make models of nature They tried to explain patterns in nature without resorting to myth or supernatural Greeks on Planetary motion: Underpinnings of the greek geocentric model: Earth at the center of the universe Everything revolved around us 5 planets easy to see by eye: Mercury, Venus, Jupiter, mars, and Saturn, all known to greeks Review: over a period of 10 weeks, Mars appears to stop, back up, then go forward Most advanced geocentric model was that of Ptolemy (100-170), the Ptolemaic Model: Accurate enough to be used for 1500 years Arabic translation of Ptolemy’s work named Almagest (greatest compilation) He does have planets going backwards in his model Why did Greeks reject the real explanation for planetary motion: -their inability to observe stellar parallax (i.e. the thumb bouncing between eyes) when you look at a star in janurary, then again in july, you’re on the other side of your orbit, so it’s mirrored because of this, Greeks knowledge could mean: 1. stars are so far away that stellar parallax is too small to notice with the naked eye 2. earth does not orbit sun, it’s the center with rare exceptions such as Aristarchus, the Greekes rejected the correct explanation because they didn’t think the stars could be that far away The Copernican Revolution: Proposed sun-centered model: 1543 Used modem to determine layout of solar system (planetary distances in AU) But… Model was no more accurate that Ptolemaic model n predicting planetary positions, because it still used perfect circles Tycho Brahe (1546-1601) -compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions -still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but said other planets go around sun) -Hire Kepler, who used Tyco’s observations to discover the truth about planetary motion -had a fake nose Johannes Kepler, 1571-1630 Kepler first tried to match Tycho’s observations with circular orbits An 8-arcminute discrepancy led him to ellipses Ellipse is elongated circle Eccentricity: how oval it is, 0 is a circle Kepler’s First Law: the orbit of each planet around the sun is an ellipse with the sun at one focus (3/4 of the way on one side of the major axis) Kepler’s Second Law: as a planet moves around its orbit, it sweeps out equal areas in equal times Kepler’s Third Law: more distant planets orbit the sun at slower average speeds, obeying the prelationship: P(sqrd)=a(cubed) P=orbital period in years A=average distance from sun in AU 1-24-06 Galileo solidified Coperniean revolution? He overcame major objections to Coperican view. 3 objections rooted in Aristotelian view here: 1. earth could not be moving because ovjects in air would be left behind 2. non-circular orbits are not “perfect” as heavens should be 3. if Earth were really orbiting sun, we’d detect stellar parallax Oercoming the 1st objection (nature of motion) Galileo’s experiments showed that o bjects in air would stay with a moving earth Aristotle thought all objects naturally come to rest Galileo showed that objects will stay in motions unless a force acts to slow them down (Newton’s first law of motion) Overcoming the 2nd objection )heavenly perfection) Tycho’s oberservations of comet and supernova already challenged this idea Using his telescope, Galileo saw: sunspots on sun (Imperfections), mountains and valles on moon (proving it’s not a perfect sphere) Overcoming 3rd objection (parallax) -tycho thought he had measured stellar distances so lack of parallax seemed to rule out an orbiting earth Galileo showed stars must be much farther than Tycho though, in part by using his telescope to see the Milky Way is countless individual stars If stars were much further away, then lack of detectable parallax was no longer so troubling Catholic Church ordered that Galileo recant his claim that suns orbits earth in 1622 His book on the subject was removed from church’s index of banned books in 1824 Galileo was formally vindicated by church in 1992 Hypothesis: educated guess Hallmarks of Science #1: Modern science seeks explanation for observed phehomena that rely solely on natural causes (scientific model cannot include divine intervention) Hallmarks of Science #2: Science progresses through the creation and testing of models of nature that explain the observations as simply as possible Scientific theory must: Explain a wide variety of observations with a few simple principles AND -must be supported by a large, compelling body of evidence -must NOT have failed any crucial test of its validity Understanding Motion, Energy, and Gravity (Ch 4) Speed: rate at which object moved, speed=distance/time Velocity: speed and direction, 10m/s, due east Acceleration: any chance in velocity units of speed/time, m/s(sqrd) Gravity=10 m/s-sqrd Galileo showed gravity is the same for all faling objects 1-26-06 momentum=mass x velocity net force: changes momentum, generally means acceleration angular momentum: rotational momentum of a spinning or orbiting object mass: amount of force on an object weight-the force that acts upon an object there is gravity in space weightlessness is due to a constant state of free-fall Newton: Realized the same physical laws that operate on Earth also operate in the heavens, therefore one universe Discovered laws of motion and gravity Much more: experiments with light: first reflecting telescope, calculus… Newton’s First law: A object moves at constant velocity unless a force acts to change its speed or direction Newton’s Second law: Force=mass x acceleration Newton’s third law: For every force, there’s always an equal and opposite reaction force Conservation laws in Astronomy