Chapter 20- Are There Other Galaxies

Question 1

Hubble classification

Answer 1

scheme for classifying galaxies based upon their appearance

• ellipticals
• spirals
• barred spirals
• irregulars

Question 2

elliptical galaxies

Answer 2

• elliptical shape
• no spiral arms
• red/yellow color indicates old stars
• E0-E7
• E0: perfectly rounded
• E7: highly elongated

Question 3

spiral galaxies

Answer 3

• arched lanes of stars
• young, hot, blue stars
• Sa-Sc
• Sa: large central bulged compared to arms
• Sc: small central bulge compared to arms

Question 4

why does on going star formation lead to a blue-white region running through nucleus

Answer 4

short lived blue stars outshine others

Question 5

barred spiral galaxies

Answer 5

• spiral arms originate at ends of bar shaped region running through nucleus
• SBa-SBC
• SBa: large central bulge relative to arms
• SBc: smal central bulge relative to arms

Question 6

irregular galaxies

Answer 6

• indiscriminate shapes

- large quantities of interstellar dust and gas

Question 7

what is the meaning of the “tuning-fork” diagram

Answer 7

• Hubble thought it represented an evolutionary track
• but he was wrong
• does appear to reflect the angular momentum of each galaxy

Question 8

distribution of galaxies

Answer 8

• spiral galaxies are often found in groups of galaxies
• clusters: elliptical galaxies are much more common to be in clusters
• aka 100’s of thousands

Question 9

How do we measure distance to galaxies

Answer 9

• radar
• parallax
• cepheid variables
• type Ia supernovae
• redshift and bubbles law

Question 10

Hubble law

Answer 10

• Edwin Hubble discovered that “spiral nebulae” were actually other galaxies outside the Milky Way
• he did this by using cepheid variables to measure distances
• he also measured the line-of-sight velocities of these galaxies (aka how close or far away from us)
• he did this by using the doppler shift of special lines

Question 11

what did Hubble discover after using doppler shift of spectral lines?

Answer 11

he found that more distant the galaxy is, the more (redshifted) they are moving away (faster)

Question 12

what does hubbles law tell us

Answer 12

• that the universe is expanding

- galaxies that are further away are moving faster

Question 13

what was true about galaxies history

Answer 13

• galaxies must have been closer together
• the universe was immensely denser
• the start universe is called the “Big Bang”

Question 14

hubbles law allows us to estimate what

Answer 14

• the age of the universe from the Hubble constant

- 14 billion years