Astronomy Second Midterm - Chapt. 13

Front 1

In a spiral galaxy, what are dark lanes? What are the pink regions?

Back 1

Dark Lanes = Dust

Pink Regions = Ionized Hydrogen (ex. stars would be inside these areas of ionized H)

Front 2

What is Interstellar Medium (ISM)? What types of physical states can ISM be found in?

Back 2

Gas and dust in a galaxy that is not in stars (or planets)

-hot ionized gas
-dust
-cool molecular gas

Front 3

In what regions is star formation likely to occur?

Back 3

In regions of cool molecular gas (ISM) - Giant Molecular Clouds

-massive, but large regions
-some regions are cool and dense (favours gravitational collapse - gravity with no minimal thermal pressure)
-interplay of gravity (trying to collapse it) and thermal pressure (trying to expand it)

Front 4

What type of light would you want to use to look at the MW Galaxy? Why?

Back 4

Infrared or radio telescopes (or submilimeter) , b/c it has a long enough wavelength to get through the dust from the ISM found in the galaxy (visible light is about the same size as dust, and therefore blocks out visible light very well) - long wavelengths can penetrate dust

-Gas is very cool, it emits at long wavelengths

Front 5

Are stars currently forming in the MW?

Back 5

MW is about 10 billion years old

-MUST BE! Since we can see O-type main sequence stars (that have a short main sequence life span) so they must have formed recently around a few million years ago

Front 6

What is the nearest active star-forming region?

Back 6

The Orion Nebula (A constellation in the MW galaxy where stars are currently forming)
-Ex. Trapezium Cluster in Orion Nebula has produced 5 bright O-type stars

Front 7

How many stars form in the MW each year? Is this what we expect? Why/why not?

Back 7

1M Sun/year turned into stars (about 1% of stars that we expect to form)

Bipolar outflows eject material out along rotational axis of spinning disks in molecular clouds

Front 8

What are the final stages of star formation?

Back 8

-rapid collapse stops when gas is dense enough to trap its own radiation (balance b/w gravity and thermal pressure)

-protostar slowly contracts releasing gravitational energy (so gets denser and hotter)

-core reaches 10 million degrees (get H-fusion)

-Stability achieved (equilibrium) - main sequence star born

Front 9

What is the upper limit and lower limit of stellar birth masses?

Back 9

-Upper Limit = 100M Sun (so luminous that pressure of light drivers outer layers away)

-Lower Limit = .08M Sun (H fusion is not possible b/c will never reach 10 million in core - collapse is halted by electron degeneracy pressure)

Front 10

What is a brown dwarf?

Back 10

Smaller than a star, but bigger than a Jovian planet
-have deuterium fusion but not H-fusion (therefore not a star)
-support their weight by electron degeneracy pressure

Front 11

What is electron degeneracy pressure?

Back 11

-a consequence of the exclusion principle (no two electrons can occupy the same quantum state)
- comes into play when high densities of electrons are vying fro the same quantum state
-independent of temperature
-ex. people and chairs in a room

Front 12

What is considered a low mass star? What are the stages a low mass star will go through?

Back 12

< 2M Sun

1. Pre-main sequence - collapsing from a large cloud of gas

2. Main sequence

3. Red Giant (shell H-fusion, core contraction)

4. He Flash - starts of He core fusion (horizontal branch on H-R diagram)

5. Core He Fusion (He-C)

6. "Double Shell" Red Giant (ABG star) - H shell burning, He shell burning (inert C in core)

7. Planetary Nebula (White Dwarf) - cool, stable, small

Front 13

Why do stars evolve?

Back 13

B/c of the nuclear fusion in their cores

Front 14

Why will the luminosity of the Sun go up throughout its lifetime? What will this great increase in luminosity lead to?

Back 14

B/c the fusion of heavier elements in the sun will drive its luminosity (greater temp of Sun will lead to Runaway Greenhouse Effect on Earth)

Front 15

When the Sun runs of of H in core, will it simply be able to start using He?

Back 15

NO, the sun will have its core contract to release gravitational energy to heat it up to 100 million degrees in order to fuse He (3 He --> C)
-He fusion is initially unstable

This contraction of the Sun will lead to a large expansion in the Sun's radius (turning it into a red giant)

Front 16

In the ascent to the Red Giant branch, the core contracts so much that the gas has to be partially supported by what?

Back 16

Electron Degeneracy Pressure (which depends on density and NOT temperature)
-runaway He fusion energy is put towards lifting degeneracy and expanding the core (makes it cooler and more stable for He-->C)

Front 17

At what element will fusion stop in the Sun?

Back 17

Carbon (will never reach the temperature needed for C fusion in the core

Front 18

Why do we see thermal pulses in stars?

Back 18

Very thin shells with nuclear reactions are very unstable so can see pulses in their stellar luminosities

Front 19

Why do planetary nebula have different shapes?

