Deck 17: The Nature of the Stars

Question 1

Stellar parallax is the:

A)elliptical motion of a star in a binary system, as the two stars orbit around each other.
B)difference between the apparent magnitude and the absolute magnitude of a star.
C)assumed change in the distance to a star when it dims, as it passes through an interstellar cloud.
D)apparent shift in the position of a nearby star because of Earth's motion.

Answer 1

D

Question 2

Stellar parallax is used to determine which of the following properties of a nearby star?

A)its spectral type and surface temperature
B)its rotation period
C)its apparent magnitude
D)its distance from the Sun

Answer 2

D

Question 3

The most straightforward way to measure the distance to a nearby star uses:

A)a calculation involving apparent magnitude and luminosity.
B)parallax.
C)its proper motion.
D)a comparison of apparent and absolute magnitudes.

Answer 3

B

Question 4

How many stars (other than the Sun) have a stellar parallax greater than 1 second of arc?

A)over 100
B)1
C)0
D)8

Question 5

If a nearby star shows a parallax of 0.5 arcsec, what is its distance from Earth in light-years?

A)2 ly
B)6.52 ly
C)1.83 ly
D)3.26 ly

Question 6

If a nearby star shows a parallax of 0.5 arcsec, what is its distance from Earth in parsecs?

A)2 pc
B)6.52 pc
C)1.83 pc
D)3.26 pc

Question 7

The triple star system α Centauri has the largest known stellar parallax angle, namely 0.75 arcsec. How far is this star system from the Sun in light-years? (Careful with units!)

A)0.41 ly
B)0.75 ly
C)4.33 ly
D)1.33 ly

Question 8

The triple star system α Centauri has the largest known stellar parallax angle, namely 0.75 arcsec. How far is this star system from the Sun in parsecs? (Careful with units!)

A)0.41 pc
B)0.75 pc
C)4.33 pc
D)1.33 pc

Question 9

Proxima Centauri is the nearest star beyond the Sun. Its distance is roughly:

A)10⁴ km.
B)1 pc.
C)10³ pc.
D)10⁶ pc.

Question 10

The total apparent motion against the background sky as a result of Earth's motion halfway around its orbit of a star whose distance from the Sun is 40 pc is:

A)0.0125 radian or 0.72°.
B)0.0125 arcmin.
C)0.0125 arcsec.
D)40 arcsec.

Question 11

The apparent angular movement of a nearby star against the background stars as a consequence of its motion in space is known as:

A)radial velocity.
B)tangential velocity.
C)proper motion.
D)retrograde motion.

Question 12

Proper motion of a star is defined as its:

A)apparent motion toward or away from us, measured by the Doppler shift of its spectral lines.
B)apparent motion against the background stars as a consequence of Earth's orbital motion around the Sun.
C)real motion in three-dimensional space.
D)apparent motion across our sky against the background stars.

Question 13

If a star moves in space, its proper motion appears as an angular displacement against the background stars. Its parallax motion also appears as an angular displacement against the background stars. How can we tell the difference?

A)We cannot tell the difference. This is a major uncertainty in measuring either value.
B)We cannot tell the difference, so we measure proper motions only so far away that their parallax motions are vanishingly small.
C)We cannot tell the difference, so proper motions are estimated from radial values that are unaffected by parallax.
D)We can tell the difference because parallax motions are periodic while proper motions are not.

Question 14

Suppose a star lying on the celestial equator has a proper motion, directed along the equator, equal to the rather large value of Barnard's star, namely 10 arcsec/yr. How long would it take this star to change its right ascension by 1 hour?

A)10 years
B)360 years
C)5400 years
D)9200 years

Question 15

The proper motion of a star is:

A)the diameter of the circle through which the star appears to move in the sky each year, due to the motion of Earth.
B)the angle per unit time at which the star moves across our sky against the background stars.
C)the speed of the star in km/s, measured in a direction perpendicular to the line of sight from Earth to the star.
D)the speed of the star in km/s, measured along the line of sight from Earth to the star.

Question 16

The speed of a star in km/s, measured in a direction perpendicular to the line of sight from Earth to the star, is known as:

A)tangential velocity.
B)proper motion.
C)radial velocity.
D)retrograde motion.

Question 17

What two quantities do we need to measure to calculate the tangential velocity of a star?

A)distance and radial velocity
B)distance and proper motion
C)parallax and distance
D)radial velocity and proper motion

Question 18

The star Procyon has a parallax of 0.287 arcsec and a proper motion of 1.25 arcsec per year. What is the tangential velocity of Procyon?