Conservation of momentum: Total momentum of interacting objects cannot change unless an external force is acting on them Interacting objects exchange momentum through equal and opposite forces Conservation of angular momentum: Angular momentum= mass x velocity x radius Angular momentum of an object cannot change nless an external twisting force (torque) is acting on it Earth has no torque as it orbits sun, so it’ll rotate forever 1-31-06 where do objects get their energy: energy makes matter move energy is conserved, but it can: transfer from one object to another change in form kinetic: motion radiative: light stored or potential energy can change type bu cannot be destroyed Thermal energy: A type of kinetic energy, the collective kinentic energy of many particles Temperature is the average kinetic energy of many particle in a substance Thermal energy is related to temp, but is NOT the same Boils: 212 F, 100 C, 373.15K Freezes: 32 F, 0 C, 273.15K Absolute Zero: -459.67F, -273.15 C, 0k Thermal energy is a measure of the total kinetic energy of all the particles in a substance. It therefore depends both on temperature AND density Per cubic inch, there’s 1000 times more atoms in water than in air, that’s why there’s more thermal energy in boiling water and in an oven Gravitational Potential Energy: On earth, depends on: -object’s mass (m) -strength of gravity (g) -distance object could potentially fall Mass-Energy Mass itself is a form of potential energy E=mc-sqrd A small amount of mass can release a great deal of energy Concentrated energy can spontaneously turn into particles (particle accelerators) Conservation of Energy: Energy can’t be created or destroyed, just swaped around What determines strength of gravity: Universal Law of Gravitation: 1. every mass attracts every other mass 2. attraction is directly proportional to the product of their masses 3. attraction is inversely proportional to the square of the distance between their centers F(g)=G (M1M2)/d(sqrd) Newton’s Version of Kepler’s 3rd law Complicated, see lecture 6 powerpoint Use this to calculate mass of foreign objects How do gravity and energy together allow us to understand orbits (4.5) Total orbital energy (gravitational + kinetic) stays constant if there is no external force Orbits cannot change spontaneously 8 km/s will put you in orbit, any slower you crash, faster you spin out into an ellipse, based on a mass of 1 kg escape velocity is 11 km/s 2-2-06 Chapter 5: Light and Matter Warmth of sunlight tells us light is energy We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s 1068 watts per sqr meter hits the earth’s atmosphere white ordinary light is made up of colors pass the 6 colors through another prism and it becomes white again Light and Matter interact: Emission Absorption Transmission -transparent objects transmit light -opaque objects block (absorb) light Reflection or Scattering Mirror reflects light in a particular direction Move screen scatters light in all directions Interaction between light and matter determine the appearance of everything around us Chair looks red because only the red light is coming out Windows are both mirrors and transmitters Rose is red because rose reflects red light 5.2 what is light? Light can either act like a wave or a particle Photons: particles of light Wave: pattern of motion that can carry energy without carrying matter along with it Wavelength: is the distance between two wave peaks Frequency: is the number of times per second that a wave vibrated up and down Amplitude: half the difference in height between crest and a trough Electromagnetic wave consists of oscillations in electric and magnetic fields, 300,000 km/s Wave speed=wave length x frequency Light=electromagnetic wave A light wave is a vibration of electric and magnetic fields Light interacts with charged particles through these electric and magnetic fields Wavelength x frequency =speed of light =constant Shorter the wavelength the higher the frequency, and vice versa WL=1cm, F=30ghz WL=1/2cm, F=60hz etc Each photon has a frequency Energy of a photon depends on its frequency 3.00x10>8th m/s=sped of light E=hxf H=constant, 6.626x10>-34 joule xs Spectrum, 88 octaves, we can only see 1 of them, use science to see other 87 Radio waves are light, they can be as big as football fields, they have very low energy Spectrum from smallest to biggest, most energy to least Gamma rays-x rays-ultraviolet-infared-radio Higher the photon energy: shorter its wavelength 2-7-06 Final is 5-5, 8-10am, Loeb Playhouse Midterms is here 2-14 5.3 Properties of Matter Structure of Matter Atoms made of electron cloud (10 to the -10 meter) with a nucleus in the center The electron cloud gives atoms their seize Atomic Number= # of protons in nucleus Atomic Mass Number = # of protons + neutrons Molecules consist of two or more atoms (H2O, CO2) 2000 electrons=1 proton, in terms of mass Isotope: same # of protons but different # of neutrons, isotopes are important in Astro How is energy stored in atoms? Electrons in atoms are restricted to particular energy levels Atoms have: Mass energy Kinetic Energy Potential Energy in orbiting Electrons (very important) Lowest energy it can have: ground state, 0 eV Lvl 2-10.2 eV Lvl 3-12.1 eV Lvl 4- 12.8 eV Ionization lvl- 13.6 eV The only allowed changes in energy are those corresponding to a transition between energy levels The 3 types of Spectra: Continuous (rainbow from the dense filament in a light bulb, warmer) Emission (low density, loose bunch of matter from a cloud, so you get bands of color, much colder) Absorption (rainbow with