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A planetary nebula is
an astronomical object consisting of a glowing shell of gas and plasma
formed by certain types of stars at the end of their lives. They are in
fact unrelated to planets; the name originates from a supposed similarity
in appearance to giant planets. They are a short-lived phenomenon, lasting
a few tens of thousands of years, compared to a typical stellar lifetime
of several billion years. About 1,500 are known to exist in our galaxy.
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| The Cat's
Eye Nebula |
Planetary nebulae are important objects in astronomy because they play
a crucial role in the chemical evolution of the galaxy, returning material
to the interstellar medium which has been enriched in heavy elements and
other products of nucleosynthesis (such as carbon, nitrogen, oxygen and
calcium). In other galaxies, planetary nebulae may be the only objects
observable enough to yield useful information about chemical abundances.
In recent years, Hubble Space Telescope images have revealed many
planetary nebulae to have extremely complex and varied morphologies. About
a fifth are roughly spherical, but the majority are not spherically
symmetric. The mechanisms which produce such a wide variety of shapes and
features are not yet well understood, but binary central stars, stellar
winds and magnetic fields may all play a role.
Observation
Planetary nebulae are generally faint objects, and none are visible to
the naked eye. The first planetary nebula discovered was the Dumbbell
Nebula in the constellation of Vulpecula, observed by Charles Messier in
1764 and listed as M27 in his catalogue of nebulous objects. To early
observers with low-resolution telescopes, M27 and subsequently discovered
planetary nebulae somewhat resembled the gas giants, and William Herschel,
discoverer of Uranus, eventually coined the term 'planetary nebula' for
them, although, as we now know, they are very different from planets.
The nature of planetary nebulae was unknown until the first
spectroscopic observations were made in the mid-19th century. William
Huggins was one of the earliest astronomers to study the optical spectra
of astronomical objects, using a prism to disperse their light. His
observations of stars showed that their spectra consisted of a continuum
with many dark lines superimposed on them, and he later found that many
nebulous objects such as the Andromeda Nebula had spectra which were quite
similar to this – these nebulae were later shown to be galaxies.
However, when he looked at the Cat's Eye Nebula, he found a very different
spectrum. Rather than a strong continuum with absorption lines
superimposed, the Cat's Eye Nebula and other similar objects showed only a
small number of emission lines. The brightest of these was at a wavelength
of 500.7 nanometres, which did not correspond with a line of any known
element. At first it was hypothesised that the line might be due to an
unknown element, which was named nebulium - a similar idea had led to the
discovery of helium through analysis of the Sun's spectrum in 1868.
However, while helium was isolated on earth soon after its discovery in
the spectrum of the sun, nebulium was not. In the early 20th century Henry
Norris Russell proposed that rather than being a new element, the line at
500.7nm was due to a familiar element in unfamiliar conditions.
Physicists showed in the 1920s that in gas at extremely low densities,
electrons can populate excited metastable energy levels in atoms and ions
which at higher densities are rapidly de-excited by collisions. Electron
transitions from these levels in oxygen give rise to the 500.7nm line.
These spectral lines, which can only be seen in very low density gases,
are called forbidden lines. Spectroscopic observations thus showed that
nebulae were made of extremely rarefied gas.
As discussed further below, the central stars of planetary nebulae are
very hot. Their luminosity, though, is very low, implying that they must
be very small. Only once a star has exhausted all its nuclear fuel can it
collapse to such a small size, and so planetary nebulae came to be
understood as a final stage of stellar evolution. Spectroscopic
observations show that all planetary nebulae are expanding, and so the
idea arose that planetary nebulae were caused by a star's outer layers
being thrown into space at the end of its life.
Towards the end of the 20th century, technological improvements helped to
further the study of planetary nebulae. Space telescopes allowed
astronomers to study light emitted beyond the visible spectrum which is
not visible from ground-based observatories. Infrared and ultraviolet
studies of planetary nebulae allowed much more accurate determinations of
nebular temperatures, densities and abundances. CCD technology allowed
much fainter spectral lines to be measured accurately than had previously
been possible. The Hubble Space Telescope also showed that while many
nebulae appear to have simple and regular structures from the ground, the
very high optical resolution achievable by a telescope above the Earth's
atmosphere reveals extremely complex morphologies.
[Top]
Origins
Planetary nebulae are the end stage of stellar evolution for most
stars. Our Sun is a very average star, and only a small number of stars
weigh very much more than it. Stars weighing more than a few solar masses
will end their lives in a dramatic supernova explosion, but for the medium
and low mass stars, the end involves the creation of a planetary nebula.
