chapter 27: stars and galaxies

RAT:

Summer and winter constellations are different because

a) of the spin of Earth about its polar axis

b) the night sky faces in opposite directions in summer and winter

c) of the tilt of Earth’s polar axis

d) the universe is symmetrical and harmonious

b) the night sky faces in opposite directions in summer and winter

RAT:

Polaris is always directly over

a) the North pole

b) any location north of the equator

c) the equator

d) the South pole

a) the North pole

RAT:

The star nearest Earth is

a) Proxima Centauri

b) Polaris

c) Mercury

d) the Sun

d) the Sun

RAT:

The property of a star that relates to the amount of energy per unit time it is producing is its

a) luminosity

b) apparent brightness

c) color

d) volume

e) mass

a) luminosity

RAT:

The longest-lived stars are those of

a) low mass

b) high mass

c) intermediate mass

d) infinite mass

a) low mass

RAT:

We do not see stars in the daytime because

a) the Sun blocks them

b) they simply don’t exist in the daytime part of the sky

c) skylight overwhelms starlight

d) of the lack of contrast with moonlight

e) the solar wind obscures them from view

c) skylight overwhelms starlight

RAT:

After our Sun burns its supply of hydrogen, it will become a

a) white dwarf

b) black dwarf

c) black hole

d) red giant

e) blue giant

d) red giant

RAT:

A black hole is

a) an empty region of space with a huge gravitational field

b) a small region that has the mass of many galaxies

c) the remains of a giant collapsed star

d) as large as its photon sphere

c) the remains of a collapsed star

RAT:

The shape of an activve starburst galaxy tends to be

a) elliptical

b) spiral

c) irregular

d) all of the above

c) irregular

RAT:

Scientists estimate the age of our universe to be about

a) 5000 years old

b) 1 billion years old

c) 14 billion years old

d) 42 billion years old

c) 14 billion years old

Checkpoint:

Which celestial bodies appear fixed relative to one another, and which celestial bodies appear to move relative to the others?

The stars appear fixed as they move across the sky. The Sun, the Moon, and planets move relative to one another as they move across the back-drop of the stars.

Checkpoint:

What are two types of observed motions of the stars in the sky?

One type of motion of the stars is their nightly rotation as if they were painted on a rotating celestial sphere; this is due to Earth’s rotation on its own axis. Stars also appear to undergo a yearly cycle around the Sun because of Earth’s revolution about the Sun.

Checkpoint:

Which of the following three types of motion do we NOT readily see with respect to the Moon: daily motion, yearly motion, or intrinsic motion?

The Moon rises and the Moon sets. This daily motion comes from the rota-tion of our planet. from one night to the next, we also see that the position of the Moon has shifted eastward relative to the stars. This intrinsic motion is due to the Moon’s orbiting Earth. We see no obvious yearly motion with the Moon because it travels with us as we orbit the Sun.

Checkpoint:

The temperature of Sirius is about 9400 K. What color is this star—and why?

Sirius has a slightly blue color. It emits more blue light than red light because of its high surface temperature.

Checkpoint:

What characteristic do all main-sequence stars share?

All main-sequence stars generate energy by the nuclear fusion of hydro-gen to helium.

Checkpoint:

Giants have cool surface temperatures yet are highly luminous. Does this mean that the frequency of light emitted by a giant does not depend on its surface temperature as described by Figure 27.9?

No, radiation curves hold for a giant star as for any other radiating body. Giants do have a relatively low energy output per unit surface area; they are highly luminous only because they are very large.

Checkpoint:

What do the processes of thermonuclear fusion and gravitational contraction have to do with the physical size of a star?

The size of a star is the result of these two continually competing processes. Energy from thermonuclear fusion tends to blow the star outward like hydrogen bomb explosion, and gravitation tends to contract its matter in an implosion. The outward thermonuclear expansion and inward gravita-tional contraction produce an equilibrium that accounts for the star’s size.

Checkpoint:

Why does a star shrink when its core runs out of nuclear fuel?

Outward thermal expansion and inward gravitational contraction produce an equilibrium that accounts for the star’s size. As the heat from the inner ther-monuclear reactions begins to die down, gravity predominates and the star shrinks. Upon shrinking, matter becomes compressed, which is an additional source of heat to ignite further nuclear fusion. for a star the size of our Sun, compression raises the temperature enough to fuse elements to carbon

Checkpoint:

A star can undergo a nova more than once. Can a star also go through multiple supernovae? Why or why not?

A nova is a thermonuclear explosion that occurs when a white dwarf col-lects sufficient mass from a very close neighboring star. As long as the neighboring star provides mass, this explosion can be repeated multiple times. A supernova is such an energetic release of energy that it is an end-all event occurring never more than once for a particular supergiant star.

Checkpoint:

if the Sun somehow suddenly collapsed to a black hole, what change would occur in the orbital speed of Earth?

none; nothing in the Newton’s Law of gravitation changes

Checkpoint:

What determines whether a star becomes a white dwarf, a neutron star, or a black hole?

