Planetary nebula

photo: NGC 6853,The Dumbbell Nebula

A planetary nebula is an emission nebula consisting of a glowing shell of gas and plasma formed by certain types of stars when they die. The name originated in the 18th century because of their similarity in appearance to giant planets when viewed through small optical telescopes, and is unrelated to the planets of the solar system.They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

At the end of the star's life, during the red giant phase, the outer layers of the star are expelled via pulsations and strong stellar winds. Without these opaque layers, the remaining core of the star shines brightly and is very hot. The ultraviolet radiation emitted by this core ionises the ejected outer layers of the star which radiate as a planetary 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.

Observations

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 (as it was then known) 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 hypothesized 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 500.7 nm 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 ion (O2+ or OIII) give rise to the 500.7 nm 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 detectable from ground-based observatories (because only radio waves and visible light penetrate the earth's atmosphere). 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.

Under the Morgan-Keenan spectral classification scheme, planetary nebulae are classified as Type-P, although this notation is seldom used in practice.

Origins

Stars weighing more than 8 solar masses will likely end their lives in a dramatic supernova explosion, but for medium and low mass stars on the order of a solar mass, such as our Sun, the end may involve the creation of a planetary nebula.

Stars that inevitably become planetary nebulae spend most of their 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 both a helium-burning shell and a hydrogen-burning shell surrounding it. In this last stage the star will observationally be a red giant and structurally an asymptotic giant branch star.

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 ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.

Lifetime

The gases of the planetary nebula drift away from the central star at speeds of a few kilometers per second. At the same time as the gases are expanding, the central star undergoes a two stage evolution first growing hotter as it continues to contract and hydrogen fusion reactions are occurring in a shell around the core of carbon and oxygen and then cooling as it radiates away its energy and fusion reactions have ceased, as the star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. During the first phase the central star gets hotter eventually reaching temperatures around 100,000K. Eventually it will cool down so much that it doesn't give off enough ultraviolet radiation to ionize 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 of the star.

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.

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. (The Earth's atmosphere, by comparison, contains 2.5×1019 particles per cm³.) 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 10,000 K. 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 not enough matter in the nebula to absorb all the UV photons emitted by the star, and the visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionise all the surrounding gas, and an ionization front propagates outward into the circumstellar neutral envelope.

Because most of the gas in a typical planetary nebula is ionised (i.e. a plasma), the effects of magnetic fields can be significant, giving rise to phenomena such as filamentation and plasma instabilities.About 3000 planetary nebulae are now known to exist in our galaxy.out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near the plane of the Milky Way, with the greatest concentration near the galactic center. Planetary nebulae have been detected as members in only four globular clusters: M 15, M 22, NGC 6441 and Palomar 6. However, there has yet to be an established case of a planetary nebula discovered in an open cluster.Only about 20% of planetary nebulae are spherically symmetric. A wide variety of shapes exist with some very complex forms seen. The reason for the huge variety of shapes is not fully understood, but may be caused by gravitational interactions with companion stars if the central stars are double stars. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesised that the fields might be partly or wholly responsible for their remarkable shapes.