astronomy 100 exam 3

Question 1

stellar evolution: what is stellar evolution?

Answer 1

-The field of stellar evolution describes and explains the changes that individual stars exhibit as they age

Question 2

stellar evolution: what is a nebula?

Answer 2

• any cloud of gas and/or dust in space
  -not like clouds on Earth

Question 3

stellar evolution: what is gravitational contraction?

Answer 3

• when gas is cold enough it can contract
• T ≈ 5 – 15 K (– 450 °F)

Question 4

stellar evolution: what is a protostar?

Answer 4

• when gas contracts, the rotation of gas increases and flattens out forming a disk, the center of the disk turns into a protostar

Question 5

stellar evolution: what is a Pre Main Sequence Star ( PMS)?

Answer 5

• A protostar becomes a pre main sequence star (PMS) star when mass accretion end
• a PMS star continues to contract, and therefore
  
  • temperature continues to rise
  • density continues to rise

Question 6

stellar evolution: when does fusion begin?

Answer 6

• once the center of the PMS star is hot enough, hydrogen fusion begins
• T core ≈ 107 K (18 million °F)
• note: the types of fusion reactions depend on the mass of the PMS star

Question 7

stellar evolution: what are open clusters ?

Answer 7

• open clusters are young associations of stars and typically have many hot stars
  ⇒ very luminous in ultra-violet (UV) light

Question 8

stellar evolution: what are H II regions?

Answer 8

• UV light from the hot stars is able to ionize hydrogen atoms, producing H II regions

Question 9

stellar evolution: what defines a main sequence stars?

Answer 9

• thermonuclear fusion of H → He in
  the core
  → can be either proton-proton chain or CNO-cycle
• hydrostatic equilibrium
  → pressure and gravity are balanced

Question 10

stellar evolution: what does a stars main sequence life time depend on? How much fuel do high and low mass stars consume and how do they consume their fuel?

Answer 10

• a star’s main sequence lifetime depends on how long hydrogen fusion lasts in the core
• high mass stars have a lot of fuel, but consume it rapidly
  -low mass stars consume their fuel much more slowly

Question 11

stellar evolution: what is the lifetime formula and the main sequence lifetime in years?

Answer 11

• lifetime= amount of fuel / rate of consumption
• T(ms)= 10^10 / M^ 2.5

Question 12

stellar death: what are the three categories that correspond to the main sequence once a star dies?

Answer 12

1. low mass stars (M > 0.5M)
   - these stars include late K and M spectral types
   - lower main sequence
2. intermediate mass stars ( 0.5M < M < 8M)
   - includes late B to early K
3. high mass stars ( 8M < M)

Question 13

stellar death: what is happening when fusion is taking place and stops in a low mass star? what happens to the temperature and density?

Answer 13

-when hydrogen fusion stops, the star’s entire mass
has been converted into helium
- when fusion stops, equilibrium is lost, and gravitational contraction resumes
- temperatures increase
- densities increase

Question 14

stellar death: how do low mass stars stop contracting? what is the star classified as after?

Answer 14

• low mass stars continue to contract until the density becomes so great (≈106 g/cm3) that electron degeneracy is achieved, which stops the contraction
• once the low mass star is dead, it is considered a helium white dwarf

Question 15

stellar death: what is the pauli exclusion principle?

Answer 15

• pauli exclusion principle: no two identical fermions
  (e.g. electrons, protons, neutrons) can occupy the same
  quantum state (e.g. energy) at the same time

Question 16

stellar death: what is the formula for normal gas pressure? what does pressure for normal gas vary with?

Answer 16

• PV= Nkt
  -p: pressure
  -v: volume
• N: number of particles
• k: temperature ( kelvin )
• t: boltzmann constant
• normal gas pressure varies with temperature

Question 17

stellar death: what is the formula for degenerate gas? what does degenerate gas vary with? what is the typical density for degenerate matter?

