THE PLANETARY NEBULAE

Strip

From Jim Kaler's STARS

In which Hubble images are compared with century-old observations.

Latest entry: NGC 6309.

Planetary nebulae, some of the loveliest objects in the sky, are complex shells of gas that have been ejected by dying advanced giant stars like Mira. They were discovered as a set and given their name by William Herschel in the late 1700s. The story is given in the text for the "first planetary," NGC 7009. See the table that follows this introduction.

The nebulae, which can be over a light year across, are ionized and made to glow by ultraviolet radiation from their central stars, which are the stripped-down old nuclear-burning cores of stars that were once much like the Sun. In effect, they are the precursors of dense white dwarfs that are basically balls of carbon and oxygen, the result of aeons of nuclear fusion. When the outer husks of the giants are finally gone via strong winds, the old cores first heat at constant luminosity (which can be thousands of times that of the Sun) to temperatures that can hit 200,000 Kelvin. See the comments that follow the table below. After nuclear fusion shuts down and most of the old hydrogen envelope is stripped away, they then cool and dim as they head for the white dwarf realm on the HR diagram. Indeed, the cooling stars are in effect already white dwarfs. All the nebulae are expanding with typical speeds of 20 -30 kilometers per second. Eventually the nebulae, some loaded with the by-products of nuclear fusion, dissipate into space (to be come fodder for new stars), leaving the lonely white dwarfs behind. Beautiful but ephemeral, the whole show is over in under 10,000 years.

The nebulae are shaped by hot winds from the stars ramming into mass lost while the stars were advanced giants. The intricate structures can be exceedingly complex, the result of double star action, rotation, the effects of stellar magnetic fields, or other causes: no one really knows.

The pages here present a contrasting picture between two great sets of planetary images made nearly a century apart: those brought to us by Heber Doust Curtis in the Publications of the Lick Observatory, Volume 13, Part III, 1918, and those made using the Hubble Space Telescope. The first set, the first extensive compendium, was observed with Lick Observatory's Crossly Reflector. It includes all then-known objects north of 34 degrees south declination and us gives a sense of the visual view through the telescope. Some of them are photographic images, while others are composite drawings made from photos of different exposures so as to reproduce a great dynamic range, which was impossible with the photographic emulsions of the time. The Curtis images have north always to the top; the Hubble images are then brought into the best alignment.

While the Hubble set reveals both the true natures and the beauty of these intricate and extraordinary structures, it also demonstrates the high quality and accuracy of the work of the distant past. These pages are meant to honor both. Many thanks to Lick Observatory for permission to reproduce the images and to the observers and technicians at STScI.

THE NEBULAE

The table presents, in order:
CATALOGUE POPULAR NAME CONSTELLATION FEATURES
NGC 2022 ... Orion Double shell with large outer envelope
NGC 2371-2 ... Gemini Prominent lobes with separate names; outer halo; jets
NGC 2392 Eskimo Gemini Intricate double shell; one of the brightest central stars.
NGC 2440 ... Puppis Very hot star; chemically enriched; local dust?
NGC 3242 Ghost of Jupiter Hydra Double ring; once thought to be "non-thermal"; jets
NGC 6210 ... Hercules Bizarre shape; two sets of jets
NGC 6309 Box Nebula Ophiuchus Complex looping structure with outer envelope
NGC 6369 Little Ghost Ophiuchus Faint, heavily-dimmed ring; large outer structure
NGC 6537 Red Spider Sagittarius Extraordinary, huge hourglass flow with small core; extremely hot central star
NGC 6543 Cat's Eye Draco Very intricate shells with huge envelope; first central star observed; first observed spectroscopically; jets
NGC 6572 Blue Radquetball Ophiuchus Compact and very bright with intricate internal detail
NGC 6578 ... Sagittarius Small, double shell; heavily obscured
NGC 6720 Ring Nebula Lyra Messier 57; probably the most famed of all; see location
NGC 6741 Phantom Streak Aquila Compact oval; high temperature nucleus
NGC 6751 ... Aquila Irregular ring with radial fingers
NGC 6790 ... Aquila Vrey small, "stellar"; probable low mass
NGC 6818 Little Gem Sagittarius Oval with outside ring and anomalous central star
NGC 6826 Blinking Nebula Cygnus Complex inner structure inside a giant halo.
NGC 6853 Dumbbell Vulpecula Messier 27; among the largest and brightest; see location
NGC 6881 ... Cygnus Bright core with huge extended "wings"; uncertain star brightness
NGC 6884 ... Cygnus Twisted inner ring; central star barely detectable
NGC 6886 ... Sagitta Hot central star, near temperature turnaround, unseen against bright background
NGC 7009 Saturn Nebula Aquarius Herschel's discovery object; prominent ansae
NGC 7027 ... Cygnus One of most studied; local dust distorts visual view; very hot star; carbon rich
NGC 7293 Helix Aquarius Closest classical nebula; rings made of dense filaments
NGC 7662 Blue Snowball Andromeda Classic double-ring nebula
IC 418 Spirograph Lepus Low excitation, "cool" star; interior ring
IC 3568 Lemon Slice Camelopardalis Oddly smooth and round; interior shell
IC 4593 ... Hercules Low excitation; multiple jets; giant halo
IC 4997 ... Sagitta Stellar; young; changing spectrum and structure

