THE Milky Way 2 Flashcards
what is the diffuse band of light across the sky
Milky Way
what is the “milky light” caused by?
- due to stars
- Galileo, 1610
why does our galaxy have a very flattened (disk-like) shape?
to see the concentration of stars
why did Herschels think that sun was approx at center of MWG?
they counted roughly same number of stars in all directions
what did Kapteyn discover about motion of stars and how?
that they were due to orbit of stars about Galaxy center
-doppler shift
globular clusters
extremely compact star cluster
star cluster
stars bound together by gravity
what did Harlow Shapley do in order to get distances to globular clusters?
used RR Lyrae variable stars
RR Lyrae stars have predictable what….?
luminosities
-allows us to measure distances into space
what did the discovery that globular clusters were offset from Sun tell astronomers?
sun was at edge of Galaxy and galaxy was much larger than Kapteyn estimated
What did the Kapteyn and Shapley models neglect the effects of in terms of measuring distances to stars?
neglected the effects of dust that lies between stars (interstellar medium)
what color brightness does dust reduce more?
it reduces blue light brightness more than red light brightness
what is the effect of dust blocking blue light better than red light?
stars look redder than they would if there was no dust
what is a very reliable measure of temperature?
spectral type
what kind of wavelength makes us able to see past the dust?
infrared wavelengths
what is the temperature of dust?
cold, 10-20 Kelvin
at what wavelengths does thermal dust emission occur?
very long infrared or radio wavelengths
interstellar medium contains more gas or dust?
GAS
does gas produce visible light?
no
- too far away from stars
- too cold
energy difference in atomic hydrogen atoms?
energy diff is small, so can be readily excited by collisions in cold interstellar medium
how are molecules formed in the denser regions of the interstellar medium?
the atomic gas can undergo chemical reactions
when were radio spectral lines produced by quantized rotation of molecules detected?
1960s and 1970s
does it take little or lots of energy to set a molecule into rotation?
takes little energy
-this emission can be produced by very cold molecular gas
where do stars form?
in the dense, molecular clouds
how do individual stars and planetary systems form?
gas in molecular clouds fragments and collapses
visible material in MWG by mass…
90% stars
10% gas/dust (interstellar medium)
MWG structure
spiral galaxy with a central bar
what is the spiral pattern a result of?
spiral density waves
“the wave pattern moves thru that galactic disk, but what 3 things do not remain in the spiral arms?
stars, gas, and dust
why do stars, gas, and dust bunch up?
due to the spiral density wave
what do the spiral arms represent a mixture of?
crowding of stars and interstellar medium
what is the spiral density wave?
moves around galaxy, compressing stars/gas/dust and rarifying
what does gas compressed in the spiral density wave do?
initiates the gravitational collapse of the gas to form dense molecule clouds and eventually stars
what do spiral arms represent?
waves of star formation in the gaseous disk
what kind of stars delineate the arms well? (3 characteristics)
young, luminous, massive stars
who discovered the two stellar populations and when?
Walter Baade, 1940s
Pop I and Pop II
population I stars
young, disk stars
-abundant in elements heavier than H and He
population II stars
old, bulge and halo stars
what do the two stellar populations have differences in?
age, size, color, orbits, amounts of heavy elements
Population I star orbits?
orbits confined to the disk of the Milky Way
Population II star orbits?
often have elliptical orbits that extend into the halo
recycling in the galaxy
MWG recycles material from gas to stars back to gas and the process repeats
-enriches MWG in fusion products (heavy elements)