dark lines because it’s absorbing light from the light bulb, given energy from warmth, but it absorbs certain energies for certain atoms, the dark spots can tell us what’s in the cloud) Change in temp affects intensity Downward energy pattern produce specfic fingerprints Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths Peak has to be in un-visible light to be in infared 2-9-06 spectrascopes break up light into its primary colors color of light tells us the temp of an object heats up through the colors of the spectrum Properties of Thermal Radiation: 1. Hotter objects emit more light at all frequencies per unit area 2. Hotter objects emit photos with a higher average energy. Why don’t we glow in the dark? People only emit light invisible to our eyes Which is hotter? Blue star THIS IS ON EXAM 2: Telescopes, Ch 6: Refraction: refraction is the bending of light when it passes from one substance into another Your eye uses refraction to focus light Refraction at sunset: Sun appears distorted at sunset because of how light bends in earth atmosphere The focal plane is where light from different directions comes into focus Image behind a single convex lens is actually upside down Cameras use CCDs Midterm: 1. E 2. B (100 Billion in Milky Way) 3. D 4. B 5. C 6. C 7. C 8. E (230 Million Years to orbit the milky way galaxy) 9. A (most of the mass in the Milky Way is in the halo, dark matter) 10. D 11. A (Copern, Kepler, Galil) 12. A (Baghdad) 13. D Galileo discovered Newton’s first law of Motion 14. B stellar parallax 15. E all of the above (keppler’s 3rd law) 16. D 17. C 18. C 19. C 20. B 21. D (pro positive, elect negative) 22. C (look this up) 23. E (electrical) 24. B 25. A (9.8 m/s-sqrd downward) 26. D 27. B 28. C (reviews Newton’s laws) 29. D 30. D (universal law of gravitation) 31. B 32. C 33. B 34. D 35. D (cloud=absorption) 36-40 is telescopes 2-16-06 2 most important parts of telescope 1.light-collecting area 2.angular resolution Light collecting area: A telescope’s diameter tells us it’s light collecting area: pi(diameter/2)>sqrd Angular res: the minimum angular speperation that the telescope can distinguish Ultmate res comes from interference between waves, bigger telescopes have less intereference The lomit on angular resolution is known as the diffraction limit Refracting telescope: focuses light with lenses (very long, large heavy lenses) Reflecting telescope: focuses light with mirrors (much larger diameter, these are modern) Imaging: taking pictures of sky Spectroscopy: breaking light into spectra Timing: measuring how light output varies with time Astro detectors record only one color at a time, takes several imagses to make a ful one Colors can also represent energy levels Spectroscopy: breaks up light before it hits the defractor Best conditions for ground viewing: Calm, high, low light, dry Light Polution: scattering of human-made light in atmosphere is a growing problem for astro Turbulent air flow in earth’s atmosphere distorts view, causing starts to twinkle Adaptive optics: tries to counteract wind turbulance Satalite dish is essentially a telescope for observing radio waves Radio telescope is like a giant mirror that reflects radio waves to a focus (biggest telescopes are radio) VLA-very large array (telescopes working together) Sun: Ch 14 Sun shines because nuclear energy, e=mc-sqrd, burns for 10 billion years Gravitational equilibrium: energy provided by fusions maintains pressure Contraction of sun stopped when fusion began Structure: Radius: 6.9 x 10tothe8th Mass: 2x 10tothe30th Luminocity 3.8 x 10tothe 26th watts solar winds, corona, chromosphere, convection zone, radiation zone, core 2-21-06 14.2 Nuclear Fusion in the Sun gravitational and equilibrium keeps core hot and dense enough to release energy through nucear fusion fission: big nucleus splits into smaller pieces (power plants) fusion: small nuclei stick toegehr to make a bigger one (sun, stars) sun releases energy by fusing 4 hydrogen nuclei into 1 helium nucleus hydrogen is fused by the proton-proton chain Sun is losing mass, but it’s huge, so it’s ok Slight rise in core temp leads to a rapid rise in fusion energy, the core would expand and cool, solar thermostat keeps burning rate steady Decline in core temp causes fusion rate to drop, so core contracts and heats up Rise in core temp causes fusion rate to rise so core expands and cools down How does energy from fusion get out of the sun Takes rougly 10 million years for a photon to escape Convection (rising hot gas) takes energy to surface Bright blobs on photosphere are where hot gas is reaching surface We learn about the inside of the sun by: Making mathematical observations Patterns of vibrations Observations of solar neutrinos can tell us what’s happening in the core Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number 2-23-06 4 protons go in, 1 helium atom comes out neutrino discovered in 1932, doesn’t have a charge, bit of uncharged nucleus solar neutrino problem: early attempts failed to find them predicted number, recently we’re able to find the right number but its changed, earlier experiments weren’t sensitive to the changed form 14.3 Sun-Earth Connection solar activity is like weather: sunspots, solar flares, solar prominences, all related to magnetic frields sunspots are cooler than other parts of the sun’s surface: 4000k, regions with strong mtic field magnetic fields, sun spots are size of the earth Zeeman effect: We can measure magnetic fields in sunspots by observing the splitting of spectral lines Charged particles spiral along Loops of bright gas often connect