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74 Cards in this Set
- Front
- Back
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What is the consequence of temperature drop/rise at the core of the sun?
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Will cause the rate of fusion to decrease/ increase and then the core will contract/ expand and get hotter/colder and then when this happens it will balance out again
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How do we define apparent brightness?
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How bright a star appears to us; it is measured as the amount of power (energy per unit area) that reaches us
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What is another phrase for absolute brightness?
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Luminosity; the total amount of power the star emits into space
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What do we use to measure a star's apparent brightness?
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A CCD or detector that records how much energy strikes the surface
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What are the difficulties in detecting apparent brightness?
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(1) Making sure the detector is calibrated
(2) For ground based telescopes, accounting for light absorption by the earth's atmosphere |
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When we measure the apparent brightness of a star in visible light, what can we calculate?
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The stars visible-light luminosity
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What do the terms total luminosity and total apparent brightness refer to?
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They describe the luminosity and apparent brightness we would measure if we could detect photons across the entire EM spectrum
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What is a problem with calculating a star's luminosity from it's apparent brightness?
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The inverse square law for light works best only if starlight passes uninterrupted to the earth, so any interstellar dust that scatters or absorbs light will give skewed results
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What is the stellar parallax angle?
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Half the star's annual back and forth shift
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What is the stellar parallax formula?
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d (in parsecs)=1/p (in arcseconds)
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What is the diameter of the milkway?
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100,000 light years
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What is the range of luminosities of stars (with the sun as the standard luminosity)?
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10^-4 to 10^6 as luminous as the sun
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What is the absolute magnitude of a star?
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It is a way for describing a star's stellar luminosity. It is calculated as the apparent magnitude (on the scale devised by Hipparcos) that a star would have if it were at a distance of 10 parsecs
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What are the 4 fundamental properties of a star?
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Temperature, size, age and brightness
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Why is the surface temperature of a star emphasized over it's actual core temperature?
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Surface temperature is the only thing that can be measured
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Why is it easier to measure a star's surface temperature than it's luminosity?
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Because distance does not impact the way we measure a star's surface temp
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The color of a star is based on...
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...the portion of visible light where it emits this most photons. Where a star lies on the visible spectrum is dependent on it's temperature (more red light for cooler stars, more blue light for hotter stars)
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How do astronomers measure surface temperature?
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By comparing it's apparent brightness in two different colors of light (and seeing which is stronger, thus where it lies on the spectrum)
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What are the two ways that astronomers measure a star's surface temperature?
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Spectral Lines and Color
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Why are temperatures determined from spectral lines more accurate than those determined by color?
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Because interstellar dust can affect the color of a star
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If a star displays spectral lines of highly ionized elements, what does that tell us about the star?
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That very hot at the surface, because it takes a high temperature to ionize atoms
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If a star displays spectral lines of molecules, what does that tell us?
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The star's surface temperature is cool because molecules break apart into atoms unless at relatively cool temps
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What are the spectral types for a star?
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OBAFGKM (oh be a fine girl kiss me)
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The system that classifies a star by it's spectral type is related to what property...
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...the surface temperature of the star
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What did Cecilia Payne-Gaposchkin discover about spectral lines?
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That differences in spectral lines reflected changes in the ionization levels of emitting atoms (e.g. O stars have weak hydrogen lines because at their temp., most of the hydrogen is ionized)
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Is mass easier to measure than luminosity or temperature?
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No, it is more difficult
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What is the most dependable method for measuring a star's mass?
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Newton's version of Kepler's Third Law but it is limited by the fact that we can only use this law when we can observe one object orbiting another
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What are the different types of binary stars?
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Eclipsing binary, Visual Binary, Spectroscopic binary
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What is a visual binary star system?
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A pair of stars that we can see orbiting each other
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What is a eclipsing binary star system?
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A pair of stars that orbit in the plane of our sight and whose apparent brightness varies as one star moves in front of the other, etc.
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What is a spectroscopic binary system?
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Neither visual nor eclipsing, a pair of stars that are binary and we can detect this by observing doppler shifts in the spectral lines
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What are the differences between single and double lined spectroscopic binaries?
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When we can detect two sets of line shifting back and forth, that is doubleshifted. When we can detect only one, that is single lined
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For a binary system, what do we need to know in order to apply Newton's Version of Kepler's third law to determine mass?
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Orbital period and separation between the stars
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Why is it difficult to measure the separation between binary stars?
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Because we usually cannot measure the separation directly, astronomers need to use doppler shifts to calculate speed but doppler shifts only tell you the velocity towards or away, so that can is only part of the velocity. With eclipsing binaries, the doppler shift is very useful however
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What are the stars in the upper right called?
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Supergiants
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What are the luminosity classes?
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(I) Supergiants, (III) Giants, (V) Main sequence, 2 and 4 are in between
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What are the two ways of classifying a star?
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Spectral Type (OBAFGKM), Luminosity Class (based on luminosity but also tells about the radius)
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What is the significant correlation for star's on the main sequence?