Detailed Comments

The nebulae are roughly 90 percent hydrogen, 10 percent helium (though the helium can be enriched by nuclear processes in the predecessor star). They are ionized by ultraviolet light from the hot central stars, which must be hotter than about 26,000 Kelvin to have sufficiently energetic photons to ionize the dominant hydrogen. Recombination of electrons and protons, plus collisional excitation of other atoms by energetic free electrons, then produces the nebular radiation, which consists almost entirely of emission lines. (Cooler stars inhabit dusty planetaries-to-be called proto-planetary nebulae.) The central stars heat the nebular gas to temperatures of around 10,000 Kelvin (which reflects only the velocities of the atoms and free electrons in the gas, and not the nebula's radiant power) with a range between 8000 and 20,000 Kelvin. While the hydrogen emissions are strong, particularly the H-alpha line in the red part of the spectrum, most nebulae are dominated by the collisionally-produced lines of doubly ionized oxygen first seen by William Huggins in NGC 6543 and not identified as such until 1928.

As bright as many seem to be, the nebulae are very rarefied. Densities range from a maximum of only a million atoms and electrons per cubic centimeter for the most compact objects down to under 10 for the huge ones that are dissipating into interstellar space. By contrast, the air you breathe has roughly 10**19 (1 followed by 19 zeros) molecules per cubic centimeter. The nebular gas would in fact make an excellent "vacuum" in the physics lab. Were you inside a planetary nebula, you would not know it. Nebular temperatures, densities, and chemical compositions are all found from ratios of the emission lines. Much the same can be said for "diffuse nebulae" such as the Orion Nebula

The hotter the star, the higher the level of ionization. Central star temperatures are commonly calculated by using the intensity of nebular emission lines to estimate the amount of ultraviolet radiation from the star and then comparing that to the amount of visual radiation derived from the visual magnitude. Such magnitudes can be difficult to measure because of the brightness of the nebulae. As the stars become hotter, more and more radiation is shifted into the ultraviolet, the nebulae brighten while the visual light fades, and the stars become nearly impossible to see against the background. Star temperatures can also be derived through the nebular emission lines alone.

The biggest problem with planetary nebulae lies in their distances, which are commonly very difficult to estimate. Many essentially have no known values. Only a handful is close enough for standard parallax. A small number of distances are also derived by comparing the angular expansions with the measured expansion velocities, and a few are also found from the degree of interstellar dust absorption. The fallback is to derive statistical distances based on the clearly incorrect assumption that all nebulae have common parameters (ionized masses etc). Almost anything goes. Stellar luminosities, which depend on the distances, are then also typically very uncertain as well.

Masses are uncertain. The average of the ionized nebulae seems to be around a couple tenths of a solar mass, but the range around that must be enormous. Young nebulae like IC 418 are at the low end, as the ionizing ultraviolet light from the central star is still working its way into the mass lost by the predecessor giant. Younger objects are thus surrounded by huge amounts of neutral molecular dusty gas. In older nebulae, the stellar ultraviolet has reached the outer boundary of the dense portion of the nebula, and we see more or less the whole thing. Nevertheless, many planetaries, even the older ones, are surrounded by huge outer shells created by early giant-star mass loss. Given that solar-type stars lose half of themselves back into space and more massive stars as much as 80 percent of more, the complete planetary structure can contain far more than a solar mass.

As a set, the planetary nebulae represent the transition state between the immense advanced giant stars (and Mira variables) and the tiny white dwarfs, which are the giants' old nuclear burning cores. The shapes of the nebulae help us to understand how giant stars lose their outer envelopes through their powerful winds (which can be millions of times stronger than the wind from the Sun). The nebulae are commonly enriched in helium, nitrogen, carbon, and other elements (as by-products of nuclear fusion), and thus give us a chance of probing how stars enrich their outer envelopes and thereby enrich interstellar space with new elements for use in later stellar generations. Generally, beyond a critical threshold that seems to fall around 0.65 solar masses, the higher the mass of the central star (which is the old nuclear-burning core of a star that started with a much larger mass) the greater the enrichment.

Return to STARS.

Learn and See More

Planetary Nebulae and the Future of the Solar System
A Planetary Nebula Sampler
Gallery of Planetary Nebula Spectra
Gallery of Planetary Nebula Images
Planetary Nebulae
Valid HTML 4.0! Copyright © James B. Kaler. All rights reserved. Unless otherwise indicated, the text is the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. Opening image: Hubble view of NGC 6543. Last updated 27 June, 2012. Thanks to reader number drupal statistics.