sunspot pairs Magnetic activity also causes solar prominances that erupt high above sun’s surface Magnetic activity causes solar flares that sen bursts of x-rays and charged particles into space Charged particles streaming from Sun can disrupt electrical power grids and can disable communicant satalies Every 11 years we get more, then none, then a lot, etc (sunspot cycling) Sunspots have something to do with winding and twisting of sun’s mag 2-28-06 How do we measure stellar luminoscities (How much energy a star is putting out, measured in watts) Apparent brightness: Amount of starlight that reaches earth (energy per second per square meter) Luminosicty passing through each sphere is the same: Area of sphere (4pi(radius)sqrd) Divide luminosity by area to get brightness Lumin=4pi(dist)sqrd x brightness Parallax and distance P=parallax angle D(in parsecs)=1/p(in arcseconds) D (in lightyears)=3.26x(1/p(in arcseconds)) 1. hotter objects emit mor elight per unit area at all frequencies 2. hotter objects emit photons with a higher average energy stars very from 3,000k to 50,000k, out sun is 5800k level of ionization also reveals a star’s temp lines in a star’s spectrum correspond to a spectral type that reveals it’s temp (hottest) O B A F G K M (coolest) orbit of a binary star system depends on strength of gravity types of binaries: visual binary (we can directly view orbital motions) eclipsing binary (we measure periodic eclipses) spectroscope binary (determine orbit by measuring Doppler shifts, object moves closer, wavelength shortens, lengthens as it goes away) about half of all starts are in binary systems direct mass measurement are possible only for stars in binary star systems: need 2 of 3: 1. orbital period 2. orbital seperation 3. orbital velocity 3-2-06 H-R diagram: x axis: temp, y axis: luminosity Most stars fall somewhere on the main sequence of the H-R diagram Stars wither lower T and higher L than main-seq stars must have larer radii, giants and supergiants Stars with higher T and lower L than main-seq stars must have smaller radii, white dwarfs Stars classification includes spectral type, lum class 1 (brightest)-5(main-seq) Sun: G2V (in between g and k and it’s main sequence) Sirius: A1V Betelgeuse- M2 1 (super giant, close to super nova) H-R can show: Temp, colo, spectral type, lum, radius Main-seq stars are fusing hygrogen into helium in their cores like the sun Lum main-seq stars are hot (blue) Less lum ones are cooler (yellow or red) The mass of a normal, hydrogen burning star determines its lum and spectral type Core pressure and temp of higher-mass star needs to be larger in order to balance grav Higher core temp boosts fusion Star that’s 10 times our sun, it burns 10,000 times faster Our sun will be 10 billion years 10 times smaller than our sun=100 billion years Off Main-Seq: Go from giants to white dwarfs Why do properties of some stars vary? Variable stars: can’t get balance, not enough radiation from the core, brightness changes due to expansion/compressing, cycles in days Most pulsating stars inhabit an instability strip on the H-R diagram Star Clusters: Open cluster: a few thousand loosely packed stars, in galaxy Globular cluster: up to a million or more stars in a dense ball bound by gravity, above or below plane Age=where stars are going off main seq Pleiades now has no stars with live expectancy of less than 100,000 years Oldest global clusters are 13 billion years old 3-7-06 Thursday after spring break is exam, Tuesday is review Stars form in dark clouds of dusty gas in interstellas space Gas between stars is called interstellar medium Absorption lines in spectra determine composition 70% H, 28% He, 2% heaver element in our region of Milky Way Molecular Clouds Most of the matter in star-forming clouds is in the form of molecules These molecular clouds have a temp of 10-30 K and a density of about 300 molecules per cubic cm Most of what we know about molecular clouds comes from observing emission lines of CO Interstellar Dust: Tiny solid particles of interstellas dust block our view of stars on the other side of a cloud Particles are <1 icrometer in size and made of elements like C, O, Si, and Fe Interstellar reddening: stars viewed through edges of cloud look redder because dust blocks (shorter wavelength) blue light better than (longer wavelength) red light Long-wavelength infrared light passes through a cloud more easile than visible light Observations of infrared light reveal stars on the other side of the cloud Visible light is often trapped inside the cloud of a new star, infrared lets it out Dust grains that absorb visible light heat up and emit infared light of even longer wavelength Long wavelength infrared light is brightest from regions where many stars are forming Why do stars form? Gravity vs pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons A typical molecular cloud must contain at least a few hundred solar masses for gravity to overcome pressure Resistance to gravity: A cloud must have even more mass to begin contracting if there are additional forces opposing gravity Both magnetic fields and turbulent gas motions increase resistance to grav Fragmentation of a cloud: Gravity within a contracting gas cloud becomes stronger as the gas becomes denser Gravity can therefore overcome pressure in smaller pieces of the cloud causing it to break apart into multiple fragments, each of which may go on to form a star 3-9-06 midterm 2 is 3-23 in class, practice exam is up, review is 3-21, material: ch 6, 14, 15, 16 first stars: carbon and oxygen nt yet made w/o CO to cool, clouds that