THE PLANETARY NEBULAE
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.
Watch these pages grow!
The table presents, in order:
- NGC (New General Catalogue) number followed by IC (Index
Catalogue);
- Popular Name (if any);
- Constellation of residence
- Significant features including Messier (M) number (M 27 and M 57).
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.
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
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 15 March,
2009. Thanks to reader number  .
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