The internal state of an atom can change by absorbing or emitting bits
of light. In a warm gas or plasma the electrons are frequently shuttling
back and forth from one state to another.
Some of these states are longer lived than others, though, because
of extenuating circumstances. For instance, many transitions from an
excited state to the ground state occur in nanoseconds, but some can
last for tens of seconds or longer.
Measuring the true lifetime of the longer-lived of these transitions
is difficult for the simple reason that even when a sample of atoms
is dilute, an atom is being bumped so often that de-excitations come
about before the state decays radiatively.
When even the best laboratory vacuum on Earth is still too crowded
for making such delicate measurements, persistent scientists turn to
outer space. Tomas Brage of Lund University (Lund, Sweden), Philip Judge
of the High Altitude Observatory at NCAR (Boulder, CO), and Charles
Proffitt of the Computer Science Corporation (Baltimore, MD) resort
to viewing excited atoms in the planetary nebula NGC3918 where, amid
the wreckage of a dying star, there is enough energy to excite atoms
but a density low enough (a few 1000 per cubic centimeter) that mutual
pumping isn't a problem (see figure).
Using the Hubble Space Telescope, the three scientists looked at the
emissions of excited triply ionized nitrogen atoms and observed a lifetime
of 2500 seconds for one particular hyperfine transition.
Why is this state so robust? Brage
(firstname.lastname@example.org, 46-46-222-7724) says that angular momentum
can be preserved in this transition only if, in addition to the electron
emitting an ultraviolet photon, the nucleus itself flips over. Other
than adding to basic knowledge about atomic physics, studies like these
should provide spectroscopic information for studying the deaths of
et al., Physical Review Letters, 31 December 2002.)