AST 100

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AST 100
2012-05-05 09:16:56
milky way galaxies cosmic distances age structure dark matter fate universe

Unit 4 study guide
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  1. Basic Stats (type, size, mass, # of stars)
    • Type: a spiral galaxy
    • Size: 100,000 lyrs in diameter
    • Mass: 150 X 109 solar masses
    • # of stars: 100-400 billion stars
  2. Structure (3 major components)
    • Bulge: diameter of 16,000 lyrs; population II; center is a black hole with a black hole causing close stars to have an orbit of 50-100 years
    • Disk: 100,000 lyrs in diameter and 1,000 lyrs thick; population I; contains bright stars that form in the spiral arms; stars form in open clusters
    • Halo: diameter is about 200,000 lyrs in diameter; population II; form in global clusters; 90% lie within 100,000 lyrs
  3. Population I and II and there types of orbit
    • Population I: disk population; orbit galactic center in about the same plane and direction
    • Population II: spheroidal population; orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy (go all over)
  4. Rotation Rates and dark matter
    rotation speed of spiral arms: 125 km/s
  5. Star Formation (rates and where)
    • active star-forming regions, marked by the presence of hot, massive stars and ionization nebulae, are found preferentially in spiral arms
    • spiral arms represent regions where a spiral density wave has caused gas clouds to crash into eachother
    • ->compressing them and making formation more likely
    • active star formation occurs in the disk
  6. Hypothesized formation from giant H and HE clouds
    • A protogalactic cloud contains only hydrogen and helium gas
    • Halo stars begin to from as the protagalactic cloud collapses
    • Conservation of angular momentum ensures that the remaining gas flattens into a spinning disk
    • Billions of years later, the star-gas-star cyle supports ongoing star formation within the disk. The lack of gas in the halo precludes star formation outside the disk.
  7. Three basic types of galaxies
    • Spiral: have prominent disks and spiral arms
    • Elliptical: rounder and redder than spiral galaxies and contain less cool gas and dust
    • Irregular: neither disklike nor rounded in appearance
  8. The Hubble "tuning fork"
    • Edwin Hubble invented a system for classifying galaxies that organizes the galaxy types into a diagram shaped like a tuning fork
    • Elliptical galaxies appear on the "handle" at the left designated by an E and # (the larger the # the flatter the elliptical galaxy)
    • Two forks show spiral galaxies; S for regular spirals; SB for barrel spirals; bulge size decreases from a-c, while the amount of dusty gas increses
    • Irregular galaxies are Irr
  9. Kinds of stars found in the different types of galaxies
    • Spiral galaxies: look white because they contain stars of all different colors and ages
    • Elliptical galaxies: look redder because old, reddish stars produce the most of their light
    • Irregular galaxies: look white because they contain stars of all different colors and ages
  10. Distances - standard candle measuring methods
    know luminosity and use apparent brightness to find distance
  11. Measuring distances - completing the "measuring chain"
    • To measure distances, you can use Hubble's law
    • more distant galaxies are moving away faster supporting the hypothesis of an expanding universe at an accelerating rate
    • standard candle: knowing luminosity and apparent velocity to find distance
    • Cepheid period luminosity relation: can get luminosity with graph; get apparent brightness and find distance
  12. Large scale galactic recession (expansion of the universe)
    • universe is expanding at 22 km/s/a million lyrs
    • eternal expansion
    • expansion of the universe is speeding (dark energy)
  13. Two hyptheses of expansion
    fixed and expanding
  14. Hubble's Law and Hubble's Constant
    • V = HO x d
    • V- velocity
    • HO- Hubble's constant
    • d- distance
    • Found that the universe is expanding by using spectra (1929)
    • allows us to determine a galaxy's distance from the speed at which it is moving away from us, which we can measure from its doppler shift
  15. The observable universe and lookback time
    • distance btwn galaxies always changing
    • Look back time: the time it has taken for the galaxy's light to reach us
    • the expansion of the universe during that time stretches the light coming from the galaxy, leading to a cosmological redshift directly related to the galaxy's look back time
  16. Age of the universe and how this is estimated
    • combining distance measurements with velocity measurements, tells us Hubble's constant, and the inverse of Hubble's constant tells us how long it would have taken the universe to reach it's present size if the expansion rate had never changed
    • age of univers = 14 billion years
  17. Large scale structures
    • galaxies distributed in gigantic chains and sheets surround great voids
    • these large scale structures trace their origin directly back to regions of slightly enhanced density earlier in time
  18. The Big Bang concept
    • The early univers was filled with radiation and elementary particles
    • It was so hot and dense that the energy of radiation could turn into particles of matter and anitmatter, which then collided and turned back into radiation
  19. What is it - ordinary and extraordinary matter
    • Ordinary matter: baryonic matter- electrons, protons, and neutrons (not enough ordinary matter to account for all the dark matter)
    • Extraordinary matter: nonbaryonic matter- undiscovered particles called WIMPs
  20. What is the evidence for dark matter?
    • In galaxies: applying Newton's laws of gravitation and motion to these orbits leads to the conclusion that the total mass of a galaxy is far larger than the mass of its stars; b/c no visible light = dark matter
    • In clusters: from galaxy orbits, from the hot temperature of the hot gas in clusters. and gravitational lensing predicted by Einstein; total mass of galaxy cluster = 50x the mass of its stars, implying the huge amount of dark matter
  21. What are the effects of dark matter?
    • most of galaxy's mass = dark matter
    • gravity of that dark matter is probably what formed protogalactic clouds and then galaxies from slight density enhancements in the early universe
  22. Competing hypotheses of expansion
    • The Big Crunch: a recollapsing universe (critical density)
    • Critical Universe: stops expanding and hangs there
    • Expansion never stops: keeps expandinng at the same rate
    • Accelerating universe: rate is increasing because of dark energy
  23. Dark Energy and its Role
    • Dark energy: whatever may be causing the expansion of the universe to accelerate
    • The unknown force causing the universe to expand at an accelerating rate