formed had to be considerably warmer than todays first stars must have therefore been more massive for grav to overcome pressure simulations of early star formations suggest the first molecular clouds never cooled below 100K, making stars of approx 100M-sun trapping of thermal engery as contraction packs, molecules and dust particles come together, cloud becomes more opaque thermal energy builds center forms a protostar, when contraction slows matter from the cloud continues to fall onto the protostar until either the protostar or a neighboring star blows the surrounding gas away cloud’s rotation on star birth: nedular theory of solar system formation illustrated importance of rotation conservation of ang. Momentum: rotation speed of cloud affects rotation of star collision between particles flattens cloud out collision also reduces up and down motion jets are observed coming from centers of disks around protostars no rotation means no planets how does nuclear fusion begin in a newborn star protostar looks starlike after surrounding gas is blown away, but its thermal energy comes from grav contraction, not fusion contraction must continue until the core becomes hot enough for nuc fus contaction stops when energy released by cure fusion balances energy radiated from surface, star is now main-sequence luminocity model shows sun requires 30 mill to go from proto to current 3-21-06 Exam: ch 6 (telescopes), 14 (sun), 15 (surveying stars), 16, (star birth) HR-surface(x), luminosity (y) Higher mass stars form faster, lower mass from more slowly Fusion and Contraction: Fusuion will not begin in a contracting cloud if some sort of force stops contraction before the core temp rises above 10k Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation Degenerancy Pressure: laws of quantum mechanics prohibit two electrons from occupying the same state in the same place Thermal pressure: Depends on heat contest, main form of pressure in most stars Degenerancy pressure: [articles can’t be in same state in same place, doesn’t depend on heat content Degnerency pressure halts the contraction of objects with <0.08M(sun) efore core temp become hot enough for fusion Starlike objects not massive enough to start fusion are brown dwarfes A brown dwarf emits infrared light because of heat left over from contraction Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous Radiation pressure: Photons exert a slight amount of pressure when they strike matter Very massive stars are so luminous that the collective pressure of photos drives theyr matter into space Model of stars suggest that radiation pressure limits how massive a star can be w/o blowing itself apart Observations have not found stars more massive than about 150M(sun) Stars more than 150M(sun) would blow up, less than .08M(sun) can’t sustain fusion Observations of star clusters sho that star formation makes many more low-mass stars than high-mass stars Exam: 1. 2. 3. 4. 5. 6. 7. 8. B C A B D A D D 9. D 10. D 11. A 12. B 13. E 14. E 15. E 16. D 17. D 18. E 19. D 20. B 21. E 22. A 23. C 24. D 25. E (luminosity) 26. ? 27. B 28. B 29. C 30. A 31. C 32. C 33. E 34. A 35. D 36. B 37. C 38. D 39. D 40. C 3-28-06 Star Stuff The mass of a main sequenc star determines its core pressure and temp Stars of higher mass have higher core temp and more rapid fusions, making those stars both more luminous and shorter lived Stars of lower mass have cooler cores and slower rates of fusion, giving them smaller luminosities and longer lifetimes Our knowledge of life stories of stars comes from comparing mathematical models of stars with observations Reminder: star clusters are particularly useful because they contain stars of different mass that were born at the same time What are the life stages of a low-mass star? Star remains on the main sequence When Star can’t fuse H in core what happens? Core shrinks and heats up Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over Broken Thermostat: As core contracts, H begins fusing to HE in a shell around core Luminosity increases because the core thermostat is broken, the increasing fusion rare in the shell does not stop the core from contracting Helium fusion does not begin right away because it requires higher temps than hydrogen fusion, larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon When helium fuses in a low temp star:, helium fusion increases very rapidly Helium Flash: Theromsat is broken in low-mass red giant because degenerency pressure supports core Core temp rises rapidly when helium fusion begins Helium fusion rate skyrockets until thermal pressure takes over and expands core Helium burning stars neigher shrink nor grow because core therm is temp fixed Life track after helium flash: Models show that a red giasnt should shrink and become less luminous after helium fusion begins in core Observations of star clusters agree Helium burning stars are found in the horizontal brance of the HR What happens when core runs out of He? He fuses in shell around core Double shell burning: After core He dusion stops, He fuses into carbon in a shell around carbon core and H fuses to He in a shell around He layer This double shell burning stage never reaches equilibrium fusion rate periodically spikes upward in a series of thermal pulses With each spike, convection dredges csrbon up from core to surface of star Double shell burning ends with a pulse that ejects H and He into space as a planetary nebula Core left behind becomes a white dwarf End of fusion: Fusion progresses no further in a low mass star because the core temp never grows hot enough for fusion of