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UA / Physics / ASTR 170 / Where do stars tend to form?

Where do stars tend to form?

Where do stars tend to form?


School: University of Arizona
Department: Physics
Course: The Physical Universe
Professor: Daniel marrone
Term: Fall 2018
Tags: astronomy
Cost: 50
Name: ASTR 170 - Final Study Guide
Description: This is study guide covering new material that was studied after Midterm #3.
Uploaded: 12/07/2018
12 Pages 136 Views 2 Unlocks

jacoblacy (Rating: )

ASTR 170 B1 – Final Study Guide 

Where do stars tend to form?

Chapter 15: The Milky Way Galaxy 

I What it looks like

a Holds over 100 billion stars and some 100 billion  galaxies

b Spiral galaxy from spiral arms 

i about 100,000 light-years in diameter

ii disk is about 1000 light-years thick

1 Sun is located in disk

c Interstellar Medium

i Cloud of interstellar gas and dust

II The orbits of stars in galaxy

a Depends on location

b Orbits in Disk 

i Like merry-go-round

1 Bob up and down, which makes up the  

What are the two key differences between halo and disk stars?

thickness of the galaxy

ii General orbits are from gravitational  

attraction to galactic center If you want to learn more check out Who was orpheus?
If you want to learn more check out When does l (v) becomes a multiple of v?

iii Bobbing comes from the pull of gravity in the  

disk itself

iv Timing

1 Each orbit takes about 200 million years

2 Up and down motion takes few 10’s  

million years

3 Orbit velocities are the same for close  

and distant stars

a So close stars finish first

c Orbits in the Halo 

i Relatively random and neighboring stars can  

circle in opposite direction

How the galaxy formed?

ii Also bob up and down but much higher and  lower

1 Makes halo more puffier

d Orbits of Bulge Stars 

i Difficult to measure because they are more  distant

ii Have a mix of variety of orbits making it  Don't forget about the age old question of What are the implications of the author's findings or arguments?


e Stellar orbits and mass of the galaxy

i Using Kepler’s third law we can determine  

mass of galaxy within a particular orbit

III Galactic Recycling

a Star-Gas-Star Cycle

b Gas from dying stars

i Result of planetary nebulae and supernovas ii Gas is ejected into space at high speeds from strong stellar winds

1 If it sweeps up surrounding interstellar  Don't forget about the age old question of Who is the first to scientifically study imagery?

material it forms a bubble

iii Supernovae generate shock fronts 

1 Abrupt, high-gas-pressure walls moving  

faster than sound waves can travel

iv Bubbles can form to giant bubbles and erupt  out of the disk

v Only gravity will be able to stop it, and it gets pulled back into the halo and disk

vi Supernovae can cause cosmic rays 

1 Made up of electrons, protons, and  

atomic nuclei

2 Travel close to the speed of light

3 Cause genetic mutations in living  


c Cooling and Cloud Formation

i Atomic hydrogen gas

1 Cooler gas, result of bubbles cooling

2 Emits a 21-centimeter line in  


3 Total amount is about 5 billion solar  We also discuss several other topics like What is the difference between the central route and peripheral route?


ii Gravity pulls blobs of gas together that grows denser We also discuss several other topics like State the difference between a hypothesis and a theory.

1 Blob cools and contracts to form clouds

iii Process of cloud forming takes long so it  

takes up a lot on star-gas-star cycle

iv Clouds contain small amount of interstellar  dust

1 Dust grains-tiny, solid flecks of carbon  

and silicon minerals

2 Form in winds of red giant stars

3 Responsible for absorbing visible light,  

that allows us not to see past the disk

v Completing the Cycle

1 Molecular clouds form new stars that  

use the gas

2 Although not all gas is recycled because  

it gets trapped in brown dwarfs or dead  

corpses of stars

IV Where do stars tend to form?

a Ionization Nebulae 

i Glow because electrons are raised to high  

energy levels or ionized

ii Striking colors come from spectral lines  

produced by particular atomic transitions

iii Blue and Black Tints

1 Blue Tint allows Starlight to reflect from  

dust grains

a Reflection nebulae

2 Black regions are dark, dusty gas clouds  

that block our view of stars beyond  


b Spiral Arms 

i Contains young stars and also new forming  stars

ii Spiral pattern of star formation is from spiral  density waves 

1 Packs gas cloud closer together which  

increases gravity within

V History of the Milky Way

a What halo stars tell us

i Two key differences between halo and disk  


1 Main-sequence turnoff points indicate  

that they are old (12 billion years)

