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ASTR102

Kira Bollen Astronomy Inglés 116 fichas

ASTR102

Fichas del conjunto (116)

  1. 1What is life?- definition is controversial
    - NASA: A self-sustaining chemical system capable of Darwinian evolution
    - organisms maintain homeostasis, are composed of cells, undergo metabolism, can grow, adapt to their environment, respond to stimuli, and reproduce
  2. 2How old is the earth4.5 billion years old
  3. 3How old is life on earth?earliest life forms probably about 3.85 billion years ago, shortly after the end of heavy bombardment
    -evidence from fossils, carbon isotopes
  4. 4Panspermia theorylife could have migrated to earth through exchange of rock from other planets
  5. 5Necessities for life- nutrient source
    - energy (sunlight, chemical reactions, internal heat)
    - liquid water (or possibly some other liquid)
  6. 6Extremophiles- very high temps (121 oC)
    - very low temps (-20 oC)
    - acidic environments
    - high salinity
    - UV radiation
    - very high pressure
    - most are bacteria
  7. 7Jupiter's moons of interest for life- Europa (sub surface ocean)
    - Ganymede
    - Callisto
  8. 8Saturn's moons of interest for life- Enceladus (subsurface ocean)
    - Titan (lakes of liquid ethane/methane, dunes of hydrocarbon), has an atmosphere
  9. 9Habitable zone- region around a star where liquid water can be sustained on a planet's surface
    - boundaries still debated
    - inner edge ~0.95 AU
    - outer edge ~ 2 AU
  10. 10Habitable planetEarth sized planets in the habitable zone
  11. 11Hot JupitersJupiter mass planets in close orbits
  12. 12Star system constraints for habitable planets1. Old enough to allow time for evolution (rules out high mass stars - 1%)
    2. Stable orbits (might rule out binary/multiple star systems - 50%)
    3. Stable host star - no flare stars (ex. Proxima Centauri)
    - probably still billions of potentially habitable planets
  13. 13Places to search for life- surfaces of Earth like planets
    - moons with habitable surfaces
    - Super Earths and water worlds in habitable zones
    - sub-surfaces
  14. 14The Drake Equation- number of civilizations with whom we could potentially communicate
    = N(HP) x f(life) x f(civ) x f(now)
    - we do not know the values for this equation
  15. 15N(HP)total number of habitable planets in galaxy
  16. 16f(life)fraction of habitable planets with life
  17. 17f(civ)fraction of life-bearing planets with civilization at some point
  18. 18f(now)fractions of civilizations around now
  19. 19Additional parameters of the Drake Equationf(moon): stabilizing axis tilt
    f(Jupiter): planetary dynamics
  20. 20SETISearch for Extraterrestrial Intelligence
    - looks for deliberate radio signals from extraterrestrials
  21. 21CETICommunication with Extraterrestrial Intelligence
  22. 22Difficulties of interstellar travel- more efficient engines are needed
    - energy requirements are enormous
    - ordinary interstellar particles become like cosmic rays
    - social complications of time dilation
  23. 23Fermi's Paradoxplausible arguments suggest that civilizations should be common
    - why haven't we detected them?
