“Space-the
final frontier. These are the voyages of the Starship Enterprise. Our mission:
to boldly go where no-one has gone before.” –Captain Jean-Luc Picard
In its own
way, this has been the mission statement of human kind since the
beginning. At first, “space” was the far
bank of the river, then the other side of the mountain, then the
ocean…..eventually the heavens.
There are
many things out there in space, one of which is a phenomenon called a planetary
nebula (nebulae for plural).
What Is This Thing?
Planetary
nebulae are what forms when a star no more than four times the size of the Sun
comes to the end of its life-cycle. The star’s internal gravity becomes so
intense that the star explodes, blowing the outer layers into space, which are
then lit up by the heat of the star’s core. Due to that intense heat, these
nebulae do not last long at all, perhaps only about 50,000 years. Scientists believe that our sun may grow and
go through this process in about 5 million years.
It is
predicted that there are approximately 10,000 planetary nebulae now in the
Milky Way, and 2,000 have been discovered. However, only four have been
officially catalogued: the Dumbbell, Ring, Little Dumbbell, and Owl Nebulae.
The last of these-the Owl Nebula-was discovered in 1751.
Despite its
name, planetary nebulae have nothing to do with planets. The name was coined in
the 1780’s by William Hershel, because it reminded him of the rings of Uranus.
What Does It Look Like?
Planetary
nebulae are fairly small-only about 1 light year in diameter, although there is
a range of sizes as the nebula ages. NGC 3918, for instance, is only 3/10 light
year in diameter, while the Helix Nebula is over 2.5 light years. NGC 3918 is
calculated to be only 3,000 years old, so it is still fairly young.
Nebulae are
quite hot (as expected, being a former star). The temperature range is 8,000 to
23,000 K, with an average of around 10,000 K.
It is also
thought that planetary nebulae only last about 50,000 years. Since most expand
at 20-30 km/s, after that time the density of particles are too spread out to
be seen.
The
Vorontsov-Velyaminov Scheme
In 1934, Russian
astrophysicist Boris Vorontsov-Velyaminov came up with this way of categorizing
planetary nebulae based on their morphology. There are six styles:
- Stellar image (I).
- Smooth disk (II) (a,
brighter toward center; b, uniform brightness; c, traces of
a ring structure).
- Irregular disk (III) (a,
very irregular brightness distribution; b, traces of ring
structure).
- Ring structure (IV).
- Irregular form, similar to a
diffuse nebula (V).
- Anomalous form (VI).
There also
can be combinations. For instance, there can be two rings (a “4 + 4”).
With all
these kinds of nebula, who knows what else is out there. Will astronomers and
their colleagues in space science ever find a planetary nebula that actually
has a planet? Probably not, but it is fun to think about. As Captain Picard
says, “Make it so.”
