astronomy 100 exam 3 Flashcards
stellar evolution: what is stellar evolution?
-The field of stellar evolution describes and explains the changes that individual stars exhibit as they age
stellar evolution: what is a nebula?
- any cloud of gas and/or dust in space
-not like clouds on Earth
stellar evolution: what is gravitational contraction?
- when gas is cold enough it can contract
- T ≈ 5 – 15 K (– 450 °F)
stellar evolution: what is a protostar?
- when gas contracts, the rotation of gas increases and flattens out forming a disk, the center of the disk turns into a protostar
stellar evolution: what is a Pre Main Sequence Star ( PMS)?
- 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
stellar evolution: when does fusion begin?
- 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
stellar evolution: what are open clusters ?
- open clusters are young associations of stars and typically have many hot stars
⇒ very luminous in ultra-violet (UV) light
stellar evolution: what are H II regions?
- UV light from the hot stars is able to ionize hydrogen atoms, producing H II regions
stellar evolution: what defines a main sequence stars?
- thermonuclear fusion of H → He in
the core
→ can be either proton-proton chain or CNO-cycle - hydrostatic equilibrium
→ pressure and gravity are balanced
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 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
stellar evolution: what is the lifetime formula and the main sequence lifetime in years?
- lifetime= amount of fuel / rate of consumption
- T(ms)= 10^10 / M^ 2.5
stellar death: what are the three categories that correspond to the main sequence once a star dies?
- low mass stars (M > 0.5M)
- these stars include late K and M spectral types
- lower main sequence - intermediate mass stars ( 0.5M < M < 8M)
- includes late B to early K - high mass stars ( 8M < M)
stellar death: what is happening when fusion is taking place and stops in a low mass star? what happens to the temperature and density?
-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
stellar death: how do low mass stars stop contracting? what is the star classified as after?
- 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
stellar death: what is the pauli exclusion principle?
- 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
stellar death: what is the formula for normal gas pressure? what does pressure for normal gas vary with?
- PV= Nkt
-p: pressure
-v: volume - N: number of particles
- k: temperature ( kelvin )
- t: boltzmann constant
- normal gas pressure varies with temperature
stellar death: what is the formula for degenerate gas? what does degenerate gas vary with? what is the typical density for degenerate matter?
- 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
stellar death: what is happening to an intermediate mass star as fusion occurs? what is happening to the temperature and density?
- 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
stellar death: what is en envelope?
-envelope: all interior parts of the star outside of the core
- increasing pressure causes the envelope to expand
- envelope cools and appears more red
stellar death: what is the sub giant phase?
- 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
stellar death: what is a red giant? what are the characteristics of a red giant?
- 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
stellar death: what is a red giant? what are the characteristics of a red giant?
- 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
stellar death: what occurs inside a red giant ( in order)?
- 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
stellar death: what is the classification of a star that was a red giant and moved up and right on the H-R diagram?
- red supergiant ( class luminosity l )
stellar death: what are the characteristics of a red supergiant?
-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
stellar death: how does a red super giant become a planetary nebula?
-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
stellar death: what is a planetary nebula?
- 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)
stellar death: what elements are planetary nebulas composed of? what happens to the density of the nebulas as they expand?
- 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
stellar death: what happens after the the nebula disputes into the interstellar medium? what does the nebula become?
- all that remains is the core of the progenitor star, exposed directly to space, which is now a carbon-oxygen white dwarf
stellar death: what are the characteristics of a carbon oxygen white dwarf?
- 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
stellar death: what is the chandrasekhar limit?
- 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
stellar death: what is a type 1a supernova?
- 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
stellar death: what is a characteristic a of type 1a supernova? what happens to the white dwarf after the type a1 supernova explosion?
- 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)
stellar death: what are the characteristics of high gas stars?
- 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
stellar death: what is stellar death and when does it occur?
- 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
stellar death: when does a type 2 supernova explosion occur?
- 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
stellar death: what is the supernova remnant?
- 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
collapsed objects: what are neutron stars and their characteristics?
- 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
collapsed objects: what to neurons stars do? what happens to it’s rotation?
- 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
collapsed objects: what to neurons stars do? what happens to it’s rotation?
- 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
collapsed objects: what is a pulsar? how are they most commonly detected and how can a x-ray pulsar be seen?
- 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
