CHAPTER 7 | ATOMS AND STARLIGHT 141
- If a star has a surface temperature of 20,000 K (2.00 104 K), at
what wavelength will it radiate the most energy? - Infrared observations of a star show that it is most intense at a
wavelength of 2000 nm (2.00 103 nm). What is the temperature of
the star’s surface? - If you double the temperature of a blackbody, by what factor will the
total energy radiated per second per square meter increase? - If one star has a temperature of 6000 K and another star has a
temperature of 7000 K, how much more energy per second will the
hotter star radiate from each square meter of its surface? - Electron orbit transition A produces light with a wavelength of
500 nm. Transition B involves twice as much energy as A. What
wavelength light does it produce? - Determine the temperatures of the following stars based on their
spectra. Use Figure 7-7c.
a. medium-strength Balmer lines, strong helium lines
b. medium-strength Balmer lines, weak ionized-calcium lines
c. strong TiO bands
d. very weak Balmer lines, strong ionized-calcium lines - To which spectral classes do the stars in Problem 7 belong?
- In a laboratory, the Balmer beta line has a wavelength of 486.1 nm.
If the line appears in a star’s spectrum at 486.3 nm, what is the star’s
radial velocity? Is it approaching or receding? - The highest-velocity stars an astronomer might observe have velocities
of about 400 km/s (4.00 102 km/s). What change in wavelength
would this cause in the Balmer gamma line? (Hint: Wavelengths are
given on page 133.)
Learning to Look
- Consider Figure 7-3. When an electron in a hydrogen atom moves from
the third orbit to the second orbit, the atom emits a Balmer-alpha
photon in the red part of the spectrum. In what part of the spectrum
would you look to fi nd the photon emitted when an electron in a
helium atom makes the same transition? - Where should the police car in Figure 7-12 have parked to make a good
measurement? - The nebula shown below contains mostly hydrogen excited to emit
photons. What kind of spectrum would you expect this nebula to
produce? - If the nebula in the image here crosses in front of the star, and the
nebula and star have different radial velocities, what might the spec-
trum of the star look like?
sequence. By classifying a star, the astronomer learns the temperature
of the star’s surface.▶ (^) Long after the spectral sequence was created, astronomers found the
L dwarfs (p. 135) and T dwarfs (p. 135) at temperatures even cooler
than the M stars.
▶ (^) A spectrum can tell you the chemical composition of the stars. The
presence of spectral lines of a certain element shows that that element
must be present in the star, but you must proceed with care. Lines of a
certain element may be weak or absent if the star is too hot or too cool
even if that element is present in the star’s atmosphere.
▶ (^) The Doppler effect (p. 137) can provide clues to the motions of the
stars. When a star is approaching, you observe slightly shorter wave-
lengths, a blueshift (p. 138), and when it is receding, you observe
slightly longer wavelengths, a redshift (p. 138). This Doppler effect
reveals a star’s radial velocity, Vr (p. 139), that part of its velocity
directed toward or away from Earth.
Review Questions
- Why might you say that atoms are mostly empty space?
- What is the difference between an isotope and an ion?
- Why is the binding energy of an electron related to the size of its orbit?
- Explain why ionized calcium can form absorption lines, but ionized
hydrogen cannot. - Describe two ways an atom can become excited.
- Why do different atoms have different lines in their spectra?
- Why does the amount of blackbody radiation emitted depend on the
temperature of the object? - Why do hot stars look bluer than cool stars?
- What kind of spectrum does a neon sign produce?
- Why are Balmer lines strong in the spectra of medium-temperature
stars and weak in the spectra of hot and cool stars? - Why are titanium oxide features visible in the spectra of only the
coolest stars? - Explain the similarities among Table 7-1, Figure 7-7c, Figure 7-8, and
Figure 7-9. - Explain why the presence of spectral lines of a given element in the
solar spectrum tells you that element is present in the sun, but the
absence of the lines would not necessarily mean the element is absent
from the sun. - Why does the Doppler effect detect only radial velocity?
- How can the Doppler effect explain shifts in both light and sound?
- How Do We Know? How is the world you see around you determined
by a world you cannot see?
Discussion Questions
- In what ways is the model of an atom a scientifi c model? In what ways
is it incorrect? - Can you think of classifi cation systems used to simplify what would
otherwise be complex measurements? Consider foods, movies, cars,
grades, and clothes.
Problems
- Human body temperature is about 310 K (3.10 102 K, or 98.6°F).
At what wavelength do humans radiate the most energy? What kind of
radiation do we emit?
T. Rector, University of Alaska, and
WIYN/NURO/AURA/NSF