Download PHYS 175 Fall 2014 Final Recitation Ch. 16 The Sun

PHYS 175
Fall 2014
Final Recitation
Ch. 16 The Sun
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describe the fusion process taking place in the present day Sun
In a low-mass star, such as the Sun, the proton-proton chain fuses H into He. This process
releases more energy than the kinetic energy required to bring the protons close enough for
fusion to take place. This energy is carried off in the form of neutrinos and gamma rays (high
energy photons).
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describe a photon produced by fusion in the Sun’s core and how the energy from that
photon eventually escapes the Sun’s outer atmosphere
Photons released in the core (where fusion takes place) collide almost instantaneously with
other core constituents. This energy gradually flows outward, until the density of the sun
decreases sufficiently to allow for radiative diffusion of the energy. Again, the photons still
undergo many collisions, but fusion has ceased. The collisions continue near the outer layers of
the Sun, where convection (like in a boiling fluid) dominates the energy transfer and matter
flow. Once out of the convection zone, photons in the Sun’s atmosphere are free to escape into
space. The process of this energy transfer can take as much as 170,000 years, from core to
atmosphere.
Ch. 17 The Stars
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review the inverse square law for stellar intensities
Here I’m just looking for easy math that demonstrates the inverse square falloff with distance.
Some examples, when you move a distance of 4 farther away, the intensity falls off by a factor of
16, etc.
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plot the evolutionary track of the Sun on a blank H-R diagram
Ch. 18 Star Birth
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discuss the differences between dark nebulae, emission nebulae and reflection nebulae
Emission nebulae: clouds of excited gas, caused by UV stimulation from nearby O & B stars in
the nebula’s H atoms. Emissions are in the visible, red range.
Reflection nebulae: dust and gas reflect the blue light from nearby O & B stars.
Dark nebulae: star forming region, can contain a protostar. No visible emissions, but strong IR
emissions from them.
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label the following on an HR diagram: red giants, blue giants, supergiants, main
sequence, white dwarfs, red dwarfs
Ch. 19 Stellar Evolution
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what determines when a star is on the main sequence? What happens to cause it to
move off?
A star is on the main sequence when it is converting H into He. This happens by either the p-p
chain (low mass stars) or the CNO cycle (high mass stars).
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what are Cephied variable stars and why are they so useful?
These stars have a known relationship between period and luminosity. They are one of the
standard candles used on the cosmic distance ladder. Since they are found at distances that
overlap other techniques on the ladder, we know that the estimates of distance are valid (we
have calibrated the technique).
Ch. 20 Star Death
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describe the characteristics of white dwarfs and neutron stars
A white dwarf is the carbon core lefty over from a low mass star at the end of its H- and Heburning life. These objects are supported by electron degeneracy pressure, are very hot and not
very luminous due to their small size. A neutron star is smaller than a white dwarf and is made
up entirely of neutrons, supported by the neutron degeneracy pressure. Neutron stars often
have radiation jets emanating along their magnetic axes – if this axis is mis-aligned with the
spin axis, it can appear as a pulsar, when observed from a distance.
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what is the “dividing line” between the two?
The Chandrasekhar limit of 1.4M¤ governs whether or not a white dwarf core remnant will
collapse into a neutron star (above this limit, the electron degeneracy pressure is overcome by
the gravitational collapse). Above about 2-3M¤ the neutron degeneracy pressure is overcome
and a black hole is formed.
- how are the heavy elements formed?
Once Fe beings to form via fusion, the reaction consumes more energy than it produces, so
fusion ceases in the core very quickly. Massive stars can get to this stage. Elements heavier
than Fe are formed when the shockwaves from a supernova event cause fusion in the stellar
mass that has been ejected from the star’s outer layers.
Ch. 21 Black Holes
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describe gravitational redshift
This type of redshift is caused by the distortion of spacetime near a massive object. In order to
be observable, the density of the object must approach that required for a black hole. The
redshift caused by gravity becomes infinite at an event horizon, so we can never observe matter
falling in to one.
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what are the main results of special relativity?
The speed of light is the same for all observers, regardless of their relative motion. Space and
time are both aspects of a single spacetime, and the perception of that spacetime is dependent
upon the observer’s frame of reference. While special relativity only takes into account relative
motion (not gravity), it correctly predicts many observed distortions of mass, length and time.
Ch. 22 The Milky Way
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why is radio astronomy so important for observations of our own galaxy?
Gas and dust between the stars limits our ability to observe stars in the galactic plane from our
vantage point within the galaxy. Using radio telescopes, we can peer into the very galactic core.
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how does dark matter effect the observable matter in the galaxy?
Without invoking a dark matter halo, astronomers cannot explain the rotational speeds of the
stars with our galaxy (and others). As you move out from the galactic core, speeds should fall
off as Kepler predicted; however, this falloff is not observed. The dark matter halo explains the
observed speeds, but we still don’t exactly know what makes up this dark matter.
Ch. 23 Galaxies
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describe the basic characteristics of elliptical, spiral and irregular galaxies
Elliptical: among the most massive galaxies, few younger stars (Pop. II and old Pop. I), about
20% of observed galaxies, stars formed quickly
Spiral (barred and grand design): most common (~77%) type observed and span less mass
scales, star formation active in spiral arms, supported by density waves, have dark matter halos,
central bulges host elliptical-like star populations and supermassive black holes
Irregular: least commonly observed (~3%), host star formation, possibly the result of galactic
collisions
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how does the Hubble Law work? when does its use become more uncertain?
Discuss the image above, and how the certainly falls off a bit (but not badly) as the distance
increases (this why ages are discussed in redshifts (known precisely), rather than the uncertain
actual age, which depends on the value of Ho).
Ch. 24 Quasars and Active Galactic Nuclei
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what is an AGN?
what are its characteristics?
Quasars are one class of AGN. They are essentially a combination of supermassive black hole at
the center, accretion disk and lobes of matter blown off in a dipolar fashion along the oppositely
directed magnetic field lines. They may or may not have active radio emissions and may reside
in spiral or elliptical host galaxies.
Ch. 25 Cosmology
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what is the cosmic microwave background? why is it important?
It is the expected blackbody radiation remnant from the Big Bang event that occurred ~13.7
billion years ago. It is at a peak temperature of 2.725 K, and is present in all directions when
looking with a microwave-tuned radio dish. There are fluctuations in the radiation, but these
are very small compared to the background. This remnant radiation gives us string clues as to
the nature of the very early universe and its subsequent evolution.
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discuss the following plot and what it means for the expansion of the universe
This plot was the winning evidence for the 2011 Nobel Prize in Physics. It shows that the
expansion of the universe is actually speeding up when compared to earlier epochs. The data
are best fit with a line that clearly (despite some uncertainty) puts the emphasis on an increasing
rate of expansion.