Physics: Lecture 20

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  1. Definition of Galaxy

    - how big
    - what does it contain besides stars
    a very large collection of stars held together by gravity

    • > 107 - 1010
    • usually contains gas and other matter as well
  2. What are some important aspects of stars
    • Colors
    • Spectra
    • Distance + Flux = Luminosity
    • Structure (shape, morphology (form))
  3. The Milky Way

    - How many stars
    - why, in pictures of the galaxy, does light appear to be smooth, how transparent?
    - elements of the galaxy
    - what type of galaxy
    • - 100 billion (1011) stars
    • - stars are so far away and so close together that their light overlaps and looks smooth (continuous emission of light from billions of stars) --> opaque
    • - bulge (direct center), disk, spiral arms, halo (actually a "stellar halo"- a very sparse, faint collection of very old metal poor stars, globular clusters
    • - spiral galaxy
  4. Distances in the Galaxy

    - from sun to center - on what arm of galaxy?
    - width
    - length of disk
    • - 28,000 ly, 8 kpc - sagittarius arm
    • - 1,000 ly or 3000 pc
    • - 100,000 ly or 30, 000 pc
  5. The different components of a galaxy have different ....
    • colors (sizes --> blue = massive)
    • motions
    • chemical compositions
    • origins
  6. Spiral Galaxies
    • - flat disk with bulge at center
    • - size of bulge and prominence of spiral patters vary
    • - disk contains inteterstellar medium (ISM) of cool gas and dust
    • - larger bulge, less ISM, and therefore less star formation
    • - dust lanes --> gas --> active star formation
  7. Shape depends on
    orientation (somewhat), formation
  8. Disk Component vs Spiral Component of Spiral Galaxies
    • disk component
    • - flat disk where stars follow orderly almost circular orbits around center.
    • - Always contains an interstellar medium (gas and dust)

    • Spheroidal component
    • - bulge and halo together.
    • - Bulge rotates, but not much, more like stars moving back and forth,
    • - usually little ISM
    • - those with larger bulges (the Sombrero Galaxy) usually contain less interstellar gas and dust, and less current star formation
    • - bulge dominates, but still has a clear disk.
    • - spiral pattern is visible, but tightly wound.
  9. How are massive stars produced
    gas in the disk collapses
  10. Rotation of the whole galaxy takes how long
    - 100 million years,

    (bulge rotates, but not much, more like stars just moving back and forth)
  11. Elliptical Galaxies (or just "ellipitcals")
    • - all bulge
    • - very little gas
    • - mostly low-mass stars
    • - "late type"- has evolved to the point where there is no gas left for making new stars!
    • - random motion
  12. Irregular Galaxies - in Between Elliptical and Spiral
    • - "lenticualrs"or S0 Galaxies
    • - like ellipticals but usually a bit fatter, usually lack spiral arms
    • - tend to have less cool gas than normal spirals but more than ellipticals

    • - bulgeless, low mass --> more chaotic
    • - very high star formation rate
    • - no bulge or spiral patterns
  13. Barred Spiral Galaxies
    • - many galaxies have "bars" - linear arrangements: straight bar cutting across the center, with spiral arms curling away from ends of bar
    • - the Milky Way - b/c our bulge is somewhat elongated
  14. The Hubble Sequence
    • - Hubble invented a system for classifying galaxies (diagram looks like a tuning fork)
    • - Ellipticals or spheroids (more massive, gas poor, older) on handle then split into Spirals on to (normal or barred) and Disks (Both are less massive, gas rich, ongoing star formation)
  15. Dwarf Galaxies
    • - most numerous type of galaxy in universe
    • - most giant galaxies probably made up of merged dwarf galaxies
  16. Mathematical Insight 20.1: Standard Candles
    • The apparent brightness of a light source decreases with the square of its distance away from us.
    • (apparent brightnesss = luminosity / 4π x (distance)2)

    • We can always measure and objects apparent brightness (flux)
    • If an object is close enough to measure its distance through parallax, then we can calculate the luminosity using inverse square law (above)

    But if the object is a standard candle (already know luminosity) - we can solve the inverse square law to find distance

    distance = √(luminosity / 4π x (apparent brighness))
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Physics: Lecture 20
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