Answer 17

• P = p^ 5/3
• P: pressure
• p: density
• degenerate gas depends on density
• the typical density for degenerate matter is 10^6 g/cm^3

Question 18

stellar death: what is happening to an intermediate mass star as fusion occurs? what is happening to the temperature and density?

Answer 18

• as fusion continues throughout the star’s main-sequence lifetime, eventually H in the star’s core is exhausted and, hydrogen fusion continues in a shell surrounding the contracting He core
• hydrogen fusion stops and equilibrium is lost ⇒ gravitational contraction of the core resumes
• temperature rises
• density rises

Question 19

stellar death: what is en envelope?

Answer 19

-envelope: all interior parts of the star outside of the core
- increasing pressure causes the envelope to expand
- envelope cools and appears more red

Question 20

stellar death: what is the sub giant phase?

Answer 20

• also known as luminosity class IV
• a star’s subgiant phase is a period of transition following its main sequence phase as core contraction continues, eventually temperatures in the core become hot enough for helium fusion to begin the triple alpha process
  -triple alpha process: 3 He → Carbon + Energy
• the results produce oxygen: C + He → O + Energy

Question 21

stellar death: what is a red giant? what are the characteristics of a red giant?

Answer 21

• an intermediate mass stars is classified as a red giant once core fusion stops and equilibrium is sustained
• is characterized by core He fusion, shell H fusion, extended envelope
  -as a red giant, a star’s stellar wind increases by 10^7 ×
  ⇒ Red giant wind, with ∆M ≈ 10^6 M per year

He fusion lasts from ≈ 106 years to 109 years depending
on the mass of the star

Question 22

stellar death: what is a red giant? what are the characteristics of a red giant?

Answer 22

• an intermediate mass stars is classified as a red giant once core fusion stops and equilibrium is sustained
• is characterized by core He fusion, shell H fusion, extended envelope
  -as a red giant, a star’s stellar wind increases by 10^7 ×
  ⇒ Red giant wind, with ∆M ≈ 10^6 M per year
• he fusion lasts from ≈ 106 years to 109 years depending on the mass of the star

Question 23

stellar death: what occurs inside a red giant ( in order)?

Answer 23

• he fusion to continue in a shell around C/O core
• h fusion to continue in a shell around He-fusion shell
• total luminosity of star’s interior causes envelope to
  expand even more, and cool
• cooling temperature → drives luminosity down
• increasing radius → drives luminosity up

Question 24

stellar death: what is the classification of a star that was a red giant and moved up and right on the H-R diagram?

Answer 24

• red supergiant ( class luminosity l )

Question 25

stellar death: what are the characteristics of a red supergiant?

Answer 25

-aka an Asymptotic Giant Branch (AGB) star - radius of a Red Supergiant is ≤ 1500 R (or ≤ 7 AU)! - when the Sun becomes a supergiant, R > 1 AU

Question 26

stellar death: how does a red super giant become a planetary nebula?

Answer 26

-as the envelope expands to become a supergiant, it becomes gravitationally unbound, and is lost to space. the detached expanding envelope becomes a planetary nebula

Question 27

stellar death: what is a planetary nebula?

Answer 27

- planetary nebula: expanding shells of gas ejected by dying intermediate-mass stars; “powered” by UV light from exposed stellar core and by shock fronts from collision with ambient interstellar medium (ISM)

Question 28

stellar death: what elements are planetary nebulas composed of? what happens to the density of the nebulas as they expand?

Answer 28

- planetary nebula composition is mostly H and He, but they are enriched with the products of fusion: C, N, O, S, and others - as the planetary nebula continues to expand, their densities decrease, and they dissipate into the interstellar medium

Question 29

stellar death: what happens after the the nebula disputes into the interstellar medium? what does the nebula become?

Answer 29

- all that remains is the core of the progenitor star, exposed directly to space, which is now a carbon-oxygen white dwarf

Question 30

stellar death: what are the characteristics of a carbon oxygen white dwarf?