A)68 km/s
B)21 km/s
C)1.7 km/s
D)16 km/s

Question 19

The star Ross 128 has a parallax of 0.301 arcsec, a radial velocity of 13 km/s, and a proper motion of 1.40 arcsec per year. At what total velocity does Ross 128 travel through space, relative to the Sun?

A)655 km/s or 2.36 million km/hr (1.47 million mph!)
B)22 km/s or 79,200 km/hr (49,500 mph!)
C)72 km/s or 259,200 km/hr (162,000 mph!)
D)26 km/s or 93,600 km/hr (58,500 mph!)

Question 20

Neptune is about 1.5 times as far from the Sun as is Uranus, and it is about the same size. About how much radiant energy does Neptune receive from the Sun, compared to Uranus?

A)44%
B)67%
C)100%
D)150%

Question 21

Light, leaving a point source, spreads out so that the apparent brightness, b, of light per unit area varies with distance d according to which law ( $\alpha$ means "proportional to")?

A)b ? 1/d².
B)b = constant.
C)b ? 1/d.
D)b ? d².

Question 22

Suppose that, at night, the brightness of a light bulb is measured from a certain distance and then the light bulb is moved to a distance twice as far away. How bright will the light appear compared to the earlier measurement?

A)1/16 as bright
B)1/2 as bright
C)1/8 as bright
D)1/4 as bright

Question 23

The intensity of sunlight per square meter reaching Venus is approximately what fraction of the intensity of sunlight at Earth's orbital distance?

A)1/0.721
B)about the same
C)0.721 times
D)(1/0.721)²

Question 24

What is the intensity of sunlight per square meter reaching Saturn compared to the intensity at Earth's orbital distance?

A)1/91
B)91 times brighter
C)1/9.5
D)1/3.1

Question 25

If Mercury is at 0.4 au, the Moon is at 1.0 au, and the inverse-square law holds, how much more light falls on a unit area of Mercury's surface than on an equivalent area of the Moon?

A)6.25 times more
B)0.4 times as much
C)2.5 times more
D)16 times more

Question 26

Which of the following statements is TRUE of the population of stars in our neighborhood?

A)There are many more bright than faint stars.
B)There are about equal numbers of stars of various brightness.
C)There are more stars of intermediate brightness and fewer fainter or brighter stars.
D)There are many more faint than bright stars.

Question 27

The luminosity of a star is:

A)its apparent magnitude.
B)the total energy emitted at all wavelengths toward Earth.
C)the total energy emitted at all wavelengths into all space from its whole surface.
D)the total energy emitted by the star within the sensitive range of the eye, in the so-called V filter band.

Question 28

The luminosity of a star is:

A)the energy output of 1 m² of its surface space at all wavelengths.
B)its brightness when measured from Earth.
C)its total energy output emitted at all wavelengths into all space.
D)its brightness when measured from a distance of 10 pc, or 32.6 ly.

Question 29

A luminosity function is a plot of the:

A)luminosity of stars in a given volume as a function of their distance from us.
B)number of stars in a given volume as a function of their distance from us.
C)number of stars in a given volume as a function of their luminosity.
D)parallax angles of stars in a given volume as a function of their luminosity.

Question 30

Figure 17-5 shows a typical luminosity function. This shows that:

A)faint stars are more numerous.
B)stars like the Sun are rare.
C)stars like the Sun are most numerous.
D)the number of bright stars decreases linearly with increasing luminosity.

Question 31

The absolute magnitude of the Sun is +4.8 and its luminosity is 3.9 × 10²⁶ watts. The luminosity of a star whose absolute magnitude is +3.8 is:

A)2.9 × 10²⁶ W.
B)9.8 × 10²⁶ W.
C)3.9 × 10²⁷ W.
D)1.6 × 10²⁶ W.

Question 32

The absolute magnitude of the Sun is +4.8 and its luminosity is 3.9 × 10²⁶ watts. The absolute magnitude of the star $v$ (Nu) Hydrae is -0.2. What is the luminosity of $v$ Hydrae?

A)3.9 × 10²⁴ W
B)2.0 × 10²⁷ W
C)3.9 × 10²⁸ W
D)7.8 × 10²⁵ W

Question 33

By approximately how many magnitudes is the star Sirius fainter than the full moon in our sky? (See Figure 17-6.(in the text))

A)2
B)5
C)15
D)11

Question 34

Two stars that differ from each other by five magnitudes have a ratio of brightness of:

A)5.
B)25.
C)10.
D)100.

Question 35

What is the magnitude of a star that appears twice as bright as a 3.0 magnitude star?

