Amber Reed's Quiz

Chapter 15 Galaxies and the foundation of modern cosmology 

15.1 

What are the three major types of galaxies? 
-our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away
-a galaxy's age,its distance ad the age of the universe are all closely related
-the study f galaxies is thus intimately connected with  cosmology-the study of the structure and evolution of the universe 
-disk component:stars of all ages,many gas clouds
-spheroidal component: bulge and halo, old stars, few gas clouds
-barred spiral galaxy:has a bar of stars across the bulge
-lenticular Galaxy:has a disk like a spiral galaxy but much less dusty gas(intermediate between spiral and elliptical)
-Elliptical galaxy:a spheroidal component,virtually no disk component
-red yellow color indicates older star population 
-irregular galaxy:neither spiral or elliptical. 
-Blue white color indicates ongoing star formation
-Spiral galaxies, elliptical galaxies and irregular galaxies
-Spirals have both disk and spheroidal components; ellipticals have no disks

How are galaxies grouped together?
-spiral galaxies are often found in groups of galaxies(up to a few dozen galaxies per group)
-elliptical galaxies are much more common in huge clusters of galaxies(hudreds to thousands of galaxies)
-Spiral galaxies tend to collect into groups of up to a few dozen galaxies
-Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies

15.2 distance of galaxies

How do we measure the distances to galaxies?
-brightness alone does not provide enough information to measure distance 
-Step #1 is to determine size of solar system using radar
-Step #2 determine distances of stars out to a few hundred light -years using parallax
-luminosity passing through each sphere is the same
-Step#3 apparent brightness of star cluster's main sequence tells us its distance
-knowing a star cluster's distance we can determine the luminosity of each type of star within it
-cepheid variable stars are very luminous
-apparent brightness of the white dwarf supernova tells us the distance to its galaxy (up to 10 billion light years)
- The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system
-Using parallax and the relationship between luminosity, distance and brightness, we can calibrate a series of standard candles
-We can measure distances greater than 10 billion light years using white dwarf supernovae as standard candles

What is Hubble's Law?

-before Hubble, some scientists argued tat "spiral nebulae" were entire galaxies like out Milky Way, whereas there scientists maintained they were smaller collections of stars within the Milky Way
-the faster a galaxy is moving away from us , the greater its distance
-Hubble settled the debate by measuring the distance to the andromeda galaxy using Cepheid variables as standard candles
-Hubble also knew that the spectral features of virtually all galaxies are redshifted=> they're all moving away from us
-by measuring distances to galaxies, Hubble found that redshift and distance are related  in a special way
Velocity=H0 x distance
-distances of the farthest galaxies are measured from redshifts
-we measure galaxy distances using chain of interdependent techniques


How do distance measurements tell us the age of the universe?
-Hubble's constant tells us the age of the universe because it relates  and distance of all galaxies
-the expansion rate appears to the same everywhere in space
-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
-not proved but consistent with all the observations to date
-distances between faraway galaxies change while light travels 
-astronomers think in terms of look-back time rather than distance 
-expansion stretched photon wavelength, causing a cosmological redshift directly related to look-back time
-measuring a galaxy's distance and speed allows us to figure out how long the galaxy took reach its current distance 
-measuring Hubble's constant tells us that amount of time:about 14 million years

15.3 Galaxy Evolution

How do we observe the life's histories of galaxies?
-deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages

How did galaxies form?
-Our best models for galaxy formation assume that gravity made galaxies out of regions of the early universe that were slightly denser than their surroundings 
-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 stars
-supernova explosions from he first stars kept much of the gas from forming stars
-leftover gas settled into spinning a disk
- conversation angular momentum 

Why do galaxies differ?
-spin:initial angular momentum of protogalactic cloud could determine the size of the resulting disk 
-density:elliptical galaxies could come form dense protogalactic clouds that were able to cool and form stars before gas settled into a disk
-observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe 
-we must also consider the effects of collisions 
-collisions were much more likely early in time, because galaxies were closer together 
-many of the galaxies we see at great distances do indeed look violently disturbed
-the collisions were observe nearby trigger bursts of star formation  
-Some of the differences between galaxies may rise from the conditions in their protogalactic clouds
-Collisions can also play a major role because they can transform two spiral galaxies into elliptical galaxy 
-modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical 
-collisions may explain why elliptical galaxies tend to be found where galaxies are closer together 
-giant elliptical galaxies at the centers of the clusters seem to have consumed a number of smaller galaxies
-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 starbursts  galaxies can drive galactic winds 

15.4 Quasars and other active Galactic Nuclei 

What are quasars?
-if the center of a galaxy is unusually bright, we call it an active galactic nucleus
-quasars are the most luminous examples
-galaxies around quasars sometimes appear disturbed by collisions 
-active galactic nuclei are very bright objects seen in the centers of some galaxies and quasars are the most luminous type
-quasars powerfully radiate energy over a wide range of wavelengths,indicating that they contain matter with a wide range of temperatures
-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 

What is the power source for quasars and other active galactic nuclei?
-The only model that adequately explains our observations holds that supermassive black holes are power source
-jets are thought to come from twisting of magnetic field in the inner part of the accretion disk 

Do supermassive black holes really exist?
-Observations of stars and gas clouds orbiting the centers of galaxies indicate that many galaxies,perhaps all of them, have supermassive black holes


Amber Reed

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