Module 13

Question 1

What are the three main types of galaxies classified by astronomers?

Answer 1

• Spiral galaxies
• Elliptical galaxies
• Irregular galaxies

These classifications are based on their appearance and structural features.

Question 2

What is the Hubble eXtreme Deep Field?

Answer 2

An image showing thousands of galaxies in a small patch of sky taken by the Hubble Space Telescope over 23 days.

This image helps estimate the total number of galaxies in the observable universe.

Question 3

How many galaxies are estimated to be in the observable universe?

Answer 3

Well over 100 billion large galaxies and many more small ones.

This estimation is based on representative patches of the sky observed by telescopes.

Question 4

What differentiates spiral galaxies from elliptical galaxies?

Answer 4

Spiral galaxies have a disk component and spiral arms, while elliptical galaxies lack a significant disk component and have only a halo.

Elliptical galaxies usually appear more rounded and redder due to older stars.

Question 5

What are the two primary components of a spiral galaxy?

Answer 5

• Disk component
• Halo component

The disk contains stars and gas clouds, while the halo has older stars with random orbits.

Question 6

What characterizes irregular galaxies?

Answer 6

They do not fit into the spiral or elliptical categories and often appear disorganized with young, massive stars.

Examples include the Magellanic Clouds.

Question 7

What is the significance of galaxy color in relation to star formation?

Answer 7

Galaxies in the blue cloud have active star formation and young stars, while those in the red sequence contain older stars with little ongoing star formation.

This distinction helps in understanding a galaxy’s evolutionary path.

Question 8

What does the tuning fork diagram created by Edwin Hubble represent?

Answer 8

A classification system for galaxies based on their shapes and components, with elliptical galaxies on the left and spiral galaxies on the right.

It shows the gradual change from elliptical to spiral characteristics.

Question 9

Fill in the blank: Most galaxies in the red sequence are ______.

Answer 9

elliptical in shape.

They are typically redder due to the presence of older stars.

Question 10

What type of galaxies are often found in loose collections of up to a few dozen?

Answer 10

Spiral galaxies with ongoing star formation.

Their environment influences their star formation activity.

Question 11

How do astronomers infer the history of galaxies?

Answer 11

By observing different galaxies at various life stages and considering their distance and age.

This approach allows for understanding the evolution of galaxies over time.

Question 12

What is the typical size range of dwarf galaxies?

Answer 12

Less than 1 million stars.

These are much smaller compared to giant galaxies with over 1 trillion stars.

Question 13

True or False: The Milky Way Galaxy is classified as a spiral galaxy.

Answer 13

True.

It exhibits characteristics typical of spiral galaxies.

Question 14

What does the presence of hot, young stars in a galaxy indicate?

Answer 14

Active star formation is occurring.

This is typically found in spiral and irregular galaxies.

Question 15

What is a lenticular galaxy?

Answer 15

A type of galaxy that has a disk and halo but lacks spiral arms.

It is considered an intermediate class between spiral and elliptical galaxies.

Question 16

What role does a galaxy’s environment play in its evolution?

Answer 16

It influences star formation rates and the transition between galaxy types.

Galaxies in denser environments may evolve differently compared to those in isolation.

Question 17

What is the relationship between galaxy size and star formation cessation?

Answer 17

Larger galaxies tend to stop star formation sooner than smaller ones.

This is indicated by their position in the red sequence of galaxy color and luminosity diagrams.

Question 18

What does the red sequence in galaxies suggest about star formation?

Answer 18

Star formation shuts down sooner in larger galaxies than in smaller ones

The red sequence refers to a group of galaxies that have ceased star formation and appear redder in color.

Question 19

What are galaxy groups?

Answer 19

Loose collections of up to a few dozen galaxies

An example is our Local Group.

Question 20

What distinguishes galaxy clusters from groups?

Answer 20

Galaxy clusters contain hundreds or thousands of galaxies and can be over 10 million light-years across

Question 21

What percentage of large galaxies in the central regions of clusters are elliptical galaxies?

Answer 21

About 50%

Elliptical galaxies represent only about 15% of large galaxies found outside clusters.

Question 22

What role does a galaxy’s local environment play in its classification?

Answer 22

It helps determine whether a galaxy ends up as a spiral or elliptical galaxy

Question 23

What is parallax in astronomy?

Answer 23

The apparent shift in a star’s position as Earth orbits the Sun

Question 24

What is radar ranging?

Answer 24

A method where radio waves from Earth bounce off planets or objects to measure distance

Question 25

What are standard candles?

