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33 Cards in this Set
- Front
- Back
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Stellar Parallax
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the apparent shift of an object relative to some distant background as the observer's point of view changes
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Parallax Angle
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Half of the min angle a star appears to be displaced due to Earth's rotation
The farther away the object is, the smaller the parallax angle |
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Distances
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60 arcsec = 1 arcmin (60"=1')
60 arcminute = 1 degree (60'=1 degree) 1 arcsec = 1/3600 deg (1"=1/3600 degrees) = Thickness of a credit card, one football field away 1.5 arcsec is very small The parsec (parallax second) The distance of an object that has a parallax angle of 1 arcsec from earth. Distance (pc) = 1/parallax angle (arcsec) There are ~30 stars within 4 pc of the Sun |
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Telscope Resolution of Parallax
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Ground based telescope limit: ~30 pc
Adaptic optics limit: ~120 pc Hipparcos (High-precision Parallax Collecting Satellite) limit: ~200 pc (measures parallax in milliseconds of arc) |
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Star formation
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If gas pressure > gravity, material remains dispersed in a cloud (nebula)
If gravity > gas pressure, the cloud contracts and collapses (star formation begins) |
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Gravitational attraction
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Increases as the number of atoms (the mass of the cloud) increases
For a cool (~100 K) cloud, need about 10^57 atoms for gravitational attraction to prevent dispersion. |
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How does collapse begin?
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Must be triggered by an external event
Collision between two nebulae A shock wave from star formation A shock wave from a supernova Dense cores too cold to resist gravity |
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Interstellar cloud
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Temp: ~10 K (very low)
Size: Huge, 10 s of pc across Density: ~10^9 particles/m^3 This is very low density, equivalent to the best vacuum achieved in scientific labs. Must be massive enough (many times mass of sun) to contract/collapse Molecular cloud near M20 Concentrations of gas suggest that collapse may be beginning A Doppler shift in absorption lines will indicate contraction |
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Interstellar cloud collapse
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Accompanied by fragmentation (into 100s to 1000s of contracting pieces ) due to instability
Results in formation of multiple stars |
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Cloud fragment
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Temperature: ~10K (still low, due to radiation), but center heats up
Volume: Up to ~100x the size of the solar system Density: ~10^12 to 10^18 particles/m^3 Mass: 1-2 solar masses |
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Protostar
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Hot dense center of a cloud fragment
A photosphere has formed on the fragment (i.e. fragment is opaque) Gravity > gas pressure, so radius decreases while its density increases The Orion nebula: a molecular cloud surrounds the emission nebula “Knots” of protostars in the cloud Intense radio emissions may be the protostars and nearby gas and dust cloud could become planets |
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Protostar evolution
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Continues to shrink and heat, but not hot enough for nuclear fusion reactions
Highly luminous (~100 Luminosity) due to the release of gravitational energy, not radiation May be plotted on the H-R diagram As it evolves, its luminosity decreases and temperature increases |
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T-Tauri star
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G, K or M-class protostars exhibiting violent surface activity and strong stellar winds.
Strong jets of matter (up to 40% of total mass) ejected. Named after first protostar observed at this stage. After T-Tauri phase, the rate of contraction slows down and the rate of stellar evolution slows down |
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Main Sequence star
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After ~10 million years of contracting
Central temperature is now high enough for nuclear fusion to occur T increased and hydrogen fusion begins 4 H nuclei → 1 He nucleus Hydrostatic equilibrium established The main sequence is reached when H is burning (fusing) in the core and the star is in hydrostatic equilibrium (stable) The main sequence is the end point of the “prestellar” evolutionary track Stars do not evolve along the main sequences Stellar mass varies along the main sequence >85% of known stars are low-mass (<1 M), main sequence stars |
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High mass stars
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Form from high-mass cloud fragments
Evolve more quickly Spend less time on the main sequence Ex. O class stars form in ~1 million years |
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Low mass stars
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Form from low-mass cloud fragments
Evolve more slowly Spend more time on the main sequence Ex. M-class stars form in ~1 billion years |
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The zero-age main sequence
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The region of the H-R diagram where stars (of any mass) plot at the onset of hydrogen fusion in their core. Predicted by theory. As fusion proceed, stars move off the ZAMs
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Failed stars: Brown dwarfs
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A low mass cloud fragment
Gravitational force is not high enough to compress and heat the star’s core Reaches hydrostatic equilibrium before nuclear fusion begins Small faint and cool objects → difficult to detect |
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Star cluster
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A group of stars that formed at the same time from the same cloud of ISM. The stars differ in mass but are roughly the same.