A)0.75
B)2.25
C)2.75
D)3.35

Question 36

Two stars orbit each other in a binary system. Each star by itself has an apparent magnitude of 4.0. What is the apparent magnitude of the two seen together as a single object?

A)0.75
B)2.0
C)3.25
D)8.0

Question 37

The star γ Phoenecis has an apparent magnitude of 3.4 and an absolute magnitude of -4.6. The North Star (Polaris) has an apparent magnitude of 2.0 and an absolute magnitude of -4.6. Assuming that no light has been absorbed or scattered by interstellar dust, we can say for sure that:

A)both stars are the same distance away from Earth.
B)Polaris appears fainter in our sky than γ Phoenecis.
C)Polaris is closer to Earth than γ Phoenecis.
D)Polaris is farther away from Earth than γ Phoenecis.

Question 38

A star with an apparent magnitude of m = +2.5 is at 100 pc from Earth. What is its absolute magnitude, M?

A)+7.5
B)-2.5
C)-7.5
D)-47.5

Question 39

The star β Arietis has an apparent magnitude of +2.7 and a distance of 52 ly. What is its absolute magnitude, M?

A)+6.2
B)-0.9
C)+1.7
D)+3.7

Question 40

Several optical glass filters are used to select specific portions of a star's light for photometry. Which of the following filters most closely matches the sensitivity of the eye, with peak wavelength sensitivity at about 550 nm? (I = infrared, B = blue, U = ultraviolet, V = visual)

A)I
B)B
C)U
D)V

Question 41

Measurements of the brightness of a distant star through the three appropriate filters indicate that the star is brightest in U, less bright in B, and faintest in V. What conclusion can be drawn from this information, assuming no absorption of light between the star and Earth?

A)This information is insufficient to allow a conclusion to be drawn about star surface temperature.
B)The star has an intermediate temperature, close to the Sun.
C)The star has a very low surface temperature.
D)The star has a very high surface temperature.

Question 42

The ratio of the brightness of a star at two different colors, blue and visual-b_V/b_B-is a direct measure of what property of the star?

A)distance from Earth
B)surface temperature
C)luminosity
D)radius

Question 43

A star with a ratio of brightness b_V/b_B of 1.00 has a surface temperature of about: (See Figure 17-9.(in the text))

A)24,000 K.
B)10,000 K.
C)4500 K.
D)7500 K.

Question 44

A star whose ratio of visual and blue apparent brightnesses is b_V/b_B = 5 (see Figure 17-9(in the text)) has a surface temperature of about:

A)12,000 K.
B)It is not possible to determine the star's temperature with this information alone, because the value of either b_V or b_B is also needed.
C)6000 K.
D)3600 K.

Question 45

Sirius has a brightness through a blue filter of b_B = 9.4 × 10^-8 W/m². What does this tell you about Sirius?

A)Sirius is a particularly bright star.
B)Sirius is a particularly hot star.
C)Sirius is a particularly cool star.
D)This number, by itself, tells you nothing about Sirius.

Question 46

What is the color of a star whose ratio of visual and blue apparent brightnesses, b_V/b_B, is 1.8?

A)blue-white
B)yellow-white, similar to that of the Sun
C)white
D)red

Question 47

The color ratio b_B/b_U has the following values for four stars. Which value indicates the coolest surface temperature?

A)0.5
B)1.0
C)2.5
D)4.0

Question 48

A particular star is brighter seen through a blue filter than through a yellow filter. Which of the following surface temperatures is possible for this star? (See Figure 17-9.(in the text))

A)3000 K
B)15,000 K
C)4500 K
D)6000 K

Question 49

A particular star is equally bright when viewed through a blue filter and a yellow filter. What is the approximate surface temperature of this star? (See Figure 17-9.(in the text))

A)9000 K
B)15,000 K
C)3000 K
D)It is not possible for a star to be equally bright at two different wavelengths.

Question 50

What effect does interstellar dust have on the apparent color of a star seen through the dust?

A)The dust makes the star look brighter than it really is, but leaves the color of the star unchanged.
B)The dust makes the star look bluer than it really is.
C)The dust makes the star look fainter than it really is, but leaves the color of the star unchanged.
D)The dust makes the star look redder than it really is.

Question 51

A red filter passes light at the long wavelength end of the visible spectrum as shown in Figure 17-7. Star 1 and Star 2 are viewed through identical red filters, and Star 1 appears brighter through the filter than does Star 2. What can be determined from this information?