  24. 24Possible solutions to Fermi's Paradox1. We are alone (life/civ is rarer than we might have guessed)
    2. Civilizations are common but interstellar travel is not (more difficult than we think, no desire for it, civilizations destroyed themselves before accomplishing)
    3. There IS a galactic civilization we have yet to meet
  25. 25Lighta wave of electric and magnetic fields, moving at the speed of light
  26. 26Star Population I- bluer (composed of old and young stars)
    - many in open clusters
    - higher metallicity (elements heavier than helium = <2%)
    - found in the disk, corotating
  27. 27Star Population II- redder (composed of old stars)
    - low metallicity (as low as 0.02%)
    - found in the halo
    - many in globular clusters (some lone stars with fast orbital velocities)
    - halo stars
  28. 28Walter Baade- German born astronomer in the US
    - observations at Mt. Wilson observatory
    - observed the Andromeda galaxy and classified stars into two populations (also applies to Milky Way)
  29. 29Ionization nebulaeindicate active star formation as gas is excited and ionized by xrays from newly formed OB stars
  30. 30Parts of the Milky Way- halo
    - disk
    - spiral arms
    - bulge
    - globular clusters
  31. 31Our speed relative to the galactic centre220 km/s
  32. 32Our orbital period (one galactic year)227 million years
  33. 33Age of the sun in galactic years20
  34. 34total mass within the orbit100 billion solar masses in the disk enclosed by our orbit
  35. 35Disk- flat pancake shape
    - ~30 kpc in diameter
    - 300 pc in height
    - mostly young stars, gas, dust, mainly in spiral arms
    - some old stars
  36. 36Bulge- spherical shape
    - outer parts hard to distinguish from halo
    - ~3 kpc in diameter
    - mainly old stars, gas, dust
  37. 37Halo- very large, spherical distribution
    - about 40 kpc in diameter
    - mainly old stars and globular clusters
  38. 38Disk star orbits (ex. Sun)orbit in circles with the same orientation, except for a little up and down motion
  39. 39Halo star orbitstravel high above and far below the disk on orbits with random orientation
  40. 40Bulge star orbitsrandom orientation
  41. 41Three theories of MW rotation1. Solid body: the galaxy rotates like a merry-go-round (outer stars go faster)
    2. Keplerian: all stars rotate under control of a central 'lump', like the planets orbiting the sun (inner stars go faster)
    3. Something in between
  42. 42Actual MW rotationSomething in between: the stars move under the combined force of all the other stars and gas -- the stars at the centre move a little faster than the sun, but not nearly as much as Kepler's laws would imply
    - There is differential rotation (stars closer to the centre orbiting faster, stars further out orbiting slower)
  43. 43star-gas-star cycleatomic hydrogen clouds -> molecular clouds -> star formation -> nuclear fusion in stars -> returning gas -> hot bubbles -> atomic hydrogen clouds
  44. 44'returning' gas to the galaxy- stellar winds
    - core collapse and white dwarf supernovae via gas (slow) and cosmic rays (fast)
  45. 45Galactic fountaingas expands in the direction of least pressure (away from the disk)Galactic fountain
  46. 46second+ generation star formationnew star generations form with more heavy elements than the previous generation
  47. 47Galactic recycling- star makes new elements by fusion
    - dying stars expel gas and new elements, producing hot bubbles
    - hot gas cool, allowing atomic hydrogen clouds to form
    - further cooling permits molecules to form, making molecular clouds
    - gravity forms new stars (and planets) in molecular clouds
  48. 48Can galactic recycling go on forever?No
    - eventually all of the galactic material will end up locked away in white dwarfs, neutron stars, and black holes
    - not for more than 1 trillion years from now though
  49. 49How can we tell when there are halo stars in the disk?- high proper motion that is not in the direction of the disks rotation
    - much lower metallicity than other nearby stars
  50. 50Galaxy formationinitial spherical structure (halo) left behind old stars before the disk was formed from gas friction and cooling
  51. 51Nebula(e)latin for 'cloud'
  52. 52The nebular hypothesis- early 20th century
    - Shapley
    - there is only one Milky Way: this huge stellar system is the entire universe
    - the spiral nebulae are likely solar systems in formation, they are small satellites of our own galaxy
  53. 53reasons for the nebular hypothesis1. If the nebulaes were outside the galaxy they should be distributed randomly around the sky (but there is a 'zone of avoidance' in which there are none)
    2. Apparent rotation of the spirals (measurements ended up being faulty)
    3. 'Novae' seen in the spirals - brightness seemed to imply they were nearby (but they were actually more distant supernovae instead)
  54. 54Island universes hypothesis (multiple galaxies)- Curtis
    Evidence:
    - more novae in andromeda than in entire MW
    - doppler shifts imply galaxies are receding
    - dark lanes in spirals similar to dust clouds in MW
  55. 55Uses of 21 cm hydrogen line radio signal1. look for regions of neutral hydrogen in the galaxy