The mass of a star is the principal factor that determines its fate. Stars that are about as massive as the Sun, and those that are less massive, evolve to become white dwarfs; stars with masses of 10MSun or greater evolve to become neutron stars; the most massive stars of about 40MSun or greater ultimately become black holes.

Checkpoint:

Is it possible for one type of galaxy to turn into another?

Yes, and this occurs as two symmetrically shaped galaxies collide to form an asymmetrically shaped irregular galaxy.

Checkpoint:

Are there any quasars found within the Milky Way galaxy?

No. A quasar is the active galactic nucleus of a galaxy as it appeared toward the beginning of the universe. All quasars are billions of light-years away from our galaxy.

Checkpoint:

Cosmologists now estimate that there are more than 200 billion galaxies within our observable universe. How does this compare to the number of stars in our own galaxy, the Milky Way?

Recall from the beginning of this chapter that astronomers estimate that there are about 100 billion stars in our galaxy. Assuming every galaxy had 100 billion stars, that would mean there are about 1022 stars in our observable universe, which is about the same as the number of water molecules in a drop of water. As large as the observable universe is large is as small as the fundamental building blocks of our body are small. As humans we are nicely situated between these two extremes.

early astronomers divided the night sky into groups of stars called?

constellations

the group of seven stars we not call the?

Big Dipper

The names of the constellations today carry over mainly from the names assigned to them by what early astronomers (name three)?

Greek, Babylonian, Egyptian

the great beat is a large constellation also called the?

Ursa Major

True or False

To some cultures, the constellations stimulated storytell-ing and the making of great myths; to other cultures, the constellations honored great heroes, such as Hercules and Orion; to yet others, they served as navigation-al aids for travelers and sailors

True

True or False

The stars are at different distances from Earth. However, because all the

stars are so far away, they appear equally remote.

True

The stars are at different distances from Earth. However, because all the

stars are so far away, they appear equally remote. This illusion led the ancient Greeks and others to conceive of the stars as being attached to a gigantic sphere surrounding Earth, called?

The celestial sphere

when we look upward to the celestial bodies, there are three types of motion that we can perceive. what are they?

daily motion

yearly motion

intrinsic motion

daily motion, sometimes called _________, is a consequence of the daily eastward rotation of Earth on its axis

diurnal motion

True or False

With daily motion, stars change their positions relative to each other

False; they don’t!

True or False

The North Star appears nearly stationary because it lies very close to the projection of Earth’s rotational axis.

True

Yearly motion, sometimes called __________, is a consequence of Earth’s revolving around the Sun

revolutionary motion

Earth’s orbiting the Sun results in a continuous shift in our nighttime view of the universe. Because of the direction of our orbit, the constellations appear to migrate toward the

a) north

b) east

c) south

d) west

d) west

(this shift is very subtle that it takes several weeks for the change to become readily apparent. however the constellations in the night sky are compeltely different)

true or false

after a total of 12 months, the same constellations reappear because Earth has completed an entire orbit

true

in addition to daily and yearly motion, there is _________ motion, in which the celestial bodies appear to move relative to each other

intrinsic

_______ motion results from the motion of the celestial bodies themselves

a) daily

b) intrinsic

c) yearly

b) intrinsic

planet is derived from the Greek term for ____________

wanderer

the ancients viewed the planets as wandering stars

do stars themselves hvae intrinsic motion?

yes; they are so far away, however, that this motion is not apparent on the time scale of a human life; intrinsic movement of stars results in new patterns of stars

true or false

the constellations we see today are quite different from the ones that appeared to our earliest ancestors

true

true or false

some stars on the celestial sphere are actually much farther away than others

true

astronomers measure the vast distances between Earth and the stars using?

light years

one light-year is the distance that light travels in 1 year, nearly 10 trillion km

the diameter of the Milky Way galaxy is about how many light years?

100,000 light years

about three-fourths of the interstellar material from which a star forms is ________; one fourth is ______; and no more than 2% of the material from which a star forms consists of heavier chemical elements

hydrogen

helium

stars shine brilliantly for millions or billions of years because of?

the nuclear fusion reactions that occur in their cores

if you look into the night sky, you will see that stars differ in two very visible ways. what are they?

brightness and color

__________ relates to how much energy a star produces

a) brightness

b) luminosity

a) brightness

how does the inverse square law correlate?

the intensity of light diminishes as the reciprocal of the square of the distance from the source

apprent brightness is the brightness of a star as it appears to our eyes. ________, on the other hand, is the total amount of light energy that a star emits into space

luminosity

the most luminous stars are about a ______ times as luminous as the Sun, while the dimmest stars produce about ______ times as much energy per second as the Sun

million

1/10,000

besides apparent brightness, stars also vary greatly in?

color

do stars come in every color of the rainbow?

yes

a star’s color directly tells you about its?