- Age - Chemical composition - Distance from earth |
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Open clusters
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a loose collection of hundreds to thousands of stars, formed from one molecular cloud
Stars span a few pc and may disperse over time. Mostly young stars (M45, in Taurus) The Pleiades open cluster ~ 100 Ma old ➢ 3000 stars ➢ Includes brown dwarfs |
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Globular clusters
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Tightly bound, spherical collection of thousands or millions of stars.
Stars span ~50 pc and are gravitationally bound (do not disperse) Mostly old stars Ex. Omega Centauri 10 million stars 150 ly in diameter 5000 pc from Earth Features: Globular clusters Lack upper-main-sequence stars Massive stars are no longer on the main sequence All globular cluster stars are >10 Ga |
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Scientific model criteria
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Model must fit the observations
Model must make verifiable predictions Model must be aesthetically pleasing |
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90% of a star’s life is spent on the main sequence (~10 billion years for a Sun-like ~1 M, star) Requirements:
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• Hydrogen burning (nuclear fusion) occurs in the star’s core - Requires T of 10^7 K
• Hydrostatic equilibrium is maintained (gravity and pressure are balanced) At birth, mostly hydrogen and a little helium After 5 billion years, helium is forming and hydrogen is burning off After 10 billion years, more helium is forming and is starting at the core Eventually… All the H in the core is consumed The star’s nonburning core is now He-rich; it shrinks and heats H fusion continues outside the core The pressure-gravity balance shifts, and the star leaves the main sequence |
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A stars size (diameter or radius) changes
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Begins in equilibrium
If a star heats up… …it expands…. …to regain equilibrium → Red giants |
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Red Giant (AGB)
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Expanded big star; cool.
When H is depleted the core The star’s core is now He-rich (H poor) Core T is not high enough (10^8) for He fusion, so star’s core is nonburning. Its not at that temperature now, but it needs to be. H fusion 10^7 K (H → He) He fusion 10^8 K (He → C) C fusion high K Gravity > pressure, so the core shrinks and heats Hydrogen-shell-burning occurs (outside the core) Hydrogen shell burning H burning increases gas pressure Outer layers of the star expands The core has shrunk and the shell has expanded Expansion causes cooling → (right on H-R diagram) Star’s luminosity increases as expansion continues Star becomes sub giant then a red giant |
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Red giant star properties
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Highly lumionous: 100s to 1000s of times original (main-sequence) luminosity
Very large : ~100 M |
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H-R star types and roman numerals
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la: bright supergiants
lb: supergiants II: bright giants III: giants IV: subgiants V: main sequence stars |
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The helium flash
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The red giant’s core continues to shrink and heat; T eventually increases to 10^8 K
Onset of He fusion is violent – a runaway explosion causes the core T to rise sharply. Within a few hours, the energy causes the core to expand and the stars to cool The onset of He fusion causes rapid expansion of the star: Star cools and luminosity goes down and surface temperature goes up. (down and to the left on HR diagram) Star moves out of the red giant region When equilibrium is reestablished the core burns He into C Ends evolution among the red giant branch |
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Red Giant
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He and H burning causes expansion
Nonburning C core shrinks and heats If T reaches 6 x 10^8 K, C fusion occurs For a solar mass (~1 M) star, this wont happen |
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Death of a solar mass star
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Star turns into a planetary nebula
Envelope is cool, expanding at 10 s of km/s Exposed core is luminous, shrinking and heating (this ionizes surrounding gases) Near the end of the red giant’s life Fusion reactions may produce heavier elements (C, O, Ne, Mg…) |
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Planetary Nebula
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“Planetary” name is misleading (but historic)
Compare to emission nebulae Similar mechanism (ionized, glowing) Different origin and evolutionary stage (birth vs. death) Gas cloud gradually disperses, leaving behind a central star (white dwarf) Dense matter Core density increases to ~10^10 kg/m^3 The Pauli Exclusion Principle: Electrons behave as incompressible spheres (Two electrons cannot exist at the same place at the same time) When core is ~size of Earth, pressure stops contraction and core cools |
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White Dwarf
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11. White Dwarf
C core becomes visible; shines due to stored heat (no nuclear fusion) Luminosity decreases dramatically The HST has verified existence of white dwarfs Planetary nebula → White dwarf |
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Black dwarf
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White dwarf cools to become a black dwarf
As T decreases, white dwarf → yellow → red Gets colder (ultimately to absolute zero) denser and dimmer Core remains tightly packed – roughly the size of Earth |
