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The stars parallax angle increases as the star gets |
closer to the earth |
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How many light-years away must a star be for its parallax angle to be one arcsecond |
3.3 light-years |
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Why can’t the distances to very distant stars be measured using this method |
The parallax angle is so small that it is too difficult to measure it |
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If the orbital radius of the earth were greater than its actual value, the parallax angle to nearby stars would be |
higher |
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If the orbital radius of the earth were greater than its actual value, the maximum distance a star could be away from earth for its distance to be determined using the parallax method would be |
higher than the actual upper limit |
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Stars that have the highest apparent brightness are |
very luminous and nearby |
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massive -star supernova are not good standard candles because |
they have different luminosities ( since they have different masses) |
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Which two things are needed to determine a standard candles distance from earth |
apparent brightness and luminosity |
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About how long would it take for a radar pulse to travel round trip from earth to Neptune and back again to earth |
8 hours |
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In general, how does the velocity of a galaxy depend on its distance from the earth |
the greater the distance, the higher the velocity |
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Relationship between hubble’s constant and the age of the universe |
the higher Ho, the younger the universe |
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Astronomers believe that nearly all galaxies are moving away from us because |
the space in between the galaxies is expanding |
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Suppose we lived in a universe that was shrinking rather than expanding. If you measured the velocities of many galaxies, what would you find |
all galaxies are moving toward you |
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One reason hubble’s constant isn’t exactly known today is |
the exact distance of a galaxy is hard to measure |
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Which of these galaxies would you most likely find at the center of a large cluster of galaxies |
a large elliptical galaxy |
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What baseline distance must we know before we can measure parallax |
the earth-sun distance |
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Which cosmic distance measurement techniques are considered standard candle techniques |
-main-sequence fitting -white dwarf supernovae -Cepheids |
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Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true |
We would not be able to measure distances of distant galaxies |
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Why do we use Hubble’s law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases |
We have not observed white dwarf supernovae in most galaxies |
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We determine the distance of a cepheid by |
Determining its luminosity from the period-luminosity relation and then applying the inverse square law for light |
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Which kind of object is the best standard candle for measuring distances to extremely distant galaxies |
a white dwarf supernova |
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Why do virtually all galaxies in the universe appear to be moving away from our own |
a galaxy in the local group |
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Which of these galaxies is most likely to be oldest |
a galaxy in the local group |
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When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching earth, we are seeing that galaxy as it was when the universe was |
4 billion years old |
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Which of these items is a key assumption in our most successful models for galaxy formation |
some regions of the universe were slightly denser than others |
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the luminosity of a quasar is generated in a region the size of |
the solar system |
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Why do astronomers hypothesize that a massive black hole lies at the center of M87 |
A very small region at the center of M87 releases an enormous amount of energy |
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Disk component of spiral galaxies |
-Stars of all ages -Many gas clouds |
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spheroidal component of spiral galaxies |
-Bulge and halo -Old stars – few gas clouds |
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why does ongoing star formation lead to a blue-white appearance |
short-lived blue stars outshine others |
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barred spiral galaxy |
has a bar of stars across the bulge |
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Lenticular galaxy |
Has a disk like a spiral galaxy but much less dusty gas |
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elliptical galaxy |
-All spheroidal component, virtually no disk component -Red-yellow color indicates older star population |
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irregular galaxy |
-Neither spiral or elliptical -Blue-white color indicates ongoing star formation |
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how are galaxies grouped together |
-Spiral galaxies are often found in groups of galaxies (few dozen galaxies per group) -Elliptical galaxies are much more common in huge clusters of galaxies ( hundreds to thousands ) |
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how do we measure the distances to galaxies |
-Determine size of solar system using radar -Determine distances of stars out to a few hundred light-years using parallax -Apparent brightness of star cluster’s main sequence tells us its distance |
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which kind of stars are best for measuring large distances |
high-luminosity stars |
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cepheid variable star |
-Very luminous -Light curve of a cepheid variable star shows that its brightness alternately rises and falls over a 50-day period -Stars with longer periods have greater luminosities |
