- The Envelope of the stars is only loosely bound because the
AGB star has a large radius. During the shell flashes the envelope
is subjected to bursts of photons and energy.
With some complications glossed over, the envelope and as much as 50%
of the stellar mass is detached from the star and expelled
into space leaving the AGB star very hot core exposed.
- This is called a Planetary Nebula.
Note that the name is a misnomer. There are several PN that
can be barely resolved with the naked eye as is the case for
the brighter planets. So these objects do not "twinkle" like
the stars and they (PN and planets) got lumped together.
- The core of the former AGB star is very hot - the
surface temperature can be 150,000K. It is supported against gravity
by e- degeneracy and has a very high density of 1000 kg/cm3
or about 1 ton per thimblefull. We will come back to this object
in a little bit.
- The high temperature of the "central star" (it is not
REALLY a star as there is no fusion energy source)
means it has a Planck curve that peaks way out in the UV
and produces many UV and every soft X-ray photons. These
collide with the H, He, C and O atoms in the former envelope
that we now call a PN. These atoms get ionized, and on recombination
the e- drop through the energy levels giving off various
lower energy photons (that add up in energy to the original
UV or X-ray ionizing photon) as they head for the ground state.
- This is a lot like a fluorescent light
- What kind of spectrum do we expect to see
from a Planetary Nebula?
Lots of emission lines from the e- cascades
- The Halpha line is usually strong which gives some PN a
reddish color. There is also a strong line in the green
part of the spectrum that was not recognized from studies
of terrestrial sources and originally attributed to a new
element called "nebulium". This was later recognized as
the emission from a "forbidden" transition of doubly ionized
Oxygen.
Some electron orbits are "meta-stable" and rather than immediately
de-exciting via emission of a photon from these starts the
e- sit in these particular excited states for a long time before
dropping down and emitting a photon. On Earth, before the
"forbidden" photon can be emitted, the atom is collisionally
de-excited. It is only because the density in the PN is
very low that an atom can float around long enough in the excited state
for a photon-emission de-excitation to occur.
- PN often appear as a ring - why?
It is a geometrical effect.
- PN play an important role in the Chemical Enrichment
of the Galaxy.
In the RGB and AGB phases of evolution
the convection zone of stars extends deep into the center
of the star and the freshly minted He, Carbon and Oxygen
products of fusion reactions are mixed up to the surface
of stars. PN envelopes are therefore composed partly of
"new" elements.
- PN expansion velocities are around 20 km/sec and
after around 30,000 years the envelopes are so thin that
they become invisible. These gas has become a part of the interstellar
medium to be later incorporated into new stars.
PN return between 0.05 and 0.6 MSun of partly enriched
material back to the ISM.
- In the Galaxy there are around 30,000 PN.
- Planetary Nebulae ejection probably occurs only for stars with M < 3MSun
For these stars it never gets hot enough in the core to
have Carbon or Oxygen fusion occur. So the "central star"
of PN is not a star at all.
