Chapter 15 home work 13 Surveying the Stars

Question 1

What is the approximate chemical composition (by mass) with which all stars are born?

half hydrogen and half helium
98% hydrogen, 2% helium
95% hydrogen, 4% helium, no more than 1% heavier elements
three quarters hydrogen, one quarter helium, no more than 2% heavier elements

Answer 1

three quarters hydrogen, one quarter helium, no more than 2% heavier elements

Question 2

According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?

Its apparent brightness will decrease by a factor of 3.
Its apparent brightness will decrease by a factor of 9.
Its apparent brightness will increase by a factor of 9.
Its apparent brightness will increase by a factor of 3.

Answer 2

Its apparent brightness will decrease by a factor of 9.

Question 3

Star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5. Which star is brighter in our sky?

Star B
The two stars have the same brightness in our sky, but Star A is closer to us than Star B.
There is not enough information to answer the question.
Star A

Answer 3

Star A

Question 4

From hottest to coolest, the order of the spectral types of stars is _________.

	ABCDEFG
	ABFGKMO
	OMGABFF!!
	OBAGFKM
	OBAFGKM

Answer 4

OBAFGKM

Question 5

Our Sun is a star of spectral type _________.

S
G
F
M

Answer 5

G

Question 6

Astronomers can measure a star’s mass in only certain cases. Which one of the following cases might allow astronomers to measure a star’s mass?

The star is of spectral type G.
We know the star's luminosity and distance.
The star is of spectral type A.
The star is a member of a binary star system.

Answer 6

The star is a member of a binary star system.

Question 7

How is the lifetime of a star related to its mass?

More massive stars live much longer lives than less massive stars.
More massive stars live much shorter lives than less massive stars.
More massive stars live slightly shorter lives than less massive stars.
More massive stars live slightly longer lives than less massive stars.

Answer 7

More massive stars live much shorter lives than less massive stars.

Question 8

What is the common trait of all main sequence stars?

They are in the final stage of their lives.
They are all spectral type G.
They all have approximately the same mass.
They generate energy through hydrogen fusion in their core.

Answer 8

They generate energy through hydrogen fusion in their core.

Question 9

What is a white dwarf?

It is a star that follows a period-luminosity relation.
a main sequence star of spectral type F, which tends to look white in color
the remains of a star that ran out of fuel for nuclear fusion
It is a type of star that produces energy by gravitational contraction.

Answer 9

the remains of a star that ran out of fuel for nuclear fusion

Question 10

Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is __________.

more luminous, hotter, larger, and more massive
more luminous, hotter, smaller, and less massive
less luminous, cooler, larger, and more massive
less luminous, cooler, smaller, and less massive

Answer 10

less luminous, cooler, smaller, and less massive

Question 11

Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are __________.

hotter and larger in radius
cooler and larger in radius
cooler and smaller in radius
hotter and smaller in radius

Answer 11

cooler and larger in radius

Question 12

Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are __________.

hotter and larger in radius
cooler and larger in radius
cooler and smaller in radius
hotter and smaller in radius

Answer 12

hotter and smaller in radius

Question 13

Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category. Red Giant or Supergiant stars

Very cool but very luminous
Found in the upper right of the H-R diagram
The majority of starts in our galaxy
The sun for example
The hottest and most luminous stars
Very hot but very dim
Not much larger in radius than earth.

Answer 13

Very cool but very luminous

Found in the upper right of the H-R diagram

Question 14

Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category. Main-sequence stars
Very cool but very luminous
Found in the upper right of the H-R diagram
The majority of starts in our galaxy
The sun for example
The hottest and most luminous stars
Very hot but very dim
Not much larger in radius than earth.

Answer 14

The majority of starts in our galaxy
The sun for example
The hottest and most luminous stars

Question 15

Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category.White dwarfs 
Very cool but very luminous
Found in the upper right of the H-R diagram
The majority of starts in our galaxy
The sun for example
The hottest and most luminous stars
Very hot but very dim
Not much larger in radius than earth.

Answer 15

Very hot but very dim

Not much larger in radius than earth.

Question 16

Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore:

Both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.
Both stars have the same luminosity, but the apparent brightness of Star A is four times that of Star B.
Both stars have the same apparent brightness, but the luminosity of Star B is four times that of Star A.
Both stars have the same luminosity, but the apparent brightness of Star B is twice that of Star A.

Answer 16

Both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.

Question 17

Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?

Rigel has a higher core temperature than Sirius.
Sirius has a higher core temperature than Rigel.
Sirius has a higher surface temperature than Rigel.
Rigel has a higher surface temperature than Sirius.