heavier elements (some Hefuses to C to make oxygen) Degenereacy pressre supports white dwarf against gravity Sun’s luminosity will rise to 1,000 times it’s current level, too hot for life on earth Have to go to titan, a moon of Saturn, for life to continue Sun’s radus will grow to neat current radius of earth’s orbit Life stages: H in core H fusion in shell around contracting core (red g) He fuses in core (horizontal branch) Double shell burning (red giant) Low mass star dies: Ejection of H and He in planetary nebular leaves behind an inert white dwarf High mass stars: CNO cycle High mass main seq stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen and catalysts Greater core temp enables H nuclei to overcome greater repulsion Ife stages of High-mass stars Late life stages of high mass stars are similar to low mass H core fusion H shell burning He core fusion (super giant) Elements with even numbers protons are everywhere in universe, because of helium capture Big bang made of 75% H, 25% He-stars make everything else Heliium fusion can make C in low-mass stars CNO cycle can change C into N and O Core temp in stars with >8m(sun) allow fusion of elements as heavy as iron Advanced reactions in stars make elements like Si, S, Ca, Fe Advanced nuclear burning proceeds in a series of nested shells Ion is a dead end for fusion because nuclear reactions involving iron do not release energy (Fe has lowest mass per nuclear particle) evidence for helium capture: higher abundances of elements with even numbers of protons Supernova Explosion: Core degenerancy pressure goes away because electrons combine with protons, making neutrons and eutrinos Neutrons collapse to the center forming a neutron star Energy and neutrons released in supernova explosion enable elements heavier than Fe to form, including Au and U 4-4-06 Iron builds up in core until degenerancy pressure can no longer resist gravity Core then suddenly collapses, creating supernova explosion Supernova: Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos Neutrons collapse to center, forming a neutron star Energy and neutrons released in supernova explosion enable elements heavier than iron to for, inclusng Au and U Energy released by collapse of core drives outer layers into space Crab Nebula is the remnant of the supernova seen in 1054 AD Closest supernova in last 4 centuries was seen in 1987 We got hit with neutrinos when this supernova 150,000 light years away Rings around Supernova 1987A Supernova’s flash of light caused rings of gas around the supernova to glow Impact of Debris with Rings More recent observations are showing the inner ring light up as debris crashes into it Algol consists of a 3.7m(sun) main star and a .8M(sun) stars Smaller star is the sun Stars are close enough that matter can flow from sub giant onto main-sequence Bizarre Stellar Graveyard What is a White Dwarf: White dwarfs are the remaining cores of dead stars Electron degeneracy pressure supports them against gravity White dwarfs cool off and grow dimmer with time White dwarfs with same mass as sun are about the same size as earth Higher mass white dwarfs are smaller It’s pretty fucking dense White Dwarf Limit Quantum mechanics say that electrons must move faster as they are squeezed into a very small space As a white dwarf’s mass approaches 1.4M(sun) its electrons must move at nearly the speed of light Because nothing can move faster than the speed of light, a white dwarf cannot be more massive than 1.4 M(sun) the whie dwarf limit, Chandrasekhar limit What happens to a white dwarf in a close binary system Accretion Disks: Mass falling towards a white dwarf from its close binary companion has some angular momentum The matter therefore orbits the white dwarfin an accretion disk Friction between rings of matter transfers angular momentum outward and causes disk to heat up and glow If no friction, gas would orbit indefinetly Nova: Temp of accreted matter eventually becomes hot enough for hydrogen fusion Fusion begins suddenly and explosively causing a nova The nova star temporarily appears much brighter Explosion drives accreted matter out into space When white dwarf accretes enough matter to rreach 1.4M(sun)—it explodes Two types of supernova: Massive star supernova: White dwarf supernova Light curve shows how luminocity changes over time Nova or Supernova: Supernovae are MUCH MUCH more luminous (10 million times) Nova: H to He fusion of a later of accreted matter, white dwarf left intact Supernova: complete explosion of white dwarf, nothing left Difference in supernovae: Light curve, spectra 4-6-06 A neutron star is the same size as a small city But 300 times more mass than earth Discovery of Neutron Stars Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission comng from a single part of the sky The pulses were coming from a spinning neutron star-a pulsar Pulsar at center of Crab Nebula pulses 30 times per second Pulsars A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis The radiation beams sweep through space like lighthouse beams as the neutron star rotates Why Pulsars must be Neutron Stars Circumference of NS=2(pi)(radius) ~60km Spin Rate of Fast Pulsars ~ 1000 cycles per second Surface Rotation Velocity ~ 60,000 km/s ~20% speed of light ~escape velocity from NS anything else would be blown to pieces pulsars spin fast becase core’s spin speeds up as it collapses into neutron star Pulsars could appear to other civilizations as not pulsars, they’re not in plane Matter falling toward neutron star forms an accretion disk, just as in a white dwarf binary Accreting matter adds angular