A typical star weighing less than about twice the mass of the Sun spends
most of its lifetime shining as a result of nuclear fusion reactions
converting hydrogen to helium in its core. The energy released in the
fusion reactions prevents the star from collapsing under its own gravity,
and the star is stable.
After several billion years, the star runs out of hydrogen, and there is
no longer enough energy flowing out from the core to support the outer
layers of the star. The core thus contracts and heats up. Currently the
sun's core has a temperature of approximately 15 million K, but when it
runs out of hydrogen, the contraction of the core will cause the
temperature to rise to about 100 million K.
The outer layers of the star expand enormously because of the very high
temperature of the core, and become much cooler. The star becomes a red
giant. The core continues to contract and heat up, and when its
temperature reaches 100 million K, helium nuclei begin to fuse into carbon
and oxygen. The resumption of fusion reactions stops the core's
contraction. Helium burning soon forms an inert core of carbon and oxygen,
with a helium-burning shell surrounding it.
Helium fusion reactions are extremely temperature sensitive, with reaction
rates being proportional to T40. This means that just a 2% rise
in temperature more than doubles the reaction rate. This makes the star
very unstable - a small rise in temperature leads to a rapid rise in
reaction rates, which releases a lot of energy, increasing the temperature
further. The helium-burning layer rapidly expands and therefore cools,
which reduces the reaction rate again. Huge pulsations build up, which
eventually become large enough to throw off the whole stellar atmosphere
into space.
The ejected gases form a cloud of material around the now-exposed core of
the star. As more and more of the atmosphere moves away from the star,
deeper and deeper layers at higher and higher temperatures are exposed.
When the exposed surface reaches a temperature of about 30,000K, there are
enough ultraviolet photons being emitted to ionise the ejected atmosphere,
making it glow. The cloud has then become a planetary nebula.
[Top]
Lifetime
The gases of the planetary nebula drift away from the central star at
speeds of a few kilometres per second. At the same time as the gases are
expanding, the central star is cooling as it radiates away its energy -
fusion reactions have ceased, as the star is not heavy enough to generate
the core temperatures required for carbon and oxygen to fuse. Eventually
it will cool down so much that it doesn't give off enough ultraviolet
radiation to ionise the increasingly distant gas cloud. The star becomes a
white dwarf, and the gas cloud recombines, becoming invisible. For a
typical planetary nebula, about 10,000 years will pass between its
formation and recombination.
[Top]
Galactic Recyclers
Planetary nebulae play a very important role in galactic evolution. The
early universe consisted almost entirely of hydrogen and helium, but stars
create heavier elements via nuclear fusion. The gases of planetary nebulae
thus contain a large proportion of elements such as carbon, nitrogen and
oxygen, and as they expand and merge into the interstellar medium, they
enrich it with these heavy elements, collectively known as metals by
astronomers.
Subsequent generations of stars which form will then have a higher initial
content of heavier elements. Even though the heavy elements will still be
a very small component of the star, they have a marked effect on its
evolution. Stars which formed very early in the universe and contain small
quantities of heavy elements are known as Population II stars, while
younger stars with higher heavy element content are known as Population I
stars
[Top]
Characteristics
A typical planetary nebula is roughly one light year across, and
consists of extremely rarefied gas, with a density generally around 1000
particles per cm³ - which is about a million billion billion times less
dense than the earth's atmosphere. Young planetary nebulae have the
highest densities, sometimes as high as 106 particles per cm³. As nebulae
age, their expansion causes their density to decrease.
Radiation from the central star heats the gases to temperatures of about
10,000K. Counterintuitively, the gas temperature is often seen to rise at
increasing distances from the central star. This is because the more
energetic a photon, the less likely it is to be absorbed, and so the less
energetic photons tend to be the first to be absorbed. In the outer
regions of the nebula, most lower energy photons have already been
absorbed, and the high energy photons remaining give rise to higher
temperatures.
Nebulae may be described as matter bounded or radiation bounded. In the
former case, there is so much matter around the star that all the UV
photons emitted are absorbed, and the visible nebula is surrounded by a
shell of un-ionised gas—hence the radiation is "bounded" by the matter. In
the latter case there are enough UV photons being emitted by the central
star to ionise all the surrounding gas.
Because most of the gas in a typical planetary nebula is ionised (ie. a
plasma), the effects of magnetic fields can be significant, giving rise to
such phenomena as filamentation, plasma double layers, synchrotron
radiation, and plasma instabilities.
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