Answer 17

Rigel has a higher surface temperature than Sirius.

Question 18

The approximate main-sequence lifetime of a star of spectral type O is _________.

3 million years
300 million years
10,000 years
10 billion years

Answer 18

3 million years

Question 19

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

By measuring stellar radii with very powerful telescopes.
By measuring the masses and spectral types of main-sequence stars in binary systems.
By comparing stars with the same spectral type but different luminosities.
By using computer models of hydrogen fusion and stellar structure.

Answer 19

By measuring the masses and spectral types of main-sequence stars in binary systems

Question 20

Before we can use parallax to measure the distance to a nearby star, we first need to know __________.

the month in which the star is observed
the distance to the nearest star besides the Sun
the Earth-Sun distance
the Sun's mass

Answer 20

the Earth-Sun distance

Question 21

Which of the following is a valid way of demonstrating parallax for yourself?

Get a camera, and photograph a person who is running back and forth.
Hold up your hand in front of your face, and alternately close your left and right eyes.
Hold up your hand in front of your face, and move it slowly back and forth.
Look up at a star, and notice how it moves toward the western horizon during the night.

Answer 21

Hold up your hand in front of your face, and alternately close your left and right eyes.

Question 22

What is the cause of stellar parallax?
Stellar parallax is caused by
the gradual motion of stars in the local solar neighborhood.
the varying speed of Earth in its orbit around the Sun.
the gradual change in the patterns of the constellations over thousands of years.
Earth’s orbit around the Sun.

Answer 22

Earth’s orbit around the Sun.

Question 23

The more distant a star, the __________.

smaller its parallax angle
slower its parallax occurs
faster its parallax occurs
larger its parallax angle

Answer 23

smaller its parallax angle

Question 24

Suppose that a star had a parallax angle of exactly 1 arcsecond. Approximately how far away would it be, in light-years?

1 light-year

3. 3 light-years
2. 1 light-years
8. 7 light-years

Answer 24

3.3 light-years

Question 25

Consider a relatively nearby, single star, that is, a star that is not a member of a binary system and has no known orbiting planets. Listed below are a few properties of this star. Classify each property as either something that we can observe or measure directly (with the aid of a telescope and instruments such as cameras or spectrographs) or something that we must infer indirectly (and hence is correct only if all of our assumptions are correct).
Observer directly

color
parallax angle
spectral type
apparent brightness
luminosity
surface temperature
mass
radius

Answer 25

color
parallax angle
spectral type
apparent brightness

Question 26

Consider a relatively nearby, single star, that is, a star that is not a member of a binary system and has no known orbiting planets. Listed below are a few properties of this star. Classify each property as either something that we can observe or measure directly (with the aid of a telescope and instruments such as cameras or spectrographs) or something that we must infer indirectly (and hence is correct only if all of our assumptions are correct).
Infer indirectly

color
parallax angle
spectral type
apparent brightness
luminosity
surface temperature
mass
radius

Answer 26

luminosity
surface temperature
mass
radius

Question 27

From Part A, you know that surface temperature is a stellar property that we infer indirectly. What must we measure directly so that we can infer a star’s surface temperature?

spectral type
apparent brightness
parallax angle
mass

Answer 27

spectral type

Question 28

Which of the following must be true if we are to infer (calculate) a star’s luminosity directly from the inverse square law for light?
Check all that apply.

We have measured the star’s apparent brightness.
No interstellar gas or dust absorbs or scatters light between us and the star.
The star must be a member of a binary system.
We have measured the star’s spectral type.
We have measured the star’s distance.

Answer 28

We have measured the star’s apparent brightness.
No interstellar gas or dust absorbs or scatters light between us and the star.
We have measured the star’s distance.

Question 29

We found that mass must be inferred for the star described in Part A. However, we can measure a star’s mass directly if __________.

it is unusually high in mass
we know its spectral type
it is near enough for us to measure its distance with parallax
it is a member of an eclipsing binary system

Answer 29

it is a member of an eclipsing binary system

Question 30

You should now see that the reason the mass of the star in Part A must be inferred is that the star has no known orbiting objects, which means we cannot apply Newton’s version of Kepler’s third law. Which of the following must be true if the star’s inferred mass is to be accurate?
Check all that apply.

We have determined that the star is a main-sequence star.
We have measured the star’s velocity.
The star must be located within the Milky Way Galaxy and not in another galaxy.
We have measured the star’s spectral type.

Answer 30

We have determined that the star is a main-sequence star.

We have measured the star’s spectral type.