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standard candles |
-Because the period of a cepheid variable star tells us its luminosity, we can use these stars as standard candles -White dwarf supernovae can also be used as standard candles |
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red shifted galaxies |
-They are moving away from us -Redshift of a galaxy tells us its distance through hubble’s law -Distances of the farthest galaxies are measured from redshifts |
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hubble’s constant |
-Tells us the age of the universe because it relates velocities and distances of all galaxies -age= distance/velocity -Measuring Hubble’s constant tells us that amount of time: about 14 billion years |
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cosmological red shift |
-Light waves stretch to longer wavelengths as the universe expands -Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time |
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how do we study galaxy evolution |
Deep observations show us very distant galaxies as they were much earlier in time |
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modeling galaxy formation |
-Matter originally filled all of space almost uniformly -Gravity of denser regions pulled in surrounding matter -Denser regions contracted, forming protogalactic clouds -H and He gases in these clouds formed the first star |
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Conservation of angular momentum in modeling galaxy formation |
-Supernova explosions from the first stars kept much of the gas from forming stars -Leftover gas settled into a spinning cloud |
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conditions in protogalactic cloud |
-Spin: initial angular momentum of protogalactic cloud could determine the size of the resulting disk -Density: elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk |
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distant red elliptical’s |
Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe |
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giant elliptical galaxies |
Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies |
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starburst galaxies |
-Are forming stars so quickly that they will use up all their gas in less than a billion years -The intensity of supernova explosions in starburst galaxies can drive galactic winds |
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effect of collisions on galaxy formation |
-Collisions were much more likely early in time, because galaxies were closer together -Collisions we observe nearby trigger bursts of star formation -Two spiral galaxies can merge to make an elliptical -May explain why elliptical galaxies tend to be found where galaxies are closer together |
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What is the evidence for supermassive black holes at the centers of galaxies |
-If the center of a galaxy is unusually bright, we call it an active galactic nucleus -The most luminous examples are called quasars |
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What can you conclude from the fact that quasars usually have very large redshifts |
-They are generally very distant -They were more common early in time -Galaxy collisions might turn them on -Nearby galaxies might hold dead quasars |
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Quasars |
-Galaxies around quasars sometimes appear disturbed by collisions -Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures -Accretion of gas onto a supermassive black hole appears to be the only was to explain all the properties of quasars |
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energy from a black hole |
-The gravitational potiential energy of matter falling into a black hole turns into kinetic energy -Friction in the accretion disk turns kinetic energy into thermal energy ( heat) -Heat produces thermal radiation ( photons) |
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jets |
Thought to come from the twisting of a magnetic field in the inner part of the accretion disk |
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black holes in galaxies |
-Many nearby galaxies- perhaps all of them- have supermassive black holes at their centers -These black holes seem to be dormant active galactic nuclei -All galaxies may have passed through a quasar-like stage earlier in time |
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galaxies and black holes |
-The mass of a galaxy’s central black hole is closely related to the mass of its bulge -The development of a central black hole must somehow be related to galaxy evolution |
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what is the evidence for supermassive black holes at the centers of galaxies |
-Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type -The only model that adequately explains our observations holds that supermassive black holes are the power source |
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Why do we think the growth of central black holes is related to galaxy evolution |
-Observations of stars and gas clouds orbiting the centers of galaxies indicate that many galaxies, perhaps all of them, have supermassive black holes -The masses of the black holes are closely related to the properties of their home galaxies, suggesting a connection between the black hole and galaxy evolution |
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radio galaxies |
-contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at near light speed -The lobes of radio galaxies can extend over hundreds of millions of light-years don’t appear as quasars because dusty gas clouds block our view of the accretion disks |
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active galactic nucleus |
-Can shoot out blobs of plasma moving at nearly the speed of light -The speed of ejection suggests that a black hole is present |
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characteristics of Active galaxies |
-Luminosity can be enormous -Luminosity can vary rapidly ( comes from a space smaller than solar system) -They emit energy over a wide range of wavelengths ( contain matter with wide temperature range) -Some drive jets of plasma at near light speed |
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cosmological principle |
-The universe looks about the same no matter where you are within it -Matter is evenly distributed on very large scales in the universe -No center and no edges -Distances between faraway galaxies change while light travels -Astronomers think in terms of lookback time rather than distance |
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what are the three major types of galaxies |
-spiral galaxy -elliptical galaxy -irregular galaxy |
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