Assignment 6 PHY2390
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School
University of Ottawa *
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Course
2390
Subject
Astronomy
Date
Jan 9, 2024
Type
Pages
4
Uploaded by BarristerLapwing168
Chapter 9
Question 5
How did our galaxy form and evolve
The oldest open star clusters suggest our galaxy's disk is around 9 billion years old, while
the oldest globular clusters are at least 13 billion years old, this means our galaxy began forming
at least 13 billion years ago. Observing stellar populations are what gives important clues about
the formation of the galaxy: the halo (Population II stars) is formed before the disk (Population I
stars). Population II stars are stars that have less heavy elements and Population I stars like the
Sun are richer in metals. The monolithic collapse model, proposing a single gas cloud forming
the galaxy, now includes mergers with other galaxies.
Question 7
Why didn't astronomers before Shapley realize how large the galaxy is?
Astronomers before Shapley did not realize how large the galaxy is primarily due to
interstellar gas and dust.
Question 8
What is the evidence that our galaxy contains a large amount of dark matter?
Dark matter is “ Nonluminous matter that is detected only by its gravitational
influence. (p. 210)”. The evidence that our galaxy contains a large amount of dark matter is that
even objects far from the center of the milky way have high orbital speeds. The velocities are
larger than would be expected without taking into consideration the effect of dark matter. This
indicated a large amount of dark matter
Question 19
Why must astronomers use infrared telescopes to observe the motions of stars around SgrA *?
Sagittarius A* is “The powerful radio source located at the core of the Milky Way
Galaxy. (p. 220)” in other words, this is the nucleus of the milky way. To observe Sgr A* and the
stars around it astronomers use infrared telescopes because the nucleus isnt visible at visual
wavelengths due to space gas and dust. However, radio, infrared and x-ray radiation can penerate
the gas and dust.
Learning to look 2
Why are the spiral arms in the galaxy below blue? What colour would the halo be if it were
bright enough to see in this photo?
The spiral arms are blue because there are many young hot stars and young massive stars
emit blue light. If the halo was visible it would be red because it would typically contain older
stars more specifically population II stars.
Chapter 10
Question 2
How do astronomers measure the distances to galaxies, and how does that allow the sizes,
luminosities, and masses of galaxies to be determined?
Astronomers measure the distances to galaxies by using objects of known luminosity, like
Cepheid variable stars and type Ia supernovae. They estimate galaxy distance by dividing its
apparent recession rate by the Hubble constant, as per the Hubble law which states that there is a
“linear relation between the distances to galaxies and the apparent velocity of recession. (p.233)”
This distance information helps with determining galaxy sizes, luminosities, and masses
of galaxies. Luminosity is related to distance and apparent brightness (inverse square law of
light) and size can be determined with a good measurement of apparent distance. To determine
the mass of a galaxy we analyse orbital motion of its stars. We measure the speed of stars in the
galaxy to find the gravity in the galaxy, knowing that gravity depends on mass and distance you
find the galaxy's mass.
Question 5
What is the energy source for active galaxies, what can trigger the activity, and what does that
reveal about the history of galaxies?
The energy source is supermassive blackholes
Question 7
What is the difference between an Sa and an Sb galaxy? Between an S0 and an Sa galaxy?
Between an Sb and an SBb galaxy? Between an E7 and an SO galaxy?
a)
Sa vs Sb galaxies: Sa galaxies have larger nuclei, less gas and dust, and less hot, bright
stars then Sb galaxies. Sb galaxies also have more open arms they Sa galaxies.
b)
S0 vs Sa galaxies: S0 is elliptical shaped galaxy while Sa is spiral galaxy with a disk and
spiral arms
c)
Sb vs SBb galaxies: SBb galaxies have less gas and dust then Sb galaxies. Also the spiral
arms of SBb galaxies reach more outwards then the Sb category(look at image below)
(
https://www.e-education.psu.edu/astro801/content/l9_p3.html
)
d)
E7 vs S0 galaxies: E7 is an elliptical galaxy and has the most slim shape(it's the most
stretched circle for the scale of elliptical galaxies). Wheras S0 galaxies have an obvious
disk and nuclear buldge but the arms are so tight that it looks elliptical S0 galaxies are
considered the transition between elliptical and spiral galaxies.
Question 8
Explain how the rotation curve method of finding a galaxy's mass is similar to the method used
to find the masses of binary stars.
Both methods are similar because they use Kepler's thris law; however, for binary stars
the distance used is the separation of the two stars, whereas to find the mass of a galaxy the
distance used is the distance to the galactic center. Both methods will determine the sum of
masses in the system that is being measured.
Question 17
What evidence can you cite that galactic cannibalism really happens?
“The Milky Way Galaxy is currently snacking on the two Magellanic Clouds that orbit
around it. Its tides are also pulling apart two other small satellite galaxies, the Sagittarius Galaxy
and Canis Major Dwarf Galaxy” (Ghose et al.,p.240)
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