The Milky Way Flashcards
milky way
diffuse band of light that we see across the sky
-Galileo, 1610 w/ telescope
shape of Milky Way galaxy
-very flattened (disk-like) system to see the concentration of stars
where are we located in the Milky Way
sun is approx. in center
William and Caroline Herschel
English astronomers (1780s) -counted stars, found we were in center since # of stars was same in each direction
size of Milky Way Disk
diameter of disk was about 10 times its thickness
Dutch astronomer Kapteyn
- early 1900s
- noted that motion of stars were not random: due to orbit of stars about Galaxy center (Doppler shift)
Kapteyn 1922
used star counts and stellar motions to determine sun was in center
Kapteyn Model
15,000 pc, but most stars were within a few kpc of center
star cluster
stars bound together in groups by their gravity
globular cluster
extremely compact star cluster
Harlow Shapley
- 1920s
- used RR Lyrae variable stars (predictable luminosities) to find that glob clusters were distributed well offset from Sun
Shapley’s Model
shows that Sun was at the edge of the Galaxy and galaxy was much larger than thought
Discrepancy b/w Kapteyn and Shapley Models
- both wrong!
- did not account for effects of dust that lies between stars
interstellar medium
dust lying b/w stars
-dust grains=effective in blocking visible star light…distorted view of Milky Way
interstellar dust
dust grains=small particles of matter
-can only see stars near the Sun (probs w/ Kapteyn/Shapley models)
dust’s effect on brightness
reduces brightness of background stars
-reduces blue light more than red light
Reddening
stars look redder than they should if there is no foreground dust
reliable measure of temperature
spectral type
thermal dust emission
dust=10-20 K
-emission occurs at very long infrared or radio wavelengths
Gas in the Interstellar Medium
medium contains more gas than dust
-near hot star, can become hot enough to produce emission line spectrum (visible light emission)
radio wave from cold interstellar gas
atomic hydrogen atoms in space emits radio spectral line emission at wavelength of 21 cm due to spin or electron and proton (1951)
collisions in cold interstellar medium
energy difference is small=readily excited by collisions
molecular clouds
in denser regions of interstellar medium, atomic gas undergoes chem reactions to form molecules
how are radio spectral lines produced
By the quantized rotation of molecules (1960s/70s)
why can radio spectral lines be produced by very cold molecular gas?
takes lil energy for collisions to set a molecule into rotation
where do stars form?
dense, molecular clouds where gas in interstellar medium is molecular rather than atomic
how do stars form?
gas in molecular clouds fragments and collapses to form individuals stars/planetary systems
visible material in Milky Way (%)
90% stars
10% gas and dust
Structure of Galaxy
- spiral structure w/ central bar
- spiral pattern is result of spiral density waves
- wave pattern moves thru galactic disk, but stars/gas/dust do not remain in spiral arms
spiral arms
stars, gas, and dust bunch up in spiral arms due to spiral density wave
spiral density wave
moves around the galaxy alternatively compressing the stars, gas, and dust (spiral arm) and rarifying (inter-arm region)
gas compressed in spiral density wave
initiates gravitational collapse of the gas to form dense molecule clouds…eventually stars
what do spiral arms represent?
waves of star formation in the gaseous disk
-young massive stars delineate the arms
Walter Baade
discovered two stellar populations:
- Pop I=young disk stars
- Pop II=old, bulge and halo stars
what are the differences b/w Population I and Population II?
differ in color, age, shape, orbits, amounts of heavy elements
Population I
less than a few billion years old
- still have big blue stars
- more abundant than H and He
Population II
over 10 billion years old, red stars
Population I Orbits
confined to the disk of the Milky Way
Population II Orbits
elliptical orbits that extend into the halo
What does the Milky Way recycle?
its material from gas to stars to gas and this process repeats
-enriches Milky Way in fusion products