momentum to a neutron star, increasing its spin Episodes of fusion on the surface lead to x-ray bursts Matter accreting onto a neutron star can eventually become hot enough for helium fusion Sudden onset of fusion produces a burst of x-rays Black Holes: Black holes don’t suck A black hole is an object whose gravity is so powerful that not even light can escape it When you shrink an object, escape velocity increases Surface of a black hole. The surface of a black hole is the radius at which the escape velocity equals the speed of light This sphereical surface is known as the event horizon The radius of the event horizon is known as the Schwarzschild Radius The even horizon of a 3M(sun) black hole is also about as big as a small city Event hor. is larger for black holes of larger mass Black hole’s mass strongly warps space and time in the vicinity of the event horizon 4-11-06 a black hole’s mass strongly warps space and time in the vicinity of the event horizon. No Escape: Nothing can escape from within the event horizon because nothing can go faster than the speed of light No escape means there is no more contact with something that falls in. the in-falling object increases the hole mass, and changes the spin or electric charge of the hole but otherwise loses its identity Neutron Star limit: Quantum mechanics say that neutrons in the same place cannot be in the same state Neutron degeracy pressure can no longer support a neutron star against gravity if its mass exceeds 3M(sun) This means massive stars that become supernovae can make a black hole if enough mass is in, or falls onto, core Singularity Beyond the neutron star limit, no known force can resist the crush of gravity As far as we know, gravity crushes all the matter into a single point known as singularity What would it be like to visit a black hole If the sun shrank and became a black hole, its gravity would be different only near the event horizon Black Holes don’t suck! Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshirt Time passes more slowly near the event horizon Blacks holes are very small Black hole with same mass as a sun wouldn’t be much bigger than a college campus Tidal forces near the event horizon of a 3M(sun) black hole would be lethal to human Tidal forces would be a gentler near a super massive black hole because it’s radius is much bigger Do black holes really exist Verification: Need to measure mass Use orbital properties of companion Measure velocity and distance of orbiting gas It’s a black hole if it’s not a stay and its mass exceeds the neutron start limit (>3M(star)) Some x-ray binaries contain compact objects of mass exceeding 3M(sun) which are likely to be black holes Our Galaxy: Dusty gas clouds obscure out view because they absorb visible light This is interstellar medium that makes new star systems We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters Stars in disk tend to orbit in same direction 230 million year orbit center of galaxy has roughly 1x10tothe11th M(sun) Galactic recycling: how is gas recycled in our galaxy Star-gas-star cycle Recycles gas from old stars into new star systems High-mass stars have strong stellar winds that blue bubbles of hot gas Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae Multiple supernovae create huge hot bubbles that can blow out of disk Gas clouds cooling in the halo can rain back down on disk Atomic hydrogen gas forms hot gas cools, allowing electrons to join with protons Molecular clouds form next, after gas cools enough to allow new elements Gas cools off to eventually make new stars and planets 4-13-06 physics.purdue.edu/astro/ast263/quiz_rev.html Where do stars tend to form in our galaxy? Ionization nebulae are found around short lived high mass stars, signifying active star formation Halo: no ionization nebulae, no blue starts ->no star formation Disk: ionization nebulae, blue stars -> star formation Much o star formation in disk happens in spiral arms Spiral arms are waves of star formation Whirlpool Spiral arms are waves of star formation: Gas clouds get squeezed as they move into spiral arms Squeezing of clouds triggers star formation Young stars flow out of spiral arms Halo stars formed first, then stopped Disk stars formed later, kept forming Halo stars firmed first as gravity caused cloud to contract Remaining gas settled into spinning disk Stars continue to form in disk as galaxy grows older 4-18-06 What lies in the center of our galaxy? Stars appear to be orbiting something massive but invisible…a black hole? Orbits of stars indicate a mass of about 4 million M (sun) x-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chucks of matter about to fall in Hubble Deep Field: Our deepest images of the universe show a great variety of galaxies, some of them billions of light years away A galaxy’s age, it’s distance, and the age of the universe are all closely related The study of galaxies is this intimately connected with cosmology, the study of the structure and evolution of the universe Hubble Ultra Deep Field (2002) Spiral galaxy Elliptical galaxy Irregular galaxy Disk component: stars of all ages, many gas clouds Spherical component: bulge, halo, old stars, few gas clouds Why does ongoing star formation lead to blue-white appearance? Short-lived blue stars outshine the others Barred-spiral galaxy: has a bar of stars across the bulge Milky Way: barred spiral Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) Elliptical galaxy: all spheroidal component Hubble Galaxy classes: spheroid dominates, disk dominates Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies) Elliptical