2 Halo stars contain smaller proportions of

heavier elements

ii Based on differences, they are divided into 1 Disk population (Population I) 

a Stars follow orbital pattern of the  


b Includes both young and old stars

c Heavy element proportion near 2%

2 Halo Population (Population II) 

a Orbit the center with many  

inclinations crossing the disk

b Old stars that are low in mass

c Heavy-element proportions as low  

as 0.02%

iii Halo doesn’t contain cold dense molecular  clouds

1 Clouds are warm and molecules are  

spread out

a Makes it hard for stars to form here

b How the galaxy formed

i Protogalactic cloud

1 Helium and gas w/ larger amount of dark

matter (like molecular cloud)

2 Functions like a present-day star-forming


3 Began with 10% amount of heavy

elements it has today

a Star-gas-star cycle increased it

VI The Galactic Center

a The center is believed to be a black hole

i Using Kepler’s third law it tells us that the  

center has a mass of about 4 million solar  

masses, packed into a region of space

b Observations come from X-ray, Infrared, and radio  telescopes because it cannot be visibly seen

Chapter 16: A Universe of Galaxies 

I. Hubble’s Classification Scheme

a. Spiral Galaxies 

i. Look like flat white disks with yellowish  

bulges at their centers

ii. Disks filled with cool gas and dust, with  

hotter ionized gas, and usually spiral arms

iii. Made up with two components

1. Disk component

a. Consists of stars and dusty gas  

clouds, following an orbit around  

the galactic center

2. Halo component

a. Includes both halo and central  


iv. Barred spirals

1. Appear to have straight bar of stars  

cutting across the center, with arms  

spiraling off the ends

b. Elliptical Galaxies 

i. Redder, rounder, and elongated

ii. Contain very little cool gas and dust  

compared to spiral galaxies

iii. Contain very hot ionized gas

iv. Made up only one component

1. Halo component

a. Look like halo with bulge and no  


v. Most common in the universe

c. Irregular galaxies 

i. Appear neither disk-like nor rounded

ii. Contain many young, massive stars

iii. Were more common when universe was  younger

d. Galaxy Classes

i. Elliptical galaxies are classified by letter E 1. Followed by a number, the bigger the  number the flatter/elongated the galaxy

ii. Spiral galaxies are classified by letter S (ordinary spirals) and SB (barred spirals) 1. Followed by lowercase letter a, b, or c 2. Going from a to c

a. The amount of dusty gas increases,

while bulges size decreases

iii. Irregular galaxies are classified by Irr e. Blue cloud

i. Contain numerous hot and young stars ii. Tend to be spiral or irregular galaxies f. Red sequence

i. Contain only older stars

ii. Are elliptical galaxies

II. Hubble’s Law

a. Speed of galaxies depends on their distances

b. Found out that universe is expanding because  

galaxies are moving away from us

c. All galaxies are moving away from us = redshifted  d. We can find distances by using Doppler shifts then  determine velocity

III. Active Galactic Nuclei (AGN’s)  

a. Unusually bright centers of galaxies

b. The most luminous of these are called Quasars 

i. found at very great distances

IV. Radio Galaxies

a. Unusually strong radio-wave emission

b. Comes from radio lobes, which are result of jets V. AGN’s and Quasars

a. Both have broad emission lines

i. Rapid motion in gas

b. Luminous Nuclei

c. Powered by supermassive black holes

Chapter 17: The Big Bang Theory 

I. Conditions in Early Universe

a. Big Bang Theory

i. Scientific theory of what universe was like  

early in time

b. We began as a hot and dense collection of matter  and radiation

c. Observations show we are cooling and expanding  over time

II. Particle Creation and Annihilation

a. Universe was so hot, photons could transform into  matter (E=mc^2)

b. Electron-anti-electron pair

i. Two photons collide to create a negatively  

charged electron and positively charged anti  

electron (AKA positron)