A)Star 1 is hotter than Star 2.
B)Star 2 is hotter than Star 1.
C)Star 1 is more luminous than Star 2.
D)Nothing can be concluded from this fact alone.

Question 52

Each of the following could cause a star to appear red EXCEPT ONE. Which is the EXCEPTION?

A)The star's surface may be relatively cool.
B)The star's light may pass through long regions of dust on its way to us.
C)The star may be moving away from us at a high rate of speed.
D)The star may be relatively small.

Question 53

From which feature of light from a nearby star is the surface temperature determined most precisely?

A)relative distribution of the continuum light in the spectrum
B)relative strengths of emission lines in its spectrum
C)relative strengths of absorption lines from different atoms (e.g., H, Ca) and molecules (e.g., TiO)
D)Doppler shift of its spectral lines

Question 54

Which characteristic of a star's spectrum is most important in determining its spectral classification?

A)the number of prominent spectral lines in its spectrum
B)the ratio of bright emission lines to dark absorption lines
C)the identification of the elements producing the spectral lines and their relative strengths
D)the Doppler shift

Question 55

In the spectral sequence of star types, each category has been divided into 10 intervals. The Sun is classified as:

A)M9.
B)A1.
C)G2.
D)O1.

Question 56

As the spectral classification changes from F1 to F9, the main change in the star being characterized is that it becomes:

A)hotter.
B)cooler.
C)larger.
D)younger.

Question 57

Why is it that stars with low surface temperature, much cooler than 10,000 K, show very weak or no hydrogen Balmer lines in absorption?

A)To absorb at Balmer wavelengths, atoms need to have electrons in the n = 2 level, and electrons will not be excited to this level by collisions at these low temperatures.
B)The stellar gas is so cool that there is no radiation to be absorbed at the wavelength of Balmer lines.
C)Hydrogen atoms have to be hot enough to be ionized to show Balmer absorption.
D)Hydrogen atoms will have no electrons at any energy level at these low temperatures.

Question 58

Which molecule produces the spectral absorption bands that are prominent in the spectrum of the cool M-type star in Figure 17-12?

A)CH₄-methane
B)H₂O-water vapor
C)TiO-titanium oxide
D)HCl-hydrogen chloride

Question 59

The spectrum of a particular star shows absorption lines due to calcium, iron, carbon, and titanium oxide. Which of these substances is classified as a "metal" by astronomers?

A)only iron
B)only calcium, iron, and carbon
C)all four (calcium, iron, carbon, and titanium oxide)
D)only calcium and iron

Question 60

On the basis of the graph of absorption line strengths in spectra of stars presented in Figure 17-12, what is the spectral type of a star with the following spectral pattern: very strong H, weaker Mg II and Si II, and no He I or Ca II lines?

A)G
B)B
C)K
D)A

Question 61

On the basis of the graph of absorption line strengths (in Figure 17-12), what is the spectral type of a star with the following spectral pattern: very strong He I lines, moderately strong H lines, weak Si III lines, and no Ca II, Fe II, or He II lines?

A)M
B)F
C)B
D)A

Question 62

On the basis of the graph of absorption line strengths (in Figure 17-12), what is the spectral type of a star with the following spectral pattern: very strong Ca II lines; weaker Fe I lines; equal strength but weaker Fe II and Ca I lines; and no He I, He II, or TiO lines?

A)A
B)F
C)K
D)O

Question 63

Brown dwarfs of spectral type T have surface temperatures below 1300 K. We are also familiar with a group of objects that emit infrared radiation at even lower temperatures, typically a few hundred kelvins. These objects are called:

A)white dwarfs.
B)stars of spectral type M.
C)brown dwarfs of spectral type Q.
D)planets.

Question 64

When the spectrum of a particular star is photographed and analyzed, it is found that the spectrum contains absorption bands of the metal hydrides methane (CH₄) and ammonia (NH₃). Based on this information, the star is:

A)an M-type star.
B)actually a white dwarf.
C)a true star, cooler than spectral class M.
D)actually a brown dwarf.

Question 65

A particular star is found to have a very faint companion. When the spectrum of the companion is photographed and analyzed, it is found to contain absorption bands of the metal hydrides methane (CH₄) and ammonia (NH₃), :but no water (H₂O). Based on this information, the companion is a brown dwarf of spectral class

A)K.
B)T.
C)M.
D)L.

Question 66

A particular star is found to have a very faint companion. When the spectrum of the companion is photographed and analyzed, it is found to contain absorption features of neutral potassium (K) and water (H₂O). Based on this information, the companion is a brown dwarf of spectral class:

A)T.
B)L.
C)K.
D)M.