Back 19

Due to their thermal pulses ejecting mass

-Fusion in the H, and He shells becomes unstable (due to their thinness) - during AGP
-the envelope is ejected and forms a planetary nebula
-the exposed core is hot, but completely supported by degeneracy pressure (white dwarf is stable at any temp)

Front 20

What is the Chandrasekhar limit?

Back 20

A fundamental limit to how massive white dwarfs can be (1.4 Solar Masses)
-If greater than this will become unstable and collapse (supernova)

-Stars with Solar Masses >8/9 on main sequence will end up with cores (white dwarfs) greater than this limit, so won't leave behind white dwarfs

Front 21

What will happen to high-mass stars at the end of their lives?

Back 21

-Can access all nuclear fuels, leaving inert iron core at end of their lives
-iron core is too massive, so collapses and supernova explosion occurs

Front 22

What does the evolution of a high mass star look like on a H-R diagram?

Back 22

High mass stars will evolve of the H-R diagram off the main sequence horizontally to the right (so they get cooler, not more luminous, and not more massive)

Front 23

What is the CNO cycle? In what type of stars does it occur in?

Back 23

Massive stars on the main sequence can turn H--> He using the CNO cycle

-uses a C catalyst to produce He (C + H --> O --> O16 --> C + He)

Front 24

What happens after H is exhausted as nuclear fuel in massive stars?

Back 24

No Red Giant Phase, No degenerate electrons

Carbon fusion begins right away

Ex. End up with an "onion" like structure to the star with shells of different nuclear fusions

Front 25

As you go through the nuclear fuels up to Fe (most tightly bound atomic nucleus), the amount of time the fuel can be "burned" gets progressively?

Back 25

Faster!

-In a 25 Solar Mass star, takes 7 Mill years to burn H, takes only 1 day to burn last element Si

Front 26

Once nuclear fusion has ended with Fe, can massive stars produce energy via nuclear fission?

Back 26

NO

-there isn't enough radioactive heavy elements in massive stars & no free neutrons at center of star for fission to occur

Front 27

Why can't white dwarfs more massive than the Chandrasekhar limit be supported by electron degeneracy pressure?

Back 27

Electrons can only move as fast as the speed of light (they cannot go faster to create a pressure to support these massive cores, so the cores collapse)

Front 28

What is the process of the core-collapsing into a supernova for a high-mass star?

Back 28

-iron core (> billion degrees) leads to Fe nuclei broken apart by high energy gamma rays
-tons of neutrinos are produced/escape, which cools the core and allows energy to escape
-in a fraction of a second the core collapses
-releases 100 times the energy (gravitational potential energy) our Sun will radiate in its entire life

Front 29

What happens after the supernova? (for stars with ~25 Solar Masses to begin with)

Back 29

Electron degeneracy pressure halts the collapse when the core reaches the size of an asteroid (20 km) --> forms a Neutron Star

-the core tends to collapse beyond its stable radius, so it "bounces back" to the radius that electron degeneracy pressure can support

-also the neutrino pressure exerted pushes neutron star radius out

Front 30

What is considered a metal by astronomers? How are all metals created? What is the metallicity of the Sun and how will it compare to future generations of stars?

Back 30

-Anything heavier than He
-Metals are created from H & He through actions of our stars
-Metallicity of Sun is 2%
-Future gens of stars will have greater metallicity

Front 31

In terms of the abundance distribution of elements in the galaxy, we see a peak at Fe...why? If Fe is the end state of nuclear fusion, why do we get elements heavier than Fe in the galaxy?

Back 31

Fe is the end state of fusion in stars (so we see a peak in its abundance)

Heaviest naturally occurring elements were made in Supernovas
-Supernovas produce lots of free neutrons, which can be combined to an existing nucleus to make it heavier (but it takes only about 11mins for neutrons to decay)
-So neutrons are added in a R-process (rapid neutron capture)
-Neutrons added very quickly in a zig-zag patter (add some neutrons, a few decay, so add some more, and so on)

Front 32

Supernova in the MW are very rare or common?

Back 32

Rare...b/c massive stars are very rare

About 1 every 50-100 years

Front 33

In which stellar populations do supernova occur?

Back 33

Very young star clusters, near the gas and dust of their formation & intermediate-age stellar populations

Front 34

There is a class of observed supernova in which they do not fit our ideal picture of a supernova (occur in older stellar pops, ejected envelopes are devoid of H, ad peak brightness curves differ)...which stars are these?

Back 34

Binary Stars!
-Gravitationally bound pair of 2 or more stars
-If they have different masses, they will evolve at different rates
-high mass star evolves first, and over fills its Roch load, so get mass transfer to lower-mass star (mass transfer to white dwarf, may push it over Chandrasekhar limit, and supernova)