14.May.98 class: still MISSING TRIPOD AND OPTICS...

• DQ from Ch.14

• DQ from Ch.15

Ch.14: our Milky Way Galaxy

Distance from Sun to center of MW galazy can be found by

• finding center of Galaxy by observing orbits of globular clusters in halo
• then observing Cepheids in the center (Sagittarius)
• use Henrietta Leavitt's period-luminosty relation to find the absolute brightness from measured pulsatoin period of Cepheids
• compare absolute to apparent brghtness to find distance
• d~28,000 LY

The mass of the Galaxy inside the Sun's orbit can be found from Kepler's 3d law:

• Find Sun's speed about the center of the Galaxy by measuring our motion with respect to the "background" of the abverage position of the halo globular clusters: v~230 km/s
• calculate the orbit period T~230 MY (there appear to be evidence for increased asteroid impacts on this timescale, perhaps as we return to an area of denser debris in the galaxy)
• Mass inside Sun's orbit works out to about 10¹¹ solar masses

Dark Matter:

• recall Keplerian velocity curve (for central point mass)
• compare to velocity curve in Fig.14-12 p.318 (Vera Rubin's story...)
• Calculate mass inside 60,000 LY
• What percent of galactic mass is outside Sun's orbit? Not a central point mass!

21 cm radiation

• due to spin flips: e energy is lower when spinis anti-aligned with nuclear spin
• Ex: freq. of 21 cm light?
• Ex: MW rotation toward and away p.319: what Doppler shift in wavelength?

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Discussion Questions from Ch.14: RQ 3, 5, 6, 8 (p.322)

3. What observations led Harlow Shapley to conclude that we are not at the ceenter of the Galaxy?

5. How do hydrogen atoms generate 21-cm radiation? What do we learn from it?

6. Why do we beieve in dark matter?

8. Why no O and B stars in glubular clusters?

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Ch.15: Galaxies

Spiral density waves (C.C. Lin and Frank Shu) v stars > v arms. Stars (cars) move through waves (around truck = moving traffic jam)

O and B stars trace arms because bright stars burn out fast and never get far from their birthplace. Lifetime ~ 105 years < rotation period ~ 108 years.

Elliptical galaxies are old - tend to lose structure with age, or due to collisions and tides...

Collisions are about as rare as bullets hitting midair in a war

What's the difference betwen globular clusters (p.263) and dwarf ellipticals (p.330? both have bout 106 stars.

Scales:

• MW ~ 105 ly
• local group ~ 106 ly= 1 Mly
• 50 Mly to virgo cluster
• 300 Mly to Coma Berenices
• Great Wall 650 Mly long (Margaret Geller, Harvard and John Huchra)

DARK MATTER : there's bout 10 times as much as what we see, from rotation curves.

Hubble Law was based on Slipher's data (janitor). Wendy Freedman's value best accepted: 75 km/s/Mpc. Depends on independent distance measurements, but can be used to find distances if you believe H.

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Discussion Questions from Ch.15: DQ 20, 22; RQ 4, 5, 7 (p.344)

20 Hubble galaxies represent evolutionary sequence?

22 Explain spiral arms.

4 Self-propagating star formation -> spiral arms?

5 Density wave theory

7 Naked eye galaxies?

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Deriving

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Maintained by E.J. Zita