Answer 25

Objects whose luminosity is known and can be used to calculate distance based on apparent brightness

Question 26

What is the inverse square law for light?

Answer 26

A law that relates the brightness of an object to its distance from the observer

Question 27

What are Cepheid variable stars?

Answer 27

A special class of luminous stars that pulsate in size, causing changes in brightness

Question 28

What is Leavitt's law?

Answer 28

The relationship between a Cepheid's pulsation period and its luminosity

Question 29

What is the significance of white dwarf supernovae in distance measurement?

Answer 29

They have nearly the same luminosity, making them valuable as standard candles for large distances

Question 30

What was the Great Debate in astronomy?

Answer 30

A debate over the nature of spiral nebulae, whether they were part of the Milky Way or separate galaxies

Question 31

Who were the main participants in the Great Debate?

Answer 31

Harlow Shapley and Heber Curtis

Question 32

What was Shapley's argument in the Great Debate?

Answer 32

Spiral nebulae were gas clouds internal to the Milky Way

Question 33

What was Curtis's position in the Great Debate?

Answer 33

Spiral nebulae were separate islands of stars

Question 34

Fill in the blank: The distance to spiral nebulae was crucial for determining whether they were _______.

Answer 34

separate galaxies

Question 35

What was the outcome of the Great Debate?

Answer 35

Neither Shapley nor Curtis scored a convincing victory

Question 36

What was the rotation period claimed for the spiral nebula M101?

Answer 36

100,000 years ## Footnote This implied a relatively small and nearby structure, or else the rotation speed would exceed the speed of light.

Question 37

What was the actual classification of the 1885 Andromeda 'nova' observed by Shapley?

Answer 37

Supernova ## Footnote This classification explained its surprising brightness.

Question 38

Who was invited to join the staff at the Mount Wilson Observatory in 1917?

Answer 38

Edwin Hubble ## Footnote He delayed his acceptance to serve in World War I.

Question 39

What significant discovery did Hubble make regarding Cepheids?

Answer 39

He discovered that some stars in the Andromeda spiral were Cepheids ## Footnote This allowed him to determine their luminosities using Leavitt's law.

Question 40

What law did Hubble use to estimate the distance to the Andromeda Galaxy?

Answer 40

Leavitt's law ## Footnote It relates the period of Cepheid variables to their luminosity.

Question 41

How did Hubble's measurements regarding the Andromeda Cepheids affect his conclusions?

Answer 41

He underestimated the true distance by about half ## Footnote This was due to not knowing about the two types of Cepheids.

Question 42

What did Hubble's discovery about the Andromeda 'nebula' signify for the universe?

Answer 42

It proved that the Andromeda 'nebula' is a separate galaxy ## Footnote This changed the understanding of the universe from a single galaxy to many galaxies.

Question 43

What did Hubble discover about the movement of galaxies in 1929?

Answer 43

The more distant a galaxy, the greater its redshift and the faster it moves away ## Footnote This led to the conclusion that the universe is expanding.

Question 44

Who made the redshift measurements that Hubble initially used?

Answer 44

Vesto Slipher ## Footnote These measurements were made years earlier at the Lowell Observatory.

Question 45

What is Hubble's law formula?

Answer 45

v = H₀ * d ## Footnote Where v is the recession speed, d is the distance, and H₀ is Hubble's constant.

Question 46

What are the two practical difficulties encountered when using Hubble's law?

Answer 46

* Galaxies do not obey Hubble's law perfectly * Distances depend on the accurate measurement of Hubble's constant

Question 47

What did the Hubble Space Telescope aim to measure regarding Hubble's constant?

Answer 47

An accurate value of Hubble's constant ## Footnote This was crucial for understanding distances to galaxies.

Question 48

What chain of techniques is used to measure cosmic distances?

Answer 48

* Radar ranging * Parallax * Cepheid variables * Distant standards * Hubble's law

Question 49

What implication does the expansion of the universe have on its history?

Answer 49

It suggests that the universe must have had a beginning ## Footnote This led to the concept of the Big Bang.

Question 50

What does the Cosmological Principle imply about the universe's expansion?

Answer 50

The universe is not expanding into a void ## Footnote It suggests a more complex structure of cosmic expansion.

Question 51

What is the significance of Hubble's discovery for our understanding of the universe?

Answer 51

It radically changed our view, revealing that the Milky Way is just one of many galaxies ## Footnote This opened up new avenues for astronomical research.

Question 52

What did Hubble underestimate in his distance calculations?

Answer 52

The luminosities of Cepheids ## Footnote This was due to the fact that he observed the brighter type of Cepheid.