The Milky Way

Question 1

milky way

Answer 1

diffuse band of light that we see across the sky

-Galileo, 1610 w/ telescope

Question 2

shape of Milky Way galaxy

Answer 2

-very flattened (disk-like) system to see the concentration of stars

Question 3

where are we located in the Milky Way

Answer 3

sun is approx. in center

Question 4

William and Caroline Herschel

Answer 4

English astronomers (1780s) 
-counted stars, found we were in center since # of stars was same in each direction

Question 5

size of Milky Way Disk

Answer 5

diameter of disk was about 10 times its thickness

Question 6

Dutch astronomer Kapteyn

Answer 6

• early 1900s

- noted that motion of stars were not random: due to orbit of stars about Galaxy center (Doppler shift)

Question 7

Kapteyn 1922

Answer 7

used star counts and stellar motions to determine sun was in center

Question 8

Kapteyn Model

Answer 8

15,000 pc, but most stars were within a few kpc of center

Question 9

star cluster

Answer 9

stars bound together in groups by their gravity

Question 10

globular cluster

Answer 10

extremely compact star cluster

Question 11

Harlow Shapley

Answer 11

• 1920s
• used RR Lyrae variable stars (predictable luminosities) to find that glob clusters were distributed well offset from Sun

Question 12

Shapley’s Model

Answer 12

shows that Sun was at the edge of the Galaxy and galaxy was much larger than thought

Question 13

Discrepancy b/w Kapteyn and Shapley Models

Answer 13

• both wrong!

- did not account for effects of dust that lies between stars

Question 14

interstellar medium

Answer 14

dust lying b/w stars

-dust grains=effective in blocking visible star light…distorted view of Milky Way

Question 15

interstellar dust

Answer 15

dust grains=small particles of matter

-can only see stars near the Sun (probs w/ Kapteyn/Shapley models)

Question 16

dust’s effect on brightness

Answer 16

reduces brightness of background stars

-reduces blue light more than red light

Question 17

Reddening

Answer 17

stars look redder than they should if there is no foreground dust

Question 18

reliable measure of temperature

Answer 18

spectral type

Question 19

thermal dust emission

Answer 19

dust=10-20 K

-emission occurs at very long infrared or radio wavelengths

Question 20

Gas in the Interstellar Medium

Answer 20

medium contains more gas than dust

-near hot star, can become hot enough to produce emission line spectrum (visible light emission)

Question 21

radio wave from cold interstellar gas

Answer 21

atomic hydrogen atoms in space emits radio spectral line emission at wavelength of 21 cm due to spin or electron and proton (1951)

Question 22

collisions in cold interstellar medium

Answer 22

energy difference is small=readily excited by collisions

Question 23

molecular clouds

Answer 23

in denser regions of interstellar medium, atomic gas undergoes chem reactions to form molecules

Question 24

how are radio spectral lines produced

Answer 24

By the quantized rotation of molecules (1960s/70s)

Question 25

why can radio spectral lines be produced by very cold molecular gas?

Answer 25

takes lil energy for collisions to set a molecule into rotation

Question 26

where do stars form?

Answer 26

dense, molecular clouds where gas in interstellar medium is molecular rather than atomic

Question 27

how do stars form?

Answer 27

gas in molecular clouds fragments and collapses to form individuals stars/planetary systems

Question 28

visible material in Milky Way (%)

Answer 28

90% stars | 10% gas and dust

Question 29

Structure of Galaxy

Answer 29

- 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

Question 30

spiral arms

Answer 30

stars, gas, and dust bunch up in spiral arms due to spiral density wave

Question 31

spiral density wave

Answer 31

moves around the galaxy alternatively compressing the stars, gas, and dust (spiral arm) and rarifying (inter-arm region)

Question 32

gas compressed in spiral density wave

Answer 32

initiates gravitational collapse of the gas to form dense molecule clouds...eventually stars

Question 33

what do spiral arms represent?

Answer 33

waves of star formation in the gaseous disk | -young massive stars delineate the arms

Question 34

Walter Baade

Answer 34

discovered two stellar populations: 1. Pop I=young disk stars 2. Pop II=old, bulge and halo stars

Question 35

what are the differences b/w Population I and Population II?

Answer 35

differ in color, age, shape, orbits, amounts of heavy elements

Question 36

Population I

Answer 36

less than a few billion years old - still have big blue stars - more abundant than H and He

Question 37

Population II

Answer 37

over 10 billion years old, red stars

Question 38

Population I Orbits

Answer 38

confined to the disk of the Milky Way

Question 39

Population II Orbits

Answer 39

elliptical orbits that extend into the halo

Question 40

What does the Milky Way recycle?

Answer 40

its material from gas to stars to gas and this process repeats -enriches Milky Way in fusion products