surface temperature

which is hotter: a blue star or a yellow star?

blue star

which is hotter: a red star or a yellow star?

a yellow star

astronomers use ______ to measure the temperature of stars

color

all objects with a temperature emit energy in the form of?

electromagnetic radiation

why do stars have different colors?

they have different colors because they emit different frequencies of electromagnetic waves in the visible range

the radiation curves show that the hotter a star is, the ________ the wavelength of its peak frequency and the bluer it looks than the red ones.

a) shorter

b) longer

a) shorter

the sun is in which part of the spectrum of stars?

middle of the visible spectrum; that is why it appears yellow

Early in the 20th century, Danish astrono-mer Ejnar Hertzsprung and American astronomer Henry Norris

Russell did just this. They produced a diagram known as the?

Hertzsprung-Russel diagram or the H-R diagram

the H-R diagram is a plot of the luminosity versus what?

the surface temperature of stars

On the H-R diagram, are luminous stars near the top or the bottom? are dim stars towrd the top or bottom?

luminous stars are near the top

dim stars are toward the bottom

hot bluish stars are toward the __________

cool reddish stars are toward the ___________

a) right side; left side

b) left side; right side

b) left side; right side

on the H-R diagram, most stars are plotted on the band that stretches diagonally across the diagram. this band is called?

the main sequence

true or false

stars on the main sequence, including our Sun, generate energy by fusing hydrogen to helium

true

true or false

the hottest main-sequence stars are the brightest and redest stars

false; they are the brightest and bluest stars

the coolest main sequence stars are how bright and what color?

they are dim and they are red

toward the upper right of the diagram is a distinct group of stars called?

the giant stars

(these stars clearly do not follow the pattern of the hydrogen-burning main-sequence stars)

the fact that the giants are both much cooler and much brighter than the Sun tells us that these stars must also be?

much larger than the sun

(these are the giant stars on the upper right of the H-R diagram)

above the giants on the H-R diagram are a few rare stars called?

supergiants

they are even larger and brighter than the giants

true or false

red giants and supergiants are stars nearing the end of their lives

true

eventually the giants and supergiants compress to form the slightly smaller what?

blue giants

(upper left of the diagram)

which stars are so dim that they cannot be seen with the unaided eye?

a) red giants

b) super giants

c) blue giants

d) white dwarfs

c) blue giants

while blue giants are very small and really dim, they are?

really hot; they are hotter than the sun, which makes them blue or white

some blue giants luminosities are low. to be so hot and radiate so little light, these stars must be very small—-they are called?

white dwarfs

_________are typically the size of earth or even smaller yet they have a mass comparable to that of the Sun

a) red giants

b) blue giants

c) supergiants

d) white dwarfs

d) white dwarfs

the density of a white dwarf is?

a) the same as any other blue giant

b) really low

c) really high

d) insignifant because it varies

c) really high

the nebular theory explains how the Sun formed from an expansive, low-density cloud of gas and dust called a?

nebula

over time what happens to the nebula?

it flattens, heats, and spins more rapidly as it graivtationally contracts

true or false

eventually, the center of the nebula becomes dense enough to trap infrared radiation so that this energy is no longer radiated away

true

the hot central bulge of a nebula is called a?

protostar

as the density of gaseous particles in a protostar increases, matter is crunched up. what is fusing? what is it forming?

hydrogen nuclei begin fusing to form helium nuclei

(this is thermonuclear reaction, converting hydrogen to helium)

does thermonuclear fusion release a lot of energy?

yes, enormous amounts of radiant and thermal energy

the ignition of nuclear fuel marks the change from ________ to _______

protostar to star

outward-moving radiant energy and the gas accompanying it exert an outward pressure called ____________ on the contracting matter

thermal pressure

when nuclear fusion occurs fast enough, thermal pressure becomes strong enough to halt the?

gravitational contraction

when does the star’s size stabilize?

it stabilizes when the thermal pressure balances inward gravitational pressure

true or false

all stars are born the same way from a contracting nebulae

true

do star all progress their lives the same way?

no

what mainly determines the stages a star will go through from birth to death?

a star’s mass

for example: a protostar with a mass less than 0.08 times the mass of the sun never reaches the 10 million K threshold needed for sustained fusion of hydrogen

true or false

most stars have masses not very different from that of the Sun

true

such stars inhabit a central place on the main sequence of the H-R diagram

true or false

if you plot the life-cycle stages of average stars on an H-R diagram, they trace a round-about curve similar to the one for our Sun

true

what star was born about 4.5 billion years ago at position 1, when the fusion of hydrogen ignited

the sun

true or false

the Sun will spend most of its lifetime about some 10 billion years outside of the main sequence, with thermal pressure keeping gravity at bay

no; inside the main sequence

speaking more generally, a star’s hydrogen-burning lifetime lasts for a period of a few million to a projected ______million years, depending on the mass

50 million

the universe itself is thought to be only about how hold?

14 billion years old