THE PLANETARY NEBULAE
In which Hubble images are
compared with Curtis's historic century-old set of Lick
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 collective 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, the central stars 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
The pages here present a contrasting picture between two
great sets of planetary images made 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 nebulae north of 34 degrees south declination and us gives a sense of the
visual view through the telescope. Some of the old pictures are
photographic, 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 table presents, in order:
- NGC (New General Catalogue) number followed by IC (Index
- 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
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
Gallery of Planetary Nebula Spectra
Gallery of Planetary Nebula Images
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 16 December, 2013. Thanks to reader number .