astro ch 16

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The stars parallax angle increases as the star gets

closer to the earth

How many light-years away must a star be for its parallax angle to be one arcsecond

3.3 light-years

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

If the orbital radius of the earth were greater than its actual value, the parallax angle to nearby stars would be


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

Stars that have the highest apparent brightness are

very luminous and nearby

massive -star supernova are not good standard candles because

they have different luminosities ( since they have different masses)

Which two things are needed to determine a standard candles distance from earth

apparent brightness and luminosity

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

In general, how does the velocity of a galaxy depend on its distance from the earth

the greater the distance, the higher the velocity

Relationship between hubble’s constant and the age of the universe

the higher Ho, the younger the universe

Astronomers believe that nearly all galaxies are moving away from us because

the space in between the galaxies is expanding

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

One reason hubble’s constant isn’t exactly known today is

the exact distance of a galaxy is hard to measure

Which of these galaxies would you most likely find at the center of a large cluster of galaxies

a large elliptical galaxy

What baseline distance must we know before we can measure parallax

the earth-sun distance

Which cosmic distance measurement techniques are considered standard candle techniques

-main-sequence fitting -white dwarf supernovae -Cepheids

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

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

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

Which kind of object is the best standard candle for measuring distances to extremely distant galaxies

a white dwarf supernova

Why do virtually all galaxies in the universe appear to be moving away from our own

a galaxy in the local group

Which of these galaxies is most likely to be oldest

a galaxy in the local group

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

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

the luminosity of a quasar is generated in a region the size of

the solar system

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

Disk component of spiral galaxies

-Stars of all ages -Many gas clouds

spheroidal component of spiral galaxies

-Bulge and halo -Old stars – few gas clouds

why does ongoing star formation lead to a blue-white appearance

short-lived blue stars outshine others

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

elliptical galaxy

-All 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

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 )

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

which kind of stars are best for measuring large distances

high-luminosity stars

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

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

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

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

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

how do we study galaxy evolution

Deep observations show us very distant galaxies as they were much earlier in time

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

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

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

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

giant elliptical galaxies

Giant elliptical galaxies at the centers of 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 starburst galaxies can drive galactic winds

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

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

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


-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

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)


Thought to come from the twisting of a magnetic field in the inner part of the accretion disk

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

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

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

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

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

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

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

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

what are the three major types of galaxies

-spiral galaxy -elliptical galaxy -irregular galaxy

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