Question 67

Brown dwarfs give out energy due mainly to:

A)nuclear fusion reactions.
B)the Kelvin-Holmholtz contraction.
C)radioactivity.
D)nuclear fission reactions.

Question 68

Brown dwarf spectral classifications, in order of increasing temperature, are:

A)L, T, and Y.
B)T, L, and Y.
C)Y, T, and L.
D)Y, L, and T.

Question 69

The relationship between the luminosity of a star, L, its surface temperature, T, and its radius, R, is given (with $\sigma$ = Stefan-Boltzmann constant) by:

A)L = 4 $\pi$ $\sigma$ R⁴T ².
B)L = L = 4 $\pi$ $\sigma$ R²T ².
C)L = L = 4 $\pi$ $\sigma$ RT ².
D)L = L = 4 $\pi$ $\sigma$ R²T ⁴.

Question 70

A particular star has 12 times the radius of the Sun and only 60% of the Sun's surface temperature. What is the star's luminosity in solar units ( $L_{\odot}$ )? (See Box 17-4.)

A)51.8 $L_{\odot}$
B)18.7 $L_{\odot}$
C)22.6 $L_{\odot}$
D)7.2 $L_{\odot}$

Question 71

The radius of a typical supergiant is how many times the radius of a typical white dwarf?

A)10
B)10³
C)10⁵
D)10⁷

Question 72

A particular star has a radius half that of the Sun and a luminosity equal to 60% of that of the Sun. What is the star's surface temperature? (See Box 17-4.) The surface temperature of the Sun is 5800 K.

A)7220 K
B)6650 K
C)4660 K
D)3610 K

Question 73

Figure 17-14 sketches a number of stellar properties: radius, temperature, chemical composition, spectral type, luminosity, and distance. It shows how these may be calculated from three measured quantities:

A)parallax angle, apparent brightness, and absolute magnitude.
B)parallax angle, apparent brightness, and spectrum.
C)absolute magnitude, spectrum, and parallax angle.
D)apparent brightness, spectrum, and absolute magnitude.

Question 74

The Hertzsprung-Russell diagram is a statistical plot of which of the following two stellar parameters?

A)luminosity and surface temperature
B)radius and mass
C)luminosity and mass
D)mass and surface temperature

Question 75

Which two vital parameters are used to describe the systematics of a group of stars (e.g., cluster) in the Hertzsprung-Russell diagram?

A)luminosity and radius
B)surface temperature and mass
C)luminosity and surface temperature
D)mass and apparent magnitude

Question 76

What fraction of the stars in the night sky are main-sequence stars?

A)almost none of them, less than 1%
B)relatively few of them, about 20%
C)almost all of them, about 90%
D)roughly half of them, about 55%

Question 77

Using Figure 17-15a, what can you conclude about the star Mira compared to the Sun?

A)Mira is hotter and bluer but intrinsically fainter than the Sun.
B)Mira is hotter than the Sun and intrinsically brighter.
C)Mira is cooler, redder, and intrinsically fainter than the Sun.
D)Mira is cooler and redder but intrinsically brighter than the Sun.

Question 78

Where are brown dwarfs located on the H-R diagram?

A)upper right
B)upper left
C)lower right
D)lower left

Question 79

The following are parameters of stars that astronomers obtained from their measurements. Which one of these conclusions is obviously erroneous, on the basis of the positions of these alleged stars on the Hertzsprung-Russell diagram in Figure 17-15a? ( $L_{\odot}$ and $R_{\odot}$ are the luminosity and radius of the Sun, respectively.)

A)luminosity = 1 $L_{\odot}$ , radius = 1/10 $R_{\odot}$ , temperature = 20,000 K; conclusion: white dwarf star
B)luminosity = 1/100 $L_{\odot}$ , radius = 1/100 $R_{\odot}$ , temperature = 20,000 K; conclusion: white dwarf star
C)luminosity ? 10⁴ $L_{\odot}$ , radius $R_{\odot}$ 100 ? , temperature 5000 K; conclusion: a supergiant star
D)luminosity = 1 $L_{\odot}$ , radius = 1 $R_{\odot}$ , temperature = 6000 K; conclusion: main-sequence star

Question 80

In the Hertzsprung-Russell diagram of Figure 17-15a, which of the following is the correct sequence of stars in order of increasing intrinsic brightness?

A)Procyon B, Sirius B, the Sun, and Deneb
B)the Sun, Procyon B, Deneb, and Sirius B
C)Sirius B, Deneb, Procyon B, and the Sun
D)Deneb, the Sun, Sirius B, and Procyon B