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Astronomy 100-09 > astronomy 100 exam 3 > Flashcards

astronomy 100 exam 3 Flashcards

1
Q

stellar evolution: what is stellar evolution?

A

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

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2
Q

stellar evolution: what is a nebula?

A
  • any cloud of gas and/or dust in space
    -not like clouds on Earth
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3
Q

stellar evolution: what is gravitational contraction?

A
  • when gas is cold enough it can contract
  • T ≈ 5 – 15 K (– 450 °F)
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4
Q

stellar evolution: what is a protostar?

A
  • when gas contracts, the rotation of gas increases and flattens out forming a disk, the center of the disk turns into a protostar
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5
Q

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

A
  • 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
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6
Q

stellar evolution: when does fusion begin?

A
  • 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
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7
Q

stellar evolution: what are open clusters ?

A
  • open clusters are young associations of stars and typically have many hot stars
    ⇒ very luminous in ultra-violet (UV) light
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8
Q

stellar evolution: what are H II regions?

A
  • UV light from the hot stars is able to ionize hydrogen atoms, producing H II regions
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9
Q

stellar evolution: what defines a main sequence stars?

A
  • thermonuclear fusion of H → He in
    the core
    → can be either proton-proton chain or CNO-cycle
  • hydrostatic equilibrium
    → pressure and gravity are balanced
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10
Q

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?

A
  • 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
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11
Q

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

A
  • lifetime= amount of fuel / rate of consumption
  • T(ms)= 10^10 / M^ 2.5
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12
Q

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

A
  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)
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13
Q

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

A

-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

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14
Q

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

A
  • 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
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15
Q

stellar death: what is the pauli exclusion principle?

A
  • 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
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16
Q

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

A
  • PV= Nkt
    -p: pressure
    -v: volume
  • N: number of particles
  • k: temperature ( kelvin )
  • t: boltzmann constant
  • normal gas pressure varies with temperature
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17
Q

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

A
  • 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
18
Q

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

A
  • 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
19
Q

stellar death: what is en envelope?

A

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

20
Q

stellar death: what is the sub giant phase?

A
  • 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
21
Q

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

A
  • 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

22
Q

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

A
  • 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
23
Q

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

A
  • 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
24
Q

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

A
  • red supergiant ( class luminosity l )
25
Q

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

A

-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

26
Q

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

A

-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

27
Q

stellar death: what is a planetary nebula?

A
  • 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)
28
Q

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

A
  • 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
29
Q

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

A
  • all that remains is the core of the progenitor star, exposed directly to space, which is now a carbon-oxygen white dwarf
30
Q

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

A
  • 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
31
Q

stellar death: what is the chandrasekhar limit?

A
  • 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
32
Q

stellar death: what is a type 1a supernova?

A
  • 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
33
Q

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

A
  • 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)
34
Q

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

A
  • 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
35
Q

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

A
  • 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
36
Q

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

A
  • 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
37
Q

stellar death: what is the supernova remnant?

A
  • 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
38
Q

collapsed objects: what are neutron stars and their characteristics?

A
  • 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
39
Q

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

A
  • 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
39
Q

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

A
  • 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
40
Q

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

A
  • 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

Astronomy 100-09 (3 decks)

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