galaxies are much more common in huge clusters of galaxies Elipitcal galaxies don’t have a disc Brightness alone doesn’t dictate distance Step 1: determine size of solar system via radar Step 2: determine distances of stars to a few-hundred light years out Step 3: apparent brightness of star’s cluster’s main sequence tells us its distance Step 4: because of the period of a Cepheid variable star tells us its lumin, we can use these stars are standard candles Luminosity passing through each sphere is the same, divide luminosity by area to get brightness Brightness=lumin/(4pi (distances)sqrd) Distance=lumin/sqrt(4pi x bright) A standard candle is an object whose luminosity we can determine without measuring distances Knowing a star’s cluster distance we can determine the lumin of each type of star within it What kind of stars are ebst for measuring long distance? High lum stars Cepheid variable stars are very luminous Cepheid bariable stars with longer periods have greater lumin White dwarf supernovae can be also used as standard candles 4-20-06 How do we measure the distances to galaxies? Step 5: Apparent brightness of white-dwarf supernova tells us the distance to its galaxy (up to 10 billion light years) Tully-Fisher relation: Entire galaxies can also be used as standard candles because galaxy lumin is related to rotation speed We measure galaxy distances using a chain of interdependent technique Hubble’s Law Hubble calculated the distance to Andromeda using big telescope The further something is away from us, the faster it’s moving away from us (used dopplar shift) By measuring distances to galaxies, hubble found that redshift and distance are related in a special way Hubble’s law: velocity=H(knot) x distance, h(knot) is the slope of the line or Hubble’s constant Distance=velocity/h(knot) Redshift of a galaxy tells us its distance through Hubble’s Law Cosmological princple: Universe looks the same no matter where you are within it Matter is evenly distributed on very large scales No center no edges Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies Age=distance/velocity 1/H(knot) distances between farawy galaxies chance while light travels astronomers think in terms of lookback time rather than distance expansion Dark matter, Dark energy, and the fate of the universe Dark matter: an undetected form of mass that emits little or no light, but whose existence we infer from it’s gravitational influence Dark energy: an unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate Contents of the universe: 4.4%, .6 inside stars, 3.8 outside stars Dark matter, 25%, dark energy 71% 4-25-06 The final is comprehensive, it’s worth 200 points ½ questions is material covered since midterm 2, ch 17-19, 20, 22, 23 ½ the questions will be on the same material that was the first 2 midterms, 1, 3, 4, 5, 6, 14, 15, 16 2nd half of lecture on Thursday will be a review stuff in center of galaxy has less orbital velocity than stuff in the middle most of Milky Way’s mass seems to be dark matter mass within sun’s orbit: 1x10to11th M (sun) total mass approx: 10th12th M (sun) broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting these galaxies also have dark matter we measure velocities in a cluster from their Doppler shifts cluster are about 50 times the mass clusters contain large amounts of x-ray emitting hot gas, temp of hot gas (particle motions) tells us cluster mass 85% dark matter, 13% hot gas, 2% stars gravitational lensing, the bending of light rays by gravity, can slo tell us a cluster’s mass Massive Compact Halo Objects Massive compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a small chunk of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any solar system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. In astrophysics, WIMPs, or weakly interacting massive particles, are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism they cannot be seen directly, and because they do not interact with the strong nuclear force they do not react strongly with atomic nuclei. Early universe must’ve been extremely hot and dense Photons converted nto particle-antiparticle pairs and vice-versa Early universe was full of particles and radiation because of its high temp What’s the history of the universe according to big bang? 4-27-06 dark matter is still pulling things together after correcting for hubble’s law, we can see that galaxies are flowing toward the densest regions of space Maps of galaxy positions reveal extremely large structures: superclusters and voids Does the universe have enough kinetic energy to escape its own gravitational pull? Fate of universe depends on the amount of dark matter Lots of dark matter: recollapsing Critical density of the universe: just stops Not enough dark matter: expansion continues forever Amount of dark matter is approx 25% of the critical density suggesting fate is eternal expansion (coastal universe) Expansion appears to be speeding up (Dark energy?) Estimated age depends on both dark matter and dark energy Brightness of distant-white dwarf supernovaes tell us how much universe has expanded since they exploded Sinking to center of the earth-electromagnetism 4 forces known in the universe: strong force, electromagnetism, weak force, gravity cosmic microwave background-radiation left over from the big bang was deteced by Penzias & Wilson in 1965 1. C 2. E 3. A 4. C 5. C 6. D 7. C 8. D 9. D 10. A 11. C (everest) 12. B 13. C 14. ? 15. B 16. A 17. D 18. C 19. D 20. B 21. A 22. C 23. D 24. B 25. C 26. C 27. A 28. B 29. A 30. A (velocity and distance) 31. C 32. E 33. C 34. E