Lecture 25. Celestial Distances

Lecture 25. Celestial Distances

A. Class Business

  1. TWO activities today! (= short lecture!)

  2. DON'T follow schedule given at class start... Read only what's explicitly assigned in class!

  3. Reading for Fri.:
    1. HANDOUT
    2. Section 21.2
    3. Box on p.444

  4. STAR PARTY Friday, 7/30, 10:00 p.m., Museum of Rockies

  5. TWO TALKS* -- TODAY!

  6. turn in OUT-OF-CLASS ACTIVITY papers next MONDAY, 8/2

B. Stars' Names

  1. Greek and Arabic names -- e.g., Altair, Alcor, Aldeberan
  2. ranking by brightness -- Greek letter in name, e.g. -Centauri
  3. numbered by RA from start of constellation -- 51 Pegasi
  4. more catalogs: HD #'s, HR #'s, etc.
  5. Charles Messier's "fuzzy things that aren't comets:" M #'s
      -- now know most are nebulae or galaxies
  6. New General Catalog: NGC #'s

C. Astronomical Distances

Q: HOW BIG IS THE UNIVERSE?

Q: How can we tell how far away things are?

  1. PARALLAX: near to 1000 ly (from < 4 ly until 1800's!)

  2. H-R DIAGRAM: similar range; uses luminosity classes*

  3. OPEN STAR CLUSTERS (for Fri.): proper motion gives disance, hence absolute luminosity
      -- e.g., Hyades cluster

  4. RR LYRAE: near to 2 million ly (in clusters)

  5. CEPHEIDS*: near to 60 million ly

  6. SUPERNOVAE ("secondary"): several Mpc
      -- about as far as observed!




D. Interstellar Gas & Dust

WE CAN SEE BEAUTIFUL CLOUDS IN THE NIGHT SKY. WHAT ARE THEY?

  1. GAS CLOUDS lie between stars --
      too boring to have you read, but still should know!
    • gas is hydrogen (neutral & ionized) and helium
    • detected by spectral lines (excited by nearby stars' radiation)
        -- if visible, then called EMISSION NEBULAE
    • these gases CONDENSE to form stars ("stellar nurseries")
        -- this is what the solar nebula was made of!
  2. Large clouds of DUST also lie between stars
    • dust ABSORBS or SCATTERS light, causing EXTINCTION
        -- if scatter is visible, called REFLECTION NEBULAE
    • (gases don't absorb enough; must be dust)
    • blue light scatters most, so scattering causes REDDENING
        -- blue removed from light ("DE-BLUE-ING")
    • similar to red sunset, but GAS, not dust, scatters in sky
    • (where does blue light go? to making the sky blue!)
  3. APOD!*

E. Cosmic Rays

VERY HIGH ENERGY PARTICLES REGULARLY HIT THE EARTH. WHERE DO THEY COME FROM?











F. Extrasolar Planetary Systems