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ASTA02H3 Lecture Notes - Orbital Speed, Spiral Galaxy, Brown Dwarf

Course Code
Parandis Khavari

of 4
Astronomy - Session 15
Milky Way (Galaxy), dark matter, black hole at the center of the Galaxy
Galileo confirmed that the white band (the Milky Way) was made of stars
determined the shape of the Milky Way through star distribution
wheel or disk
not a sphere because the stars were not uniformly distributed
Herschel worked with his sister, Caroline
thought that the Sun was close to the center of the Milky Way
thought that the shape of the Milky Way was a disk
bigger model than Hersc
thought that the Sun was 600 pc away from the center
thought that the shape of the Milky Way was a disk
Why were both Herschel and Kapteyn wrong about the shape of the Milky Way?
Interstellar Dust
Interstellar Dust
obstructs view, esp. towards the center of the Galaxy
interstellar gas
emission can be seen from very far away
dust extinction
dimming of starlight by dust
interstellar reddening
blue light is almost completely absorbed
can only probe galaxy through
infrared light
radio waves
galaxy is probed in 21cm lines of Hydrogen
Shapley (1915)
measured distance between globular clusters
known that globular clusters were not uniformly spread
studied small magellanic clouds
satellite galaxies orbiting Milky Way
noticed stars regularly changing brightness
located at approx. same distance
relationship between intrinsic brightness and pulsation period
the brighter a star, the larger the period
identified stars called Cepheid variables after  which was the first one of these
to be identified
the period of pulsation is from one to sixty days
absolute magnitude luminosity
   
core of yellow giants
core of red giants
find a Cepheid variable in a globular cluster to find distance
through the period-luminosity relationship
how do we know that the Milky Way is a spiral galaxy
galactic disk
galactic nucleus
where the black hole is
surrounded by galactic bulge
galactic bulge
full of gas and dust
star formation taking place
galactic halo
where globular clusters live
very old because no interstellar dust
elliptical and random orbit
Milky Way rotation
rotation of the Galaxy
Sun is approx. 8 kpc (kilo parsecs) from the center of the Galaxy
given the orbital speed and the distance of the Sun, we can calculate the orbital period of the
  
  
circumference divided by velocity
  
only 5% of the age of the Sun
therefore, the Sun has only made 20 orbits around the center of the Galaxy