    2. use its doppler shift to infer velocity of these regions
  56. 56Two kinds of spirals1. Grand design (MW)
    2. Flocculent
  57. 57Density waves (as causation of spiral arms)- Lin and Shu
    - the wavefront (the dense part) moves through space at a different speed from the individual particles that compose it
    - each particle follows its own elliptical orbit and a density wave arises where these pile up
    - when the gas is compressed, it can collapse into stars
    - these stars are left behind as the wave progresses beyond them to form new stars out of the gas ahead
  58. 58formation of the MW (spiral galaxies)- halo formed first and remaining gas collapsed to disk structure
    - protogalactic cloud contains only hydrogen and helium
    - halo stars begin to form as protogalactic cloud collapses
    - conservation of angular momentum makes remaining gas flatten into spinning disk
    - billions of years later, galactic recycling forms stars in the disk (lack of cold gas in the halo precludes star formation outside the disk)
    -
  59. 59Main types of galaxies1. Spiral (arms come right out of centre)
    1.2. Barred Spiral (central rotating bar creates trailing spiral arms)
    2. Elliptical
    3. Irregular
    4. Lenticular (in-between galaxies - not quite spiral, not quite elliptical)
  60. 60Spiral galaxies- ~75% of galaxies
    - old stars in a halo (Pop. I)
    - young stars (Pop. I and II)
    - +gas
    - +dust in a flat disk structure
  61. 61Elliptical galaxies- ~20% of galaxies
    - more football shaped
    - yellow/red in colour
    - mainly old stars (Pop. II)
    - very little gas
  62. 62Irregular galaxies- ~5% of galaxies
    - many young stars (mostly Pop. I)
    - a lot of gas
    - mostly defy ordering into sub-classes
  63. 63Dwarf galaxies- usually elliptical (few irregular, no spiral)
    - "dwarf spheroidals"
    - satellite galaxies (often found in orbit of other galaxies)
    - Missing Dwarf problem: prediction of more than are observed - but better observations = no missing dwarves
  64. 64Galaxy collision near misses can result in:- tides: galaxies pull each other apart
    - density waves
    - starbursts
    - mergers
    - large galaxy "eats" smaller companions
  65. 65Galaxies are often found in:Groups: 2-30 (usually spirals)
    Clusters: 30-300 (many ellipticals)
    Superclusters: clusters of clusters
    - and clusters can collide
  66. 66Virgo supercluster- ~100 groups of galaxies within a 30 mpc diameter
    - we are near an edge of the cluster
    - more galaxies near the centre (they are bound to each other gravitationally)
  67. 67Laniakea- ~100,000 galaxies
    - yellow in photo
    - means "immense heaven" in Hawaiian
    - defined in 2014
    - drifting apart (may not be bound)
  68. 68Copernican principle- the "mediocrity principle"
    - the Earth is not in a privileged location
  69. 69Cosmological Principleon large scales (clusters on clusters) the universe is homogenous and isotropic
    - it's about the same density everywhere
    - it looks about the same no matter which direction you look
    - there are NO privileged locations (everyone is as mediocre as we are)
  70. 70Olber's paradoxthe cosmological principle seems to suggest that there is no 'edge' to the universe , so why is the night sky dark?
    Answer: because there is a distance beyond which light has not yet had time to reach us (outside the cosmic horizon; 13.6 Gyr)
  71. 71quasarsquasi-stellar radio sources
    - appear to be point-like because they are outshining their host galaxies
  72. 72radio galaxiesgalaxies exhibiting big radio features extending outside the visible component
    - appear to be from high energy electrons being spat out at the speed of light
  73. 73Blazars- objects that have extremely bright and variable optical emissions and are radio loud
    - fast variability means must be very compact
    - luminosity of a galaxy from something the size of the solar system
  74. 74What powers the AGN?- L= 10^14 L(Sun) implies very powerful energy source
    - steady energy source: young and old galaxies exhibit AGNs
  75. 75formation of elliptical galaxies- must have formed early since they seem to uniformly contain stars that are about the same age
    - monolithic collapse: all the gas in a galaxy sized cloud instantly collapses into stars
  76. 76galaxy merger conditions- need high density of galaxies
    - low relative velocities (if high, galaxies cant grab onto each other)
    - mergers happen more in small groups
  77. 77elliptical galaxy formation (2)1. low-mass ellipticals (not dwarfs) are formed by major mergers of gas-rich (disk + irregular) galaxies at high z/early time
    2. these occur most often in regions with many galaxies
    3. merger causes gas to collapse into stars and can activate the central black hole, causing gas to be expelled
    4. larger galaxy can then merge with another companion - this time there is no gas
  78. 78evidence for mergerssupermassive black hole binaries (bc black holes are common at galaxy centres; two galaxies centres merged)
  79. 79missing galaxies- theory predicts more massive and light galaxies
    where are they?