Answer 30

- temperature (T) ≈ few × 10^5 K (initially) - radius (R) ≈ 1 R⊕ - density (ρ) ≈ 106 g/cm^3 - as the white dwarf cools, it slowly fades into obscurity, the white dwarf is supported by electron degeneracy pressure, they have a mass limit

Question 31

stellar death: what is the chandrasekhar limit?

Answer 31

- chandrasekhar limit: M < 1.4M - if the mass is ≥ 1.44 M then gravity is stronger than electron degeneracy and the white dwarf will collapse - this can happen as a result of mass accretion onto a white dwarf in a binary star system

Question 32

stellar death: what is a type 1a supernova?

Answer 32

- if a white dwarf collapses, the temperature rapidly rises, but pressure does not increase, all of the carbon & oxygen undergoes simultaneous fusion, and the white dwarf explodes as 1a type supernova

Question 33

stellar death: what is a characteristic a of type 1a supernova? what happens to the white dwarf after the type a1 supernova explosion?

Answer 33

- total energy released in the explosion ≈ 2 × 10^51 ergs in a few seconds, and since 1 L ≈ 4 × 10^ 33 ergs/ - the white dwarf is completely destroyed in the Type Ia supernova explosion, its mass returned into interstellar space as an expanding supernova remnant (SNR)

Question 34

stellar death: what are the characteristics of high gas stars?

Answer 34

- these stars have the shortest main sequence lifetimes - after exhausting their core hydrogen, the core undergoes multiple phases of contraction, getting ever hotter, and triggering the fusion of heavier elements - the final product of fusion is Iron (Fe) - the Fe core contracts and is supported by electron degeneracy pressure

Question 35

stellar death: what is stellar death and when does it occur?

Answer 35

- when the core mass > 1.4 M, degeneracy can no longer support the weight of the core, and the core begins a rapid collapse - in one second the temperature reaches 10 billion °F ⇒ surge of γ-rays begins to break down Fe nuclei

Question 36

stellar death: when does a type 2 supernova explosion occur?

Answer 36

- when the shockwave reaches the star’s photosphere the star explodes in a type 2 super - the total energy released in the core collapse is in excess of 10^ 53 ergs – 100 times more than the energy released in a Type Ia SN

Question 37

stellar death: what is the supernova remnant?

Answer 37

- SNRs from Type II SNe are rich in H and He, with many heavy elements - SNR expansion velocities are typically a few thousand km/s, and produce shock waves through the interstellar medium (ISM) ⇒ this can trigger new star formation

Question 38

collapsed objects: what are neutron stars and their characteristics?

Answer 38

- if the progenitor star’s mass is below 20 M, the collapsed core survives as a neutron stars - characteristics 1.) composed almost exclusively of neutrons, except for a thin crust of atomic nuclei and free electrons 2.) supported by NEUTRON DEGENERACY PRESSURE 3.) average densities of about 4 – 6 × 1014 g/cm3 4.) radii of about 10 km (6.2 miles) 5.) observed masses 1.1 M ≤ M ≤ 2.7 M 6.) surface gravities ≈ 200 billion times Earth 7.) magnetic field > trillion times Earth’s 8.) surface temperatures ≈ 1 million °F

Question 39

collapsed objects: what to neurons stars do? what happens to it's rotation?

Answer 39

- neutron stars are stellar remnants – dead stars; they do not generate energy but rather radiate energy as they slowly cool - during its collapse, a neutron star’s rotation increases dramatically due to conservation of angular momentum

Question 39

collapsed objects: what to neurons stars do? what happens to it's rotation?

Answer 39

- neutron stars are stellar remnants – dead stars; they do not generate energy but rather radiate energy as they slowly cool - during its collapse, a neutron star’s rotation increases dramatically due to conservation of angular momentum

Question 40

collapsed objects: what is a pulsar? how are they most commonly detected and how can a x-ray pulsar be seen?

Answer 40

- pulsar: particles accelerated along the magnetic axis emit highly collimated radiation, i.e. a beam of radiation - pulsars are most commonly detected at radio wavelengths - if there is mass accretion onto the neutron star, an x-ray pulsar might be seen