Detecting Interstellar Reddening
The Balmer Decrement
In the Bohr
model of the hydrogen atom there are many distinct energy levels,
between which electrons can transfer if they emit or absorb the proper
amount of energy. Upward moves require absorption of energy, while
downward ones release energy. Downward electron transitions that end
on the second energy level are called the Balmer
series, and are important in optical astronomy, since these are
the only transitions that involve visible light. The first three of
these are called H
,
H
, and H
,
for the transitions from 3-2,
4-2, and 5-2, respectively. When many ionized hydrogen atoms are
recombining, as in a planetary nebula where atoms are being ionized
and recombining all the time, the captured electrons cascade down
through the energy levels, emitting photons of the appropriate
wavelengths as they fall. The likelihood of any particular downward
jump is dictated by atomic constants, and thus the ratios of all
possible transitions can be calculated. This leads to the "Balmer
decrement," the well known ratios among the intensities of the Balmer
lines, where H is the
strongest line, H
is
weaker, H is weaker still,
and so on. Under typical conditions in planetary nebulae these ratios
are (from Osterbrock, Astrophysics of Planetary Nebulae and Active
Galactic Nuclei, University Science Books, 1989):
H/H =
2.87 and H/H =
0.47
The Phenomenon of Interstellar Reddening
Thus, the Balmer decrement, the intensity ratios of Balmer lines in
all planetary nebulae, should be roughly the same. However, this is
not what is observed. Interstellar
reddening produced by micron-sized dust particles selectively dims
shorter-wavelength, bluer light more than it does longer-wavelength,
redder light, leading to Balmer line ratios that differ systematically
from the theoretical predictions. A planetary nebula lying behind a
cloud of interstellar dust will be observed to have the intensity
ratios H/H more
than 2.87, and
H/H less
than 0.47. The
more dust, the greater the disparity between the observed and
theoretical Balmer decrements. Turning this concept around, from the
size of the discrepancy between observed and theoretical Balmer
decrements, astronomers can infer the amount of interstellar
reddening, and therefore, dust, between us and a given planetary
nebula.