    - supernovae feedback: periods of star formation yield supernovae which heat gas and suppress star formation and can deplete gas of small galaxies by ejecting it
    - AGN feedback: AGN activity can lead to gas outflows and star formation suppression in large galaxies
  80. 80SMBH formation hypothesis: Remnants of Pop. III starsthe first stars were likely more massive due to their inefficiency at cooling -- a 250 Msun star would very quickly form a ~100 Msun SMBH
  81. 81SMBH formation hypothesis: Gas dynamicslarge angular momentum could hold up a large amount of gas against gravity, leading to a rapidly rotating supermassive 'star'. when the hydrogen in its core runs out, it could collapse into a black hole
  82. 82SMBH formation hypothesis: stellar dynamicsdense clusters of stars could have formed in the centres of galaxies. star-star collisions would lead to rapid formation of superstars which then explode as supernovae
  83. 83Pillars of the big bang- the expanding universe
    - primordial nucleosynthesis
    - the cosmic background radiation
  84. 84Hoyle's Steady State Universe- space isn't expanding: galaxies are flying apart from one another
    - as galaxies become more separated, new galaxies form in the space between
    - this means the universe can be infinite and eternal: no need for a special 'place' in time
  85. 85the hot big bang model- the universe was once in a hot dense state and cooled as it expanded
    - the cooling and expanding changed the state of matter, leading to eras of:
    1. dissociated quarks, electrons, and light
    2. dissociated protons, neutrons (and electrons) and light
    3. nuclei and electrons
    4. neutral atoms
    - once gravity took over (because gas had time to collapse into larger objects leading to larger gravitational attraction), stars, galaxies, and clusters began to form
    - the Big Bag framework doesnt tell us how the universe wound up in that hot dense state to begin with
  86. 86primordial nucleosynthesis ("where did the helium come from?")- dissociated nuclei hot enough for nuclear fusion (fuses hydrogen into helium)
    - there is a brief window in time when nuclear fusion was possible
    ongoing challenge: where did the primordial lithium go?
  87. 87cosmic background radiationera of recombination (protons and electrons recombined into neutral hydrogen and helium)
  88. 88the afterglow of the big bang1. the light comes from everywhere and is travelling in every direction
    2. the light an observer sees at a given time comes from a specific distance
    3. the spectrum is red shifted as the universe expands
  89. 89transplanckian problemat some point, the density becomes large enough to form a black hole, but things are so small that quantum mechanics should be important
    - how does that change things?
  90. 90questions/problems with expansion1. where did the density enhancements that led to galaxies come from?
    2. why is the large-scale universe nearly uniform?
    3. why is the geometry of the universe flat?
  91. 91properties dark matter must have- cold: kinetic energy (temperature) low enough that it doesnt fly out of galaxies (why neutrinos dont work)
    - dark (transparent): does not interact with light
    - matter: behaves and clusters like matter, not like light
  92. 92% of dark matter in the universe85%
  93. 93MACHOSMassive Compact Halo Objects
    - brown dwarfs
    - rocks
    - planets
  94. 94WIMPweakly interacting massive particle
    - between 1 and 1000 proton masses
    - interacts via a new force similar to the weak nuclear force
    - produced in the very early universe (before nucleosynthesis)
    - not orbiting with us around the galactic centre
    - their orbits around the MW are random
  95. 95direct detection of dark matter- go deep underground to avoid cosmic ray contamination
    - clean lab (no radioactivity)
    - heavy nuclei (easier to measure recoils)
    - larger target better, but you pay in sensitivity
  96. 96indirect detection of dark matterlook for gamma rays, neutrinos, cosmic rays from galaxy WIMPS annihilating into lighter particles
  97. 97some direct detection experiments in Ontario- SuperCDMS (super cooled germanium detector)
    - DEAP3600 (liquid argon super cooled germanium detector)
    - PICO (you can hear the dark matter)
  98. 98annual modulationas the earth orbits the sun, our relative velocity with respect to the dark matter changes
    - we therefore expect more collisions in the summer and fewer in the winter
  99. 99other dark matter candidates- asymmetric dark matter: like protons and electrons, the early universe produces more dark matter than anti-dark matter
