Planetary Nebulae: What Are They?
“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 one 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.”