ii. Electron = matter, antielectron = anti matter III. Fundamental Forces

a. Gravity 

i. Increases w/ mass, holds planets, stars, and  galaxies together

b. Electromagnetic Force 

i. Depends on charge of particle

ii. Stronger than gravity

iii. Responsible for chemical and biological  


iv. Neutral charges causes this force to lose out  or not be needed

1. Large scales like planets are neutral

c. Strong Force 

i. Binds protons and neutrons together in  

atomic nuclei

ii. Operates over extremely close distances like  weak force

d. Weak Force 

i. Important role in nuclear fission and fusion

ii. Affects weakly interacting particles

1. Neutrinos

e. Electroweak Force 

i. Happens under very high temp

ii. Electromagnetic force combined w/ weak  


f. GUT Force 

i. Electroweak force combines w/ strong force ii. GUT = grand unified theories

1. Models that predict this merge

IV. Cosmic Microwave Background

a. 1965

i. Amo Penzias and Robert Wilson calibrate  

sensitive microwave antenna and find  

unexpected “noise” everywhere

ii. Physicists at Princeton predict characteristics  of radiation left over from the Big Bang

1. These two notes were compared and  

discovered the cosmic microwave  


b. Consists of microwave photons

i. Observing it we see the universe how it was  like at 380,000 years old

c. 1990’s

i. NASA Satellite tests it called cosmic  

background explorer (COBE)

ii. Wilkinson Microwave Anisotropy Probe  

(WMAP) and European Planck Satellite map  

temperature of it

d. Helium Formation

i. High Temp prevented helium fusion but after  the universe cooled and gamma rays weren’t  as powerful helium began to form

ii. Neutrons were highly outnumbered by  

photons this is why mostly hydrogen was left  


1. 75% hydrogen and 25% helium

V. The Big Bang and Inflation

a. Density Enhancements

i. Tiny quantum ripples can be characterized by a wavelength corresponding to their size

ii. Before period of inflation there was a tiny  

quantum fluctuation just before

b. Uniformity

i. Before inflation occurred, temp and density  equaled

ii. Nothing was moved by inflation but instead  

separated through expansion

c. Geometry of Universe

i. Flat (critical) 

1. Similar to how balloon functions

2. Overall density of matter plus energy is  

close to critical density

ii. Spherical (closed) 

1. Average density is greater than critical  


iii. Saddle Shaped (open) 

1. Average density is less than critical  


VI. Olbers Paradox

a. Named after German astronomer Heinrich Olbers b. So many stars block the path/line of sight from  seeing across the galaxy

i. Stars reflect the sunlight

ii. Why the sky is dark at night

c. Big conclusion is that the universe changes over  time

Chapter 18: Cosmology 

I. Cosmological Principle

a. Universe is homogenous (uniform)  

b. Looks the same in all directions (isotropics)

c. There is no center or preferred location

d. Extension of Copernican Revolution

II. Observational Evidence  

a. Four Expansion Models of the Universe

i. Recollapsing Universe

1. Expansion would stop and then start to  

close or act in reverse

ii. Critical Universe 

1. Gravitational attraction would not be  

strong enough to reverse it, but instead it  

would slow it down

iii. Coasting Universe 

1. Galaxies would move apart at approx.  

speed they are today

2. No force would change the speed, so it  

would “coast”

iv. Accelerating Universe 

1. Expansion would accelerate with time

2. Galaxies would continue to move apart  

from each other

3. Best model to describe the universe

b. Age of the Universe

i. Models that explain temperature variations in  

the cosmic microwave background can predict  

age of universe

ii. Oldest stars appear to be 13 billion years old, so it has to be about 14 billion years old

III. Dark Matter

a. Name of unseen mass whose gravity governs the  observed motions of stars and gas

b. Evidence of it

i. Total mass of galaxy is much larger than mass  

of its stars so it must be from surrounding  


c. WIMPs

i. Undiscovered subatomic particles in dark  


Chapter 19: Life in the Universe

I. Evidence for other Planetary System

a. Jupiter’s moons Europa, Ganymede, and Calisto;  Pluto; Neptune’s moon Triton are all places that  subsurface lakes and oceans of water were found on i. Life needs

1. Source of nutrients, energy, and liquid like  


II. Stars that life could exist on

a. Solar like stars (F, G, K) 

i. Large habitable zones

ii. Long lived

b. Orphan Planets 

i. Do not orbit star

ii. Have thick enough atmospheres

III. Catastrophes that spark life

a. Killer Comets and Asteroids

IV. Detecting Extraterrestrial Intelligence


i. Search for extraterrestrial intelligence

ii. Detects signals, like radio or laser  

communications, from outer civilizations

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