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Group A—Continued Group B—Continued
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6. The star Arcturus is 36.7 light-years away from Earth with a 6. The North Star (Polaris) is 4.1 × 10 meters away from Earth
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luminosity (L) of 3.64 × 10 watts. Ignoring the effects of the and has a luminosity (L) of 8.7 × 10 watts. Ignoring the effects
atmosphere, what is the brightness (B) of Arcturus, in watts per of the atmosphere, what is the brightness (B) of Polaris, in watts
square meter, as observed from Earth? per square meter, as observed from Earth?
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7. The star Vega is 2.4 × 10 m away from Earth with a luminosity 7. The star Spica is 262.1 ly away from Earth with a luminosity (L)
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(L) of 1.9 × 10 W. Ignoring the effects of the atmosphere, what of 7.4 × 10 W. Ignoring the effects of the atmosphere, what is
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is the brightness (B) of Vega, in W/m , as observed from Earth? the brightness (B) of Spica, in W/m , as observed from Earth?
8. The solar panel array on a spacecraft requires a minimum 8. The solar panel array on a spacecraft requires a minimum
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brightness of 150 W/m to operate the spacecraft. What is the brightness of 4.0 × 10 W/m to operate the spacecraft. What is
maximum distance in meters this spacecraft can travel from the the maximum distance in meters this spacecraft can travel from
Sun and still operate? the Sun and still operate?
9. The brightness of the Sun as observed from Mars can be used to 9. The brightness of the Sun as observed from the Moon can be
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design a solar panel array at a colony on Mars. Mars is 2.3 × 10 km used to design a solar panel array at a colony on the Moon. The
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from the Sun. The array can convert energy from the brightness Moon is 1.5 × 10 km from the Sun. The array can convert
of the Sun into electricity at an efficiency factor of 40 percent. energy from the brightness of the Sun into electricity at an
The power produced by the array is the efficiency factor times efficiency factor of 40 percent. The power produced by the array
the product of the brightness (B) and the area of the array (A): is the efficiency factor times the product of the brightness (B)
P required = (Efficiency)BA. Ignoring atmospheric effects, what is and area of the array (A): P required = (Efficiency)BA. What is the
the necessary area of the solar panel array, in square meters, to necessary area of the solar panel array, in square meters, to
produce 10 kW? produce 35 kW?
10. What is the temperature of a star emitting light with a peak 10. What is the temperature of a star emitting light with a peak
wavelength of 6,550 angstroms? wavelength of 4,570 angstroms?
11. A star emits light with a peak wavelength of 6,050 angstroms. 11. A star emits light with a peak wavelength of 8,900 angstroms.
What type of star is this and what is its color? What type of star is this and what is its color?
12. What is the peak wavelength, in angstroms, emitted from a 12. What is the peak wavelength, in angstroms, emitted from a
type F star with a temperature of 7,000 K? type A star with a temperature of 9,200 K?
13. Based on interpretation of Figure 14.8, what is the peak 13. Based on interpretation of Figure 14.8, what is the peak
wavelength, in angstroms, emitted from Betelgeuse? wavelength, in angstroms, emitted from Arcturus?
14. In 1987 a star exploded in a supernova. Imaging from an orbiting 14. Imaging from an observatory documented the diameter of an
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observatory documented the diameter of the expanding gas expanding gas cloud to be 4.2 × 10 km in 1995 and 5.0 × 10 km
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cloud to be 8.8 × 10 km in 1999 and 1.4 × 10 km in 2008. in 2003. What is the rate of expansion of the supernova gas cloud
What is the rate of expansion of the supernova gas cloud in in kilometers per year?
kilometers per year?
15. The rate at which two galaxies are moving away from each other 15. The rate at which two galaxies are moving away from each other
can be determined from Hubble’s law (see “A Closer Look” on can be determined from Hubble’s law (see “A Closer Look” on
p. 365). This speed can be estimated by multiplying the distance p. 365). This speed can be estimated by multiplying the distance
between the galaxies by the Hubble constant of 20 km/s/Mly. The between the galaxies by the Hubble constant of 20 km/s/Mly. The
formula is v = H 0 d where H 0 is the Hubble constant and d is the formula is v = H 0 d where H 0 is the Hubble constant and d is the
distance between the galaxies. A galaxy is 2 Mly away from the distance between the galaxies. A galaxy is 10 Mly away from the
Milky Way. How fast is this galaxy moving away from the Milky Milky Way. How fast is this galaxy moving away from the Milky
Way in kilometers per second? Way in kilometers per second?

14-25 CHAPTER 14 The Universe 375