    - sterile neutrino: a massive counterpart to the ordinary neutrino
    - axion: an extremely light boson that could form quantum waves the size of a galaxy
    - primordial black holes: formed in the very early universe, would not have interfered with the thermal big bang
  100. 100possible scenarios for the fate of the universe1. recollapsing universe (too much matter, expansion reverses, collapses)
    2. critical universe (universe asymptotes to zero expansion but never quite stops)
    3. coasting universe (faster expansion)
    4. accelerating universe (a type of matter with negative pressure that causes the expansion to accelerate with time)
  101. 101best standard candles for obtaining distances to distant galaxiesType 1a supernovae (white dwarf supernovae)
  102. 102possible causes of accelerated expansion- cosmological constant: something fundamental about the geometry of the universe
    - dark energy: some exotic type of matter with negative pressure that fills space
  103. 103ΛCDM: the concordance modelΛ = cosmological constant/dark energy
    CDM: cold dark matter
  104. 104the end times- in 5 billion years the sun will explode, leaving behind a planetary nebula and a white dwarf (no earth)
    - Andromeda will crash into MW (4-8 Gyr)
    -the local group will merge together (~100 billion years)
    - as matter becomes too dilute to hold back the expansion of the universe, it will eventually start growing exponentially
    - galaxies will recede from each other at a very fast pace
    - in a few hundred billion years low mass stars will start to die and galaxies will glow fainter
    - in 100 trillion years star formation will stop completely
    - in ~10^19 years, all stellar remnants coalesce into black holes
    - in 10^100 years black holes evaporate and and resulting radiation is diluted out into the expanding universe
  105. 105gravitational waves- alternately stretch and compress space in the plane perpendicular to the direction of travel
    - most are faint (undetectable)
    -
  106. 106what produces gravitational waves?the motion of mass (so pretty much everything)
    - need very fast motion of very heavy objects to have a chance to see them (pulsars, black holes)
    - different frequencies depending on what produces them
  107. 107kilonovaneutron star collision (merging neutron stars)
    - only way to produce gold, lead, platinum, uranium
  108. 108detecting gravitational wavesLIGO (laser interferometry gravitational observatory)
    1. GW approaches earth at speed of light
    2. GW detected at one LIGO location
    3. GW detected at second LIGO location
    - if signals are the same and separated by t=d/c then it was a GW
  109. 109cosmic rays- mostly protons travelling near the speed of light
    - electrons
    - muons
    - positrons
    - pions (quark- anti-quark particle)
    - kaon
    - lambda
    - some heavier nuclei as well
    - more cosmic rays come from west than east
    - positive particles get deflected left (West) in the North-South magnetic field
  110. 110origins of cosmic raysZwicky: ejecta of supernovae
    - CR deflected by small magnetic fields in the interstellar medium, and bounce around the galaxy (towards us sometimes)
    - we cant tell which direction a CR initially came from
    - the sun's magnetic field deflects and attenuates the cosmic ray flux
  111. 111observing cosmic raysto observe highest energy cosmic rays we need to:
    1. cover large area OR
    2. go to space
  112. 112moon's muon shadowmoon creates a CR shadow where cosmic rays dont hitmoon's muon shadow
  113. 113neutrino productionproduced in every reaction involving the weak nuclear force: radioactive decay, nuclear reactors, particle accelerators, cosmic rays hitting the atmosphere, fusion in the sun, fusion in stars, supernovae, kilonovae, jets from black holes, cosmic rays hitting interstellar medium
  114. 114neutrinos- not charged
    - extremely weak interacting
    - allow us to see inside astronomical objects
    - 3 flavours: tau, muon, electron
    - neutrinos arent blocked
  115. 115"seeing" neutrinos- count decay products
    - look for scintillation light
    - look for Cerenkov lgiht
  116. 116IceCube Neutrino Observatory- located in South Pole
    - 1450 m beneath surface
    neutrino detecting:
    1. neutrino arrives
    2. hits ice/water
    3.DOMs see Cerenkov light from electrons, muons
    - best at looking down (neutrinos come through the earth but CR are blocked)

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Astronomy Fichas sobre ASTR102 , creado por Kira Bollen el 18/04/2018.
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