HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
RQ2: What is the difference between an isotope and an ion?
Isotopes have different numbers
of neutrons, but the same
number of protons.
Ions have different numbers of
electrons. In astronomy, it’s
usually an atom with electrons
stripped off.
HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
RQ3: Why is the binding energy of an electron related to the
size of its orbit?
Electrons are
attracted to to the
protons in the
nucleus. The ones
that are closer to the
nucleus are more
strongly attracted and
therefore more tightly
bound to the nucleus.
HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
RQ4: Explain why ionized calcium can form absorption lines,
but ionized hydrogen cannot.
Absorption happens when an
electron at a lower energy level
absorbs a photon and jumps up to a
higher energy level. It requires the
atom to have electrons. Since
hydrogen has only one electron, if it
is ionized then it won’t have any
electrons, so it cannot form
absorption lines.
HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
RQ5: Describe two ways at atom can become excited.
Absorption of a photon: the energy of the photon goes into the
electron and pushes it up to a higher energy level.
Thermally: two atoms collide and some of the energy of the
collision goes into pushing an electron into a higher energy
level.
HW 23 Answers
Read §7-2, §7-3 and §7-4; (158) RQ 4-8
RQ6: Why do different atoms have different lines in their spectra?
Electrons in an atom can only absorb or emit photons which have
energies that exactly match the energy differences in the atomic
energy levels. Each atom has its own unique set of energy
levels.
HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
(a) What are appropriate units for B–V ? Magnitudes (unitless)
(b) For stars A through G, find their color index and temperature using
the given B and V values and the graph on the back of the homework
sheet.
Star B
V
Color index
Temperature
B–V
A
3.5
3.3
0.2
7900 K
B
14.2
14.2
0.0
8900 K
C
7.1
5.9
1.2
4200 K
D
10.4
10.45
–0.05
9400 K
E
0.1
0.2
–0.1
11,200 K
F
4.7
3.9
0.8
5400 K
G
11.9
10.8
1.1
4500 K
7
HW 5-2 Answers
(140) RQ 2-6; Supplemental Questions 3 and 4
Supp 4: What kind of spectrum would you expect to record if you
observed molten lava (through a spectrograph)?
Molten lava glows because it is hot, so it produces blackbody
radiation, which has a continuous spectrum (full rainbow).
In practice, to view molten lava you must look through gases
boiling out of the lava. What kind of spectrum might you see in
that case?
The gasses will tend
to absorb certain
wavelengths and so
you’ll see an
absorption
spectrum.