Astronomy Questions

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Introduction to Astronomy (ASTR& 101)

Spring 2016

Homework 6 Due Monday May 16

(1) When did astronomers first begin to study the structure of our galaxy using

radio telescopes? Research this topic on the web and write a one half page

summary about the people and the observations that made the first radio

maps of the Milky Way.

Be careful to avoid plagiarism and reference your sources.

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(2) Briefly summarize the Shapley-Curtis debate. Were there any definitive

conclusions drawn from this debate?

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(3) The force that keeps the Sun in orbit around the center of the Galaxy is the

gravitation pull of all the matter interior to the Sun’s orbit. We can

estimate the total mass of all of this matter using a derivation of Newton’s

form of Kepler’s third law:

2rv

G M 

The value for r is the distance of the Sun to the center of the

galaxy,26,000 light years.

The value for v has been estimated to be 220 km/s.

The value for G is the gravitational constant you have used for

other calculations in this class.

Using these values, calculate the estimated mass of all of the galaxy interior

to the Sun’s orbit.

Remember that you will have to convert r from light years to meters and

convert v from km/s (kilometers per second) to m/s (meters per second).

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(4) Astronomers have been able to calculate our Solar System’s speed around

the center of the Galaxy by looking at objects outside of our Galaxy. Using

this value, you were able to calculate the mass of the galaxy within the

Sun’s orbit around the galactic center.

Does this knowledge of the Sun’s velocity in its orbit tell us anything about

the mass of the galaxy outside the Sun’s orbit? Explain your answer.

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(5) When astronomers identify the location of black holes by observing orbiting

companion stars, how do they know that these stars are not really orbiting

pulsars or white dwarfs?

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(6) Why did 19th century astronomers think that the “spiral nebulae” were part

of our Milky Way galaxy?

(7) What is the Local Group of galaxies? What are some of the prominent

galaxies in this group?

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(8) Some galaxies in the Local Group exhibit blueshifted spectral lines. Why is

this not a violation of Hubble’s law?

(9) The galaxies with the least amount of gas and dust are:

(a) barred-spiral

(b) elliptical

(c) irregular

(d) spiral

16101 Greenwood Ave. North • Shoreline, WA 98133-5696 • (206) 546-4101

(10) What type of galaxy do we usually find at the centers of galaxy clusters?

(a) barred spiral

(b) elliptical

(c) irregular

(d) spiral