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Between surface temperature and luminosity (and also mass)
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What variable does the rate of fusion depend on?
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A star's mass (dont know how yet)
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How do stellar masses move along the main sequence?
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Downward
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Why does luminosity depend directly on mass?
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The weight of a star's outer layers determines the nuclear fusion rate at it's core and more weight means more fusion is needed to achieve gravitational equilibrium (have to balance I think)
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What is the consequence of mass, surface temperature and luminosity being related to stars on the main sequence?
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We can estimate a main sequence stars mass just by it's spectral type
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What is the relationship between mass and temperature on the main sequence?
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Dont know (learn though)
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Why can star's remain on the main sequence for only a limited amount of time?
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Because they are born with only a small amount of core hydrogen and thus can only sustain hydrogen fusion for a short period of time
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What two reasons explain why massive stars are so rare?
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(1) They burn out quick
(2) Fewer of them are born in the first place |
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What is the most important thing to know about mass and how it relates to stars?
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The mass of star determines it's surface temperature and luminosity (and thus it's age) during it's main sequence lifetime
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How would classify giants and supergiants?
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Cool surface temperature, huge radius, very bright, they have lost most of their hydrogen and face an energy crisis as gravity crushes it
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What are white dwarfs?
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When a giant ejects its outer layers, all that remains is a dead core (in which nuclear fusion has ceased). White dwarfs are these remnants and they are hot because they are exposed cores, small but dense and massive
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What happens with variable stars?
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Their atmosphere alternates between expansion and contraction (which changes their luminosity) and energy output
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Where is the instability strip?
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Between the main sequence and the red giants
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What is the upper portion of the instability strip known as?
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Cepheid variable stars and they help us figure out distances of far away galaxies (dont know how)
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Why do stars form in groups?
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Because a single interstellar cloud has enough material to form multiple stars (so they often do)
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Why are star clusters useful?
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Because they are all about the same distance away and they all formed at about the same time (huge for astronomers)
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What are the two types of clusters and what are the main differences between them?
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Modest sized open clusters (younger stars and fewer of them) and tightly packed globular clusters (lots of old stars)
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Does a Tenant normally have a duty to repair? Are they liable to third parties who are injured?
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yes. Must maintain premises and make ordinary repairs. Must not commit waste. Liable to third parties T invited who are injured.
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What is the point at which a star cluster diverges from the main sequence?
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That is the star's main sequence turnoff point and it is categorized by it's spectral type
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What is useful about the main sequence turnoff for a cluster?
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The age of the cluster is equal to the lifetimes of stars at the main sequence turnoff
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What, at the most basic level, explains star birth?
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Physical processes that occur in gas clouds
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What are the types of gas clouds needed for stars to be born?
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Need to be cold and dense and they are called molecular clouds (allow molecules to form), typically between 10-30 K
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The interstellar medium is...
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...the gas and dust that fill the spaces between stars (composed mostly of hydrogen and helium)
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What explains differences between the chemical composition of the interstellar medium?
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Temp and density
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Interstellar dust...
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...are tiny, solid grains of elements heavier than helium found in the interstellar medium. Compose about 1 percent of the total mass
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What wavelengths of light do interstellar dust have the greatest effect on?
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Blue light more so than red because the shorter wavelengths of blue light are closer to those of interstellar dust (around UV)
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What is interstellar reddening?
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Around the edges of molecular cloud it appears redder because most of the blue light is blocked (absorbed), so it looks redder (not great explanation)
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Stars form when...
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...gravity causes a molecular cloud to contract and the contraction continues until the central object (eventually the core) becomes hot enough to sustain nuclear fusion
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What distinguishes molecular clouds from other places in the universe and explain why stars can form there?
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They are the only place where gravity can win the battle over gas pressure (because gas isnt moving that fast because its cold)
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Thermal pressure is dependent on...
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...only temperature and not density
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What is the complex explanation for why stars form in molecular clouds?
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They are much denser (force of gravity is greater) but they are cold so their thermal pressure is weaker, thus gravity can win the battle
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What happens as gravity makes a gas cloud shrink?
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The cloud converts some of the clouds gravitational potential energy into thermal energy and then this thermal energy escapes in the form of photons and doesnt halt the build up of gravity
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What is the minimum cloud size necessary for stellar formation?
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100 times the mass of the sun
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Do stars form in the turbulent or quiet sectors of molecular clouds?
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Quiet
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What role do magnetic fields play in the formation of stars?
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Magnetic fields are threaded through an interstellar cloud and force ionized particles to move around it's field, thus preventing them from interacting with other particles and slowing down the gravitational collapse
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What is the difference in chemical composition between older stars and newer stars?
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Older stars contain far fewer percentages of metals and thus their formation must have taken place at higher temperatures (around 100 K), because they did not contain many molecules and could not release their heat
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Describe the process of formation for the first stars...
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...low composition of metals and molecules, difficult to release their heat, formed in hotter conditions, tended to be massive (30-100 Msun), and then died out fast and as supernovas created the interstellar medium that allowed further star generation
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